Section D

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SECTION D

Animal Growth and Meat Production

Section D

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CONTENTS

SECTION D: ANIMAL GROWTH AND MEAT PRODUCTION

Page

D.1 INTRODUCTION ...................................................................................... 1

D.2 GROWTH CHARACTERISTICS ............................................................... 6

D.3 BODY AND CARCASS COMPOSITION CHARACTERISTICS ................ 14

D.4 MEAT QUALITY CHARACTERISTICS .................................................. 24

D.5 FACTORS AFFECTING CHARACTERISTICS OF IMPORTANCE

TO MEAT PRODUCTION ...................................................................... 30

D.6 CARCASS CLASSIFICATION ............................................................... 44

D.7 CONCLUDING COMMENTS FOR SECTION D ...................................... 47

D.8 REVIEW ITEMS AND QUESTIONS ....................................................... 48

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SECTION D ANIMAL GROWTH AND MEAT PRODUCTION

D.1 INTRODUCTION

The topics of meat production and animal growth are considered together

because the former is very dependent on the latter, but it is important to

appreciate that animal growth patterns and processes are also very important to

most other forms of animal production (e.g. milk and fibre production), a point that

is made in a number of other sections of this Study Guide. In this section animal

growth and meat production will be considered together by first identifying and

explaining characteristics of importance for successful and profitable meat

production, and then by considering in broad terms how certain factors affect

those characteristics.

Some important features of a generalised system of meat production are

illustrated as a flow diagram in Figure D.1. This diagram divides the overall

process into four sections or levels from top to bottom, and within each level a

selection of key productive characteristics are shown in bold on the left side of

the diagram. In addition selected inputs in addition to the meat animal are

shown as feeding into the main flow from the right, and outputs other than meat

are shown as arrows leading out from the main flow to the left. Points to note

about Figure D.1 include the following:

At the population level the important characteristics from the meat production point of view are the genotypes and phenotypes of the parents of the animals destined for meat production and the fecundity of those parents in terms of the number of offspring produced. If more offspring are produced per parent per year, then fewer parents will be needed to produce the same number of meat-producing animals and the breeding-animal overheads associated with each kilogram of meat produced will be lower, other things being equal. Often other things are not equal, however, so that in assessing the value of increased fecundity to meat production effectiveness, it is necessary to take into account the tendency for higher peri-natal losses and lower pre-weaning growth rates associated with higher lambing percentages, for example. Fecundity and other aspects of fertility as important productive characteristics are covered in Section A of 17.254.

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A particularly important question at the population level concerns the extent to which the productive characteristics of the offspring are determined by the genetic material received from their parents. Heritability estimates provide a measure of this extent, and typical examples of heritability estimates for a number of the characteristics to be discussed below were given in Section C (Table C.7).

Figure D.1 A flow-diagram depicting a generalised meat-production

system

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The individual-animal-level section within Figure D.1 covers the period from when the animal is born through to the production of a carcass following normal dressing procedures at a meat plant. As such, this section includes all the post-natal growth of the animal and the associated growth characteristics, the most important of which are the rate of growth (the amount of weight gained per day), the efficiency of growth (the amount of weight gained per unit of feed consumed), the composition of the growth in terms of the main tissues and organs, and the extent of growth (usually expressed in terms of a mature weight). These are discussed in Section D.2 except for the composition of growth which is covered in Section D.3.

The only input specified at the individual animal level in Figure D.1 is feed.

It is the relationship between the amount of this input and the output in terms of animal growth that is expressed in various ways as the efficiency of growth.

Outputs shown at the individual animal level include not only the carcass

on the main flow path, but also the non-carcass components that are removed during dressing (skin, head, feet, and a range of internal organs and structures that make up the viscera), and other products that are produced during the life of the animal. These will include items such as wool (or other fibres), milk, and velvet, and will not be considered further in this section.

There are five performance indicators listed within the individual-animal-

level section of Figure D.1, with the first four of these referring to two of the key growth characteristics of growth rate and growth efficiency as they apply during the pre-weaning and post-weaning periods. Aspects of these two characteristics will be covered in Section D.2. The fifth characteristic is dressing-out percent which is included as a composition characteristic (Section D.3).

The carcass-level section in Figure D.1 is concerned with the yield of

saleable meat from the carcass, with the key performance indicator being expressed as the yield of saleable meat as a percentage of carcass weight or saleable meat yield (SMY%). The outputs other than saleable meat from the carcass are bones and excess fat, so it follows that one carcass may have a lower SMY% than another either because it contains more bone relative to saleable meat, and/or because it contains a higher proportion of excess fat. Although the SMY% of a carcass is probably its most important composition characteristic, there are other aspects of carcass composition that are also of commercial importance, and these will be discussed in Section D.3.

The final section in Figure D.1 is at the consumer level. This can be

considered the most important level in many respects, as there is little point in producing large quantities of saleable meat if its value to consumers is not sufficient to make the whole meat-production exercise profitable.

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At the consumer level the two steps shown in Figure D.1 encompass, first, the yield of meat consumed relative to the amount purchased, and secondly the conversion from the weight of the product to the value of the product. The first step is concerned with losses during cooking (drip loss and water loss), and the losses due the removal of some parts prior to consumption (primarily excess fat and bone). The second step is much more complicated as it encompasses all those factors that influence the price per unit of meat that is consumed, and in Figure D.1 these are encompassed in the term meat quality. Aspects of meat quality, and of some of the wide range of characteristics that contribute to it are considered in Section D.4.

In summary this introductory section has set out the main features of a meat-

production system as a flow diagram, and has used this diagram to identify

groups of key characteristics that will influence the overall effectiveness of the

system. Some of these characteristics have been covered in other sections of

this paper, and those to be considered in the remainder of this section are done

so within three categories and subsections as follows:

D.2 Growth characteristics

D.3 Carcass and body composition characteristics

D.4 Meat quality characteristics

In considering each individual characteristic it is useful to consider a series of

questions, the answers to which would provide a good understanding of the

nature of that characteristic. These questions are listed in general terms in Table

D.1, along with explanations of why they are of interest. For some of these

questions the importance and the answer is obvious and clear-cut, while for

others they are less clear. With our current level of knowledge many of the

questions can be given fairly complete answers, but for others there are still

many unknowns, particularly for questions relating to the physiological basis of

variation in the growth and body composition characteristics.

In the following sub-sections the first six questions in Table D.1 will be considered

for the main characteristics separately, and then question 7, and to a lesser

extent question 8 will be considered in general terms in Section D.5.

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Table D.1 Pertinent questions about important characteristics of

meat-producing animals

Question Importance

1. How is the characteristic defined?

A clear definition of each characteristic is essential before the remaining questions can be addressed in an informed manner. Unfortunately the definitions of a number of key characteristics are not the same in all situations.

2. Why is the characteristic of importance?

The importance needs to be explained in commercial as well as biological terms.

3. How can the characteristic be measured?

A characteristic must be measurable to permit an evaluation of its importance, and of how it varies in different situations. For most characteristics there are many alternative methods of measurement, and new and improved methods are continuously being developed.

4. How are the measurements commonly expressed?

Here again many alternative approaches are often available, and misleading conclusions can easily be drawn if comparisons are made between results expressed in different ways.

5. What patterns of change are shown for the characteristic as an animal grows and develops?

A knowledge of these patterns is very important because of the commercial importance of the characteristics to be considered.

6. Are there important interrelationships between this and other productive characteristics?

Some such relationships will be positive (desirable) while others will be negative, so it is important to be aware of them when setting out to make changes in any particular characteristic.

7. What factors are responsible for variation in the characteristic?

This is a large question with clear commercial implications. The factors can broadly be divided into those due to different types of animal and those due the environment the animal exists in. For meat quality characteristics, the post mortem environment also needs to be considered.

8. What is the physiological and/or anatomical basis of variation in the characteristic?

Another big question that overlaps with the previous one. There are more unknowns here than for most of the other questions.

D.2 GROWTH CHARACTERISTICS

The three growth characteristics to be considered are described in Table D.2,

and are discussed for each characteristic individually in the text that follows.

(a) Growth Rate: This is the growth characteristic that is most widely

measured and monitored on farms. This is because all that is needed is set

of scales, and because the information provided enables the farmer to make

more informed management decisions about feeding, and about selection of

animals for breeding and slaughter.

