Regression of Mature-Equivalent Production on Age at Calving

Regressions of mature-equivalent yield on age at calving were computed separately for each breed lactation number for 198,088 first lactations, 136,038 second lactations, and 85,013 third-lactation records of five major dairy breeds. Holstein records were also analyzed separately by geographical regions. The total and intra-herd-year-season regressions of mature-equivalent production on age were frequently very different. The withinregressions were usually much larger than the total regressions. This was due to a confounding of age with herd production. On a within-parity basis, the present Dairy Herd Improvement Association age factors failed to adjust the intraherd regression of production on age to zero. Of the 30 intraherd regressions of milk production on age, 24 were positive. Age-factor errors may in some instances lead to serious misranking of sires where age and regional distributions of daughters of bulls are widely different. '~ ,~ ~0 ",~ e4~ k"$ ~ ,~'~ O0 ~ ~ ~OOO ~J & * P < .05.