The true protein digestibility in the small intestine of different ruminant feeds were measured using in situ and in vitro techniques. Two steers with average body weight of 450 kg and fitted with ruminal cannulas were used for in situ incubation of different feeds. The following feedstuffs were evaluated: animal and vegetable protein concentrate, energy concentrate, fiber by-products, and forage. Protein truly digested in the small intestine was estimated by pepsin or pepsin/pancreatin incubation with or without previous ruminal incubation. Out of 30 evaluated feeds, ruminal incubation decreased the true protein digestibility in the small intestine of 24 feeds, increased that of feather meal, black oat, 2.5-mm particle size corn grain, and oat and Tifton hays and had no effect on sunflower meal. These results showed that the small intestinal digestibility of rumen-undegradable protein from most analyzed feeds was lower than that of the original feed protein. With the exception of black oat, all remaining feeds (29) had greater true small intestinal protein digestibility after incubation with pepsin plus pancreatin showing the contribution of abomasal digestion on feed protein. Because true small intestinal protein digestibility differed among feeds, is important to take it into account on diet formulation in order to accurately determine the requirements of metabolizable protein of ruminants.
feed evaluation; intestinal digestion; in vitro digestibility; in situ incubation