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Table D.2 Key growth characteristics of meat-producing animals

Growth Characteristic

General Definition and Description Examples of Expressions

Growth rate The change in size per unit time. Size is most often measured in terms of live weight, but growth rates can also be in terms of changes in carcass weight, in lean meat weight, or in some linear measurement such as height per day.

1. Average daily gain (ADG) = (Final weight - Initial weight)

Number of days.

2. Weight per day of age = Weight / Age in days. This expression is useful when only one weight is available.

3. Age-corrected weight. ADG values are used to calculate the weight at a specified age (e.g. 200-day weight for cattle).

4. Relative growth rate = ADG/weight. Expressed as a proportion or percentage per day.

Growth efficiency

The gain in weight relative to the quantity of feed required to make that gain. More efficient growth is that which requires less feed to achieve the same weight gain. Measures of feed intake are commonly in terms of the dry matter (DM) content or the ME content of the feed.

1. Efficiency of feed utilisation = ADG/ Feed intake per day.

2. Feed conversion ratio (FCR) = Feed intake per day / ADG.

Growth extent or mature weight

The weight of an adult (mature) animal. The problem is to determine when an animal has reached its mature size or weight. Excess fat (e.g. more than about 20% fat) should not be included as part of mature weight.

1. Live weight at maturity after adjusting for excess fat and time of year.

2. Frame-size scores (e.g. hip height at a set age or weight) are useful as indexes of mature weight, with a high frame-size score being indicative of a higher mature weight.

Unfortunately there are several different ways in which growth rate

information is expressed (see Table D.2) and it is important that the

distinction between these be understood. For example, why is an ADG

value from birth always less than a weight per day of age? And why is an

age-adjusted weight a measure of growth rate even though it is in terms of a

weight value rather than a weight per unit time?

Patterns of change in growth rate as an animal grows and develops from

conception to a newborn, and then on to maturity, show a characteristic

underlying sigmoid pattern, but during post-natal growth this underlying

pattern is often obscured by seasonal or other changes in the quantity and

quality of feed that is offered.

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Q.1. Table D.3: Calculate growth rates in the four ways outlined in Table

D.2 for bulls 2 and 3 in the following table. Values are given for Bull 1 as an

example.

Bull number

1 2 3

Birth weight (kg) 32 38 28

230-day weight (kg) 196 217 182

390-day weight (kg) 362 402 327

ADG from birth to 230d (kg/d) 0.713

ADG from birth to 390d (kg/d) 0.846

ADG from 230d to 390d (kg/d) 1.038

Relative growth rate at 390 days (%/d) 0.287

Weight/day of age at 230d (kg/d) 0.852

Weight/day of age at 390d 0.928

200-day weight 174.6

400-day weight 372.4

This means that under New Zealand pastoral conditions, for example, there

are clear seasonal changes in growth rate, with animals usually showing the

highest growth rates in late spring when pastures are of high quality and in

plentiful supply. When growing healthy animals are offered a consistent,

adequate supply of a high quality feed, however, they will usually show the

characteristic sigmoid (S-shaped) growth curve such as that shown in

Figure D.2 for a cattle-beast with a mature weight of 600 kilograms. These

growth curves are typical for most species (cats, dogs, sheep, cattle,

horses) as all animals show an initial period of accelerating growth, which

takes place mainly prior to birth or around birth, followed by a period of

decelerating growth until the animal approaches its mature weight

asymptotically. In all species, once mature weight is achieved, energy

intake above maintenance, pregnancy or lactation requirements will result in

excess energy being deposited as predominately fat which can result in a

live weight above the expected mature weight.

The point at which the accelerating growth changes to decelerating growth

is termed the inflexion point and it has been shown to usually be at about

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one third of mature weight for farm animals. The slope at any point on the

growth curve in Figure D.2 is equal to the ADG in kg/day. This means that

the ADG increases up to the inflexion point and declines after that point to

zero at maturity. Relative growth rates in contrast normally decline from

shortly after conception. According to the simulated curve in Figure D.2 the

animal did not reach its mature weight until it was almost five years old,

which is about what has been shown in practice for real cattle.

Figure D.2 An idealised growth curve for a cattle-beast

This curve was produced by computer using the Richards growth function which is one of many that have been used to express animal weight as a function of time.

The aim in most meat-production systems is to have the animals growing as

fast as possible, because this means that target weights are reached more

quickly (or heavier weights are achieved after a set period of time), and the

efficiency of growth will generally be greater (explained below), but there are

some situations where faster growth rates may not lead to a more efficient

or a more profitable system. Examples of such situations include:

1. If expensive feed supplements have to be used in order to achieve the

higher growth rates. It may be better to accept a period of slower growth if it is known that a plentiful supply of cheaper feed will be

Section D

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available in the near future (e.g. the flush of spring pasture relative to winter supplements).

2. If fast growth rates require the use of later-maturing breeding stock

with heavier mature weights. Because they are larger, it will be necessary to have lower stocking rates, so the costs and benefits need to be carefully weighed up for each situation. A common way to gain some of the high growth potential of larger breeds, is to use them as sires only, and have breeding females of a relatively small breed.

3. In some cases very high growth rates brought about by feeding high

levels of a high quality diet may result in the animals growing less efficiently. This means that it may sometimes be more profitable to restrict the feeding level to, say, 85-90% of the ad lib intake. Possible reasons for this effect include (1) a decreased digestion of the feed (due to faster movement through the gut), (2) a higher proportion of fat in the weight gain (particularly with pigs), and (3) an increase in metabolic rate and therefore in maintenance requirements.

(b) Growth Efficiency: The efficiency of growth (as defined in Table D.2) is

seldom measured on the farm in the way that growth rate is because there

is no simple method to measure animal intake under grazing conditions.

Intake is commonly measured in systems where the feed is brought to the

animals (e.g. pigs, chickens, and animals on feedlots), but usually for

groups rather than individuals. Because it is difficult to measure, it is

important to have a good understanding of some of the main factors

affecting growth efficiency, and four of the most important such factors are

listed in Table D.4. A knowledge of these four effects will often make it

possible to predict whether one animal (or group of animals) is growing

more or less efficiently than another. Growth rate and weight effects are

illustrated by the efficiency values that can be calculated from the data in

Table D.5.

Efficiency of growth usually declines with increasing animal age and weight

until it reaches zero when growth ceases at maturity. The decline with

growth can be attributed to the increased weight, the increasing proportion

of fat in the gain, and, for post-inflexion-point growth, the decline in growth

rate (Table D.4). It should be emphasised that this is an overall pattern, and

that periods of faster growth due to better feeding will normally be

associated with improved efficiency.

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(c) Mature Weight or the Extent of Growth: This is an important growth

characteristic because it determines how much feed will be required by

mature animals on the farm and because the mature weight of an animal as

a genetic trait is associated with other growth characteristics (e.g. growth

rate and composition of growth) earlier in its life. It has already been noted

that accurate information on mature weights is difficult to obtain because

adjustments should be made for differences in fatness and for different

seasons of the year. Animals with higher mature weights have both

disadvantages and advantages relative to similar animals with lower mature

weights. Therefore, whether one is preferable to the other will depend on

the particular situation being assessed. The main points to consider are:

1. Animals that are still on the farm when mature, such as breeding

stock, will eat more if they are larger because of higher maintenance requirements. Therefore feed costs per head will be higher and stocking rates lower. [Usually a disadvantage].

2. Heavier breeding animals will generally be worth more when sold at

the end of their breeding life. [Usually an advantage]. 3. Larger animals will generally produce more products other than meat

when this is an important aspect of the system (e.g. wool in the case of sheep). [Usually an advantage].

4. Animals with a higher mature weight usually grow faster at any set

weight. This is partly because they will be at a lower proportion of their mature weight, but also because growth potential at any set proportion of mature weight tends to be proportional to mature weight. This also applies to their offspring. [Usually an advantage].

5. Animals with a higher mature weight will usually be less fat at any

particular weight. This will also be because they are at a lower proportion of their mature weight which means that they can be taken to heavier weights before they become overfat. [Usually an advantage].

