enotypic and genetic correlations between milk and productive life. Additive genotypes and phenotypes for milk yield and fertility in the first three lactations were simulated for 40000 cows, the daughters of 400 sires. In each lactation cows below a certain level for fertility were culled (involuntary culling) and the rest were ranked on milk yield and a certain proportion was culled (voluntary culling). Involuntary culling was around 15% and 30% of the cows entering the lactation, for the 1st and later lactations, respectively. In the base situation, voluntary culling was 15%, 5% and 10% of all cows starting the 1st, 2nd and 3rd lactations, respectively. Several alternatives with decreasing voluntary culling (10%, 5%, and 0% in 1st lactation and no voluntary culling in any lactation) were studied. Length of productive life was measured els the number of lactations (1-4). The estimates of genetic correlation between milk yield in 1st lactation and length of productive life decreased from 0.77 for the base situation to -0.47 for the alternative without any voluntary culling. The implication of the results is that in a real data set, one could expect a favorable genetic correlation between milk yield in 1st lactation and length of productive life if there is at least some voluntary culling for milk yield.
Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 17. Genetics and breeding of dairy and beef cattle, swine and horses, , 77–80, 1994
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