Does β-hydroxybutyrate concentration influence conception date in young postpartum range beef cows?

Cows in negative energy balance after calving often have reduced reproductive performance, which is mediated by metabolic signals. The objec- tive of these studies was to determine the association of serum metabolites, days to first postpartum ovula- tion, milk production, cow BW change, BCS, and calf performance with conception date in spring-calving 2- and 3-yr-old beef cows grazing native range. In Exp. 1, cows were classified by conception date in a 60-d breeding season as early (EARLY; conceived in first 15 d of breeding) or late conception (LATE; con- ceived during the last 45 d of breeding). Beginning on d 35 postpartum, blood samples were collected twice per week for serum metabolite analysis and progester- one analysis to estimate days to resumption of estrous cycles. As a chute-side measure of nutrient status and glucose sufficiency, whole-blood β-hydroxybutyrate (BHB) concentrations were measured 14 ± 2 d before breeding. In Exp. 2, cows were classified by subse- quent calving date resulting from a 55 ± 2 d breeding season as conceiving either early (EARLY; conceived in first 15 d of breeding) or late (LATE; conceived dur - ing the remaining breeding season). Blood samples were collected in 2 periods, 30 ± 4 d before calving and 14 ± 3 d before the initiation of breeding, to determine circulating concentrations of IGF-I and BHB. In Exp. 1, BHB and serum glucose concentrations were less (P £ 0.04) in EARLY cows than LATE cows. Serum insulin concentrations were greater (P = 0.03) in EAR- LY cows relative to LATE cows. Milk production and composition did not differ (P ³ 0.24) by conception date groups. In Exp. 2, cow age × sample period × con- ception date interaction (P 0.10) for 2-yr-old cows (in greater nutri- tional plane compared with Exp. 1) regardless of their conception date classification and sampling period. However, precalving serum BHB concentrations were greater (P < 0.01) for LATE than EARLY in 3-yr-old cows with no difference (P = 0.86) at prebreeding. Serum IGF-1 concentrations were greater (P < 0.01) for EARLY cows relative to LATE cows at precalving and prebreeding. This study indicates that blood BHB concentrations during times of metabolic dysfunctions may provide a more sensitive indicator of energy sta- tus than body condition, predicting rebreeding compe- tence in young beef cows as measured by interval from calving to conception.