6. Despite their faster growth rates, animals with higher mature weights

will usually take longer to reach a specified level of finish or fatness. [May be a disadvantage].

7. Under some circumstances the use of sires with high mature weights

may lead to an increased incidence of birth difficulties (dystocia), especially if the dam breed is smaller. [Usually a disadvantage].

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Table D.4 Factors affecting the efficiency of growth

Only four basic factors are listed in this table, but a knowledge of these four can explain many differences in the efficiency of growth that have been reported between different groups of animals.

Factor Effect on Efficiency of Growth

Explanation of the Effect

Animal weight

With increasing weight the efficiency of growth decreases (other things being equal).

With increasing weight the feed required for maintenance is higher, so more needs to be eaten to achieve the same growth rate. Hence the ratio of ADG to daily feed intake (the efficiency) declines.

To show this effect the animals being compared should be offered the same feed ad lib, and they should be growing at similar rates.

Growth rate (ADG)

With an increase in ADG the efficiency of growth will usually increase (other things being equal).

Faster growing animals will achieve a set weight gain over a shorter period, which means the maintenance requirements per unit gain will be lower. To show this effect the animals should be offered the same feed, and they should be of the same weight.

This effect will usually apply both when the animals are offered as much feed as they can eat, in which case the faster growing animals will tend to be those with bigger appetites, as well as when those growing faster are doing so because they are given more feed.

Sometimes referred to as a dilution of maintenance effect.

Proportion of fat in the weight gain

With an increase in the proportion of fat in the gain the efficiency of growth will usually be lower (other things being equal).

Because 1kg of adipose tissue contains 5 to 6 times more energy than 1kg of non-fat tissue such as muscle.

This effect is offset to some extent by the fact that feed energy is converted to energy in lipid more efficiently than to energy in protein.

Comparisons need to be made between animals with the same weight and growth rate in order to show this effect.

Maintenance feed requirements

(MR)

With an increase in MR the efficiency of growth will usually be lower (other things being equal).

This is because more feed used for MR will mean less is available for growth.

Apart from increasing with animal weight, MR may also be affected by animal activity, low temperatures, and animal gender (MRs are higher for males of some species).

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Q2: Table D.5: Estimates of feed requirements of lambs (MJ ME/d) for

lambs of three weights growing at rates from 0 to 300g/d. Calculate

efficiency values for the blank columns and draw graphs of efficiency vs

growth rate for lambs of the three weights. For lambs of 40kg, calculate the

returns per kg DM for lambs growing at 50 to 300 g/d from 40 to 45kg

assuming (1) the feed contained 10.8 MJ ME/kg DM, (2) carcass weight

gain was 43% of liveweight gain, and (3) the schedule value is 312c/kg of

carcass weight. Draw conclusions.

Growth Lamb live weight (kg)

rate 20 kg 30 kg 40 kg

(ADG) MJ ME/d Effic. MJ ME/d Effic. MJ ME/d Effic. c/kg DM

0 (maint.)

50

100

150

200

300

6.5

8.0

9.5

11.0

12.5

15.5

9.0

11.0

13.0

15.0

17.0

21.0

11.0

13.5

16.0

18.5

21.0

26.0

(Data from: Geenty, K.G.; Rattray, P.V. (1987). The energy requirements of grazing sheep and

cattle. In: Nicol, A.M. (Ed.) Feeding Livestock on Pasture (pp. 39-53). New Zealand Society of Animal Production, Hamilton.).

The trend over a number of years has been for a move towards animals

with higher mature weights for most meat-producing species, mainly

because of the advantages of faster growth rates and the fact that they can

be taken to heavier weights before becoming over-fat. A concurrent trend

for consumers to favour meat products with lower fat content has also

favoured the use of animals with genetically higher mature weights. The

major disadvantage of higher mature weights is usually the higher feed

costs or lower stocking rates for breeding animals, but the opportunity for

minimising this by using animals with large mature weights as sires rather

than dams has been widely taken up for a number of the meat-producing

species. This is particularly the case for beef cattle production where the

total income from the breeding female is from her offspring. In the case of

sheep and dairy-beef systems the disadvantage of larger breeding females

is offset to some extent by higher production of wool and milk, respectively.

Movement towards animals with genetically higher mature weights can be

achieved either by selection within a breed, by cross-breeding, or by a

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change of breed. A higher average mature weight is also a likely outcome

from selection for increased growth rates due to a positive genetic

association between these traits. Experiments have shown that selection

for increased growth rate while holding mature weight constant results in

appreciably slower genetic progress in growth rate, than when mature

weight is allowed to rise.

The question of patterns of change with normal growth and development

does not apply with respect to mature weight.

D.3 BODY AND CARCASS COMPOSITION CHARACTERISTICS

The five characteristics covered in this section are given in Table D.6. Except in

the case of dressing-out percentage, which is a simple ratio between two

weights, the composition characteristics are more complex than growth

characteristics because they are concerned with more than two variables.

Carcass composition, for example, is concerned with muscle, fat and bone, and

muscle distribution is concerned with the relative sizes of hundreds of different

individual muscles. This has led to a wider variety of methods of expression, and

this can make the subject confusing.

Composition characteristics also differ from most growth characteristics in being

more difficult to measure directly under normal commercial conditions, which

means that they are often predicted from simpler measurements (e.g. the

prediction of carcass or live-animal fat% from a simple fat depth). The

measurement of the composition of live animals presents a particular challenge

which has received a lot of attention in recent years, because an ability to do this

would enable farmers to more accurately select breeding animals that will

produce superior offspring for meat production. Similarly, more accurate

methods of rapidly assessing the composition of intact carcasses are

continuously being developed in order to improve the effectiveness of carcass

classification systems.

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Table D.6 Key composition characteristics

Composition Characteristic

General Description Examples of Ways the Characteristic may be Expressed

Dressing-out percentage (DO%) (Also known as the killing-out %, or carcass yield.)

The ratio of (carcass weight to live weight) x 100.

Variation in the definition of carcass weight (hot vs cold; with or without items such as skin, kidney & pelvic fat, head, etc) and live weight (full vs fasted wt) make comparisons between studies hazardous.

Usually expressed as a simple percentage value.

As regression relationships between carcass weight and live weight, using either raw weights, or logarithms of weights.

Proportions of muscle, fat & bone in the carcass

The key trait for meat production is the muscle percentage.

Muscle % may be higher either because fat% is lower and/or because the muscle to bone ratio (M:B) is higher.

Muscle, fat, and bone are often expressed as percentages of carcass weight, but this can cause confusion.

Bone content is better expressed as M:B because it is then unaffected by changes in fat% (which is the most variable carcass component).

Distribution of muscle over the carcass

A measure of the proportion that each muscle makes up of total muscle, but usually for simplicity muscles are combined into 5 to 10 muscle groups within specified anatomical areas.

Muscle or muscle group weights relative to total muscle weight.

Proportion of total muscle in the hindquarter.

Proportion of total muscle in the high-priced cuts.

Partitioning & distribution of adipose tissue (fat) throughout the body

Partitioning of fat refers to the way total fat is partitioned between the different carcass and non-carcass depots (Table D.10).

Distribution of fat refers to the way in which fat is distributed within a depot (particularly the subcutaneous depot).

Partitioning is expressed as a depot weight relative to total body fat, or total carcass fat, or carcass weight.

Distribution may be expressed as the relative depths or weights of fat at specified sites within a depot.

Carcass shape A measure of the compactness, blockiness, thickness or legginess of a carcass. Thicker, blockier carcasses are usually considered to have a better shape (other things being equal).

Conformation = the thickness of muscle + fat relative to a skeletal dimension (e.g. a bone length).

Muscularity = the thickness of muscle only relative to a skeletal dimension.

Fleshiness = the thickness of muscle + intermuscular (seam) fat relative to a skeletal dimension.

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(a) Dressing-out Percentage(DO%): A very important characteristic for the

farmer because it relates the live weight that can be measured on the farm

to the carcass weight that is a common basis of payment (i.e. $/kg carcass

weight). Some approximate representative values for DO% and other

carcass characteristics for various classes of livestock used for meat

production in New Zealand are set out in Table D.7.

