The nutritive value of silages. Digestion of organic matter, gross energy and carbohydrate constituents in the rumen and intestines of sheep receiving diets of grass silage or grass silage and barley.

1. Two experiments were conducted to study the digestion of organic matter, gross energy and carbohydrate constituents in the rumen, small intestine and caecum and colon of sheep given grass silage diets. Three silages made from perennial ryegrass (Lolium perenne) with formic acid as an additive were used. One was made from first-harvest grass in the spring and the others from regrowth grass cut from a single sward in either early autumn or late autumn. Expt 1 involved a comparison between the spring silage given alone or supplemented with barley (silage:barley, 4:1 dry matter (DM) basis). Expt 2 involved a comparison between the early-cut and late-cut autumn silages. 2. In Expt 1, supplementation of the silage with barley resulted in a non-significant (P > 0.05) reduction in the proportion of digestible energy (DE) and digestible organic matter digested in the rumen and an increase in the proportions digested in the small intestine. There were also pronounced effects of barley on ruminal cellulolysis and the proportion of digestible cellulose broken down in the rumen was reduced (P < 0.05) from 0.90 to 0.77. There was an increased passage of alpha-linked glucose polymers to the duodenum but even with the supplemented diet 0.91 of the dietary polymers were digested in the rumen. The molar proportion of propionic acid in the rumen tended to be reduced and there were increases in the proportions of butyric acid (P < 0.01) and acetic acid. 3. Expt 2, the digestibility of organic matter, gross energy and cellulose in the early-cut silage was higher (P < 0.01) than in the late-cut silage but there were no significant (P < 0.05) differences between silages in sites of digestion of these constituents. However, the molar proportion of acetic acid in the rumen was higher (P < 0.01) and the molar proportion of propionic acid was lower (P < 0.01) with the late-cut silage than the early-cut silage. 4. The results are discussed in relation to the voluntary intake and utilization of high-digestibility silages.