Selection Goals - Babcock Institute - University of Wisconsin–Madison

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Selection Goals - Babcock Institute - University of Wisconsin–Madison

Babcock Institute for InternationalDairy Research and DevelopmentUniversity of Wisconsin-MadisonDairyEssentials17) SELECTION GOALSMichel A. WattiauxBabcock InstituteDEFINE BREEDING GOALSBreeding goals should be consideredcarefully taking into account the particularsituation of each dairy producer.Ultimately, the goal of selection is to obtainmore desirable cows—cows that give theproducer the highest profitability. Thecharacteristics that make cows profitablemost often include the following:• Production of large amounts of milkeach lactation;• Longevity (many lactations);• Milk produced has the highestpossible market value.Production traits and milk pricingCows that produce large amounts of milkare more profitable because, overall, theyrequire less feed per unit of milk producedthan cows of lower milk production.The market value of milk is an importantfactor to include in defining the goals ofselection. The first two factors mentionedabove are biological and common to allcows in the world, but milk market pricemakes the goal of selection very differentfrom one country to the next and even fromone region to another within a country.Depending on the way milk price isestablished, the most profitable strategy fora dairy producer may be to select cows thatproduce:• The highest volume of milk withoutregard to composition;• The highest volume of milk andamount of fat;• The highest amount of fat and proteinwith no regard to the volume of milkper se;• The highest amount of fat and proteinin the minimum volume of milk.In choosing a strategy for selection,remember that selection should be set forthe long term. Goals that can remainunchanged over many years bear more fruitbecause selection has small but cumulativeeffects over successive generations of cows.In addition, the longer a goal is maintained,the greater the genetic gain over time. Forexample, it took about 20 years of selection(from 1965 to 1985) to improve the geneticmerit for milk production by 1,000 kg in theUnited States. However, even the mostconservative estimates indicate that in thenext 20 years the genetic merit for milkproduction may increase by more than6,000 kg.Longevity (lifetime production) andconformationLongevity is a desired trait for manyproducers. Longevity does not mean oldage; cows that live longer are only valuablefor their tendency to have higher life timemilk production. Selection for longevityitself is inefficient because it is influencedby many factors, most of which are nongeneticin nature. Heritability of herd life isapproximately 8% (low heritability). Inaddition, it might take as many as seven oreight years for daughters of a bull tocomplete their herd lives in order to get240 Agriculture Hall, 1450 Linden Dr., Madison, WI 53706 USA, phone: 608-265-4169, babcock@calshp.cals.wisc.edu 65


Dairy Essentials - Reproduction and Genetic Selectionreliable PTA estimates for sires. By then, ifthe sire is not dead, his genetic merit forproduction traits will likely be surpassed byyounger sires. In reality, the longevity ofcows in many herds depends primarily onthree criteria—a cow usually remains in theherd as long as she:• Remains free of serious mastitis;• Remains free of serious reproductiveproblems (ability to reproduce);• Produces at a level acceptable to thefarmer.Functional type: Udder, feet and legsFunctional type is a term that has beencoined recently to refer to the bodyconformation associated with lifetimeperformance. Functional type differs fromthe conventional "ideal" body conformationthat may be of value for producers whohave animals that get high classificationscores, win show-ring contests, and may besold as breeding stock.In general, conformation traits areinaccurate predictors of longevity.Research shows that production traits arefar better predictors of longevity than anyconformation traits. Thus selection of a bullshould be done on production first andconformation traits should come next.Actually, longevity would be automaticallyselected for by building an index thateliminates the use of bulls with low PTA forproduction and with obvious conformationor reproductive defects.Of all conformation traits, uddercharacteristics—in particular teatplacement, udder depth and fore udderattachment—are most closely associatedwith longevity. Research has shown thatcows with an intermediate udder depthstay in the herd longer than cows at bothextremes (shallow udder and deep udder).It is likely that cows with shallow udderstend to be poorer producers and cows withudders that are too deep are more prone tomastitis or physical injuries.Despite the importance that manyproducers place on foot and leg traits,lifetime performance studies suggest thatfoot and leg traits have a much smallerimpact on herd life than do productiontraits or udder traits.Production traits versus functional typeDespite the common belief that functionaltraits improve the longevity of dairyanimals, cows are rarely culled for poorconformation. There is no doubt that udderinjuries are more frequent on cows withpendulous udders, and cows with seriousleg and feet problems must sometimes beculled. However, as long as the animalsremain productive, most farmers choose tokeep them in the herd. Commercialproducers are often required to cull cowswith health, metabolic or reproductivedisorders regardless of their conformation.Thus it is important to keep in mind that,economically, production traits aregenerally much more important thanconformation traits. Most economic studiesin the United States suggest that productiontraits should receive three to five times theeconomic weight that non-yield traitsreceive in determining the priorities ofselection.Big cows versus small cowsIn the past 25 years, the size of dairy cowsin the United States has increased. As withall the other traits we have discussed so far,this change has both a genetic and anenvironmental component. Bigger cows eatmore and, so the theory goes, producemore. However, larger size is not closelyassociated with milk production. In otherwords, selection for milk production doesnot necessarily increase body size. In fact, aresearch project that started in 1968 showsthat genetic gain for milk production is asrapid for small cows as it is for large cows.However, large cows have substantiallygreater dry matter intake for maintenancerequirements. Thus, when milk production66 The Babcock Institute


