Ruminal microbial protein synthesis in sheep fed forages of varying nutritive value

Two digestion experiments were conducted to determine the factors affecting the amount and the efficiency of microbial protein synthesis (MOEFF) in sheep fed forages of varying nutritive value without and with protein supplementation. Six and five wethers in the first and second experiment, fitted with cannulae in the rumen and proximal duodenum, were utilized in a 6 x 6 and 5 x 5 Latin Square metabolism trial in the first and second experiment. Sheep were fed ground corn crop residues, oat-berseem clover or alfalfa hay at two levels of intake in the first experiment. In the second experiment, ground corn crop residue with or without alfalfa or oat-berseem, clover hay supplemented to meet DIP requirements were fed to sheep ad libitum intake. In both experiments, chromium-mordanted fiber, cobalt-EDTA, and bacterial purines were used to determine digesta flow and rate of solid passage, liquid dilution rate, and microbial protein flow into the duodenum, respectively. In experiment 1, dry matter intake, and the amounts of OM apparently and truly digested in the rumen (g/d; P \u3c .05) were different between sheep fed unsupplemented forages. Slow particulate turnover rate, liquid dilution rate, post-feeding ruminal ammonia-N and VFA concentrations, total duodenal flow of microbial-N (g/d), and MOEFF (g MCP/g 100 of OMTDR) were lower (P \u3c .05) in sheep fed corn crop residues compared to sheep fed hays. While total duodenal microbial-N flow linearly increased with increasing OMI (r2 = .97), and amount of OMTDR (r2 = .97), MOEFF was quadratically related to g NTDR/100 g OMTDR (r2 = .82) and the slow rate of particulate turnover (r2 = .91). In experiment II, increasing level of hay proportion in diets linearly increased (P \u3c .05) dry matter intake, the amounts of OM apparently or truly digested and liquid dilution rate in the rumen. Post-feeding ruminal ammonia-N concentration decreased (P \u3c .01) as the corn crop residue:hay ratio decreased in the diets. Increasing level of hay proportion in the diets linearly increased (P \u3c .01) total duodenal flow of microbial-N, but did not affect MOEFF. Total duodenal microbial-N flow was linearly correlated to OMI (r2 = .96), and amount of OMTDR (r2 = .96), but MOEFF was quadratically related to g NTDR/100 g OMTDR (r2 = .96)