Reproductive performance in cycling beef cows experiencing nutritional restriction

The effect of weight loss during the breeding season on reproductive performance was assessed in 150 beef cows. Cows were sorted by calving date, body condition score (BCS; mean = 5.5 ± 0.5), BW (mean = 510 ± 8 kg), age, and breed. Restricted (RES) cows were fed to lose 5% BW from d -30 to d 0 (start of breeding season) and an additional 10% BW from d 0 to d 60 (end of breeding season). Control (CON) cows were fed to maintain BW. All cows reinitiated cyclicity (progesterone ≥ 1 ng/mL) prior to start of the breeding season. Estrus was synchronized and cows were inseminated at estrus and 12 h later. Bulls were placed with each group after the synchronization period through the breeding season. Pregnancy was determined at d 30, 60, and 150. Measurements for BW, BCS, and blood samples for progesterone, insulin, nonesterified fatty acids (NEFA), and urea nitrogen (BUN) were collected weekly from d -30 to d 60. Body weight of RES cows decreased 56. 9 kg; whereas, CON cows increased 10.8 kg from d -30 to d 60. Insulin decreased throughout the experimental period in RES cows (d -30, 0.33 ± 0.02 ng/mL; d 60, 0.24 ± 0.02 ng/mL; P < 0.05). Concentrations of NEFA (d -30, 0.77 ± 0.12 mEq/dL; d 60, 0.95 ± 0.12 mEq/dL; P < 0.05) and BUN (d -30,20.4 ± 0.9 mg/dL; d 60, 29.1 ± 0.9 mg/dL; P ≤ 0.05) increased in RES cows. Neither estrous response (86.3 vs. 88.3%), estrous interval (2.5 vs. 2.6 days), nor conception rates (68.3 vs. 76.5%) differed between RES and CON cows, respectively. Pregnancy rates were similar on d 30 (58.9 vs. 67.5%), d 60 (89.0 vs. 93.5%), and d 150 (91.8 vs. 96.1%) between RES and CON cows, respectively. Crown-rump length did not differ between RES and CON embryos on d 45 in year 1 (25.7 ± 0.6 vs. 26.9 ± 0.6 mm) or d 30 in year 2 (11.5 ± 0.2 vs. 10.8 ± 0.2 mm, respectively). Calves from RES cows had lower BW than CON at the end of the breeding season (145.4 ± 2.9 vs. 176.8 ± 2.9 kg; P < 0.05). In conclusion, cows which had reinitiated estrous cyclicity but were losing weight during the breeding season did not have reduced reproductive performance. These results suggest lower pregnancy rates associated with poor nutrition may be more related to failure of animals to reinitiate postpartum cyclicity rather than inability to establish and maintain pregnancy.