The effect of grinding and pelleting on the digestion of Italian ryegrass and timothy by sheep

1. Primary growths of Italian ryegrass and timothy were harvested in late May, high-temperature dried and either retained in the chopped form or ground through a 2 mm sieve and pelleted. All diets were fed to four sheep fitted with re-entrant cannulas into the proximal duodenum and measurements of the sites of energy and protein digestion and the synthesis of volatile fatty acids (VFA) and microbial protein were made. 2. Grinding and pelleting significantly reduced rumen digestion of organic matter and structural carbohydrate (P less than 0.05) and the synthesis of rumen VFA (P less than 0.01), whilst significantly more digestion occurred in the hind gut, although this was not sufficient to prevent a decline in over-all digestibility on the pelleted diets (P less than 0.05). The magnitude of all responses was much larger on the Italian ryegrass diet. 3. Net microbial protein synthesis was 15% less on the pelleted diets but efficiency of microbial protein synthesis was unaffected (mean 188 g/kg rumen digested organic matter). Pelleting reduced the degradation of dietary protein from 69% to 47%, and dietary protein represented significantly more of the total protein flowing to the duodenum on the pelleted diets (chopped 28%, pelleted 41%). 4. Over-all, grinding and pelleting reduced total absorbed energy supply by 10% but increased absorbed protein supply by 15% which may contribute to some of the improvements seen in the net energy value of pelleted diets.