|Van Vleck, Lloyd|
Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/22/2000
Publication Date: N/A
Interpretive Summary: Treatment of some cows and not others with bovine somatotropin (bST) may affect genetic evaluations for milk production traits. Genetic evaluations based on test-day yields reported by dairy herd improvement programs were used to compare genetic evaluations computed considering effects of bST and ignoring such effects. Percentage increases in yield with bST treatment were 5 to 8% for test-day milk, fat and protein yields. These increases ar smaller than reported from controlled experiments. The DHI records, however, do not report dates of treatment or numbers of treatments. Correlations between genetic evaluations using models including and ignoring effects of bST treatment were .99. This result suggests that bias in genetic evaluations due to ignoring which cows are treated with bST may be small. The number of cows with bST treatment were small (5,245 test-day records) which suggests that additional analyses will be needed with more data to confirm this result.
Technical Abstract: Records from the North Carolina State Dairy Records Processing Center were used to estimate effects of bovine somatotropin (bST) treatment and (co)variance components and to predict breeding values on milk production traits. The data comprised 5,245 test-day records of bST-treated cows and 126,223 test-day records of untreated cows in first lactation for milk, fat, and protein yields. Fixed effects of bST treatment and (co)variance components of random effects (additive genetic, permanent environmental, and residual) were estimated from test-day animal models with herd-year effects or herd-test-date effects as other fixed effects. Random regression coefficients for additive genetic and permanent environmental effects were included in the models. To assess the potential for bias in genetic evaluations when some and not all cows are treated with bST, breeding values predicted by the test-day models with and without effects of bST treatment were compared for cows and sires. In the model with herd-year effects, responses to bST treatment increased with days in milk, suggesting interaction between effects of bST and days in milk. However, in the model with herd-test-date effects, the interaction between effects of bST and days in milk was small. Percentage increases due to bST treatment were from 5 to 8% for test-day milk, fat, and protein yields. Correlations between breeding values predicted from the models with and without effects of bST treatment were .99. These results suggest that bias in genetic evaluations due to ignoring bST treatment will be small.