Care needs to be taken when comparing DO% values for different species,

and for different conditions for the same species, both because of variation

in gut contents in different situations (e.g. varying time off-feed, animals on

different diets, etc.), and also because of variation in what is meant by a

carcass (or the alternative spelling carcase) for different species and in

different countries. This variation in meaning is unavoidable when carcass

has to be defined in commercial terms as That part of the animal that

remains after normal commercial dressing procedures have been

completed. An example of species differences is the fact that the carcass

includes the skin, and feet for pigs and chickens, but not for sheep, cattle,

goats or deer. An example of a country difference is the fact that the kidney

plus pelvic fat remains with beef carcasses in the USA, but is removed in

New Zealand and Australia.

Within a species, factors that have been shown to be responsible for

differences in DO% include: Level of fatness, with fatter animals generally having higher DO%. Fat partitioning patterns, with animals laying down more fat in the non-

carcass depots (see Table D.10) tending to have lower DO%. Level of muscling or M:B, with better muscling and high M:B values

being associated with higher DO%. Animal gender, with entire males having lower DO% values in some

cases, either due to low levels of fatness and/or to the contribution of the testes to the weight of non-carcass components. This effect is most apparent for ram lambs.

Higher levels of gut-fill will lead to lower DO%. This may be due a short time off feed, to high levels of nutrition, or to feed of low digestibility.

For females pregnancy (particularly the last ) will be associated with lower DO%.

Heavy hide weights relative to body weight will lead to lower DO%. For sheep a full fleece will lead to a lower DO%, other things being

equal. Weaned lambs tend to have lower DO% than milk-fed lambs at the

same weight, probably due to a better developed rumen and possibly lower levels of fatness.

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Table D.7 Approximate representative values for selected carcass

characteristics of several classes of livestock

Class Carcass weight (kg)

Dressing-out %

Carcass fat %

(Sub.):

(Inter.)C

Carcass muscle

%

Carcass M:B

Finished steer

Finished lamb

Finished pig

Goat

Red deer

Fallow deer

Broiler chicken

Heavy turkey

Rabbit

Ostrich

290A

16A

65

10A

53

30

1.5

16

1

54

52

43

73

41

57

59

72

79

50

54

21B

23B

22B

8B

10

9

15

12

8

9

0.6

1.1

3.0

0.4

60

57

61

65

71

73

61

58

67

63

4.3B

4.1B

5.2B

3.3B

5.1

5.3

2.5

2.0

3.8

2.4

A Carcass weights will tend to be higher for later-maturing breeds and strains.

B Large differences have been shown between breeds or lines within the species.

C Ratio of subcutaneous fat weight to intermuscular fat weight.

With this extensive list of factors affecting DO%, it is often difficult to predict

whether one group of animals will be better or worse than another in this

respect, but it is important for a farmer to have a good understanding of the

sort of DO% values to expect because the step/stair nature of the

schedule payment systems widely used in New Zealand can mean that

small differences in DO% will lead to quite large differences in returns if it

means a move from one carcass weight range to another.

As animals grow and develop, and carcass weights get heavier, DO%

values usually increase gradually (Figure D.3), mainly because of the

combined effects of an increase in carcass fat% and an increased M:B.

(b) Proportions of Muscle, Fat and Bone: In many respects these are the

most important composition characteristics as they determine carcass meat

yield, but unfortunately they are also some of the most difficult to measure

accurately under commercial conditions. The main standard method of

measurement is to physically dissect one or both sides of a carcass into

muscle, fat and bone using a knife or scalpel, and then to calculate the

characteristics specified in Table D.6 (muscle%, fat%, M:B) after weighing

Section D

19

each component. Muscle% is commonly referred to as the lean meat yield

(LMY%) and is a function of fat% and M:B such that:

LMY% = (100 - fat%) x (M:B / (M:B + 1)).

This equation can be used to show how LMY% changes with changes in

either fat% or M:B. An important carcass characteristic that is closely

related to LMY% is the saleable meat yield (SMY%) which differs from

LMY% in including some fat with the meat so that the comparable equation

for SMY% has excess fat% in place of fat%, and meat to bone ratio in

place of muscle to bone ratio.

An alternative standard method to physical dissection, that may be more

appropriate for some situations, is by chemical analysis of an

homogeneous sample that is obtained by thorough mincing and mixing.

The chemical analysis may be of the whole body, the carcass, a side, or any

sub-component of the body or carcass. A wide range of chemical analyses

are possible, but the most common are for water (by drying), lipid (by

solvent extraction), protein (from nitrogen content), and inorganic material or

minerals (by ashing).

It is seldom feasible to carry out a detailed dissection or full chemical

analysis of a carcass because of the time and cost involved and because it

means cutting up the carcass in a way that is likely to be commercially

unacceptable. As a result many other measurements have been used as

predictors or indicators of composition. A selection of such indirect

predictors are listed and briefly explained in Table D.8.

Section D

20

Figure D.3 Changes in composition characteristics with cattle growth

The usual commercial range of carcass weight (250 - 350kg) is shown by the box in each graph, but by showing changes over a wider carcass weight range, patterns become more apparent. The top three graphs show the change in weights of muscle, fat and bone, the second row shows corresponding percentage values, and the bottom three show changes in M:B, dressing-out % (based on an empty live weight), and gut contents (relative to full live weight). (Based on data from Moulton, C R; Trowbridge, P F; Haigh, L D. 1922: Studies in animal nutrition. II Changes in proportions of carcass and offal on different planes of nutrition. Missouri Agricultural Experiment Station, Research Bulletin; 54.)

Section D

21

Table D.8 Examples of indirect predictors of carcass or whole-body

composition

Measurement or Predictor

Explanation and Comments

1. Subjective assessments of fatness and carcass shape.

Assessments made without the aids of instruments or measuring equipment.

Provide an overall evaluation of fatness or shape rather than at specific sites.

Often suffer from imperfect repeatability between people or over time.

Repeatability can be improved by using standard photographs or pictures.

Applicable to several characteristics & to live animals as well as carcasses

2. Fat depth over a muscle.

A useful predictor of fat% and therefore of LMY% when most of the variation in LMY% is due to variation in fat% (i.e. M:B does not vary much).

Can be measured directly at the time of quartering, on intact carcasses by electronic probes, and on live animals by ultrasound.

Most accurate for pigs where most fat is in the subcutaneous depot.

For beef carcasses, fat removal by hide pullers is a problem.

Ultrasonic methods enable measurement in live animals and on the hide-on carcass, but the procedure is not yet automated.

Video-Image Analysis (VIA) can be used to assess the fat cover of a carcass, and also the fat depth after quartering.

3. Soft tissue depth over a bone.

Similar to 2 above, but some muscle may be included in the depth.

Having the hard bone surface to measure to is an advantage.

Best example is GR, as used for classifying lamb, mutton, & venison in NZ.

4. Eye-muscle area (EMA) (i.e. the cross-sectional area of M longissimus thoracis et lumborum).

An indicator of M:B or muscling, but relationships with LMY% are poor.

It should be a better predictor of LMY% when variation in fat% is low.

Measurable on the intact carcass or live animal using ultrasonic scanners.

More sophisticated scanning technologies involving X-rays (CT scans) or magnetic resonance imaging (MRI) can also be used.

When quartered, EMA may be measured by grids, by VIA, or following tracing.

5. Carcass shape measured objectively.

Not necessarily related to LMY%.

Measurable objectively by VIA, and measures such as the Fleshing Index (weight per unit length).

Table D.8 Cont. over

Section D

22

Table D.8 Cont. Examples of indirect predictors of carcass or

whole-body composition

6. Weight of kidney plus pelvic fat.

An objective measure that is available on the slaughter floor.

Other fat removed during dressing may also be weighed.

Its relationship with fat% is known to vary between breeds.

7. Total body electrical conductivity (TOBEC).

Conductivity is higher when fat % is lower.

This method has so far been applied mainly to cartons of meat.

The equipment is expensive and the item being evaluated must pass through a tunnel surrounded by a coil.

8. Bioelectrical impedance.

Also based on the different electrical properties of muscle and fat.

Still experimental, but results look promising for carcasses & live animals.

9. Yields of selected cuts.

Results are not obtained until the carcass is cut. Not yet in commercial use.

Accuracy is dependent on accurate and consistent cutting methods.

Feasibility will be enhanced by traceability requirements.

