Factors affecting intake and digestion of forages by cattle

Abstract

An experiment was conducted to measure voluntary intake, nitrogen flow and digestibility of fiber and nitrogen fractions in the gastric (G), small intestines (I) and hindgut (H) digestive segments of crossbred yearling steers (230 kg weight) fitted with "T" type cannulae at the proximal duodenum and terminal ileum offered chopped hay (2.54 cm screen) ad libitum at 0700 h (A) and 1800 h (P). Diets consisted of either: 1) first cutting bermudagrass (Bl); 2) third cutting bermudagrass (B2); 3) B2 plus 600 g soybean meal/d; 4) B2 previously treated with 3% NH₃ (B2N); 5) B2N plus SBM; 6 ) sorghum hay (S) or; 7) S plus SBM. Diets were fed to seven animals in a 3 X 7 Youden Square. Duodenal digesta and feces were sampled at -1, 3, 6 , and 9 h relative to A and P on d 1 and 2 and at 3 h post feeding on d 3, 4, 5 of a 5 d collection period. Ileal digesta was sampled at 3 h post A and P. Protein supplementation with SBM of both B2 and B2N tended to depress overall forage intake. In sorghum hays, SBM supplementation resulted in greater nitrogen digestion in the small intestine and increased NDF fecal outputs with no apparent change in ruminal digestibility of potentially digestible fiber in the foregut. Higher levels of nitrogen digestion in the small intestine resulted in increased forage intakes. Ammoniation treatment of bermudagrass hay resulted in increased forage CP levels and decreased INDF levels without altering NDF or ADL contents. Airanoniation treatment tended to increase overall NDF digestibility over the total GI tract resulting in decreased fecal NDF outputs. Supplemental protein due to ammoniation of B2 with 74% NDF content had little effect on levels of nitrogen digestion in the small intestine and consequent forage intakes. Nitrogen flow at the duodenum and digestion in the hindgut, of steers on all forage containing diets did not change appreciably with time postfeeding up to 9 h following a meal. Digestion of forage fiber of all forage diets was almost fully restricted (85-90%) to the reticulorumen and did not change with time postfeeding up to 9 h following a meal. There was an apparent diurnal variation in cellulose digestion in the hindgut with more digestion occurring at 6 h postfeeding than either 1, 3, or 9 h postfeeding.