The effect of the physical form of the diet and the level of feeding on the digestion of dried grass by sheep

Intact and rumen cannulated sheep were used to study the digestion of an artificially dried grass offered either in a chopped or a ground and pelleted form at energy intakes ranging from 1–3.2 × the energy requirement for meantenance. In each form of the grass the digestibility of all dietary constituents was depressed at high levels of feeding and, excepting ether extract, was lower in the ground and pelleted grass than in the chopped grass. The digestibility of organic matter (DOM) varied with treatment from 70.7–82.7% and for each form of grass was related to the level of feeding (LF) such that DOMC = 83.1–1.01 LF (P < 0.05) and DOMG = 84.6–4.12 LF (P < 0.001). Corresponding relation‐ships were observed between the mean retention time of food residues in the alimentary tract (MRT) and the level of feeding (LF) such that MRTC = 111.7–95.1 log LF (P < 0.001) and MRTG = 77.2–60.8 log LF (P < 0.001). Where animals were fed on ground and pelleted grass there were changes in rumen fermentation characterised by a lowered rumen pH and cellulolytic activity and by reduced concentrations of ammonia‐N and total short‐chain fatty acids in the rumen fluid. The percentage of acetic acid in the fermentation products was decreased and there were associated increases in the percentages of butyric acid and valeric acid. The results indicate that grinding and pelleting the grass was associated with a reduction in the digestibility of organic matter resulting partly from an increased rate of flow of digesta through the alimentary tract and partly from a reduction in cellulolytic activity in the rumen.