The pattern of change in LMY% as an animal grows and develops over the

normal range of slaughter weights is usually a decrease at an increasing rate as

the animal moves into the fattening phase of growth (Figure D.3). This is

because the clear increase in fat% with increasing weight more than cancels out

the relatively gradual increase in M:B. This is not invariably the case, however,

and slower decreases or increases in LMY% with growth will be more likely in the

following three situations:

1. When the animal has a low propensity to fatten (e.g. late maturing

breeds and/or entire males). 2. When the animal is at a lower proportion of its mature weight. 3. Following treatments that decrease fat growth (e.g. high-protein diets

or certain growth promotants).

(c) Distribution of Muscle: This characteristic is important because meat in

some cuts is worth more than that in other cuts. Unfortunately the scope for

improving muscle distribution in animals of the same species and at a

similar weight is limited because there is little variation between animals.

There is a tendency for heavily-muscled animals to have a slightly higher

proportion of total muscle in the high-value cuts, but the differences are

seldom more than 1 or 2 percentage points.

Section D

23

Changes in muscle distribution with growth are significant and are similar

across species. Results from one study with cattle are shown in Table D.9

within Q3 below.

(d) Partitioning and Distribution of Fat: The principal fat depots of relevance

for meat production, together with an indication of their relative values, are

described in Table D.10.

Four reasons why fat partitioning and distribution are important are:

1. If more fat is partitioned toward the non-carcass depots, dressing-out

percent will decrease ( other things being equal). 2. Partitioning of fat preferentially toward the subcutaneous depot will

mean a set fat depth will be attained at a lower carcass or body fat%. 3. Partitioning of fat toward the intramuscular fat depot, rather than other

carcass depots, may be beneficial for certain markets where marbling fat is favoured.

4. Distribution of subcutaneous fat such that more is at the classification sites means that set depths at those sites will be reached at a lower subcutaneous fat %.

Q3: Table D.9: Using the values given for percentage of total muscle

within 7 anatomical groups for cattle at birth, and the percentage change in

these values from birth to 4 years, calculate the percentage each of these

groups made up of total muscle at 4 years and enter these values in the

final column. Draw conclusions.

Muscle Group % of total muscle at birth

% change in % contribution to

total muscle

% of total muscle at 4 years of age

1. Proximal muscles of hind leg

2. Distal muscles of hind leg

3. Muscles around spinal column

4. Muscles of abdominal wall

5. Proximal muscles of foreleg

6. Distal muscles of foreleg

7 Muscles of the neck & thorax

31.5

7.1

12.2

7.0

12.7

4.5

25.0

104

70

100

135

90

60

103

Section D

24

Species differences in fat partitioning are illustrated by the (Sub):(Inter)

values in Table D.7. As an animal grows and develops the main changes

that take place in the partitioning of fat are that an increasing proportion of

total fat is normally found in the subcutaneous fat and in the kidney plus

pelvic fat, while a decreasing proportion is found in the intermuscular depot.

(e) Carcass Shape: The terminology to describe carcass shape outlined in

Table D.6 is not universally accepted and articles on carcass shape can be

confusing. This is partly because a blockier shape can arise from thicker

muscles (generally desirable) or excess fat (generally undesirable). The

term muscularity was introduced to overcome this confusion by excluding

the contribution of fat to soft-tissue, or overall carcass, thickness.

As an animal grows its shape usually becomes less leggy, thicker, and

more blocky or compact.

Section D

25

Table D.10 The main carcass and non-carcass fat depots

Depot Name(s) Anatomical Location Relative Value

Carcass Fat Depots:

Subcutaneous (also known as bark fat, or selvidge)

Intermuscular (also known as seam fat)

Intramuscular (also known as marbling fat)

Immediately under the skin and over muscles (except M. cutaneous trunci). May be present as distinct layers.

In the seams between muscles. Continuous with subcutaneous fat.

Within muscles as visible speckles of fat when the concentration is high enough.

High, up to the level at which trimming is necessary, then very low due to the cost of trimming.

As for subcutaneous fat, but more difficult to trim when in excess.

Usually of high value, especially for some markets (e.g. Japan).

Non-Carcass fat Depots:

Perinephric ( also known as perirenal or kidney fat).

Pelvic (also known as channel or retro-peritoneal fat).

Omental (also known as caul fat)

Mesenteric (also known as intestinal fat)

Thoracic (also known as heart fat)

Around the kidneys and adrenal glands.

In the pelvic canal.

Around the stomach, and fore-stomachs of ruminants.

Within the mesentery that encloses the intestines.

In the thoracic cavity (not restricted to that around the heart)

Low value as a rendered by-product.

Low value, as for kidney fat.

Low value.

Low value.

Low value.

D.4 MEAT QUALITY CHARACTERISTICS

Meat quality is a difficult concept to define accurately because it has different

meanings for different consumers or groups of consumers, and even for the

same consumer at different times or in different environments. It is further

Section D

26

complicated by the fact that the term quality has more than one meaning. As

used here it is taken to mean the level of goodness or acceptability of a

product, but in other contexts it is used to indicate the fitness for purpose of a

product in terms of how well the product meets the specifications set for it. The

distinction between these meanings is important because, whereas the former is

a key determinant of price, the latter is largely affected by price.

The characteristics contributing to quality vary at different points in the chain of

meat production as shown in Figure D.4, which causes further confusion when

considering this characteristic, because with movement back along the chain

from the consumer to the producer additional components need to be added at

each step.

Figure D.4 Components of quality at different levels of the

meat marketing chain Quality components of MEAT as it is consumed:

1. Appearance characteristics (e.g. colour). 2. Palatability characteristics(e.g. tenderness, flavour). 3. Nutritive value characteristics (e.g. protein% & lipid%). 4. Safety & wholesomeness characteristics.

Quality components of a MEAT PRODUCT at the time of purchase:

1. Those listed in the box above, plus

2. Edible meat yield. 3. Ease of preparation and cooking advice. 4. Storage requirements and other convenience aspects. 5. On-farm production system involved.

Quality Components for a CARCASS at the time of purchase by a processor:

1. All those listed in the two boxes above, plus

2. Saleable meat yield (%) from the carcass. 3. Proportion of the saleable meat in the more valuable cuts. 4. The shape of the carcass, and therefore of the cuts. 5. Processing properties of the meat (if the meat is to be processed).

Quality components of a young MEAT ANIMAL at the time of purchase by a farmer:

1. All those listed in the three boxes above, plus 2. The animals growth potential (growth rate and efficiency of growth) 3. Productivity in terms of products other than meat (wool, milk, velvet, etc.). 4. The dressing-out percentage when it reaches a finished weight. 5. Complementarity with other farm activities.

The characteristics that are listed in the lower outer-most box of Figure D.4

include some of the growth characteristics discussed in D.2, while those in the

next box up include several of the composition characteristics covered in D.3.

The items in the two inner boxes are more directly related to meat quality, with

those in the top inner-most box being intrinsic properties of the meat itself, and

Section D

27

those in the second box generally being aspects external to the meat, such as

the way it is presented and the production system involved. The edible meat

yield in that box will be a function of carcass composition, of how the product is

prepared (level of trimming and boning), and of how much weight is lost during

cooking.

The main meat quality characteristics are listed and briefly explained within five

categories in Table D.11. The two categories focussed on in this paper are the

appearance and the palatability characteristics of meat, because these are the

ones that are sometimes included in carcass classification or grading systems,

and because many of these characteristics can be influenced by the type of

animal, by the way animals are treated on the farm, and by pre-slaughter

treatments.

The nutritive value characteristics of meat are also very important, but for lean

meat they do not show very much variation, and the brief list of features of meat

as a source of nutrients for humans that is given in Table D.11, provides an

overview of the principal strengths and weaknesses of meat in this respect.

