Associative effects of mixed feeds. I. effects of type and level of supplementation and the influence of the rumen fluid pH on cellulolysis in vivo and dry matter digestion of various roughages

Abstract

1. (1) The effect of various levels and types of concentrate supplementation of roughage-based diets on dry matter degradation, rumen cellulolysis and rumen pH, as well as changes in the rumen microflora and in the proportion of volatile fatty acids (VFA) produced were investigated. 2. (2) In the first experiment it was found that the amount and proportion of barley necessary to depress rumen pH, dry matter (DM) degradation and cellulolysis depended on the availability of readily fermentable barley starch to the rumen microorganisms, and was, therefore, influenced by the degree of processing. A pH threshold of 6.0–6.1 was obtained below which cellulolysis was totally inhibited. 3. (3) When a range of fibrous roughages were supplemented with various concentrates at 65% on a dry matter basis, it was observed that the extent of depression of DM degradation was influenced by both the rumen pH and the rate of solubilisation of the concentrate supplement. This effect was further confounded by the degradability of the roughage, since the percentage reduction in the rate of cellulolysis was greatest with those roughages with a low DM degradability. 4. (4) The depression in the rate of DM degradation could be almost totally alleviated when the rumen pH levels of the sheep were maintained at 6.70 ± 0.15. The reduction in DM degradation which could not be alleviated by increasing the rumen pH was termed the ‘carbohydrate effect’. This effect increased with the degree of processing of grain supplements and was greatest with molasses. 5. (5) It was concluded that in order to maintain rumen cellulolysis when roughages are supplemented with a readily fermentable carbohydrate, the rumen pH has to be maintained above 6.0–6.1, which appears to be the cellulolysis threshold, although the value appeared to vary with the quality of the roughage.