A simulation study for optimizing the use of dairy bulls in breeding programs

Abstract

Different breeding program alternatives in dairy cattle population were studied by simulation. Traits studied were milk production and a low heritability trait that is negatively correlated with milk production, e.g. fertility. The variable factors in the study were the number of young bulls to be tested, the number of daughters per bull in progeny testing, the number of tested bulls to be used and the weights for selected traits in an overall index of the bull’s breeding value. The influence of variable factors on genetic response in milk production and fertility was studied by calculating the average of real genotypic values on both traits for all cows born in the same year and having a complete first lactation record. This was done for a 25 year period. The population structure used in simulation was like the Finnish milk recorded Ayrshire population in which there are ca. 250000 cows. The genetic response in milk production was improved by increasing the selection intensity amongst bulls. The negative effect of selection for milk yield on fertility could be decreased by giving the fertility a larger indexed weight. If the milk production had a weight of 1 and genetic correlation between traits was —0.20 then increasing the weight of fertility from 0.1 to 0.3 did not affect significantly the response in milk production.