The B allele of the K-casein gene has been shown in several studies to increase protein yield in milk by about 3%. It has also been suggested that it improves cheese yield independant of any effects on protein yield. A deterministic computer simulation of dairy cattle breeding with progeny testing was constructed to evaluate the economic gains of including selection on K-casein genotype. The simulation created annual cohorts of animals in a four path system with generation intervals of 7, 7, 6 and 5 years in the sire to sire, sire to dam, dam to sire and dam to dam paths and proportions selected of 0.05, 0.15, 0.05 and 0.9. The build up of differential quantitative gametic phase disequilibrium between genotypes and a negative correlation between quantitative genetic affects and the K-casein genotype were allowed for. If K-casein only effects protein yield, incorporation of K-casein genotype into an optimum selection index has almost no effect on genetic improvement in the cow population. With an additional 3.3% improvement in cheese yield of the BB genotype, improvements of economic genetic gain in the firs t cohort of up to 8% are possible when a high proportion of milk goes to cheese manufacture. Improvements in later cohorts and in other milk markets are substantially reduced. If the gene effect is overestimated, genetic responses are reduced, rapidly becoming less than if the genotype is ignored. It would likely be cost beneficial to genotype bulls provided that the B allele has an economic advantage other than through increasing protein yield. Genotyping of dams of sires may also be worthwhile where a high proportion of milk goes to cheese manufacture, but not else where.
Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume XIV. Dairy cattle genetics and breeding, adaptation and conservation., , 163–166, 1990
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