Protein metabolism in the rumen is the result of metabolic activity of ruminal microorganisms. The structure of the protein is a key factor in determining its susceptibility to microbial proteases and, thus, its degradability. Ruminal protein degradation is affected by pH and the predominant species of microbial population. Ruminal proteolytic activity decreases as pH decreases with high-forage dairy cattle-type rations, but not in high-concentrate beef-type rations. Accumulation of amino acid (AA) N after feeding suggests that AA uptake by rumen microorganisms could be the limiting factor of protein degradation in the rumen. In addition, there are several AA, such as Phe, Leu, and Ile, that are synthesized by rumen microorganisms with greater difficulty than other AA. The most common assessment of efficiency of microbial protein synthesis (EMPS) is determination of grams of microbial N per unit of rumen available energy, typically expressed as true organic matter or carbohydrates fermented. However, EMPS is unable to estimate the efficiency at which bacteria capture available N in the rumen. An alternative and complementary measure of microbial protein synthesis is the efficiency of N use (ENU). In contrast to EMPS, ENU is a good measurement for describing efficiency of N capture by ruminal microbes. Using EMPS and ENU, it was concluded that optimum bacterial growth in the rumen occurs when EMPS is 29 g of bacterial N/kg of fermented organic matter, and ENU is 69%, implying that bacteria would require about 1.31 x rumen-available N per unit of bacterial N. Because the distribution of N within bacterial cells changes with rate of fermentation, AA N, rather than total bacterial N should be used to express microbial protein synthesis.

A three-step in vitro procedure was developed to estimate intestinal digestion of proteins in ruminants. Dacron bags containing feed samples were suspended in the rumen for 16 h. Residue containing 15 mg of N after ruminal exposure was incubated for 1 h in 10 mL of a .1 N HCl solution containing 1 g/L of pepsin. After incubation, pH was neutralized with .5 mL of 1 N NaOH and 13.5 mL of a pH 7.8 phosphate buffer containing 37.5 mg of pancreatin were added to the solution and incubated at 38 degrees C. After a 24-h incubation, 3 mL of a 100% (wt/vol) trichloroacetic acid solution were added to precipitate undigested proteins. Preincubation of samples in the rumen did not affect (P > .05) pepsin-pancreatin digestion of residual CP in soybean meal (SBM), corn gluten meal (CGM), and blood meal (BM) and reduced (P < .05) pepsin-pancreatin digestion of residual CP in hydrolyzed feather meal (HFM), fish meal (FM), and meat and bone meal (MBM) (80 vs 70, 88 vs 81, and 82 vs 56%, respectively, for nonruminal vs ruminal preincubation). Pepsin digestion before pancreatin digestion increased (P < .05) CP digestion of all proteins tested by a mean of 23 percentage units. The pancreatin digestion step was validated using 34 duodenal samples from which small intestinal CP digestion was determined in vivo. The regression equation of in vivo estimates on pancreatin digestion had an r value of .91 (P < .001).(ABSTRACT TRUNCATED AT 250 WORDS)

1. Twelve grain mixtures, one lucerne (Medicago sativa) hay and one maize silage which had been used in mixed diets for which dietary nitrogen undegraded in the rumen (UDN) had been estimated with duodenally-cannulated cows, were studied. Total N in the feeds was fractionated into pool A (N soluble in borate-phosphate buffer), pool B (total N-(pool A+pool C)) and pool C (acid-detergent-insoluble N or residual N after 24 h incubation in protease solution).2. N solubilization in protease solution containing 6.6 units/ml (substrate-saturating enzyme concentration) indicated the presence of subfractions in pool B, with different rates of solubilization. Such subfractions were not detectable from in situ, Dacron bag, estimates of N solubilization.3. UDN was estimated using a dynamic mathematical model and rate-constants obtained from N solubilization in protease solution or in situ. For three grain mixtures tested using the protease technique the model predicted UDN values of 7, 10 and 12% compared with values of 47, 66 and 59% estimated in vivo. The full range of experimental feeds was tested using the in situ technique and UDN values predicted by the model were used to derive UDN values for twelve mixed diets. The latter values were significantly but not closely correlated with those determined in vivo (rz 0.41, P < 0.05).4. An attempt was made to simulate rumen proteolysis in vitro by choosing a protease enzyme concentration (0.066 units/ml) providing a proteolytic activity similar to that of whole rumen fluid. The experimental samples of feed were subjected to simulated rumen proteolysis for 18 or 48 h to resemble the mean retention times in the rumen for grain mixtures and roughages respectively. The residual N at the end of incubation was considered as an estimate of UDN. The UDN values estimated from simulated rumen proteolysis and those determined in vivo for twelve mixed diets were in close agreement (re 0.61, P < 0.01). 5.Simulated rumen proteolysis can serve as a simple, rapid and sensitive method to estimate UDN in a varief y of feedstuffs.

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Methanogen populations in the rumen and in model rumen systems (operated over a 240‐h period) were studied using the small subunit (SSU) rRNA phylogenetic framework for group‐specific enumerations. Representatives of the family Methanobacteriaceae were the most abundant methanogen population in the rumen, accounting for 89.3% (± 1.02%) of total archaea in the rumen fluid and 99.2% (± 1.8%) in a protozoal fraction of rumen fluid. Their percentage of archaea in the model rumen systems declined from 84% (± 8.5%) to 54% (± 7.8%) after 48 h of operation, correlated with loss of protozoa from these systems. The Methanomicrobiales, encompassed by the families Methanomicrobiaceae, Methanocorpusculaceae, and Methanospirillaceae were the second most abundant population and accounted for 12.1% (± 2.15%) of total SSU rRNA in rumen fluid. Additionally this group was shown to be essentially free living, since only a negligible hybridization signal was detected with the ruminal protozoal fraction. This group constituted a more significant proportion of total archaea in whole rumen fluid, 12.1% (± 2.1%) and model rumen fluid containing no protozoa (26.3 ± 7.7%). In contrast, the Methanosarcinales, generally considered the second most abundant population of rumen methanogens, accounted for only 2.8% (± 0.3%) of total archaeal SSU rRNA in rumen fluid.