PSII-9 - Impact of yearling weight on culling age in Angus cattle

Abstract: The economic success of cow-calf operations depends on the ability of females to remain in the herd long enough to recoup the cost of their development and maintenance. On average, a cow must wean five calves before she begins to generate a profit, resulting in the optimal cow remaining in the herd past six years of age. However, traits such as longevity are lowly heritable and influenced by multiple environmental factors. This creates a challenge for producers wishing to improve the longevity of their herds. Conversely, mature weight is a highly heritable trait, and cow size has increased over the past few decades in response to selection for growth. The objective of this study was to examine the relationship between culling age and yearling weight to determine if cow size is associated with longevity. Performance and culling records were obtained on 242 cows born from 2003 to 2014 from the Iowa State University McNay Research Farm purebred Angus herd. The largest proportion of females were culled if they failed to calve (n = 42) or were not pregnant at the time of fall pregnancy diagnosis (n = 153). Other reasons for culling included poor mothering ability, low production, soundness, disposition, and age. Adjusted 365-day yearling weights were calculated according to Beef Improvement Federation guidelines and were used to classify females by weight. The mean adjusted yearling weight was 347.41 ± 30.60 kg, and weights ranged from 265.91 kg to 443.64 kg. Females within one standard deviation of the mean were categorized as medium (316.81 – 378.01 kg), while cows weighing less than 316.81 kg were grouped as light and heavy females weighed more than 378.01 kg. Data was analyzed using R software version 4.4.1. A pairwise comparison of culling age in days and weight group showed no significant difference between light, medium, and heavy females (P > 0.05). Multiple linear regression analysis was used to test if adjusted yearling weight and birth year predicted culling age in days. An analysis of variance indicated that birth year was significant (P < 0.0001) in the model and adjusted yearling weight trended towards significant (P = 0.06). The regression coefficient for yearling weight was -1.97 days/kg, indicating that as yearling weight increased culling age decreased when birth year was accounted for. The adjusted R² value of 0.45 implies that the included predictors have substantial impact on culling age variability, although external factors not included in the model may also affect longevity. These results indicate a slight association between longevity and cow weight for this population of cows.