Effects of Strategic Supplementation of Low Quality Diets and Residual Feed Intake Classification to Optimize Performance in Gestating Beef Cattle

A wintering beef cow study was conducted to evaluate strategic supplementation of a straw-based diet with an energy-protein supplement during the last 75 days of gestation on beef cow performance and feed efficiency during gestation and subsequent pre-weaning calf performance. Gestating crossbred beef cows (predominantly Angus X Simmental; n = 118, 128 ± 31 days in gestation, 691.5 ± 235 kg BW) were evaluated on one of five nutritional regimens: 1) 70% haylage, 30% millet straw on a dry matter basis (DMB) fed ad libitum (HAYL), 2) millet straw fed ad libitum along with haylage at 0.8% DMB of cow BW (WSHAYL), 3) WSHAYL along with an energy-protein supplement once/week, 4) WSHAYL with supplementation twice/week, 5) WSHAYL with supplementation three times/week. The supplement contained 54.6% corn gluten meal, 23.4% soybean meal, 21.0% calcium propionate and 1.0% tallow, and was fed at 0.32 g/kg of BW per day DMB. Cows were allocated to ensure similar days in gestation, age, residual feed intake (RFI) classification and breed composition across nutritional regimens. Cows were evaluated throughout second and third trimesters of gestation for feed intake, BW, average daily gain (ADG), residual feed intake (RFI), body condition score (BCS), ultrasound measurement of rib fat depth, serum blood metabolite concentrations and pre-weaning calf performance traits. Cows were classified into low, medium and high RFI groups (< 0.5 SD; ± 0.5 SD; > 0.5 SD respectively based on a mean RFI of 0). Statistical analysis to examine differences in nutritional regimens and RFI classification were performed using PROC GLIMMX in SAS. Cows fed the HAYL diet had higher percentage of high RFI cows and greater DMI vs. all other nutritional regimens (P < 0.001). Serum NEFA concentrations were lower in low RFI cows (P = 0.003) while urea concentrations tended to be greater in low RFI cows (P = 0.069). Serum AST concentrations were greater in HAYL cows vs. all other diets (P = 0.001). ADG was highest in HAYL cows vs. all other nutritional regimens (P < 0.001), while ADG was greater in low vs. high RFI cows (P < 0.001). Rib fat depths increased the most over gestation in HAYL vs. all other nutritional regimens (P < 0.001). Pre-weaning calf performance traits were not significantly affected by nutritional regimen or RFI classification. RFI classification was affected by nutritional regimen while wintering cow performance was related to individual cow RFI classification.