Lifetime production of beef heifers calving first at two vs three years of age1

Records of Angus, Hereford, Shorthorn, and first-cross cows born from 1960 to 1963 were used to study the effect of age at first calving and culling policy on cumulative survival (SU), number of breeding seasons (BS), pregnancies (PG), calves born (CB) and alive at 72 h (C72) and at weaning (CW), calf weaning weights (WW), and input/output efficiency, up to 12 yr of age. The 155 cows born in 1960 and 1961 calved first at 3 yr of age (M3) and the 173 born in 1962 and 1963 calved at 2 yr of age (M2). Analyses included both actual culling of cows open in two consecutive years (AC) and imposed culling of any open cow (IC). Also, economic efficiency for no culling (NC) of cows for infertility after first calving and up to 10 yr of age was calculated. The model included cow birth year-sire breed of cow (Y-S), sires within Y-S, breed of cow's dam (D), and D x Y-S. Linear functions of Y-S were used to estimate means for M2 and M3 cows. Under AC, by 12 yr of age M2 exceeded M3 cows by 6.8% SU, 1.1 BS, 1.2 PG, 1.1 CB, 1.0 C72, .9 CW, and 138 kg WW (P less than .10). Under IC, M2 exceeded M3 cows by only .7% SU, .3 BS, .3 PG, .3 CB, .2 C72, and .1 CW; and M3 produced 24 kg more WW than M2 cows (P greater than .10). Economic efficiency was higher for M2 than for M3 cows (greater than 10% at 6 yr to greater than 5% at 12 yr culling age), regardless of culling policy. Economic efficiency was maximum when terminal age of cows was 6 to 9 yr for M2 and 8 to 9 yr for M3 cows. Repeatability of pregnancy was higher (P less than .05) for M3 (19%) than for M2 (2%) cows. Economic efficiency for M2 or M3 cows under NC was slightly poorer (2 to 3%) than under either AC or IC policies. Economic efficiency can be improved by managing heifers to calve first as 2-yr-olds under either mild or intense culling of open cows.