Submitted to: Western Section of Animal Science Proceedings
Publication Type: Proceedings
Publication Acceptance Date: April 15, 2011
Publication Date: June 20, 2011
Citation: Ujazdowski, V., Waterman, R.C., Petersen, M.K. 2011. The Role of Rumen-Protected Methionine on Amino Acid Metabolism in Late Gestation Beef Heifers in the Northern Great Plains. Western Section of Animal Science Proceedings 62:309-313. Interpretive Summary: Late gestating cattle grazing rangelands in the northern Great Plains often require supplementation in order to meet their nutritional requirements. However, the proper balance of amino acids still may not be achieved since amino acids requirements for pregnant range beef cattle are not well defined. Supplement formulation is inexact, and some supplements are utilized less effectively. Research indicates that the first limiting amino acids are most likely methionine (Met), especially when rumen microbial protein is the key supplier of amino acids to the small intestine. Forage and supplementary proteins are generally degraded extensively in the rumen, incorporated into microbial protein, and delivered to the small intestine as metabolizable protein, which becomes the primary amino acids source for the host ruminant. Researchers have concluded that pregnant beef cows exhibited improved nitrogen retention when consuming a mature forage based diet with added urea to minimize any N deficiency in the rumen while infusing DL-Met directly into the abomasum. The objectives of this field metabolism study were to primarily determine if plasma amino acids concentrations were altered due to inclusion of rumen-protected, DL-Met when supplemented to range heifers in late gestation. The second objective was to determine if these changes manifested in serum metabolites or BW measurements. By supplementing rumen-protected, DL-Met, we hypothesize that heifers will likely improve utilization of other non-limiting or less limiting amino acids. In summary, addition of methionine to supplements allowed for better utilization of branch chain amino acids. It is not determined if Branch chain amino acids were utilized by the placenta, fetus, or maternal tissues.
Technical Abstract: This study evaluated changes in plasma amino acids in late-gestating (beginning 58 ± 1.02 d prior to calving), primiparous, winter-grazing range heifers receiving a wheat middling based supplement without (CON) or with rumen-protected methionine (MET). Plasma was collected on d -2 and d 0 (start of MET supplementation just prior to individually receiving supplement at 0700 h). Plasma was sampled again on d 40, 42 and 44 prior to supplementation at 0700 h and 1100 h (4 h after receiving daily supplement). Data were analyzed with cow as the experimental unit. Continuous variables were analyzed by the main effects of treatment, date, or time and their interaction when appropriate. Comparable BW (P = 0.74) and BCS (P = 0.65) over the 44-d metabolism trial were found between both CON- and MET-fed heifers. MET-supplemented heifers had greater (P < 0.0001) plasma concentrations of methionine indicating that the rumen-protected technology successfully presented methionine to the small intestine for absorption. Notable decreases in plasma AA concentrations in MET-fed heifers after 44 d included leucine (P = 0.04), valine (P = 0.03), and serine (P = 0.05). Glycine, the most abundant amino acid in maternal blood, was greater (P = 0.05) in CON-fed heifers than MET-fed heifers. Lower glycine concentrations in MET heifers were due to the role of glycine in catabolism of methionine. Branched chain amino acids (BCAA) are instrumental in metabolism of maternal protein and there were several significant differences in BCAA between CON and MET fed heifers. Isoleucine, leucine and valine resulted in notable decreasing percent changes from d 0 to 44 (P = 0.06, P = 0.04, P = 0.03, respectively) for MET-fed heifers. These results imply methionine is a limiting amino acid in late-gestating heifers grazing dormant range forages due to improved utilization of other amino acids when supplemental methionine was provided.