Search

The objective was to estimate genetic parameters and lactation curves for cows treated or not treated with bovine somatotropin (bST) from individual test-day milk, fat, and protein yields and somatic cell score (SCS) with a cubic spline model with five knots. Test-day records of Holstein cows that calved between 1994 and early 1999 were obtained from Dairy Records Management Systems in Raleigh, North Carolina for the analysis. Estimates of heritability for individual test-days and estimates of genetic and phenotypic correlations among pairs of test-days were obtained from estimates of variances and covariances from the cubic spline analysis with REML. Estimates of genetic parameters for the average test day within each of the ten 30-d test day intervals were reported. The model included herd test-day, age at first calving, and bST treatment as fixed factors. Cubic splines were fitted for the overall lactation curve, additive genetic effects, and permanent environmental effects with five predetermined intervals between days 0, 50, 135, 220, and 305. Plots of lactation curves were used to estimate differences between cows treated with bST and cows not treated with bST. Estimates of heritability for lactation one ranged from 0.09 to 0.15, 0.06 to 0.10, 0.08 to 0.15, and 0.02 to 0.06 for test-days one to ten for milk, fat, and protein yields and SCS, respectively. Mean differences between bST treated and untreated cows of 2 to 4 kg for test-day milk yield and 0.10 to 0.16 kg test-day fat yield (smaller differences for test-day protein yield) at day 90 were maintained until about day 305 of lactation. Mean differences due to bST treatment were smaller for test-day yields in lactations two and three. Estimates of heritability were larger for lactations two and three. Estimates of heritability increased over the course of the lactation. Estimates of genetic and phenotypic correlations were smaller for test-days further apart. Small mean differences were detected between bS