Digestion site of starch from cereals and legumes in lactating dairy cows

The effect of grinding and rolling ( i.e. processing) of cereals and legumes ( i.e. source) on site of starch digestion in lactating dairy cows was tested according to a 2×2 factorial design using a dataset derived from an overall dataset compiled from four experiments conducted at our laboratory that investigated effects of various processing techniques on digestion kinetics. Data originated from four experiments with cows cannulated in the rumen, duodenum and ileum, and fed rations composed of the respective starch source and grass–clover silage balanced with soy bean meal and a mineral premix. In addition to apparent digestibilities of starch in the rumen, small intestine and hind gut, true ruminal starch digestibilties and duodenal flow of microbial starch was estimated by regression analysis. There was an interaction between starch source and processing, as the apparent and true ruminal digestibility of starch was decreased by rolling for legumes, whereas the three other source by processing combinations did not differ. The duodenal flow of microbial starch was estimated to 276 g/d as the intercept in the regression analysis. Apparent ruminal digestibilities of starch seemed to underestimate true ruminal digestibility in rations with low starch intake due to a relatively higher contribution of microbial starch to total duodenal starch flow compared to rumen escape feed starch. The small intestinal and total tract digestibility of legume starch was lower compared with starch from cereals. Rolling reduced the small intestinal and total tract digestibility of starch from both cereals and legumes compared to grinding. Reductions in ruminal starch digestion were generally not associated with increases in small intestinal starch digestion, but were associated with increases in hind gut and reductions in total tract digestion of starch. Starch from legumes had lower ruminal and small intestinal digestibilities compared to starch from barley and wheat, indicating that action of bacterial and pancreatic amylase is limited by the same factors. Rolling did not provide sufficient physical processing of legume seeds to obtain a high digestibility of starch before the hind gut.