Effect of forage to concentrate ratio in the diet on ruminal fermentation and digesta flow kinetics in sheep offered food at a fixed and restricted level of intake

The effects of four diets differing in their forage:concentrate ratio (80:20, 60:40, 40:60 and 20:80; g/100 g fresh matter) on rumen characteristics, digestibility and digesta flow kinetics were investigated. Alfalfa hay was used as forage and concentrate was composed of barley, soya-bean meal and maize. Diets were prepared by mixing all ingredients and offered to the animals as complete diets. Eight mature Merino sheep, each fitted with a rumen cannula, were offered 1.055 kg dry matter per day of the corresponding diet over two experimental periods. The daily evolution of ruminal pH, volatile fatty acids (VFA) and ammonia nitrogen (N) concentrations were measured. Digestibility was determined by total faecal collection and Cr and Co were used as markers to estimate digesta passage rates, Microbial nitrogen flow at the duodenum (MNDF) was estimated from the urinary excretion of purine derivatives (PD). The apparent digestibility of organic matter increased (P < 0.001) whereas that of all fibrous fractions decreased linearly (P < 0.05) as the proportion of concentrate in the diet increased. Rumen pH decreased linearly (P < 0.001) with increasing proportions of concentrate but total VFA concentrations were unaffected by changes in the diet (P > 0.05). Both liquid and solid digesta outflow rates from the rumen decreased quadratically (P < 0.01) as the proportion of concentrate in the diet increased. The urinary excretion of total N, urea-N and ammonia-N was unaffected (P < 0.05) by changes in the diet. In contrast, the daily urinary excretion of both allantoin and total PD increased quadratically (P < 0.05) with increasing proportions of concentrate. Consequently, the estimated MNDF increased linearly (P < 0.001) from 9.9 g/day on the high forage diet to 14.5 g/day on the high concentrate diet.