Variances, repeatabilities, and age adjustments of yield and fertility in dairy cattle.

Breeding receipts from artificial insemination were matched with records of milk yield from northeastern United States. Nine measures of yield and 10 measures of fertility were investigated. A completely random model with herd-year-season, sire, and interaction of sire by herd-year-season was applied by parity; interaction of sire by herd-year-season was not important except possibly for fertility of virgin heifers. Sires accounted for 6 to 7% of yield variation and herd-year-seasons accounted for about 45%. Measures of fertility had small sire variances, and herd-year-season variance was much smaller than for yield. Effects of age within parity were significant for yield from ordinary least-squares analysis. Age adjustment factors were obtained for measures of yield. Repeatabilities of first- and second-parity yield ranged from .40 to .61 whereas repeatabilities of fertility were .03 to .13. Repeatabilities of virgin heifer and first-parity fertility were .01 to .03, suggesting heifer and cow fertility may not be related.