quantitative ruminant nutrition--a green science

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antitative Ruminant Nutrition--A Green Science

p://www.ciesin.org/docs/004-180/004-180.html[13/08/2014 09:47:55 p.m.]

Reproduced, with permission, from:Leng, R. A. 1993. Quantitative ruminant nutrition - A green science.Australian Journal of Agricultural Research 44: 363-80.

Quantitative Ruminant Nutrition - A Green Science

R. A. Leng

Department of Biochemistry, Microbiology and Nutrition, University of New England, Armidale, N.S.W. 2351.

Abstract

Knowledge of quantitative digestion and metabolism in ruminants was developed most rapidly when isotopedilution techniques became easy to apply, facilitated by improved instrumentation and mathematical approaches.The Armidale group led by Professor E. F. Annison and Dr. D. B. Lindsay were at the forefront of thesedevelopments in the late 1950's. Since then knowledge in this area has developed at an ever increasing rate. Thedata that accumulated from the quantitative approach led to simple or complex models of animal digestion,

metabolism and growth. These in turn led to much questioning of the dogma of feed evaluation and feedingstandards as they applied in practice, especially for ruminants fed on poor quality forages. The knowledge thatdeveloped has clearly shown that the way toward substantial increases in productivity of ruminants on forage baseddiets is through the balanced nutrient approach that considers the efficiency of the rumen ecosystem and theavailability of dietary nutrients post-ruminally. With increasing emphasis on quality-beef markets at the presenttime, it seems likely that the time is ripe for application of much of this knowledge. The major breakthroughs havecome about by recognition of the nutrients required to balance a ruminant's diet where the animal depends on theend-products of rumen fermentation (i.e. on a forage-based diet). When this is achieved, the increase in efficiencyof use of nutrients lifts the overall nutrition of the animal to a level that is well above that predicted from feedingstandards, based on the metabolizable energy content of the supplement or the total diet. This understanding,together with the stoichiometry of rumen fermentation, has indicated an important approach to help ameliorate the

greenhouse effect, that is, lowering of enteric methane production per unit of feed intake or per unit of animalproducts from ruminants by strategic supplementation.

Keywords:methane production, greenhouse effect, feed conversion efficiency.

Introduction

The origins of the development of isotope dilution as a means of quantitating nutrient turnover in animals can betraced mainly to two laboratories: Professor Max Kleiber's laboratory in Davis, California, and Professor FrankAnnison's in Armidale in the mid to late 1950s. Since those early days numerous laboratories have made majorcontributions to advance knowledge of the quantitative aspects of metabolism and digestion in ruminants, not least

those scientists who pioneered the mathematical analysis of isotope dilution in primary and secondary pools in theanimal (see Rescigno and Segre 1966).

The early research centered initially on analytical technology and the major questions related mainly to thedigestive mode of the ruminant: what nutrients were important in their metabolism and how did they rank inquantitative importance (Annison et al. 1967). At that time debate raged over whether glucose was important inruminant metabolism (Annison and White 1961) and whether ketone bodies were normal substrates or toxic by-products of fat metabolism. The period was exciting. As an example of the lack of knowledge of the quantitativecontribution of circulatory nutrients to the nutrition of ruminants in the early 1960's, the following example isgiven: when radioactivity labelledB-hydroxybutyrate was first prepared by incubating (l4C)-butyrate with sheepliver slices in vitro(Leng and Annison 1963), the opportunity arose for the first time to examine ketone body

metabolism in the whole animal (Leng and Annison 1964). In the first run in which labelledB-hydroxybutyratewas infused into a mature sheep and bothB-hydroxybutyrate and carbon dioxide isolated for assay of specificradioactivity, it was estimated that over 80% of the carbon dioxide produced by the sheep arose from oxidation ofB-hydroxybutyrate (Leng, R. A., unpublished observations). This was hailed as a major breakthrough inknowledge; however, it was soon realized that a rather simple analytical error had created an enormous error in the
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estimate. The specific radioactivity of the extractedB-hydroxybutyric acid had been estimated in a two stepprocess. The radioactivity ofB-hydroxybutyrate was estimated after evaporating the extracting solvent withoutprior conversion to a salt; a proportion of theB-hydroxybutyrate apparently volatilized with the solvent. Theconcentration of the acid had, however, been determined by titration of a separate portion. The result was a verylow apparent specific radioactivity of theB-hydroxybutyrate in blood and an extremely high estimate of its entryrate. Repeating the experiment with neutralization of the solvent prior to evaporation putB-hydroxybutyrate intoperspective as a minor nutrient in ruminant metabolism.

