Effect of feeding increasing concentrations of rumen-protected carbohydrate on the growth performance and blood and plasma metabolites of receiving feedlot heifers

The objective of this experiment was to evaluate the inclusion of a rumen-protected carbohydrate (RPC) on growth performance and plasma metabolites in growing beef heifers. Crossbred heifers (n = 135; 136 ± 14 kg) were used in a 63-d experiment. Heifers were blocked by initial body weight (BW), placed into 15 pens (9 heifers/pen), and assigned randomly to 1 of 3 treatments 0, 0.5, and 1.0% of RPC. Animals were fed 82.3% of a basal diet (38.8% corn silage, 41.5% dry rolled corn, 2% mineral-vitamin premix on a dry matter basis) and 17.7% supplement. The non-processed supplement or RPC supplement composition (DM basis) were 58.1% soybean meal, 38.9% soluble carbohydrates, 2% urea, and 1% mineral salt. The non-processed supplement or RPC supplement consisted of the same ingredients, differing in the processing of the carbohydrate (i.e., protected or not from ruminal degradation). For the 0, 0.5, and 1% RPC treatments, the ratio of supplement to RPC was 1:0, 1:1, and 0:1, respectively. Body weight was measured on d 0, 21, 42, and 63. Pen dry matter intake (DMI) was measured weekly from d 7 to 63. Back fat on the 12th rib (BF) was measured at d -21 and 63. Blood samples were taken on d -21, 21, 42, and 63 from jugular vein prior morning feeding and analyzed for blood glucose concentration and plasma insulin, urea, and non-esterified fatty acids (NEFA) concentrations. Data were analyzed as a randomized complete block design with repeated measures using a mixed model of SAS (9.4). Treatment × day interaction (P ≤ 0.02) was observed for DMI, ADG, and G: F. Heifers on treatment RPC0.5 had the least DMI (P < 0.05) and the greatest G: F (P < 0.05). No differences (P ≥ 0.34) were observed in the concentrations of blood glucose, plasma insulin, plasma NEFA, plasma urea, or BF on d 63. Feeding 8.85% of RPC (treatment RPC0.5) improved G: F through lesser DMI without altering ADG, blood, or plasma metabolites.