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Abstract Twelve sheep were fed corn stover ad libitum plus a protein-mineral supplement (period 1). The diet of each sheep was then supplemented differently with corn grits so that these constituted 0, 90, 120, 150, 180, 210, 240, 270, 300, 330, 360 or 390 g/kg of the total daily food intake (period 2). All diets contained 13.8% crude protein. Stover intake and the amount of cellulose and hemicellulose digested per day declined progressively with increasing grain supplementation. Retention time of solids in the rumen decreased. Numbers of total culturable, cellulolytic and active xylanolytic bacteria per gram rumen ingesta did not change significantly with grain feeding. In both periods, the predominant cellulolytic bacteria resembled Ruminococcus albus. Other major species were Ruminococcus flavefaciens-like cocci and Butyrivibrio fibrisolvens-like rods. The number of R. albus-like bacteria did not change but those of other cellulolytic species tended to increase with increasing amounts of grain fed. The same species constituted the major hemicellulolytic bacteria, but the B. fibrisolvens-like rods were more numerous than the R. albus-like cocci. Grain supplementation did not affect the proportions of the different hemicellulolytic species. The decline in the intake and digestion of fiber when grain was fed could not be attributed to a decrease in production of cellulolytic or hemicellulolytic bacteria in the rumen. Copyright © 1984. American Society of Animal Science. Copyright 1984 by American Society of Animal Science