Bias in genetic evaluations by records of cows treated with bovine somatotropin

Records from Dairy Records Management Systems in Raleigh were used to estim ate effects of bovine somatotropin (bST) treatment and to predict breeding values for milk production traits. The data comprised 5245 test-day records of bST-treated cows and 126,223 test-day records of untreated cows in firs t lactation for milk, fat, and protein yields. Fixed effects of bST treatme nt were estimated from test-day animal models with herd-test-date as anothe r fixed factor. Percentage increases due to bST treatment ranged from 7 to 8% for test-day milk, fat, and protein yields. Random regression coefficien ts for additive genetic and permanent environmental effects were included i n the model. To assess the potential for bias in genetic evaluations when s ome and not all cows are treated with bST, breeding values predicted by the test-day model with and without effects of bST treatment were compared for cows and sires. Correlations between breeding values predicted from models with and without effects of bST treatment were 0.99. However, relatively l arge bias was found for individual animals. This result suggests that bias in genetic evaluation caused by ignoring bST treatment may be significant.