Weight gain potential affects pregnancy rates in bovine embryo recipients raised under pasture conditions

The aim of the present study was to evaluate the effect of differences in body weight gain after embryo transfer on the pregnancy rates of crossbred heifers used as recipients and raised under a grazing system. The study was performed during the dry (April to September) and the rainy (October to March) seasons. The embryos transferred were produced by in vitro fertilization. The body weight of each recipient was measured immediately before the embryo transfer and 23 to 25 days later, when the diagnosis of pregnancy was performed by ultrasonography. The associations among initial body weight (IBW), daily body weight gain (DWG), season, and pregnancy rate were evaluated using a logistic procedure that included the effect of the IBW, season, and linear and quadratic effects of the DWG. Altogether, there was no effect of season and pregnancy rates did not change between the dry and rainy seasons (42.3 vs. 45.8 %, respectively; P > 0.05). However, the pregnancy rate was greater in the recipients with daily body weight gains over 250 g/day, regardless of the season. In addition, the pregnancy rate of the recipients was better ( P < 0.04) explained by a logistic regression model that included the linear and quadratic effects of the DWG. The probability of each heifer to become pregnant according to DWG is explained by the follow equation: P ( y = 1) = (Exp (−1.06703 + 0.0108 * DWG − 0.00002 * DWG ^ 2) )/(1 + Exp (−1.6703 + 0.0108 * DWG − 0.00002 * DWG ^ 2) ). In conclusion, body weight gain potential is a critical factor for the pregnancy rates of in vitro embryo recipients managed under grazing systems.