Effect of increasing availability of water-soluble carbohydrates on in vitro rumen fermentation

The effect of adding water-soluble carbohydrate (WSC) on the microbial fermentation of fresh perennial ryegrass was examined in an in vitro RUSITEC system over two 10 day periods. Four treatment levels of WSC were used: a basal grass and a basal grass plus a sugar infusion to raise the WSC basal grass level by approximately 1.25×; 1.5× and 1.75×. The infusion was a mixture of inulin and sucrose (80:20) infused over the first 14 h of each 24 h cycle, based on a preliminary study which indicated a linear disappearance of forage WSC over this period. With increasing WSC inclusion there were linear reductions in pH and ammonia-N (NH3N) (P = 0.001) but only a small increase in total volatile fatty acid concentration. There was also a decrease in the proportion of acetate and an increase in propionate with increasing WSC inclusion (P = 0.02 and 0.009, respectively). Microbial nitrogen production was similar for the first three treatments of Basal to Basal × 1.5 but was lower at the highest level of WSC inclusion (P < 0.001). The efficiency of microbial protein synthesis increased from Basal to Basal × 1.5 (9.9, 10.8, 12.7 g N/kg organic matter apparently digested (OMAD), respectively) but at the highest level of WSC inclusion (Basal × 1.75) there was a reduction to 7.1 g N/kg OMAD. This may have been related to the low pH values (<6.0) at certain times during incubation and/or futile bacterial energy cycles as a result of the low nitrogen concentration in the vessels. With increasing WSC inclusion there were also significant reductions in OMAD from 14.4 g per day at Basal to 12.0 g per day at Basal × 1.75