Nutritional and metabolic factors influencing reproductive efficiency of grazing beef cattle

Two studies were conducted to evaluate the effects of post-weaning management of British crossbred heifers on growth and reproduction. In Exp. 1, 239 spring-born, crossbred heifers were allotted randomly to 1 of 2 treatments: (1) 36% CP containing 36% RUP (36RUP), or (2) 36% CP containing 50% RUP (50RUP). In Exp. 2, 191 spring-born, crossbred heifers were: (1) pasture developed and fed 0.9 kg/d of a 36% CP supplement containing 36% RUP (36RUP); (2) pasture developed and fed 0.9 kg/d of a 36% CP supplement containing 50% RUP (50RUP); (3) corn silage based diet in a drylot (DRYLOT). In Exp. 1, BW was not different (P ≥ 0.27) among treatments at all measurement times. However, 50RUP heifers had greater (P = 0.02; 80 and 67%) pregnancy rates than 36RUP heifers. In Exp. 2, DRYLOT heifers had greater (P < 0.01) BW at breeding than 36RUP or 50RUP developed heifers. Pregnancy rates tended to be greater (P = 0.10) for 50RUP heifers than 36RUP and DRYLOT. Retention rate after breeding yr 3 and 4 was greatest (P ≤ 0.01) for 50RUP heifers. Thus, increasing the proportion of RUP in supplements fed to heifers on dormant forage before breeding increased pregnancy rates and cow herd retention.;Cows in negative energy balance after calving often have reduced reproductive performance, which is mediated by metabolic signals. Cows were classified by conception date as early (EARLY; conceived in first 15 d of breeding) or late conception (LATE; conceived during the remaining breeding season). Whole-blood beta-hydroxybutyrate (BHB) concentrations were measured prior to breeding in Exp. 1 and prior to calving and breeding in Exp 2. In Exp. 1, BHB concentration was lower (P ≤ 0.04) in EARLY cows than LATE cows. In Exp. 2, a cow age x sample period x conception date interaction (P 0.10) for 2-yr-old cows regardless of conception date and sampling period. However, pre-calving serum BHB concentrations were greater ( P < 0.01) for LATE than EARLY in 3-yr-old cows. This study indicates that BHB concentrations may provide a sensitive indicator of energy status for rebreeding success in young beef cows.;Reports on the relationship between performance data and serum BHB concentrations prior to calving are limited. Cows were from 2 to 10 years of age (n = 381). Serum BHB concentrations were measured 30 +/- 8 d prior to breeding. Cow age, calving date, and cow BW at calving was negatively correlated ( P < 0.05) with serum BHB concentration. Calf BW at birth was positively correlated (P < 0.01) with serum BHB concentration prior to calving. Calving BCS and cow's sire were not correlated (P ≥ 0.27) with pre-calving BHB concentration. A cow age x heifer development treatment interaction (P = 0.06) occurred for pre-calving serum BHB concentration. This study does indicate that heifers developed on a restricted gain may have increased adaptive mechanism to the metabolic dysfunctions caused by increased energy demands of gestation and lactation of a first calf heifer than heifers developed at a greater rate of gain.