133. Relationship Between Mature Cow Size and Weaning Weights in Bos Taurus Cows in the Southeastern United States

Abstract The objective of this study was to investigate the relationship between different estimates of cow size and weaning outputs in commercial beef cows in the Southeast United States. Postpartum Bos taurus beef cows (n = 498; body weight: 591.6 ± 64.40 kg; body condition score: 5.3 ± 0.81) from two locations were enrolled in this study over the course of two years (2024 and 2025). Cows were exposed to an industry-standard estrus synchronization protocol followed by fixed-time artificial insemination and natural service for 60 to 85 days. At weaning, estimates of cow size were collected, including mature cow weight (MCW), body condition score (BCS), hip height (HH), and body volume (BV). Univariate analyses were performed for weaning weight (WW) and percent of cow body weight weaned (PCWW) using linear mixed-effects models. Fixed effects of the abovementioned cow size indicators as well as conception method (CM), calving day (CD), and calf sex (CS) were included in the model. The model also included the random effects of calf sire, cow age, and location. There was a quadratic association between MCW and calf WW for both years, where WW increased linearly until MCW reached 680.4 kg, followed by a plateau. Alternatively, MCW had a negative linear association with PCWW ( P < 0.01), where PCWW decreased by 4.5 and 2.2% in 2024 and 2025, respectively, for every 45.4 kg increase in MCW. Calf WW had a quadratic association (P < 0.01) with cow BCS at weaning. Calves from cows with BCS up to approximately 4.8 tended to be heavier at weaning, whereas further increases in BCS were associated with a plateau in WW. Calving days were linearly associated with calf WW (P < 0.01) and PCWW (P = 0.01). For every 21-day delay in calving, calves were 19.8 and 18.1 kg lighter in 2024 and 2025, respectively. Moreover, cows weaned 2.5% (2024) and 3.2% (2025) less of their MCW for every 21-day delay in calving. Body volume, HH, and CM were not associated with calf WW (P > 0.10). These results underscore the impact of cow size on the performance of their offspring, which could benefit cattle producers in determining the ideal cow size in Southeastern cow-calf production systems.