marker-assisted selection in wheat - ictsd

202Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishThis population consists of approximately120 000 cows of which 90 percent aremilk recorded. Approximately 20 bullsare used for general service. Each yearabout 300 elite cows are selected as bulldams. These are mated to the two to fourbest local bulls and an equal number offoreign bulls to produce approximately 50bull calves for progeny testing. At the ageof one year, the bull calves reach sexualmaturity, and approximately 1 000 semensamples are collected from each young bull.These bulls are mated to approximately30 000 first parity cows to produce about5 000 daughters, or 100 daughters peryoung bull. Gestation length for cattle isnine months. Thus the young bulls areapproximately two years old when theirdaughters are born, and are close to fourwhen their daughters calve and begintheir first lactation. At the completion oftheir daughters’ first lactations, most ofthe young bulls are culled. Only four tofive are returned to general service, and asimilar number of the old proven sires areculled. By this time the selected bulls areapproximately five years old.Various studies have shown that ratesof genetic gain by a PT scheme are about0.1 to 0.2 genetic standard deviations ofthe selection index per year (Nicholasand Smith, 1983; Israel and Weller, 2000).The PT was devised to take advantageof the nearly unlimited fertility of males.However, compared with breeding schemesfor other species, the PT has several majorweaknesses. First, for a PT system to beeffective, the population must include atleast several tens of thousands of animalswith recording on production traits andpaternity. Inaccurate recording can significantlyreduce rates of genetic gain (Israeland Weller, 2000). Second, generation intervals,especially along the sire-to-dam andsire-to-sire paths, are much longer thanthe biological requirements. The increasein generation interval reduces genetic gainper year. As artificial insemination (AI)institutes generally pay a premium pricefor male calves of elite cows, these cows areoften given preferential treatment in orderto increase their genetic evaluations (Powelland Norman, 1988). The small number ofbulls actually used for general service, andthe even smaller number of bulls used asbull sires, tends to reduce the effective populationsize, which increases inbreeding anddecreases genetic variance in the population.The effective population size of the UnitedStates Holstein population with ten millioncows has been estimated at about 100(Farnir et al., 2000). Finally, there is virtuallyno selection along the dam-to-dam path.Generally, 70-80 percent of healthy femalecalves produced are used as replacements.Various studies have suggested thatselection intensities along the dam-to-dampath could be increased by application ofmultiple ovulation and embryo transfer(MOET) and sexed semen. Costs of bothtechnologies are still prohibitively high tobe applied to the entire population, asshown below. To overcome this problemfor MOET, Nicholas and Smith (1983)proposed a “nucleus” breeding scheme. Innucleus schemes, the selection populationconsists of several hundred individuals, andbulls are not progeny tested. Instead, bullsare selected based on the genetic evaluationsof their dams and sisters, which shortens thegeneration interval on the sire-to-dam andsire-to-sire paths, but reduces the reliabilitiesof the genetic evaluations. Dams of bulls andcows are selected based chiefly on their ownproduction records, and MOET is appliedto increase the number of progeny per dam.As the selection population consists of onlyseveral hundred individuals, MOET costs