Relationship between dam's age at bull's birth and bull's genetic evaluation

After accounting for year of birth of bulls, younger dams produced superior sons. Bulls' PD, parent average, and pedigree index milk decreased with increasing age of dams at bull's birth. This appears to be due to decrease in maternal grandsire's PD milk and dam's Cow Index milk with increase in age of dam. Younger dams have an advantage of potentially higher genetic merit due to genetic trend but have less information included in their evaluations. Bull's latest parent average and pedigree index overestimated latest PD milk. This overestimation was not different with age of dam at bull's birth, suggesting that the bias in predicting bulls' PD from eventual evaluations of dams chosen at a younger age is not different than for dams chosen at a later age. Data were for 2826 bulls sampled by four AI organizations and born between 1965 and 1981. The increase in PD milk of bulls was about 27 kg/yr, but the trend was larger (45 kg/yr) during the last 5 yr. Inclusion of dam's age did not improve prediction of bull's PD milk. Maternal grandsire's PD milk was the primary variable associated with dam's age that accounted for lower bull's PD milk, mainly because older dams tended to have sires of lower genetic merit.