Influence of undegraded intake protein on reproductive performance of primiparous beef heifers maintained on stockpiled fescue pasture

The objectives of this study were to evaluate the effects of pre-and postpa rtum undegraded intake protein (UIP) supplementation on body condition scor e (BCS), BW, calf weight, milk production, serum IGF-I concentrations, and postpartum interval in primiparous beef heifers (n = 44). Heifers were main tained on endophyte-free stockpiled tall fescue (11.7% CP, 38% ADF) and ind ividually fed supplement daily beginning 60 d prepartum. Pre- and postpartu m supplements provided 19.3% CP, 83.4% TDN (UIP); 14.1% CP, 84.1% TDN (Cont rol); 21.5% CP, 81.5% TDN (UIP); and 14.6% CP, 81.4% TDN (Control); respect ively. Blood meal (146 g/d) was the source of UIP. Six heifers were removed from the study due to calf loss unrelated to treatment; therefore, postpar tum measurements are based on 19 animals per treatment. Statistical analyse s using ANOVA and a split-plot design revealed no effects of treatment (P > 0.2) on BCS, BW, calf weight, milk production, or postpartum interval. The re tended to be a treatment x time interaction on BCS (P < 0.09) with UIP h eifers having higher BCS than Control at wk 5, 7, and 9 postpartum. There w as a treatment x time interaction on serum IGF-I (P < 0.06) during the firs t 35 d postpartum. In UIP heifers, serum IGF-I was greater at calving compa red with Central: heifers (117.5 vs 92.4 ng/mL, respectively); however, the se differences were not related to changes in BCS or BW. Although serum IGF -I concentrations were increased at calving in heifers receiving UIP, there were no treatment effects on postpartum interval (P > 0.7). During the fir st 30 d postpartum, IGF-I differed (P < 0.01) among heifers with postpartum intervals defined as short, < 50 d (128.9 ng/mL); medium, 51 to 65 d (115. 2 ng/mL); and long, 66 to 130 d (52.9 ng/mL). When analyzed as a regression , al ng/mL increase in IGF-I (UP and Control heifers) at calvng (P < 0.05) and throughout the postpartum period (P < 0.01) corresponded to a decrease in postpartum interval of 0.13 d. Based on the results of this study; the i nclusion of UIP in diets for primiparous heifers and its effects on postpar tum interval warrant further evaluation.