RESUMEN
The influence of exposure to ambient winter weather conditions (WW) during the final 90 d of gestation on serum hormones, plasma substrates, and birth weight of calves was evaluated in spring-calving, primiparous beef heifers. At of 192+/-14 d of gestation, heifers were assigned by expected calving date, breed, and sire of fetus to one of two treatments. Thirteen heifers were assigned to thermoneutral environment (TN; 12 degrees C) and housed in temperature-controlled rooms. Heifers in WW (n=16) were maintained outdoors in drylots without access to shelter and given additional dietary energy when average weekly windchill fell below -6.7 degrees C. Body weights, hip-heights and samples of serum and plasma were obtained biweekly until heifers were relocated, approximately 7 d prior to expected calving. Polynomial response curves for concentrations of glucose and nonesterified fatty acids (NEFA) in plasma and cortisol in serum were not influenced by treatment. However, average concentrations of NEFA in plasma were increased (P<0.02) and glucose tended to be higher (P=0.13) in WW heifers compared to TN heifers (172.5+/-8.9 vs 136.9+/-7.7 micromol/L and 87.8+/-2.4 vs 83.3+/-2.7 mg/100ml for NEFA and glucose, respectively). Time trends of concentrations of estradiol in serum (P<0.01) and hip-height to weight ratios were different (P<0.05) for WW and TN. Birth weights of calves from TN heifers were greater (P<0.06) than calves from WW heifers (42.3+/-2.0 vs 36.9+/-1.8 kg), but average calving difficulty scores were similar for both heifer groups (3.2+/-0.5 vs 2.7+/-0.4). These data suggest that exposing spring-calving cows to reduced effective ambient temperatures during the final 90 d of pregnancy may elevate energy-yielding metabolites in plasma and alter endocrine function. These changes may contribute to reduced birth weight of calves.