LAUSR

Establishing two breeding seasons (Fall and Spring) is a common practice in the southeast USA, including VA. We performed a retrospective analysis of data from 11 years, 22 breeding seasons and 14 locations in VA to compare reproductive performance between Fall and Spring breeding seasons. A total of 30,191 breeding records (24,032 for cows and 6,259 for heifers) from the VA Department of Corrections beef cattle operations were analyzed. Every year, each cow and heifer were enrolled in estrous synchronization protocols and received fixed-time artificial insemination (TAI) and were then exposed to natural service bulls for an approximately 65-day breeding season. Fall breeding occurred from November until January, and Spring breeding from March until May. Body condition score (BCS; 1 to 9, scale) was determined at the initiation of the TAI protocol. Estrus detection before TAI was performed by scoring an estrus detection aid prior to insemination. Pregnancy rate was determined by ultrasonography between days 55-65 post-TAI and 40-50 after the end of the breeding season. Data were analyzed using the SAS GLIMMIX procedure for binary data and MIXED procedure for continuous data. Average BCS was greater (P < 0.001) for Fall cattle than Spring cattle (5.7 ± 0.02 and 5.1 ± 0.02, respectively); however, days postpartum at TAI was similar between Fall and Spring (P = 0.645; 77.8 and 76.2 ± 0.5 days, respectively). Pregnancy rate to TAI differed (P < 0.0001) between Fall and Spring, 53.0 ± 0.03% and 48.6 ± 0.05%, respectively. Interestingly, estrus expression was greater (P < 0.001) for Spring than Fall (58.4 ± 0.05% and 50.4 ± 0.06%, respectively) and also was greater for cows that became pregnant to TAI compared with cows that failed to become pregnant to TAI (P < 0.001; 61.7 ± 0.03% and 46.9 ± 0.04%, respectively). End of breeding season pregnancy rate differed between season and was greater for Fall than Spring (P < 0.001; 90.7 ± 0.04% and 87.9 ± 0.04%, respectively). Calf birth weight was similar between seasons (P = 0.478; 34.1 and 34.4 ± 0.09 kg, for Fall and Spring, respectively). However, 205-day adjusted weaning weights differed (P < 0.001) between seasons with Spring calves being heavier than Fall calves (246.3 ± 0.12 kg and 227.8 ± 0.21 kg, respectively). In conclusion, for beef cow-calf operations in VA reproductive performance is enhanced for Fall breeding seasons; however, weaning weights were greater for calves born during the Spring seasons.