Influence of body condition at calving and postpartum nutrition on endocrine function and reproductive performance of primiparous beef cows.

The influences of body condition score (BCS) at calving and postpartum nutrition on endocrine and ovarian functions, and reproductive performance, were determined by randomly allocating thin (mean BCS = 4.4 +/- 0.1) or moderate condition (mean BCS = 5.1 +/- 0.1) Angus x Hereford primiparous cows to receive one of two nutritional treatments after calving. Cows were fed to gain either 0.45 kg/d (M, n = 17) or 0.90 kg/d (H, n = 17) for the first 71 +/- 3 d postpartum. All cows were then fed the M diet until 21 d after the first estrus. A replication (yr 2; M, n = 25; H, n = 23) was also used to evaluate reproductive characteristics. Concentrations of IGF-I, leptin, insulin, glucose, NEFA, and thyroxine were quantified in plasma samples collected weekly during treatment and during 7 wk before the first estrus. Estrous behavior was detected by radiotelemetry, and luteal activity was determined based on concentrations of progesterone in plasma. All cows were bred by AI between 14 and 20 h after onset of estrus, and pregnancy was assessed at 35 to 55 d after AI by ultrasonography. Cows that calved with a BCS of 4 or 5 had similar endocrine function and reproductive performance at the first estrus. During treatment, H cows gained BW and increased BCS (P < 0.01), and had greater (P < 0.05) concentrations of IGF-I, leptin, insulin, glucose, and thyroxine in plasma than M cows. However, during the 7 wk before the first estrus, plasma concentrations of IGF-I, leptin, insulin, glucose, NEFA, and thyroxine were not affected by time. Cows previously on the H treatment had a shorter (P < 0.01) interval to first postpartum estrus and ovulation, and a larger dominant follicle (P < 0.01) at first estrus, than M cows, but duration of estrus and the number of mounts received were not influenced by nutrient intake. Pregnancy rate at the first estrus was greater (P < 0.03) for H (76%, n = 38) than for M (58%, n = 33) cows. Increased nutrient intake after calving stimulated secretion of anabolic hormones, promoted fat deposition, shortened the postpartum interval to estrus, and increased pregnancy rate at the first estrus. Concentrations of IGF-I and leptin in plasma were constant during 7 wk before the first estrus, indicating that acute changes in these hormones are not associated with the resumption of ovarian function in primiparous beef cows.