|Van Vleck, Lloyd|
Submitted to: Genetics and Molecular Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/25/2007
Publication Date: 6/30/2007
Citation: Degroot, B.J., Keown, J.F., Van Vleck, L.D., Kachman, S.D. 2007. Estimates of genetic parameters for Holstein cows for test-day yield traits with a random regression cubic spline model. Genetics and Molecular Research. 6(2):334-344. Interpretive Summary: Traditional models for genetic evaluations use data from test-day records combined into 305-day mature equivalent lactation records. Use of a test-day model has several advantages: 1) increased accuracy of genetic evaluations, 2) direct, more precise adjustments for temporary environmental effects on test-days, 3) end-of lactation yields need not be extended for culled cows or for cows with records in-progress, and 4) models could fit lactation curves for individual cows. A random regression cubic spline function provides more flexibility to produce a “good” fit compared with polynomial functions. The cubic spline model used in this study provided flexibility for estimating genetic parameters from milk, fat, and protein test-day yields and somatic cell scores (SCS). The flexibility of the model extended to estimating genetic and permanent environmental (co)variances. Estimates of heritability increased as days in milk increased for all lactations for test-day yields and SCS. Estimates of heritability were less than previous estimates reported with other types of random regression models. The smaller estimates could be because this data set contained more grade cows than registered cows. Estimates of genetic parameters are usually smaller for grade cows compared to registered cows because of a greater chance of misidentification of sires and dams for grade cows. Estimates of genetic and permanent environmental variances for test-day yields and SCS were greater for lactations two and three than for lactation one. Lactation two had estimates of variances due to genetic and permanent environmental effects in the spline function that were more variable than estimates for lactations one and three. Estimates of genetic and phenotypic correlations decreased with an increase in days between when the yields were measured. This study showed that the cubic spline model may be a suitable method of estimating genetic parameters over the course of a lactation. From this study, the estimates of genetic parameters with the cubic spline model were comparable to estimates found with other methods. The major advantage of this method is the smaller number of variance components to be estimated compared with polynomial and multiple trait methods. Further work is needed to determine the proper number and placement of the knots for days in milk and for comparison of computational time needed to set up and solve equations with other methods used to estimate genetic parameters.
Technical Abstract: Genetic parameters were estimated with REML for individual test-day milk, fat, and protein yields and SCS with a random regression cubic spline model. Test-day records of Holstein cows that calved from 1994 through 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 between test-days were obtained from estimates of variances and covariances from the cubic spline analysis. Estimates of genetic parameters at the average of test days within each of ten 30-d test day intervals are 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 and for random additive genetic and permanent environmental effects with five predetermined knots or four intervals between days 0, 50, 135, 220, and 305. Estimates of heritability for lactation one ranged from 0.10 to 0.15, 0.06 to 0.10, 0.09 to 0.15, and 0.02 to 0.06 for test-day one to test-day ten for milk, fat, and protein yields and SCS, respectively. Estimates of heritability were greater in 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.