(a) Appearance Characteristics of Meat: This group of characteristics is

particularly important at the point of sale, because at that stage it is not

possible to assess the palatability characteristics directly. Meat and fat

colour are probably the most important appearance characteristics. Meat

colour is largely determined by the concentration of myoglobin present and

by the form that the myoglobin is in. Myoglobin is a complex molecule

which is made up of a protein (with a molecular weight of about 17 kDa)

that is very similar to haemoglobin of red blood cells (except that it consists

of only one of the four sub-units of haemoglobin) plus a haem group and an

iron atom. Changes in colour may result from changes in any one of these

parts. For example, the change from the puplish-red colour of fresh-cut

meat to a bright red colour is due to the uptake of oxygen, while the

formation of a brown colour after a longer period of exposure to air is due to

the oxidation of the iron from the ferrous to the ferric form. This is different

from the formation of greyish brown pigments on cooking which is mainly

due to denaturation of the protein part of the molecule. White meats (for

example chicken and pig meat) contain less myoglobin so colour

assessment is based around normal fresh meat being a pale pink colour.

Section D

28

Yellowness of fat is mainly due to the presence of pigments such the

carotenoids which are present in particularly high concentrations in fresh

pasture compared to grain-based rations.

Colour measurements can be made subjectively or semi-subjectively by

using sets of standard colours or photographs, but can also be made

objectively by reflectance spectrophotometry or by video image analysis

(VIA).

Section D

29

Table D.11 Five categories of meat quality characteristics

Meat Quality Characteristic General Comments/Explanations

Appearance Characteristics

Lean meat colour

Fat colour Meat texture

Firmness

Composition

Red/pink (depending on species) is the favoured colour, and brown, or meat that is too dark or too pale, are the main problems.

Yellowness is usually the main problem.

A measure of fineness/coarseness on a cut surface.

The ability to maintain shape on display.

Proportions of muscle, fat & bone on display to the consumer.

Palatability Characteristics

Meat tenderness Meat flavour

Meat juiciness

Basically the forces needed to bite through and chew a sample.

Includes both meat taste as well as aroma or smell or odour.

Affected both by water content and level of salivation.

Nutritive Value Characteristics

There are too many characteristics in this category to list here, but some important features of lean meat as a source of nutrients for humans are:

It has a high ratio of protein to energy, and is said to be nutrient dense in this respect. The protein of meat is of high quality in terms of its amino-acid composition.

Lean meat is highly digestible.

It is a good source of iron & zinc and the iron, being mainly haem-iron, is particularly well absorbed.

It is a good source of most B vitamins, and particularly of vitamin B12. Lean meat usually has a low fat content (except when it is heavily marbled).

Meat contains cholesterol, but only in moderate concentrations (about 70-80mg/100g).

Meat is a poor source of calcium, vitamin C, and dietary fibre. The lipid with meat from ruminants contains 25-40% of its fatty acids as saturated fatty

acids.

Processing Properties of Meat

Water-holding capacity

Binding capacity

Emulsifying capacity

The ability of meat to retain water during processing.

The ability of meat pieces to bind in reformed products.

The capacity to form a stable emulsion with fat.

Safety & Wholesomeness Characteristics

Presence of pathogens

Presence of spoilage organisms

Presence of dangerous residues

Micro-organisms (mainly bacteria) that cause disease.

Micro-organisms that lead to spoilage symptoms (e.g. off odours).

Residues may be from a several sources (e.g. feed, medications).

As an animal grows towards maturity, meat colour gradually becomes a

deeper red (e.g. the difference between veal, and beef from a mature cow

Section D

30

or bull), fat colour is more likely to be yellow, and the texture of meat tends

to become coarser. As explained in Section D.3, the ratio of fat to muscle in

a carcass usually increases as an animal increases in weight, but it is likely

that the composition of a meat product on display will be determined more

by the extent to which fat is trimmed off during preparation, than by the

fatness of the carcass it came from. Bone decreases as a percentage of

carcass weight as the animal grows (Figure D.3), but most meat products

on display in retail shops have no bone, so it makes a minimal contribution

to appearance.

(b) Palatability Characteristics of Meat: These characteristics, and

particularly tenderness, are commonly considered the most important

determinants of overall meat quality because numerous consumer surveys

have identified them as being the characteristics of meat that consumers

are most often dissatisfied with.

The standard method of measuring palatability of meat is by the use of

sensory panels (also known as taste panels). These are made up of

groups of people who evaluate various characteristics of the meat and rate

them on some form of scoring system. Meat tenderness can also be

measured reasonably satisfactorily by mechanical devices such as the

Warner-Bratzler shear device and the MIRINZ tenderometer, both of which

measure the force required to shear through a sample of standard size that

has been cooked under standard conditions. Such mechanical measures of

tenderness are not satisfactory on uncooked meat, however, and an

accurate and quick method of assessing meat tenderness for an intact

carcass has yet to be developed.

Some components within meat have been shown to be correlated with

certain aspects of flavour in some situations, but generally the measurement

of meat flavour is more dependent on the use of sensory panels than is the

case for tenderness.

Juiciness is usually considered the least important of the three palatability

characteristics, but it can still have a major effect on the overall acceptability

of meat, and higher juiciness scores are usually associated with higher

scores for tenderness.

Section D

31

Changes in palatability characteristics as an animal gets bigger and older

are difficult to predict accurately because many of the determinants of

palatability are changing in ways that lead to some effects cancelling out

others. Over wide age ranges, a drop in tenderness and an increase in

flavour strength can be expected, but changes over shorter intervals of age

may go in either direction.

D.5 FACTORS AFFECTING CHARACTERISTICS OF IMPORTANCE TO MEAT PRODUCTION

Information on factors affecting animal growth, body composition, and meat

quality is too extensive to be covered in detail here, so the approach taken has

been to select a limited range of examples that illustrate both the way the various

characteristics can be affected, and also some of the mechanisms involved

where these are known. In considering the potential ways in which the

characteristics can be affected, it is helpful to divide the factors involved into a

number of categories according to the stage in the pathway from conception (of

the meat animal) to consumption (of the meat). An example of a split into seven

stages is shown in Table D.12. Examples of influencing factors within these

categories will be considered below for growth, composition, and selected meat

quality characteristics separately.

Section D

32

Table D.12 Categories of factors that can influence animal growth,

composition and meat quality

Category or Stage Examples of factors Comment

1. Factors associated with the type of animal chosen.

Species of animal.

Breed of animal within a species.

Genetic line within a breed.

Gender/castration class.

All these factors involve differences in the genetic makeup of the animals. Very often significant progress can be made by manipulating these factors.

2. Factors associated with the choice of the age or weight end-point.

Animal weight (either measured live weight or predicted carcass weight).

Animal age.

Most of the characteristics being considered are influenced to some extent by animal age and/or weight.

3. Factors associated with the on-farm environment.

The amount of feed offered.

The nature of the feed being offered.

Medications.

Growth promotants.

Climate, topography, light.

A very important group of factors which are often the ones that the farmer has most control over.

4. Factors associated with the pre-slaughter period (from when the animal is removed from feed to the time of slaughter).

Stresses.

Water and food supplies.

Mixing of unfamiliar animals.

Handling & transport facilities.

The preslaughter period includes a period on the farm, a period of transportation, and a period at the meat plant. Therefore many people are usually involved.

5. Factors associated with treatments during the post mortem, but pre-rigor period.

Hygiene conditions.

Temperature.

Special treatments such as electrical stimulation.

Only the meat quality characteristics are influenced by this and subsequent categories. This is a stage that is particularly important for meat tenderness.

6. Factors associated with the post rigor, but pre-cooking period.

Temperature.

Oxygen availability.

Packaging

Special treatments such as irradiation.

Of importance to meat quality characteristics only.

7. Factors associated with the cooking of the meat.

Method of cooking (e.g. wet vs dry heat)

Final cooked temperature.

Rate of heating.

A critical phase when the quality of even the best piece of meat can be ruined by inappropriate cooking.

Section D

33

(a) Factors Affecting Growth Characteristics: Some factors that influence

the rate and efficiency of animal growth are obvious and well understood.

Thus, animals that are fed more of a better diet will grow better due to an

improved supply of required nutrients at the growing tissues, and entire

males will usually grow faster than females because of androgenic

hormones. The variation in growth between similar animals offered the

same feed, however, is often more difficult to explain, although it is possible

to identify some mechanisms that, in theory, could account for such

variation. For example, the faster growth of some such animals may be due

to one or more of the following:

They may eat more because they are more competitive or because

they have larger appetites (hence the interest in learning what controls feed intake).

They may digest and absorb the feed they consume more effectively (although this is not thought to be an important source of between-animal variation).

