genetic - reproduction

7/27/2019 Genetic - Reproduction

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Interactions:Genetics andReproduction

IRM-16

Dairy IntegratedReproductiveManagement

Dr. E.R. JordanWest Virginia University

In general, genetic improvement programs havenot been directed toward increasing reproductiveefficiency in dairy cattle. Reproductive traits havesmall heritability estimates; therefore, genetic

selection requires several generations to achievemeasurable improvements.

Some reproductive traits which have been studiedare:qdays open

q services per conception

q conception rate

q semen production

q semen fertility

q calving ease

Tremendous genetic improvements have occurredin the dairy cattle population. The progress has

been due to the identification and the subsequentavailability of superior sires through artificialinsemination (Al). Since 1973 a new technology,embryo transfer (ET), has provided an additionalavenue for genetic progress. The genetic andeconomic returns from this technology remainsubject to the selection pressure placed on thedams and sires used.

In this fact sheet, the heritabilities of reproductiveperformance traits, the influence of reproductiveefficiency on genetic progress, and the effect ofembryo transfer on genetic progress arediscussed.

Reproductive Performance

Estimates of heritability for reproductive traits arelow (Table 1). For example, estimates for dystocia(difficult calving) range from .03-.15. This means 3to 15% of the variation in dystocia scores is dueto additive gene action. Animal performance is acombination of the genetic ability of an animaland the effects of the environment to which theanimal is subjected. Thus 85 to 97% of thevariation in dystocia is modified by environmentalinfluences or non-additive gene action.

Table 1: Range of Heritability Estimates ofReproductive Traits in Dairy Cattle.

Trai t HeritabilityDays open .01-.10

Dry period .15-.35

Breeding problems .00-.20

Calving interval .00-.10

Services/conception .00-.10

Age at first calving .15-.70

Dystocia .03-.15

Adapted from Mao, I.L. 1984. Variations in Dairy CattlePopulation: Causes and Consequences. Proc. National

Invitational Workshop on Genetic Improvement of Dairy CattleWisconsin p. 34.

Although reproductive traits have low heritability,progress can be made. In Sweden, the incidence ocystic ovarian disease (Fact Sheet IRM-25) was10.8% in 1954. By selecting against sire lines witha high incidence of cystic ovaries the occurrenceof cysts was reduced to 5.1% in 1961.

Because the heritability estimates for reproductivetraits are low, environmental influences dominatethe genetic influence. Management plays a keyrole in determining the animals performance for a

given reproductive trait. Cows may not be bredquickly in an attempt to increase productionrecords. The number of days open to firstinsemination varies considerably from herd to herddepending upon whether the producer startsbreeding cows at 45, 60 or 90 days postpartum.In addition to conscious effects of management,failure to detect estrus (Fact Sheet IRM-6)increases days open as a result of the missedopportunities for cows to conceive. Many studieshave shown this to be the single most costlymanagement error in dairy herds.


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Recently, the genetic relationship betweenreproductive performance and milk yield has beenexamined. Some researchers have reported adecline in days open as milk yield increased.It appears that reproductive efficiency can bemaintained at an acceptable level as long asmanagement is excellent. Therefore, economicsdictate continuing intensive selection forproduction testing.

The genetic relationship between first lactation

yield and virgin heifer fertility are complementary.Those heifers that will milk the most tend to breedeasier than those heifers that will milk less duringthe first lactation. Thus if there is an antagonisticrelationship between reproduction in lactatinganimals and milk yield, it may be caused by thestress of production.

Approximately one-fourth of the cows are culledbecause of reproductive problems. Since conceptionrates are a combination of the fertilityof the bull and cow, it is critical to evaluate themale component of reproduction. To maintainacceptable levels of semen fertility and sperm

concentration, Al studs have extensive qualitycontrol programs.

The Al studs are investigating the relationshipbetween semen volume, concentration and totalsperm and the yield of milk and milkfat. The lackof relationship indicates semen production in bullsand milk production in daughters are regulated byindependent gene complexes. Therefore, selectionfor increased milk production should not altersemen production.

Selection for semen production will yield littleresponse. The heritability of semen volume,concentration and total sperm are .12, .00 and .02,

respectively. Determining management practicesto maximize semen output from bulls with highpredicted merit will provide more rapid returnsthan selecting for semen production.

Currently, the Al studs are rating the fertility ofbulls within their respective studs. One systemranks Non-Return Rate from 1-5. The scores areinterpreted as 1 = High; 2 = Above Average;3 = Average; 4 = Below Average; and 5 = Low.Another stud does a similar ranking; however, 1 islow and 5 is high. Therefore, it is critical thatproducers familiarize themselves with the fertilityrating system(s) used by the Al stud(s) from whichthey purchase semen.

In addition, the National Association of AnimalBreeders (NAAB) ranks Holstein Al sires for theease with which their calves are born. The calvingease summary is published each July. There aretwo measures included.

The first is the probability of being easier thanbreed average. This estimate takes into accounthis estimated transmitting ability for calving easeand how accurately his transmitting ability hasbeen estimated. The sire with the greater

probability is likely to be the better choice.

