Digestive interactions in the ruminant: relationships between whole tract and stomach evaluation

Two forages, maize stover silage (MS) and alfalfa hay (AH) were combined with 0, 30 and 60% of three types of concentrate either rich in rapidly degradable starch (RS), slowly degradable starch (SS), or rich in digestible fibre (DF). Treatment effects and interactions were compared in terms of organic matter (OM), holocellulose (SC), non-structural carbohydrate (NSC) and starch digestibility in the stomach and the whole tract. Mean intakes of OM, SC, NSC and starch were 925, 472, 270 and 224 g day−1, respectively. Mean digestibilities were 47.8, 59.0, 43.5 and 78.7%, respectively, in the stomach and 68.1, 63.9, 78.5 and 96.1%, respectively, in the whole tract. The precision of the estimates of stomach digestibility was lower than that of the whole tract. Significant treatment interactions were observed at the whole tract level except for the combination AH/DF. The influences of the level of concentrate on stomach and whole tract digestibility were parallel only when diets included RS and DF. In the case of SC digestion, large interactions were observed with MS-mixed rations contrary to AH-mixed ones. For the MS diet, there was good agreement between stomach and whole tract estimations. There was no influence of concentrate level on whole tract NSC digestibility, and no general relationship between stomach and whole tract NSC digestibilities. The use of whole tract digestibility to evaluate or quantify ruminal digestive interactions could be an inaccurate criterion. A specific feed effect must be taken into account when linking stomach and whole tract digestion of OM and SC.