The early technology development and surveys of the contribution of nutrients to oxidative metabolism (Annison et

al. 1967) gave way to the use of these technologies to quantify differences between animals on different diets. Thisled ultimately to their use in research aimed at problem solving on a local and global scale.

Quantitative Digestion and Metabolism of Nutrients and Feed Evaluation

Since the early 1950s knowledge of the ruminant animal's digestion and metabolism has developed enormously,and we now have a picture of the animal which allows reasonable models of the animal's function to be describedand rates of production at times to be predicted (see Black et al. 1987a). The information on quantitative nutritionis slowly changing the approach to feed evaluation and feeding standards for ruminants. Up until 5 years ago, thesestandards were based on metabolizable energy content of a feed, and it was assumed that microbial protein to

'energy' supply was balanced, such that nutrients above maintenance requirements were used with equal efficiency.

Those of us who have attempted to apply knowledge of quantitative digestion and metabolism to the establishmentof feeding systems in the real world soon recognized that these assumptions do not, generally, apply; in particular,they do not apply to the 'forage fed' ruminant. Under production systems, nutrient availability to the rumenmicrobes often sets the upper limit of production through a low microbial growth efficiency in the rumen (whichdirectly results in a low protein to energy ratio in the nutrients absorbed). The other limitation is the amount ofdietary protein that escapes rumen digestion, but which is digested in the intestines. Ruminants require moreprotein generally (particularly in tropical climates) than arises from microbial growth in the rumen whether thelatter is efficient or inefficient (Leng 1990a).

The majority of the world's ruminants depend over their lifetime on forages that can be described only as poorquality, in which the limitations to production are a low protein supply from the microbial ecosystem and a virtualabsence of dietary bypass protein.

Microbial Growth and Volatile Fatty Acids Production in the Rumen

The most important concept that bears on the feeding strategies used for ruminants, is that microbial proteinavailable and total volatile fatty acids (VFA) produced in the rumen are inversely related (see Hungate 1966;Baldwin 1970; Leng 1982). This arises because, under the anaerobic conditions of the rumen, the feed nutrientsprovide both the substrate for microbial cell synthesis and also the potential energy as ATP generated throughconversion of feed biomass to VFA. Under variable feeding systems and prevailing production systems, this means

that in ruminants the yield of microbes relative to VFA produced is variable, that is, ATP is used with variableefficiency. Most models of the fermentative processes in the rumen suggest a pattern of relationships among theend products of fermentative digestion as shown in Fig. 1.

The efficiency of microbial growth, in the rumen (expressed as g cells produced per ATP, or per mole of VFA, orper g of carbohydrate fermented) appears to be highly variable depending on the feeding conditions. This variabilitymust be much greater in the field than is generally observed in the controlled laboratory studies. For example,responses under field conditions to supplements aimed largely at correcting nutrient deficiencies of the rumen usingmulti-nutrient blocks (see Kunju 1988; Habib et al. 1991; Hendratno et al. 1991, and Saadulah 1991) are often largeand are widespread through diverse feeding systems (see Loosli and McDonald 1968).

The factors that affect microbial growth efficiency and therefore the protein relative to VFA available for digestionand absorption are:

1. A deficiency of any microbial growth factor (e.g. ammonia, sulfur, phosphorus, amino acids, etc.) in the feed orinduced some time after feeding in rumen liquor because of rapid absorption of the nutrient. For example, ammonia
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levels decrease rapidly after feeding to reach deficiency levels after 12 h on straw based diets, even if these aresupplemented with urea.

2. The relative amounts of carbohydrate and protein that are fermented (fermentative degradation of protein andamino acids is coupled to a lower (approx. half) ATP generation than equivalent quantity of carbohydrate). A highprotein to carbohydrate ratio in the diet can lead to a relatively low microbial protein to VFA ratio in the end-products of fermentative digestion where the dietary protein is easily and rapidly fermented in the rumen.

