This experiment was conducted to evaluate factors affecting pregnancy rate of 117 crossbred Nelore x Hereford beef cows with age varying from 2 to 20 years, different gene proportion from Nellore breed (25.0, 37.5, 50.0, and 100.0%), calved between 08/11/03 and 12/23/03 and bred by natural service, between 12/10/03 and 03/12/04. The effects of July calving date (JCD), birth weight (BW) and calf sex (S) from previous calving, parity order (PO), hip height (H), weight (WM) and body condition score at the beginning of the mating season (BCS), daily gain during the mating season (DGM), gene proportion from Nellore (GPN) and calf 205-d weight (WW205) from previous calving were all investigated on the probabilities of pregnancy and conception. Data were analyzed by Logistic Regression using SAS to identify the effects of each variable. The average pregnancy rate was 43.2%. The probability of pregnancy and conception were affected by changes on JCD, WM, BCS and DGM. The pregnancy odd changing to each increase on the variable were based on the odds ratio estimative by OR = exp (b k), considering that odd is ratio between the probability of the event occur and not occur. No significant effects of BW, S, PO, H, GFN and WW205 were observed. Pregnancy rate in beef cows can be increased by improving body condition score at the beginning of breeding season and weight gain during the breeding season. Both can be achieved by pre and postpartum nutritional management.
body condition; calving date; logistic regression; feed restriction
