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).

The aim was to evaluate the effect of corn oil supplementation during postpartum anoestrus on ovarian activity, pregnancy rate, progesterone (P(4)), and lipid metabolites (cholesterol, CHO; low and high density lipoproteins; LDL and HDL, respectively) concentrations in blood of F(1) (Bos taurus x Bos indicus) grazing cows. Cows were randomly assigned to an experimental group, fed with a supplement containing 4% corn oil on dry matter basis (OG, n = 11), and a control group with the same supplement without corn oil (CG, n = 12). Both supplements contained equivalent amounts of crude protein and metabolizable energy and were fed for 34 days continuously. All cows were induced to estrous 12 days after beginning of supplementation by using a synthetic progestagen and artificially inseminated 56 h after retiring the implants. Pregnancy diagnosis was performed by transrectal palpation 45 days after insemination, evaluating simultaneously ovarian activity. P(4) and lipid metabolites (CHO, HDL, LDL) concentrations were determined in blood samples collected at 3-day intervals, from the beginning of corn oil supplementation and up to 10 days after artificial insemination. Ovarian activity was affected by treatment (p < 0.05), finding ovarian structures in 72.7% of OG cows and in 50% of CG cows. Concentration of P(4) and CHO was higher for OG with respect to CG (2.52 +/- 0.65 vs 1.88 +/- 0.62 ng/ml and 117.79 +/- 11.57 vs 85.71 +/- 12.11 mg/dl, respectively), whereas pregnancy rate and blood concentrations of HDL and LDL were not affected by treatment (p > 0.05). Addition of corn oil to the supplement stimulated ovarian activity and increased serum concentrations of progesterone and cholesterol in grazing B. taurus x B. indicus cows with low body condition score showing postpartum anoestrus.