Comparison of fermentation of diets of variable composition and microbial populations in the rumen of sheep and Rusitec fermenters. I. Digestibility, fermentation parameters, and microbial growth.

ABSTRACT

Four ruminally and duodenally cannulated sheep and 8 Rusitec fermenters were used to determine the effects of forage to concentrate (FC) ratio and type of forage in the diet on ruminal fermentation and microbial protein synthesis. The purpose of the study was to assess how closely fermenters can mimic the dietary differences found in vivo. The 4 experimental diets contained FC ratios of 7030 or 3070 with either alfalfa hay or grass hay as the forage. Microbial growth was determined in both systems using (15)N as a microbial marker. Rusitec fermenters detected differences between diets similar to those observed in sheep by changing FC ratio on pH; neutral detergent fiber digestibility; total volatile fatty acid concentrations; molar proportions of acetate, propionate, butyrate, isovalerate, and caproate; and amylase activity. In contrast, Rusitec fermenters did not reproduce the dietary differences found in sheep for NH(3)-N and lactate concentrations, dry matter (DM) digestibility, proportions of isobutyrate and valerate, carboxymethylcellulase and xylanase activities, and microbial growth and its efficiency. Regarding the effect of the type of forage in the diet, Rusitec fermenters detected differences between diets similar to those found in sheep for most determined parameters, with the exception of pH, DM digestibility, butyrate proportion, and carboxymethylcellulase activity. Minimum pH and maximal volatile fatty acid concentrations were reached at 2h and at 6 to 8h postfeeding in sheep and fermenters, respectively, indicating that feed fermentation was slower in fermenters compared with that in sheep. There were differences between systems in the magnitude of most determined parameters. In general, fermenters showed lower lactate concentrations, neutral detergent fiber digestibility, acetatepropionate ratios, and enzymatic activities. On the contrary, fermenters showed greater NH(3)-N concentrations, DM digestibility, and proportions of propionate, butyrate, isovalerate, valerate, and caproate. Values of efficiency of microbial growth were greater in fermenters compared with sheep for 7030 diets, but they were lower for 3070 diets. Differences between fermentation in sheep and fermenters can be mainly attributed to the lack of absorption in fermenters, differences in solid retention time, and compartmentalization in the Rusitec system. In general, the Rusitec system simulated more closely the in vivo fermentation of high-forage diets compared with high-concentrate diets.