Evaluation of Circulating Plasma Amino Acid Concentrations in Beef Heifers Supplemented Protein in a Low to Medium Quality Forage Diet.

Angus, Hereford, and Angus-cross yearling beef heifers (409 ± 35 kg; n=39) were used to evaluate protein supplementation strategies in low to medium quality forage diet on circulating AA concentrations. Heifers were stratified by BW and BCS and then randomly assigned to 1 of 4 protein supplements; 1) no supplementation (CON); 2) low-fat dried distillers grains (LDG, 7.4% fat; 0.8% BW); 3) high-fat dried distillers grains (HDG, 9.5% fat; 0.8% BW); and 4) cottonseed meal (CSM, 2.2% fat; 0.4% BW) to provide similar CP intake. Heifers were individually fed by means of a Calan gate system (American Calan Inc, Northwood, NH). Hay was fed twice daily 7 h apart to provide low to medium-quality chopped grass hay (8.6% CP, 70% NDF; 1.13 Mcal NE_m/kg) ad libitum and 1 h prior to hay feeding supplements were provided. Supplementation began 7 d prior to timed artificial insemination (TAI; d0) and ended 30 d later. A sub-set of heifers (3 per treatment) were randomly selected to evaluate circulating AA concentrations post-feeding on d 26, were fed protein supplements prior to AM hay feeding, and then coccygeal blood samples were collected at -2, 2, 4, 6, 8, 10, 12, and 14 h post-supplementation. Overall plasma total AA (TAA), branch-chain AA (BCAA) and essential AA (EAA) concentrations were lower (P ≤ 0003) for CON than CSM, HDG and LDG whereas overall plasma non-essential AA (NEAA) concentrations were not different (P = 0.22) among treatments. At 8 h post-feeding plasma TAA, BCAA, and EAA concentrations were lowest (P ≤ 0.03) compared to all other hours. Within h 8, HDG had greater (P ≤ 0.04) plasma TAA, BCAA, NEAA and EAA concentrations compared to CON, CSM, and LDG. Plasma EAA concentrations were lower (P ≤ 0.03) for h 6, 12 and 14 and BCAA h 2, 4 10, 12, 14 in CON heifers compared to LDG and HDG supplemented heifers, however plasma NEAA concentrations were not different (P ≥ 0.05) among treatments 4 and 14 h post-feeding. Plasma ASP and TRP were not different (P ≥ 0.33) among supplementation strategies for treatment or treatment x h. Greater plasma ARG concentrations were observed (P ≤ 0.03) in CSM vs. HDG, LDG, and CON at h 6, 10, 12, 14 h post-feeding. Overall protein supplementation increased circulating AA concentrations compared to no supplementation and the lowest AA concentrations were observed at 8 h post-feeding.