PSXIV-23 Effects of protein/energy supplementation to beef heifers grazing fall native range on growth performance and blood metabolites during mid-gestation.

Abstract The objectives of this study were to evaluate the impacts of providing a protein and energy supplement to beef heifers grazing fall pastures on growth performance and concentrations of blood metabolites during mid-gestation. Angus-based heifers (n = 30; initial body weight [BW] = 435 ± 36.0 kg) were ranked by BW and randomly assigned to graze native range pasture (CON; n = 15) or assigned to graze the same pasture with access to a protein and energy supplement (SUPP; n = 15) delivered at 0.6% of BW on a dry-matter basis using SmartFeed Pro units (C-Lock, Inc.) to control supplement access. Treatments were applied for 62 days beginning at the start of the second trimester of pregnancy. Consecutive 2-d BW were recorded at the beginning, mid-point, and end of the feeding period and average daily gain (ADG) was calculated accordingly. Blood serum samples were also collected three times during the feeding period and analyzed for concentrations of glucose, non-esterified fatty acids (NEFA), and blood urea nitrogen (BUN). Variables were analyzed for the effects of treatment, day, and the respective interaction using the MIXED procedure of SAS, using repeated measures where appropriate. Significance was considered at P ≤ 0.05 with tendencies at 0.05 < P ≤ 0.10. Heifer BW was not impacted (P = 0.85) by the interaction of treatment and day, or by the main effect of treatment (P = 0.74); however, heifer BW increased throughout the feeding period (P < 0.0001). Average daily gain tended (P = 0.08) to be greater in SUPP heifers compared with CON at 1.09 kg/d vs. 0.94 kg/d, respectively. Concentrations of NEFA were impacted by the interaction of treatment and day (P = 0.01), being similar for CON and SUPP heifers at the beginning and mid-point (P ≥ 0.95) of the feeding period, but greater for CON heifers at the end (P = 0.01) compared with SUPP. Concentrations of BUN were also impacted by the interaction (P = 0.02), being similar (P = 0.91) at the beginning but greater (P ≤ 0.001) in SUPP heifers at the mid-point and end of the feeding period compared with CON. Concentrations of glucose were not affected by the interaction of treatment and day (P = 0.11) or the main effect of treatment (P = 0.15), but concentrations increased throughout the feeding period, observed as a day effect (P = 0.03). These data indicate that protein and energy supplementation to pregnant heifers grazing low-quality fall pasture enhanced their energy and protein status, and supplemented heifers exhibited a tendency for improved ADG. The improved metabolic status of the dam may have potential programming effects on the developing fetus, which warrants further investigation to elucidate long-term impacts on the offspring.