The second is the expected percentage of difficultbirths. A direct estimate of the expected problembirths is provided when a bull is used to breedaverage heifers. An average bull would haveapproximately 11% difficult births, but it will varyfrom 1-2% for the best bulls for calving ease up to30-40% for the poorest bulls. Thus, particular careshould be taken when sires are selected for use onvirgin heifers.

When selecting sires the following guidelines canbe

1.

2.

3.

4.

5.

used:

Base primary selection on production or thecombination of production and type best suitedto the herds needs.

For open heifers, choose from the siresnormally used. However, select only thosebulls whose calves will be born with the leastexpected difficulty.

Avoid using difficult calving sires to breedcows having a history of calving problems.

Place little emphasis on calving ease forsecond and later calving.

Use sires with high fertility ratings on problembreeders.

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Genetic Progress

The four factors determining genetic progress are:accuracy of selection, selection intensity,generation interval, and genetic variation.Management skills can influence the generationinterval and selection intensity. Genetic progressand profit potential are reduced by managementdecisions that result in increases in the number ofanimals culled for involuntary reasons orlengthens the generations interval. Reproductive

performance is a major management concernsince extending the calving interval and forced orinvoluntary culling increase the generation interval,while involuntary culling also decreases theselection intensity.

By meeting the reproductive goals of a 12 monthcalving period, 1.7 services per conception, 60% ormore first service pregnancies and/or 24-27 monthsof age at first calving, producers have anopportunity to maintain optimum generationintervals and maximum selection intensity.Producers without the management skills to use Alto attain the aforementioned goals make economic

and genetic sacrifices.If heifers are not bred to superior Al sires,progress is reduced. Using genetically superiorsires increases the genetic progress 3 to 4 timesthe rate of herds using natural service. To reap thegenetic and economic rewards of Al, accurateestrous detection and high conception rates arenecessary.

As the calving interval increases, the number ofcalves born and the number of bred heifersavailable for herd replacement per year is reduced(Table 2). The end result is less voluntary culling ofcows which reduces selection intensity. Less

profitable cows must be kept.By extending the calving interval from 12 to 14months and by delaying the entry of first calfheifers into the milking herd from 24 to 27 months,approximately 5 months is added to the generationinterval. If a beef bull is used on first calf heifers,the calves cant be used for replacement heifersand the generation interval is lengthened byapproximately 4 months. The result is a major lossto genetic progress.

Embryo Transfer

Since 1973, the number of ET calves has increasedtremendously (Table 3). ETs currently account forless than 2% of registered births. However, futuregrowth may reach 500,000 by 1990.

Table 3: Number of ET Female BirthsRecorded by U.S. Breed Organizations by

Year of Birth

Year of birth Total ETs Registered

1973 101974 81975 161976 1001977 1721978 3101979 7971980 17191981 1337Total

4467

Adapted from Aitchison, T. E., 1984. Embryo TransferGeneticand Economic Opportunities, Proc. National InvitationalWorkshop on Genetic Improvement of Dairy Cattle, Wisconsin.p. 95.

Some of the many possible uses of ET are:production of replacement females, progenytesting females, production of bulls for Al progenytesting, testing for carriers of genetic recessive,exportation of embryos for profit, conversion of aherd from grade to registered animals, genetic andlaboratory research, progeny testing bulls with

fewer numbers of offspring, and commercialexploitation. To this point, ETs have not been agreat source of genetic progress, at least forproduction. However, more ETs are being used inAl sampling programs and in milking strings asreplacement heifers.

Table 2: Effect of Calving Interval on Number of Replacements in 100-Cow Herd

Calving Ave. No. Calves Bred Heifers Available forInterval Born per Year Herd Replacement per Year*

12 100 4013 92 3714 84 3415 76 31

* Assumes 80% of the female calves born survive to freshen as first calf heifers.


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Not all of the possible uses are economicallyfeasible. To produce replacement females from thetop end of the herd the cost of an embryo transfermust be in the range of $50-$100/embryo.Low estimates are in the $300/embryo range today,but transfer costs are closer to $1200 in general.Costs can be minimized when enough home-grownheifers are available as recipients.

By combining the desire to obtain replacementfemales from a superior individual in the herd and

selling male calves to the Al industry, embryotransfer can be economically justifiable.The market value for bull calves average $2,000-$3,000 currently. At this price, or the higher priceobtained in some auctions, embryo transfer maybecome extremely profitable.

Currently, using ETs to progeny test females doesnot appear to be a wise use of resources.When information on multiple offspring is available,it will be used to increase the future accuracy ofevaluation. The use of ETs to identify the malesand females which are carriers of undesirablegenetic recessives is a viable alternative.

Animals with superior genetics which have beenrecessive tested are more likely to be usedas parents for young sires.

Summary

Although most reproductive traits have lowheritability, progress can be made slowly.Management (environmental influences) plays amuch larger role in reproductive efficiency thangenetics. However, reproductive inefficiency canslow genetic progress by increasing the generationinterval and by reducing the selection pressure.

Manipulation of the reproductive processes hasresulted in the new technology of ET. Although

there has been rapid adoption of ET, many realisticopportunities to accelerate genetic progressremain.

Trade or brand names are mentioned only for information.The Cooperative Extension Service intends no endorsement norimplies discrimination to the exclusion of other products whichalso may be suitable.

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