Effects of increasing supplementation of rumen undegradable protein on plasma essential amino acid concentrations in beef cows consuming low quality forage

Abstract Text: The objective of this experiment was to determine the effects  of increasing supplementation of MP from a moderately abundant RUP source (gluten meal; 62% RUP) on plasma AA concentrations in beef cows consuming low quality forage. We hypothesized increasing MP in the form of gluten meal would positively impact reproductive function around the time of ovulation, potentially through altered circulating AA.  Non-pregnant, non-lactating mature beef cows (n = 24) were stratified by age, BCS, and BW and assigned to 1 of 3 isocaloric diets (1.06 Mcal/kg NE_m) consisting of ad libitum corn stover supplemented primarily with corn gluten meal to provide 100% MP requirements (CON), 125% MP requirements (MP125), or 150% MP requirements (MP150). Supplements were individually fed once daily, and coccygeal blood samples were taken prior to treatment initiation and on d 49 of the experimental period. Orthogonal contrasts were utilized to determine linear and quadratic treatment effects on plasma AA concentrations. At trial initiation, AA concentrations did not differ among treatments (P > 0.10). On d 49, total AA and total essential AA concentrations were not affected (P > 0.10) by treatment; however, percent essential AA tended to increase linearly (P = 0.06) with increasing MP. Total concentrations of leucine, phenylalanine, citrulline, ketogenic AA, and branched-chain AA (BCAA) all increased linearly (P < 0.04), and total urea cycle AA tended to increase linearly (P = 0.08) relative to increased MP supplementation. Total lysine and tryptophan decreased linearly (P < 0.04) and total histidine tended to decrease linearly (P = 0.09) as MP supplementation increased. When expressed as a percent of total AA, ketogenic AA, BCAA, urea cycle AA, leucine and phenylalanine increased linearly (P < 0.01), and arginine and essential proteogenic AA tended to increase linearly (P < 0.09) with increasing MP supplementation. As a percent of total AA, glycogenic AA, lysine and tryptophan decreased linearly (P < 0.04) and threonine tended to decrease linearly (P = 0.06) as MP supplementation increased. In summary, supplementing excess MP using a protein source moderately abundant in RUP may shift plasma AA concentrations and profiles when fed to beef cows consuming low quality forage. 

Keywords: branched-chain, ketogenic, urea cycle