|Van Vleck, Lloyd|
Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/30/2001
Publication Date: 1/16/2002
Citation: Degroot, B.J., Keown, J.F., Van Vleck, L.D., Marotz, E.L. 2002. Genetic parameters and responses of linear type, yield traits, and somatic cell scores divergent selection for predicted transmitting ability for type in Holsteins. Journal of Dairy Science. 85:1578-1585. Interpretive Summary: The University of Nebraska Holstein herd was divided so that one- half was mated to AI sires that were above average in estimated genetic values for final type score (overall measure of desirable conformation) and the other half to those below average. Selection was such that estimated breeding values for yield of milk, fat and protein were similar for both groups of sires. By the third generation, estimated breeding values of progeny of the above and below average bulls were significantly different for final score (basis of selection), stature, strength, body depth, dairy form, free udder attachment, rear udder height and width, udder cleft, udder depth and front teat placement. Average breeding values for milk, fat and protein yields were not significantly different. Averages for somatic cell scores were significantly different which reflects the unfavorable genetic correlation of -0.64 estimated in the analysis between final score and somatic cell score. Estimates of correlations for final score with milk fat, and protein yields were nearly zero. The study suggests selection for final type score may indirectly improve somatic cell scores (make scores smaller).
Technical Abstract: Response to divergent selection for PTAT (Predicted Transmitting Ability for Type) in Holsteins was examined. For four generations, half of the university research herd was bred to Holstein sires with PTAT > 1.50 and other half to sires with PTAT < 1.25, with nearly equal PTA for yield traits for both groups. Estimates of genetic and phenotypic correlations and heritabilities were obtained with REML. Model for type traits include fixed effects of data classified, age in days at freshening and stage of lactation at classification. Year-season when fresh was in model for yield and somatic cell scores (SCS). Animal genetic and residual effects were random. At convergence for estimates of variance components, estimates of breeding values (EBV) were obtained. Final score, milk, fat, and protein yields, and SCS had heritability estimates of 0.38, 0.13, 0.22, 0.09, and 0.38, respectively. Heritability estimates for type traits sranged from 0.04 to 0.52. Estimates of genetic correlations of final score with SCS and milk, fat, and protein yields were -0.64, 0.01, -0.18, and 0.06, respectively. Estimates of genetic correlations among linear type traits ranged from -0.77 to 1.00. Means of EBV for most type traits were significantly different between lines in third generation. Milk, fat, and protein yields were not significantly different but SCS means were significantly different. Estimate of genetic correlation between final score and SCS suggests that selection on PTAT would result in a change for SCS. In this study, divergent selection on PTAT would result in a change for SCS. In this study, divergent selection on PTAT of sires changed udder traits but had little or no effect on body, feet and leg traits.