EFFECT OF HERD-YEAR PRODUCTION LEVEL ON EXPRESSION OF SIRE MERIT

Abstract

Performance data on 110 602 first lactation Holstein cows were utilized to compare the expressions of sire proof at various herd-year production levels. Production traits analyzed were BCA milk, BCA fat and fat percentage. Herd-year averages were partitioned into six groups for each trait. Regression analysis indicated significant interactions between herd-year level and sire estimated transmitting ability (ETA) for daughter milk and fat BCA (P < 0.001). Interaction between herd group and sire proof for fat percentage was found to be insignificant. Linear contrasts were used to compare sire proof slope differences for various herd-year group comparisons. Regression coefficients for sire proof tended to increase as herd-year level increased for both milk and fat yield. The coefficient for sire fat percentage was smaller for the lowest herd-year level when compared to other herd levels. Sire proofs for milk and fat were than categorized into six and seven groups respectively. Sire group and herd-year by sire group interaction were found to account for a significant (P < 0.01) amount of variation in daughter milk and fat yield. Linear contrasts were also used to compare specific sire group differences at various herd production levels. Significant herd-year differences were observed for both daughter milk and fat yield. However specific trends were not prevalent for sire group differences as herd-year groups increased for both traits. Key words: Dairy, sire, daughter