PSVI-1 - Relationship between calving day and lifetime productivity in a population of Angus cows managed for a decade with a short breeding season as heifers

Abstract: Beef heifers that give birth early in their first calving season have a propensity to remain in the herd longer and wean more pounds of calves. This creates management strategies where fetal age is determined at pregnancy diagnosis and heifers that conceived later in the breeding season are sold. No studies have examined the influence of shortening the breeding season for heifers to less than two estrous cycles. Heifers from the commercial Angus cattle population at the U.S. Meat Animal Research Center were maintained under a short breeding season (31.5 ± 0.2 d) beginning in 2015. This allowed investigation of the influence of calving day as a heifer on lifetime productivity when the breeding season was less than two full estrous cycles. Performance data were compared between heifers that produced only one calf (n = 185) and heifers that produced two or more calves (n = 563). Data were analyzed using the MIXED procedure of SAS with performance traits as independent variables and longevity group as the fixed effect. When weights were analyzed, sex of the calf was included in the model as a random variable. Calving day as a heifer did not differ between heifers that only produced one calf and heifers that produced two or more calves (11.2 ± 0.8 vs. 11.4 ± 0.4 d, P = 0.82). Birth BW (34.7 ± 1.9 vs 31.7 ± 1.9 kg) and weaning BW (183.6 ± 6.0 vs 177.5 ± 6.3 kg) of first calves were greater for calves born to heifers that produced two or more calves compared to heifers that produced only one calf (P < 0.02). Heifers that produced two or more calves had much greater stayability than heifers that produced one calf (5.5 ± 0.08 vs 2.5 ± 0.12 y, P < 0.01). Even when the breeding season was very short, there was a tendency for an antagonistic relationship between calving day as a heifer and stayability (r = -0.08, P = 0.08). Calving day increased by 0.3 ± 0.1 d for each year a female remained in the herd (P < 0.01). While there was no difference in calving day as a heifer, there were clear maternal advantages for females that produced a second calf after being subjected to a very short breeding season as a heifer. The greatest hurdle to efficient stayability appears to be conceiving in the second breeding season even when heifers conceive very early in their first breeding season. USDA is an equal opportunity provider and employer.