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

Abstract

The effects of four diets differing in their for age: 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 ruminai 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.

Comparison of microbial fermentation of high- and low-forage diets in Rusitec, single-flow continuous-culture fermenters and sheep rumen

“…values of the ANOVA comparing both in vitro systems were 9.9, 10.9, 9.5, 6.4, 6.6, 7.8 and 8.9 for 0, 2, 4, 6, 8, 10 and 12 h after feeding, respectively. Carro et al (2000).…”

“…Ruminal dilution rate is affected by many factors, the diet fed to animals being one of them. In the in vivo trial, however, liquid dilution rate was not significantly affected by the diet, possibly due to the moderate level of feeding (Carro et al, 2000).…”

“…In vitro-in vivo comparison Since fermenters are frequently used to investigate the ruminal fermentation of different diets, we decided to analyze the differences between diets detected by the two in vitro systems used in this study, and to compare them with those detected in sheep in a previous study (Carro et al, 2000). Both SFCCF and Rusitec detected the same differences between diets found in vivo for liquid dilution rate, pH values, microbial N flow and growth efficiency (Table 4).…”

“…Whereas in vivo and in SFCCF the acetate proportion was lower and that of propionate and butyrate was higher for diet F20, in Rusitec the fermentation of diet F20 resulted in a lower proportion of propionate compared to diet F80. As a consequence of these changes, acetate/propionate ratios were lower for diet F20 compared to diet F80 in vivo (2.73 and 3.97; Carro et al, 2000) and in SFCCF (3.07 and 4.80). In contrast, acetate : propionate ratios were higher for diet F20 than for F80 (4.29 and 3.40) in Rusitec, and the values were considered unphysiological for high-concentrate diets.…”

“…Since the experimental diets had been used in an in vivo trial carried out previously, in vitro results were compared to those obtained in sheep in order to evaluate the potential of both fermentation systems (FS) for simulating in vivo ruminal fermentation. Results from the in vivo trial have been published elsewhere (Carro et al, 2000).…”