PS2-11. The Evaluation of Source and Level of Rumen Undegradable Protein on Sera and Histotroph Amino Acid Concentrations in Beef Heifers Grazing Native Rangelands.

Abstract The objective of these studies was to investigate the effects of source and level of RUP supplementation on sera and histotroph amino acid concentrations in beef heifers grazing native rangelands. Heifers (N = 64; Yr 1: n = 32; Yr 2: n = 32) were randomly assigned to one of four treatments: 1) 36% plant sourced RUP (36P, n = 8 per yr), 2) 50% plant sourced RUP (50P, n = 8 per yr), 3) 36% animal sourced RUP (36A, n = 8 per yr), and 4) 50% animal sourced RUP (50A, n = 8 per yr). Heifers were individually supplemented three times a week for 110 d and treatments were offered at a rate of 1.13 kg/hd/d. Heifers were artificially inseminated and, 7 days later, underwent uterine flushing to collect histotroph. At time of uterine flushing, a blood sample was collected. Serum and histotroph concentrations were analyzed for Asn, His, Ser, Glu, Asp, Gly, Gln, Arg, Thr, Ala, Pro, Lys, Cys, Tyr, Val, Met, Ile, Leu, Phe, and Trp using UPLC/MS. Sera and histotroph amino acid concentrations were analyzed using MIXED procedures in SAS. Analyses remain separated by year because Yr had a significant effect on concentrations (P < 0.05). In Yr 1, source, level, and their interaction did not influence sera Asn, His, Ser, Glut, Asp, Gln, Ala, Pro, Lys, Try, Val, Met, Leu, Phe, and Trp concentrations (P ≥ 0.11). In Yr 1, heifers offered the 36 level RUP diets had elevated sera Gly (P = 0.001) and Ile (P = 0.02) concentrations compared to heifers offered the 50 level diets. Histotroph Gly concentrations tended to be elevated in 36 level heifers compared to 50 level heifers, however source, level, and source x level interaction did not influence any other histotroph amino acid concentrations in Yr 1 (P ≥ 0.12). In Yr 2, heifers offered the 50 level diets had increased sera Phe (P = 0.02), Leu (P < 0.05), Met (P < 0.01), Lys (P = 0.05), and Arg (P = 0.03) concentrations compared to heifers offered the 36 level treatments. Histotroph amino acid concentration was not influenced by source, level, or source x level interaction in Yr 2 (P ≥ 0.24). In conclusion, the level of RUP in diets influenced sera amino acid concentrations, but the source, level, and their interaction did not impact amino acid histotroph concentrations. The mechanism through which level of RUP supplementation is enhancing reproduction, however, is likely not by modifying the amino acid composition of the histotroph but possibly by alterations in non-amino acid constituents in the histotroph or the microenvironment of the follicle. USDA is an equal opportunity provider, employer, and lender.