They may metabolise the absorbed nutrients more efficiently, thereby losing less energy as heat.

Their protein metabolism may be such that protein turnover rates are lower and therefore net protein gain (synthesis rate less degradation rate) shows a greater energetic efficiency.

Their bodies may be made up of a higher proportion of the metabolically less active tissues (adipose tissue, bone) and a lower proportion of the metabolically more active tissues and organs (liver, gut, brain, muscle).

They may be better able to resist the effects of disease-causing organisms or factors in the environment.

These factors may be considered as potential bottlenecks that could be

responsible for limiting the growth capacity of one animal relative to another. It is

likely that within a population of similar animals in the same environment a range

of these and other limiting factors are responsible for the variation in growth that

is typically seen. Although much is known about the metabolic pathways and

processes occurring in animal cells, there are few examples of explicit

explanations of the actual metabolic basis of variation in growth rates between

similar animals. The somatotrophic axis (Figure D.5) is an example of an aspect

of the endocrine system that appears to be closely related to animal growth, as

GH, for example, has been shown in many species to increase growth rate and

to decrease animal fatness. IGF-1 also stimulates muscle and bone growth

under certain conditions. Information such as this has led to interest in the

possibility of using substances shown in Figure D.5 as growth promotants, or in

Section D

34

using plasma levels of these hormones as an early indictor of an animals growth

potential.

Figure D.5 The somatotrophic axis

Both GH and IGF-1 have been shown to have important effects on growth-related events. Specific examples are given in the following reference. (Adapted from: Bass, J J, Clark, R G. 1989: The endocrine control and coordination of animal growth. Pp. 103-112. In Purchas, R W, Butler-Hogg, B W, Davies, A.S. (Eds.) Meat Production and Processing. NZ Society of Animal Production Occasional Publication 11, Hamilton.)

Examples of factors affecting growth rate, together with explanations of the

mechanisms involved are shown in Table D.13. Comparable examples for

growth efficiency and mature weight are not included here because these

were covered to a greater extent in Section D.2.

Section D

35

Table D.13 Examples of factors affecting animal growth rate

Numbers given following each example correspond to the numbered categories in Table D.12.

Nature of the Effect Explanation and Mechanisms Involved

Differences between breeds and between genetic lines within breeds in growth rate (1).

The physiological mechanisms are poorly understood, but much of the variation is due to the tendency for higher mature weights to be associated with faster growth.

Castrate males usually grow faster than females for some species (cattle, sheep), but often more slowly for others (pigs)(1).

Probably due to different patterns of hormone production, but the reasons for the species differences are not clear. Differences in appetite may be involved.

Followed a period of restricted nutrition, animals often grow faster than unrestricted control animals. This is a form of compensatory growth (3).

This effect is not always demonstrated, and when it is several mechanisms appear to be involved, including an increased appetite, and some changes in patterns of hormone production.

Steers grow 10-15% faster when treated with oestrogenic growth

promotants (e.g. oestradiol 17) (3).

Oestrogens appear to influence growth of muscle and bone directly as well as by increasing the production of somatotrophin in the pituitary gland.

Effects are smaller in heifers and bulls than steers, probably due to endogenous sex hormone production.

(b) Factors Affecting Composition Characteristics: All composition

characteristics of animals change appreciably as an animal grows and

develops (see Section D.3 and particularly Figure D.3), so in assessing the

influence of factors on composition it is common practice to make

comparisons at the same weight in order to eliminate differences in weight

as possible causes of differences in composition. In order to understand

how differences in composition may arise, it is helpful to know about the

processes that lead to growth within the various tissues and organs. This is

a very large subject, but two aspects that are thought to be particularly

important are:

(1) the processes involved in converting an undifferentiated stem cell

into to a cell that is committed to become part of a specific tissue such as muscle or fat or bone, and

(2) the relative dependence of the growth of a tissue on cell division vs

cell growth in size vs the accumulation of non-cellular material.

Section D

36

The former process, which is sometimes referred to as the recruitment of

cells for a particular purpose, appears to be under the control of hormones

and growth factors and could play a major role in determining the

composition of growth. A related question concerns the changes in the

ability of a tissue to recruit new undifferentiated cells as an animal grows

and develops. In the case of muscle, for example, most of the evidence

indicates that no new muscle fibres are produced after birth, but some work

suggests that there are certain growth factors that can bring about the

formation of new fibres post-natally under some circumstances such as in

an injured muscle. IGF-1 has been implicated in this role.

Point (2) above is illustrated in Figure D.6 where it is shown

diagrammatically that the same increase in growth can be achieved in three

ways. The second two options in Figure D.6 have the advantage that they

do not require cell division or the recruitment of new cells, but there is likely

to be a limit to the size cells can grow to, and extracellular material is not

usually capable of supporting further growth alone, as, apart from water,

almost all components of new tissue have to be produced within cells and

then secreted. For most tissues, growth involves a combination of the three

processes, with a change in emphasis from growth mainly by hyperplasia at

early stages to an increasing dependence on growth by hypertrophy.

Variation in the timing and extent of these changes within the various

tissues may have a major effect on the composition of growth. Good

examples of tissues with an appreciable amount of accretionary growth are

bone and blood.

Figure D.6 Three ways of achieving the same growth in size

Growth in most tissues in all species involves a combination of these options, usually with the importance of growth by hyperplasia declining as growth advances.

Section D

37

Examples of factors affecting carcass fat percentage and M:B are given in Table

D.14 together with a brief explanation of the possible mechanisms involved.

Dressing-out percent is not included in this Table because factors affecting it

were covered in Section D.3, and factors affecting the other composition

characteristics (Table D.6) are not included either. Factors affecting LMY% or

SMY% are considered indirectly rather than directly, because LMY% is a function

of fat% and M:B.

Table D.14 Examples of factors affecting carcass fat% and M:B

(Numbers following each example correspond to the categories in Table D.12)

Nature of the Effect Explanation and Mechanisms Involved

Carcass fat% at a set weight:

Animals with a heavier mature weight will tend to be less fat (1).

Attributable to the fact that they are at a lower proportion of their mature weight.

The physiological reasons for such differences are not well understood.

Clearest for breed differences with sheep and cattle.

Entire males are normally significantly less fat than those that are castrated (1).

Due to the removal of the testosterone-producing cells with castration.

Cryptorchids are usually very similar to entire males with respect to fatness as they still produce the male hormones.

Females are more fat than castrate males for some species (cattle, sheep), but less fat for others (pigs).

Animals from genetic lines where selection has been for a decreased fat depth will generally be less fat (1)

Due to the fact that measures of fatness are moderately heritable.

A number of possible mechanisms may be responsible for differences between genetic lines.

Animals that have been grown on a very high plane of nutrition may be more fat than those grown on a low plane of nutrition to the same weight (3).

This effect, which is not always apparent, is sometimes called an overflow effect because it appears that the lean-tissue growth capacity has been saturated and excess nutrients overflow into lipid synthesis.

This effect is more likely in animals with lower lean-tissue growth potential (e.g. females, early-maturing animals).

More commonly reported with pigs than with ruminants.

Table D.14 Continued over.

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Table D.14 Cont. Examples of factors affecting carcass fat% and M:B

(Numbers following each example correspond to the categories in Table D.12)

Nature of the Effect Explanation and Mechanisms Involved

Animals that have received a diet with a high protein to energy (P/E) ratio will often be less fat (3).

This effect will arise when the protein content of the diet is limiting, relative to the energy content.

Most apparent in monogastric animals and in young ruminants.

Supplying extra protein to finishing ruminants may act as a high-plane-of-nutrition effect leading to increased rather than decreased carcass fatness.

Animals treated with somatotrophin (growth hormone) will be less fat (3).

Growth hormone decreases fatness mainly by stimulating hormone-sensitive lipase in adipocytes.

The same treatment will stimulate muscle and bone growth leading to an overall increase in growth rate.

Somatotrophin is not widely used as a commercial growth promotant for meat animals although its use is legal in some countries.

Spring-born lambs at the end of their first winter will tend to be less fat than lambs of the same weight at the start of the winter (3).

The physiological mechanisms involved in this effect are not known.

The ability to build up energy supplies in the form of fat before winter may have enhanced an animals chances of survival in the wild.

