marker-assisted selection in wheat - ictsd

Chapter 12 – Marker-assisted selection in dairy cattle 203are manageable if costs are spread overthe entire national dairy industry. Ratesof genetic gain within the nucleus are thushigher than can be obtained by a nationalPT design. This gain is transferred to thegeneral population through the use of bullsfrom the nucleus population. In addition tothe greater overall rate of genetic gain, thenucleus scheme has the advantage that it isnecessary to collect data on a much smallerpopulation, which should reduce costsand increase accuracy. The disadvantagesof MOET are that overall costs and ratesof increase of inbreeding will be greaterunless steps are taken to reduce inbreeding.However, these steps will also slightlydecrease rates of genetic gain. In practice, nocountry has replaced its standard PT schemewith a nucleus breeding programme.Dairy cattle breeding indeveloping countriesThe genus Bos includes five to seven species,of which Bos taurus and Bos indicusare the most widespread and economicallyimportant. B. taurus is the main dairycattle species, and is found generally intemperate climates. Several tropical andsubtropical cattle breeds are the result ofcrosses between B. taurus and B. indicus,which interbreed freely. In the tropics,cows need at least some degree of toleranceto environmental stress due to poornutrition, heat and disease challenge tosustain relatively high production levels(Cunningham, 1989). Tropical breeds areadapted to these stresses but have low milkyield, whereas more productive temperatebreeds cannot withstand the harsh tropicalconditions, to the point of not being able tosustain their numbers (de Vaccaro, 1990).Furthermore, most tropical countries aredeveloping countries, which lack systematiclarge-scale milk and pedigree recording.A number of studies have been conductedon crosses between imported andlocal breeds in the tropics. Generally, the F 1B. taurus x B. indicus crosses are economicallysuperior to either of the purebredstrains (FAO, 1987). The heterosis effectof the F 1 cross is due to genes for diseaseresistance from the local parent, and genesfor milk production from the importedstrain (Smith, 1988; Cunningham, 1989).However, this heterosis is lost in futuregenerations if the F 1 is backcrossed to eitherparental strain. Madalena (1993) presentedan F 1 continuous replacement schemeto capitalize on its superiority. Recently,Kosgey, Kahi and van Arendonk (2005)proposed a closed adult nucleus MOETscheme to increase milk production intropical crossbred cattle.Economic considerations inapplying MAS to dairy cattleFor any new technique to be economicallyviable, overall gains must be greater thanoverall costs. This also applies to usingMAS within a dairy cattle breeding programme.However, unlike investment innew equipment, genetic gains never “wearout”, i.e. breeding is unique in that geneticgains are cumulative and eternal. Thus, asshown by Weller (1994, 2001) investmentsin MAS or other techniques that enhancebreeding programmes are economicallyviable even if “nominal” costs are greaterthan “nominal” gains.For example, consider an ongoing breedingprogramme with a constant rate of geneticgain per year. Assume that the annual rate ofgenetic gain has a nominal economical valueof V. The cumulative discounted returns toyear T, R v , will be a function of the nominalannual returns, the discount rate, d, theprofit horizon, T, and the number of yearsfrom the beginning of the programme until