After a preliminary period in which they were all fed maize straw plus a proteinmineral supplement, 18 Merino wethers were divided into six groups and fed straw, proteins and minerals as before, plug pellets containing maize grain so that these constituted 0, 78, 156, 235, 313 and 393 g/kg of the total daily intake.
The diets provided sufficient protein so that NH3 and branched-chain volatile fatty acids were not limiting for growth of the fibre-digesting bacteria in the rumen.
The intake of straw, the digestibility of cellulose and hemicellulose, and the mass of cellulose and hemicellulose digested per day declined linearly as the proportion of pellets in the diet increased above 78 g/kg. This decline was not related to the pH of the ruminal contents which was unaffected by the feeding of up to and including 235g pellets/kg diet, and which, with one exception, was only 2–6 pH-hours below pH 6 when more grain was fed.
As the proportion of pellets in the diet increased the number of cellulolytic bacteria in the rumen declined to an extent which correlated well with the decrease in mass of cellulose digested per day. There was no change in the relative proportions of the predominant genera.
There was no decrease in the number of xylanolytic bacteria in the rumen as more pellets were fed, but there was an indication of a change in the predominant genera producing diffusible xylanases.
It is concluded that some factor, in addition to nutrient limitation and pH, may play a role in the decrease in intake and digestion of roughage when starch is fed. It is suggested that starch or sugars derived from it may per se inhibit the synthesis and/or activity of the rumen cellulases and hemicellulases.