Effects of breed type and age at first calving on length of productive life in a cow-calf system in Southeastern Mexico.

The effect of age at first calving (AFC) and breed type (BG) on cow length of productive life in the herd (LPL) and cumulative kilograms of calves weaned per cow (KWC) was studied. Data on 313 Brahman, 183 Nellore, and 269 crossbred cows (undefined crosses of European × Zebu cattle) born from 1986 to 2002 were used. Three hundred and ninety-one out of the 765 cows had non-censored data. LPL curves by BG and AFC (<2.5, 2.5 to <3, 3 to <3.5, and ≥ 3.5 years of age) were obtained using life table procedures. The magnitude of the effect of AFC and BG on LPL was determined using Cox proportional hazard regression procedures. KWC data (n=359) were analyzed by a linear model that included the effects of period of birth of the cow (in years), BG, and AFC. There was no difference in the survival curves between BG (p>0.05), but there was for the hazard ratio of culling (p<0.05). Brahman and Nellore cows produced more kilograms of calves (p<0.05) than crossbred cows. AFC influenced LPL (p=0.014) and KWC (p=0.012). Cows with AFC <3 years of age stayed longer in the herd and weaned more KWC than those with a higher AFC (p<0.05).