Effects of ruminally protected choline and dietary fat on performance and blood metabolites of finishing heifers.

A 120-d finishing study utilizing 318 heifers (342 kg initial BW) was conducted to examine effects of ruminally protected choline (RPC) in diets containing graded concentrations of tallow. Heifers were blocked according to previous nutrition (full-fed or limit-fed) and allotted to 24 pens containing 11 to 15 heifers. Two pens, one within each block, were assigned to each of 12 factorially arranged treatments including dietary tallow (0, 2, or 4%) and supplemental RPC (0, 20, 40, or 60 g of product daily, estimated to supply 0, 5, 10, or 15 g/d choline postruminally). Heifers were implanted with Revalor-H and fed a finishing diet based on steam-flaked and dry-rolled corn (12.5% CP, 8% alfalfa on DM basis). Dry matter intake decreased (P < 0.10) by 5.4% when tallow was increased from 0 to 4% but was not affected by RPC. Heifers receiving 4% tallow had 7.3% lower gains than those receiving none (P < 0.10). Supplementation of RPC increased (P < 0.10) ADG, with 20 g/d resulting in an 8.6% increase. Similarly, gain efficiency improved (P < 0.10) by 7.6% with addition of 20 g/d RPC. Yield grade and kidney, pelvic, and heart fat both increased linearly (P < 0.10) with fat supplementation. The percentage of carcasses grading USDA Choice was not affected by intermediate levels of RPC but decreased with the highest level (60 g/d). Dressing percentage, hot carcass weight, marbling, and 12th-rib fat thickness were not affected significantly by either tallow or RPC. On d 90, jugular blood was collected from all heifers at 2 h postfeeding. Plasma urea and serum insulin concentrations were not affected by either tallow or RPC. Dietary tallow linearly increased (P < 0.10) NEFA, cholesterol, triglyceride, and total amino acid concentrations. Choline supplementation led to quadratic responses for total amino acids (P < 0.10), with concentrations being greatest for intermediate levels of RPC. Moderate levels of supplemental RPC improved growth performance of finishing cattle without negatively affecting carcass characteristics. Optimum performance was achieved with 20 g of product daily.