AbstractsBiology & Animal Science

The primary source of protein for milk production is of microbial origin, so increasing microbial protein supply should either increase milk protein or decrease the need for expensive bypass protein. Moreover, improving the consistency of prediction of microbial protein supply decreases the amount of protein that must be fed to the dairy cow, reducing urinary N excretion and decreasing feed costs. Methionine is co-limiting with lysine in most dairy diets. 2-hydroxy-4-(methylthio) butanoic acid (HMBi) is a methionine precursor for bacteria and might be degraded more slowly and, therefore, might be sustained longer over the feeding cycle for bacterial uptake or absorption than methionine itself. To show the effect of HMBi and methionine on bacterial population structure and protein synthesis, we used four continuous culture fermenters inoculated with rumen fluid from two lactating Holstein cows. We fed the cultures a normal dairy ration with a 50/50 ratio of concentrate and alfalfa thrice per day. Our experimental design was a 4x4 Latin square. Our treatments were control (CON), 0.11% HMBi (HMBi), 0.097% methionine (MET), and 0.055% HMBi plus 0.048% methionine (HMBi + MET). All doses were on an equivalent methionine molar basis. We dosed the stable isotopes 1-13C-methionine, 3D-methionine (methyl hydrogens labeled with deuterium) and U-13C-HMBi into the fermenters at six consecutive feedings to trace the incorporation of the carbon skeletons into microbial protein and methylation of substrates by methionine. We also infused NH3-N15 with the buffer during each period to measure efficiency of microbial protein synthesis. The data were compared using three orthogonal contrasts, all methionine treatments versus control and the linear and quadratic responses to methionine substitution by HMBi. Supplementation of methionine and HMBi had no effect on digestibilities of ADF and true OM. NDF and hemicellulose digestibility were linearly affected (P=0.04 and P<0.01, respectively) by supplementation. The flow of non-ammonia N, bacterial N, total N, and non-ammonia, non-bacterial N were not affected. Ammonia N flow tended (P=0.08) to be affected linearly. Concentration of NH3-N tended to be affected linearly (P=0.07) and quadratically (P=0.08). Peptide N was affected linearly (P=0.04) and tended to be affected quadratically (P=0.09). The proportion of bacterial N from NH3-N was affected linearly (P=0.02). Propionate production was affected linearly (P<0.01) and quadratically (P=0.05), and control was greater than the average of the three methionine treatments (P=0.01). Isobutyrate was affected linearly (P=0.05). Isovalerate tended to be affected linearly (P=0.08). Valerate was affected linearly (P<0.01) and quadratically (P=0.05), and control was less (P=0.01) than the average of the three methionine treatments. The production of total VFA was affected linearly (P=0.02). The acetate:propionate tended to be affected linearly (P=0.08) and quadratically (P=0.06). The increase in hemicellulose digestibility due to methionine…