Carcass M:B at a set weight:

Breeds developed for meat production or for draught purposes often have better M:B than breeds selected for other purposes such as milk or wool production (1).

The physiological basis of such breed differences are not well understood.

Cattle homozygous for the muscular hypertrophy (MH) or double muscling gene will have higher M:B levels (1).

Not all muscles are significantly larger in such animals, but those that are, have both more muscle fibres, and also fibres that are larger.

Double-muscled animals also have very low levels of carcass fat, but unfortunately they are more likely to have reproductive problems.

The MH gene appears to decrease the production of myostatin which is involved in limiting the recruitment of cells during embryonic muscle development.

Table D.14 Continued over.

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Table D.14 Cont. Examples of factors affecting carcass fat% and M:B

(Numbers following each example correspond to the categories in Table D.12)

Nature of the Effect Explanation and Mechanisms Involved

Sheep homozygous for the callipyge gene have higher M:B values (1).

Extra muscle growth appears to result from a reduced turnover of muscle protein because a specific protease is less active.

Sheep with the callipyge gene also have very low fat levels, but unfortunately the meat of certain cuts is significantly tougher.

Carcasses from ewe lambs have higher M:B values than carcasses of ram lambs.

This appears to be primarily due to a finer bone structure rather than thicker muscles.

The physiological mechanisms involved are not known, but are presumably associated with hormonal differences.

Comparable male/female differences are not apparent for cattle or pigs.

Carcasses from animals treated with beta-adrenergic agonists (BAAs) (e.g. clenbuterol) have higher M:B values.

These substances mimic the action of hormones of the adrenal medulla, and have their effect by inhibiting protein turnover in muscle.

BAAs are not legal as growth promotants and tend to lead to tougher meat.

(c) Factors Affecting Meat Quality Characteristics: The only two meat

quality characteristics to be considered in this section for illustrative

purposes are meat tenderness and meat colour. In order to understand the

way in which meat tenderness in particular is affected by various factors, it

is necessary to have a reasonable knowledge of basic muscle structure, of

how muscle functions in the living animal and of the changes that take place

in meat following slaughter.

Muscle structure is similar for all species of animal. A skeletal muscle (as

distinct from cardiac and smooth muscle) is made up mainly of muscle

fibres (usually >96%), but these fibres are set within a connective tissue

framework made up largely of the protein collagen (from 0.5% to 3% of

muscle weight), and some adipocytes are also present between the muscle

fibres (the marbling fat; usually < 3%, but in some cases >20%). The

connective tissue is referred to as endomysium when it surrounds

individual muscle fibres, perimysium when it surrounds bundles of fibres,

and epimysium when it surrounds the whole muscle. The epimysium is

continuous with tendons which attach the muscle to bones.

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A muscle fibre is equivalent to a cell in other tissues with an outer

membrane (the sarcolemma) and similar internal structures to most other

cells despite the fact that they often have special names (e.g. sarcoplasm

rather the cytoplasm, sarcoplasmic reticulum rather than endoplasmic

reticulum). The main differences between skeletal muscle fibres and most

other cells are:

1. A skeletal muscle fibre is a multinuclear syncitium which permits its

very long thin shape. 2. The nuclei cannot divide (a common characteristic of specialised

cells). 3. Muscle fibres contain varying amounts of the oxygen-carrying

molecule myoglobin. 4. They have a very well developed contractile system enabling them to

carry out their primary function of bringing about movement by contraction.

The contractile apparatus is highly organised, with each fibre containing

several thousand myfibrils arranged in parallel along the fibre, each of

which is surrounded by envelopes of sarcoplasmic reticulum. Each myofibril

in turn contains hundreds of thick and thin filaments running along the

length of the fibril and arranged precisely into repeating sarcomeres as

illustrated in Figure D.7. One sarcomere is the distance between adjacent Z

discs, and contraction within the sarcomere is achieved when the thick

myosin filaments slide relative to the thin actin filaments in what is referred

to as the sliding filament mode of contraction. It can be seen from Figure

D.7 that contraction (the Z discs move closer together) is achieved without

any change in the length of individual filaments. In order for contraction to

take place there must be a supply of energy in the form of ATP, and a nerve

impulse is required to bring about the release of calcium ions from within the

sarcoplasmic reticulum.

Muscle structure is similar for all animal species but species may differ in

the proportion of intramuscular fat and collagen associated with the muscle.

Further details on muscle structure and function are provided in the

reference with Figure D.7 and in almost any basic biology or physiology text.

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Figure D.7 The arrangement of filaments within a sarcomere

A single sarcomere is shown at three levels of contraction. (From: Davies, A S 1989: The structure and function of carcass tissues in relation to meat production. Pp. 43-59. In: Purchas, R W, Butler-Hogg, B W, Davies, A.S. (Eds.) Meat Production and Processing. NZ Society of Animal Production Occasional Publication 11, Hamilton.)

Changes that take place in muscle following slaughter are very important

because of the implications for several meat quality characteristics. The

sequence of some key post mortem changes that are of direct or indirect

importance to meat quality follows:

With exsanguination (bleeding) the muscle loses both its oxygen

supply as well as any external supply of nutrients. The loss of oxygen (apart from a small amount associated with the

myoglobin) causes a switch to anaerobic (glycolytic) metabolism with the end product of glucose catabolism being lactic acid, rather than the CO2 and water as in aerobic metabolism. The accumulation of lactic acid results in a drop in pH (Figure D.8).

In addition to leading to a different end product, anaerobic metabolism is also much less efficient as a means of producing ATP (by a factor of about 15 times), so ATP concentrations drop after an initial delay phase. During the delay phase ATP levels are maintained by the release of phosphate from creatine phosphate.

Apart from its role as an energy supply, ATP is also responsible for maintaining the plasticity or extensibility of muscle, and a drop in ATP concentration eventually leads to a loss in extensibility as the myosin and actin filaments lock together to form actomyosin (Figure D.8; lower graph). This loss of extensibility at the filament level causes the whole muscle to become stiff and is referred to as the onset of rigor mortis.

Muscle pH will decrease as long as glycolysis and the accumulation of lactic acid continues. This will cease either because glycogen supplies are depleted or are inaccessible in some way, or because the acid conditions prevent glycolysis (at a pH of about 5.5).

The final pH reached is referred to as the ultimate pH. Normal ultimate pH values are in the region of 5.4 to 5.6, and elevated levels

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will arise primarily when muscle glycogen levels are low at the time of slaughter. Depleted levels of muscle glycogen (< ca 1% of muscle weight) can arise from pre-slaughter stresses (due the action of adrenaline on glycogen breakdown), from excessive muscle activity (e.g. due to the riding activity of bulls), or from very poor nutrition.

Figure D.8 Changes in muscle pH and extensibility post mortem

Changes are shown for a typical longissimus lumborum (loin) muscle in a lamb carcass, with and without electrical stimulation. The rate of pH drop will be somewhat slower for beef muscle, and considerably faster for pork or chicken muscle. The time to reach a pH of 6.0 is shown because the muscle should not be chilled to less than 10

oC when

the pH is above 6, if cold-shortening and toughening is to be minimised. (For details on typical ES specifications and effects on meat tenderness, see: Chrystall, B B et al. 1989: Trends and developments in meat processing. Pp. 185-208. In Purchas, R W, Butler-Hogg, B W, Davies, A.S. (Eds.) Meat Production and Processing. NZ Society of Animal Production Occasional Publication 11, Hamilton.)

Selected examples of factors affecting meat tenderness and meat colour

(including fat colour), along with brief explanations of the mechanisms involved,

are given in Table D.15. This is only a small sample of the many effects that

have been reported. They have been included here to illustrate the fact that meat

quality can be influenced at many different points in the overall meat-production

process, but it is important to remember that only two of a large number of meat

quality characteristics are considered here.

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Table D.15 Examples of factors affecting meat tenderness and meat and fat

colour

Nature of the Effect Explanation and Mechanisms Involved

Meat Tenderness:

Cattle of the Bos indicus breeds (zebu type) produce meat that is more likely to be tough than beef from Bos taurus breeds (1).

The reason for this difference is not totally clear

Concentrations of calpastatin have been shown to be higher in Bos indicu