From the literature, the evidence seems to suggest that, if there are no deficiencies of microbial growth factors in

the rumen, the source of the carbohydrate (sugar v. starch v. fibre) and the flow rate of liquor from the rumenappear to have little influence on microbial growth yield (see, for example, Maang Chang et al. 1989).

Bacterial lysis may occur from death of bacteria when their substrate in the rumen is exhausted (Hespell 1979).Lysis under these conditions may be triggered by temperate lytic phages that already infect rumen bacteria (seeKlieve et al. 1989). Degradation in situof rumen bacteria also results through predation of bacteria by protozoa(Coleman 1975). Lysis of bacteria in the rumen can severely reduce protein to energy ratio in the nutrientsdelivered to the animal for digestion and absorption (Leng and Nolan 1984). It is emphasized here that although, attimes, microbial growth efficiency in the rumen can be high, the ratio of protein relative to energy that is availablefor digestion and absorption can be reduced markedly and can be extremely low due to this lysis. Nolan and Leng(1972), Mathieson and Milligan (1972), and Nolan and Stachiw (1979) demonstrated using stable isotopes of

nitrogen that 25-50% of the microbial protein pool in the rumen may be turning over in situand thereforeunavailable.

Although there have been numerous attempts to measure microbial and total protein entering the duodenum ofruminants on various dietary regimes, the large errors associated with such measurements have precluded asystematic examination of the factors that affect both microbial growth efficiency and the protein entering theduodenum relative to the VFA produced in the rumen and absorbed. Research with animals cannulated at therumen and duodenum and using markers have rarely given an accurate picture of the relationship between the end-products of fermentation (microbial cells entering the duodenum and VFA production). In some studies microbialgrowth in the rumen measured by these techniques have been at twice the theoretically possible rate, indicating thatthese technologies are very inaccurate and sometimes biased. In reviewing the literature, however, Czerkawski

(1986) found values for microbial growth under a wide variety of feeding conditions to be very constant betweenstudies.

Estimates of Y-ATP (g of dry microbial cells produced per unit of ATP theoretically generated in VFA formation)suggest that under good rumen conditions Y-ATP may be of the order of 10-14. Data collected from in vitrostudies suggest that Y-ATP can be extremely low on diets where key microbial nutrients are deficient (perhaps aslow as Y-ATP 4-5) and that with good conditions and when the protozoa are eliminated from the rumen ecosystemthis could go as high as Y-ATP 20 (see Veira et al. 1983). These growth efficiencies would result in protein toenergy ratios in the nutrients presented for absorption of, less than 5 to 20-30 g protein MJ of VFA energyrespectively, indicating the scope for improvement in P:E ratios is high and therefore feed utilization improvementsmay be enormous (see later).

Balancing Nutrition and Feed Conversion Efficiency

Numerous reviews are now available on the need of ruminants for more protein than is supplied by the endproducts of rumen digestion whether the diets are high in grain (0rskov 1970), molasses (Preston and Willis 1974)or forage (Leng 1990b) or even high-protein, high-digestibility ryegrass (McRae 1976; Fraser et al. 1991). Theeffect of supplementing a bypass protein to ruminants on all these diets is to induce an improvement in theefficiency of conversion of feed to liveweight gain.

Although considerable responses are recorded to supplements of digestible protein that escapes degradation in therumen in ruminants on all diets, the responses to animals on fibrous feeds have been the most dramatic. This is

discussed in more detail below.

Understanding of the stoichiometry of the rumen and the requirements for additional feed protein that provideamino acids directly to the animal has allowed a new approach of establishing feeding strategies using locallyavailable feeds. Essentially this is as follows:


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For maximum production from a particular feed, it is necessary to ensure optimal rumen condition formicrobial growth and adjust protein to energy ratios in the nutrients absorbed with a bypass protein tooptimize efficiency of utilization of the absorbed nutrients (see Leng 1990a).

This approach is now being applied on a large scale in many parts of the world. It suffers criticism because it doesnot depend on feed analysis which then becomes largely irrelevant (except for N estimation) in establishing feedingsystems. It relies on 'rules of thumb' that require a depth of appreciation of the science of ruminant nutrition fortheir effective application A more scientific approach must eventually emerge, but at the present time we have no

way of predicting the microbial growth efficiencies or the balance of nutrients absorbed by ruminant animals onany diet.

A major contribution to knowledge and production would be made by research that allowed ease of prediction of:

the efficiency of microbial growth in the rumen, andthe ratio of amino acids relative to energy in the nutrients absorbed.

