Effect of defaunation and hay:concentrate ratio on fermentation, fibre digestion and passage in the rumen of sheep

Abstract

Two rumen-cannulated wethers were used to study the effect of defaunation and dietary carbohydrate composition on fermentation, fibre digestion and passage in the rumen. According to a crossover design, sheep were either free of ciliate protozoa or had a type B protozoal population, and were fed either a diet consisting mainly of chopped hay (H) or of pelleted concentrate (C). Nylon bags containing detergent-treated hay were incubated in the rumen to measure fibre digestion. Passage was measured using chromium-mordanted hay (Cr-H) and ytterbium acetate (Yb) as markers for the particulate phase, and polyethylene glycol (PEG) 4000 as soluble marker. Total numbers of protozoa were 1.2 × 10 6 ml −1 and 0.7 × 10 6 ml −1 on Diets C and H, respectively, in the refaunated state. On Diet H, the pH, the proportion of propionate and numbers of fungal sporangia were significantly higher in the absence of protozoa, while the proportion of butyrate and ammonia concentration were significantly lower. On Diet C, the proportion of propionate and lactic acid concentration were significantly higher, and total short chain fatty acid and ammonia concentration significantly lower, in the defaunated state. On Diet H, the data clearly suggest a depressive effect of the absence of protozoa on nylon bag loss of detergent-treated hay after incubation times of 16–56 h. Statistical significance was reached only after 40 h, however, and the tendency was less marked on Diet C. Slower fractional passage rates of all three markers were found in the defaunated state on Diet H, but were only significant for PEG and Cr-H. Pool sizes on this diet were observed to be somewhat higher, but only significantly for the liquid phase. On Diet C, defaunation only resulted in an increased fractional passage rate (estimated by Yb) and an increased pool size (estimated by Cr-H). Differences in the sheep response to diet and/or defaunation occurred for several parameters, rendering interpretation difficult. The results of this study suggest, nevertheless, that defaunating the rumen of adult sheep under limited feeding conditions did not cause consistent changes in digesta kinetics. Fibre digestion in the rumen was moderately depressed.