17 - Selection GoalsTable 1: Expected relative genetic gain as thenumber of traits in a selection programincreasesNumber of traits1 2 3 4 5 6 7Relative gain (%) 100 71 58 50 45 41 38is equal, small cows are clearly moreefficient producers than large cows.A detailed study in the United States hasshown that dairy producers orientedtoward raising stock for sale at the showring prefer taller, stronger animals. Theseanimals tend to receive better finalclassification scores and rank better in theshow ring.How many traits should be included ina selection program?When selection is performed on morethan one trait, genetic gain is less rapid thanif selection is performed on one trait alone.In general, more than one trait is desirable;however, selecting for more than four tofive traits at the same time will reduce therate of genetic gain considerably.Table 1 shows the loss of selectionpressure as traits are added in a selectionprogram. For example, if the selectiondecision is based on two traits, geneticprogress of either of the two traits will beonly 71% of the progress made when one ofthe two traits is selected alone.traits. Table 2 shows the average PTA of 10sires for six traits (these sires were the top10 sires on the “net merit” index in January1995 in the United States). For example,PTA for milk yield averaged 2,274 pounds,but these top sires for milk yield also hadthe following PTA averages for other traits:type 1.03, fat percentage -0.06%, protein62.5 pounds, fat 70.4 pounds, proteinpercentage -0.04%. Table 2 shows that thehighest average PTA for a trait is obtainedby selecting the top 10 sires for a particulartrait (gray boxes).However, correlation also brings aboutchange in other traits that is sometimessignificant. In our example, selection formilk yield alone (Line 1 in Table 2) is alsoassociated with a PTA for protein yield of62.5 pounds—a genetic gain only a fewpounds short of the possible gain obtainedby concentrating all the selection pressureon protein yield (Line 4, 66.3 lbs.). Anotherinteresting observation is that putting allselection pressure on fat percentage (Line 3)would make us choose bulls among thehighest for PTA fat yield (74.9 lbs.) but lessso for milk yield (average PTA = 1,312 lbs).On the other hand, focusing selectionefforts on pounds of fat (Line 5) makes uschoose bulls that have a substantial positivePTA for fat percentage (0.078) and milkyield (1,831 lbs).Consider the correlation between traitsIn addition,when decidingwhich traitsshould be selectedand what theLine Basis of selection#relative weight ofeach trait in theselection programshould be,remember thecorrelation thatexists betweenTable 2: Average PTAs for the top 10 sires for various traits among siresavailable in the United States in January 1995Average PTA for the selected siresMilk(lbs)TypescoreFat(%)Protein(lbs)Fat(lbs)Protein(%)1 Milk yield (lbs) 2272 1.03 -0.06 62.5 70.4 -0.042 Type score 1788 2.4 -0.03 51.4 56.7 -0.023 Fat (%) 1312 0.574 0.121 50.3 74.9 0.0444 Protein (lbs) 2044 1.047 -0.01 66.3 72.8 0.0115 Fat (lbs) 1831 0.933 0.078 58.6 84.7 0.0076 Protein (%) 1293 0.376 0.096 54.9 68.8 0.067University of Wisconsin-Madison 67


Dairy Essentials - Reproduction and Genetic SelectionTable 3: Standardized weights for common indices used by the dairy industry in the UnitedStatesPTA Milk, Fat, Type-Production Production-Type Net MeritProtein (PTAMFP$) Index (TPI) 1Index (PTI) 2Milk yield 0.0546 -- -- --Fat yield 0.58 1 2 --Protein yield 1.47 3 8 --PTA MFP $ -- -- -- 10Type (final score) -- 1 -- --Udder composite 3 -- 1 -- --Functional traits -- -- 2 --Productive life -- -- 2 4Somatic cell score 4 -- -- -1 11For Holstein cows only2For Jersey cows only3See text for more details4Resistance to mastitisHow to achieve the goalsChoosing the right sire for artificialinsemination on a farm is the most effectiveand cheapest way to make progress towardgenetic goals. Sires should be chosen on thebasis of their predicted transmitting abilityfor the desired traits. Reliability should beused only to determine how intensely aparticular sire should be used.The actual price structure of milk (theprice of fat, protein and other adjustmentfactors used to calculate the price of milk)may be used in calculating an index ofselection to help in identifying the best bullto meet a selection goal. The constructionof an index of selection has been describedin this series of publications (see “Choosinga Bull”). Many of the current indices ofselection give unnecessary importance totype traits (e.g., Total performanceindex—TPI—calculated by the HolsteinAssociation). However, newer indices tryto give more importance to lifetimeproduction and resistance to mastitis(Table 3).68 The Babcock Institute

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