Armed with such a tool a considerable number of low productivity syndromes in practice could soon beunderstood.

When the two 'rules of thumb' concerning supplementation to achieve optimal microbial growth and for supply ofprotein post ruminally are applied to diets based on relatively poor quality roughages, surprisingly high levels ofproduction can be achieved. For a diversity of research results that support this concept, see Hennessy et al. 1989;Perdok and Leng 1989; Silva et al. 1989; Hennessy and Williamson 1990 and Leng 1990b.

Environmental Considerations

It is of major importance that the nutritional principles discussed above which evolved from research in the 1950'sin Armidale, have been applied by farmers to increase productivity in various parts of the world. In recent years ithas become apparent that these strategies have the potential to make a major contribution to amelioration of thegreenhouse effect by reducing enteric methane emissions from the world's ruminants (see Leng 1991).

Ruminants contribute approximately 18-20% of the global methane produced annually (Gibbs et al. 1989). Entericmethane emission is one of the few global sources of methane that can be relatively simply reduced. It is more easyto manipulate than, for instance, methane produced from marshes or in rice production. Methane accumulation inthe atmosphere requires only a slowing of emissions by 15-20% for world atmospheric concentration to stabilize.

A brief overview of the greenhouse effect is probably needed at this point in the discussion in order to present thelink between quantitative nutrition and the strategies for reducing enteric methane generation.

Environmental Change

Environmental change, over the next 50-100 years, due to the warming effect of the accumulation of gases in the

atmosphere, will clearly necessitate changes in resource allocation and utilization in the world and will directlyaffect all countries.

Although future climate change is being predicted from models, and these take into account a large number ofvariables, the useful information they generate is limited by the knowledge base and the unknowns. In fact,modeling is at present largely defining what is not known in terms of what will effect change in temperature,rainfall and sea levels, let alone sea currents, wind, sunshine hours, soil moisture, and the incidence of pests anddiseases of man, animals and plants. There are, of course, a number of skeptics who claim that global warming isnot going to occur.

There will be some beneficial effects through a warmer climate in some parts of the world, but on a global basis

disadvantages will undoubtedly outweigh any advantages. The unknowns make it essential to quickly put in placetechnologies to slow green-house gas emissions and stabilize these in the atmosphere. The real point is thatamelioration techniques put in place now have important associated benefits. In general, the strategies fordecreasing greenhouse gas emissions mean reduction in fossil fuel utilization and general reduction of the releaseof pollutants into the atmosphere. Animal production strategies that target a decreased enteric methane release


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generally lead to improved production.

The approximate reductions in anthropogenic emissions required to stabilize atmospheric concentrations of gases tocurrent levels are shown in Table 1.

The long time lag between gas released at the earth's surface mixing in the atmosphere and therefore, warming,together with the reluctance of governments to put into practice legislation to limit, in particular, carbon dioxideproduction from fossil fuels, suggests that there will be a rise in world environmental temperatures of 0.5 to 1deg.Cin the next 25-50 years (see Gibbs et al. 1989).

In the last 30 years the expansion of crop and livestock production has more than doubled throughout the thirdworld, although there are large differences between regions. Increases in human population (94 million in the lastyear), urbanization and improved income levels of the middle class have increased demand for food to such anextent that food surpluses are still rare.

Increases in animal production in the developing countries have mainly been a result of increasing animal numbers(see Jackson 1981). The lack of improvement in efficiency of ruminant production is well documented. In thedeveloping countries low productivity of cattle is amplified by late age at puberty and long intercalving intervals incows. It should be emphasized, however, that this is also a feature of ruminants fed low-quality forages in anycountry including Australia, New Zealand and South America.

New feeding strategies for animals fed on low quality forages (e.g. crop residues, tropical pastures etc.), coupledwith better genotypes, improved management and disease control, particularly in India (see NDDB 1989) haschanged this situation enormously (see Preston and Leng 1991). For example, large increases in milk productioncan be achieved in the tropics without the use of 'fossil-fuel-expensive' grain based concentrates, relying rather onby-products of agriculture (Leng 1991). These will be the only truly available feed resource for the large ruminantpopulations in these countries into the foreseeable future.

The Greenhouse Effect--A Simple Description

The greenhouse effect, or increasing world temperature, is due to the accumulation of gas in the atmosphere (Fig.

2). It is clearly ascribable to the major industrial countries, as some 50% of the increased retention of energy by theatmosphere is a result of the accumulation of carbon dioxide from combustion of fossil fuel. Industrializedcountries presently use 70% of the world's oil production, and it has been much higher in the past (Fig. 3).

The other gases that contribute to increasing temperatures arise from a variety of activities; chloroflouro-carbonshave been created by man, but will be phased out of production by legislation. Methane is an important componentof greenhouse gases in the atmosphere, and is the one most associated with animal agriculture. Methane has athermogenic effect some four to six times that of carbon dioxide.

Prior to the last two decades, world temperatures and composition of the atmosphere had changed little, but nowthere appears to be an ever-increasing rate of gas accumulation (Fig. 4). Undoubtedly contamination of theatmosphere with carbon dioxide, methane and the other greenhouse gases must be reduced or the future of the earthis threatened.

Methane concentrations in the world's atmosphere are rising rapidly, and although they contribute only 19% of theoverall warming, methane is a major component. Methane arises largely from natural anaerobic ecosystems, ricepaddies and fermentative digestion in ruminant animals (Fig. 5), whilst it is oxidized as an energy source bymicroorganisms in undisturbed soils (ref. Mosier et al. 1991). To stabilize its concentration in the atmosphere,methane production needs to be decreased by a mere 10-20%, compared with 80-85% reduction needed for theother gases (Table 1).

Animal Agriculture And The Greenhouse Effect

Animal agriculture contributes to accumulation of methane gas directly through production of methane infermentative digestion in the rumen, and indirectly when faecal materials decompose anaerobically (see Safley etal. 1992).

Ruminants in 'natural' production systems are, in general, individually inefficient. Production increases have
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depended on increasing numbers, or increasing stocking rates without much increase in individual animalproduction on the more fertile grass lands.

There is a growing appreciation that efficiency of feed utilization per unit of production of meat, milk, work etc.can be improved considerably by simple technology inputs. If applied, this could have major implications forstabilizing global atmospheric methane concentrations.

The following discussion on how to reduce enteric methane production by ruminants will deal largely with feedingstrategies that increase the efficiency of production and which eventually produce more product from fewer animals

and less total feed.

'Environment-Friendly' development of livestock production systems demands that the increased production be metby increased efficiency of production and not through increased animal numbers. An increase in numbers ofruminants would put huge pressures on many resources including forests and land that might be afforested.

Methane production from ruminants

World ruminant population densities and estimated methane production rates by animals are shown in Table 2.

Global methane emissions

Methane is accumulating in the atmosphere at a rate of 1% per annum, and it contributes about 19% to globalwarming.

Ruminant animals produce a relatively small proportion (i.e. 15-20%) of the total global emissions. However, thedomestic ruminants represent one of the few sources that can be manipulated. They are, in addition, an attractivetarget as reduction of methane is usually associated with improved productivity. It is estimated that of the entericsources of methane, beef and draught animals contribute 50%, dairy cows 19% and only 9% is from sheep (Crutzenet al. 1986).

Productivity Of Ruminants Fed 'Poor Quality' Forages

The vast majority of ruminants in developing countries and a major proportion of the national herds ofindustrialized countries are supported on the by-products of agriculture or graze forages of relatively poornutritional value.

In general, growth rates, milk production and reproductive rates in these systems are low, compared with thegenetic potential (e.g. liveweight gain is about 10% and rarely exceeds 30% of an animal's potential in a feed year).

In these systems cattle grow to maturity or slaughter weight over 3-5 years, cows produce a calf at 4-5 years, andon average every 2 years thereafter. Milk production on these feeding systems is often below 1000 L/lactation.Cows are often kept largely to produce draught oxen (e.g. in India 20 million cows are kept to provide

replacements to the 80 million draught herd), and in some specialized systems they are kept only for the productionof dung (which is valued as a fuel) and a number of other purposes (e.g. as a bank, for recreation and for religiouspurposes). It is therefore often difficult to sustain an argument that increasing the efficiency of feed utilization is theprimary mechanism for decreasing global methane generation, as it cannot be certain that numbers of livestock willnot increase when these technologies are applied.

Methane Production From Ruminants Fed 'Poor Quality' Forages

Methane output relative to product output of ruminants depends on two factors:

the efficiency of fermentative digestion in the rumen;

the efficiency of conversion of feed to product (e.g. milk, beef, draught power) which in turn depends on thebalance of nutrients absorbed.

Efficiency of Rumen Fermentation of 'Poor Quality' Forages
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In cattle on a poor quality forage, a number of essential microbial nutrients may be deficient and, microbial growthefficiency in the rumen is low. In these conditions methane produced may represent 15-18% of the digestibleenergy, but correction of these deficiencies reduces this to as low as 7%. The relationships between products offermentative digestion and the efficiency of the microbial ecosystem in the rumen is shown in Fig. 1.

Efficiency Of Feed Utilization By Ruminants Fed Crop Residues Or Other Low Protein Fibrous Feed

Research in the past 20 years has clearly illustrated that supplementation of cattle on low quality forage based dietsincreases productivity through increasing efficiency of feed utilization (Leng 1990b).

A mixture of nutrients as can be supplied for instance in a molasses urea multinutrient block lick ensures anefficient microbial digestion in the rumen. A small amount of protein meal that is directly available to the animal(i.e. bypass protein) stimulates both productivity and efficiency of feed utilization (the evidence and theory isdiscussed by Preston and Leng 1987).

Traditional feeding standards are based on the metabolizable energy (ME) content of a feed. The general andtheoretical relationship between ME/kg of feed and growth (g gain/unit of ME intake) are shown in Fig. 6. Theresults of a number of feeding trials with cattle on straw or low-quality pasture and silage-based dietssupplemented with protein meals are shown in the same figure. The efficiency of growth relative to methaneproduction assuming a relatively efficient rumen system calculated from the data in Fig. 6is shown in Fig. 7These

data clearly show the massive reduction in methane production per unit of metabolizable energy intake andliveweight gain that is possible by using strategic supplements that accommodate the requirements of the rumenorganisms and balance the absorbed nutrients to the animal's requirements.

The data in Fig. 8show the theoretical effects on methane production of balancing the rumen and the animal withprotein supplements for experimental data with growing cattle fed straw in research carried out by Saadullah(1984). This indicates the massive potential reduction in methane per unit of liveweight gain that can result fromintroducing these supplementation strategies (Leng 1991).

Provision of molasses urea blocks to draught oxen which in general receive only straw in most developingcountries will have a major effect on methane production, reducing it to perhaps half the present production rate.

Preston (1991) has recently shown that this feeding strategy also improves work capacity of buffaloes.

Milk Production From Low Quality Forages

The same principles of supplementary feeding for growth have been found to stimulate milk production of dairyanimals. The methane produced/unit milk yield under new feeding strategies that emphasize balancing the diet bysupplementation with bypass protein or traditional feeding (i.e. concentrated food) are shown in Fig. 9.Calculationsare made for local dairy cows, imported Friesians in India or Friesians under temperate country management. Thedata take into account lifetime methane production in relation to lifetime milk production. The calculations alsoinclude the effects of the supplementation on age at first calving, intercalving interval and improved milk yield ofsupplemented animals (Leng 1991).

Conclusion On The Potential to Decrease Enteric Methane Production

Large ruminants produce some 15-20% of the global production of methane. Ruminants on low quality feedspossibly produce over 75% of the methane from the world's population of ruminants. Supplementation to improvedigestive efficiency in, for example, draught animals could halve the methane production per unit of feedconsumed. Together with supplementation to improve efficiency of feed utilization and increase product output,this may reduce methane production per unit of milk or meat by a factor of 4-6. Provided animal numbersdecrease, as demand is met, the production of methane from the large populations of animals fed poor-qualityforages could be reduced to below 50%, and perhaps even to as low as 25% of its present rate.

Overall Conclusions

Knowledge of quantitative nutrition provides a powerful tool to develop concepts to undertake a wide range ofproblem-oriented research. These vary from metabolic diseases of ruminants through to amelioration of entericmethane emissions from ruminants.
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In a still rapidly expanding world, food production will need to be given some major priority in the future, and theproblems that will arise will be solved only by researchers with a breadth and depth of understanding of scientificprinciples, a concept promulgated by Frank Annison from those early days in Armidale to the present time.

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Manuscript received 24 August 1992, accepted 23 November 1992

quantitative ruminant nutrition--a green science

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