Effect of roughage source on ruminal kinetics of digestion and passage of individual feed components in mixed diets for steers.

Abstract

Total tract digestion coefficients for dry matter (DM) and neutral detergent fiber (NDF), passage rates for grain and roughage components of diets and liquid turnover were measured for 65 and 90% concentrate diets in separate 4 x 4 Latin square experiments using intact growing steers. Kinetics of in situ digestion for DM and NDF in diet ingredients, rumen pH, volume and DM distribution were determined in mature, rumen cannulated steers. Rates of passage and digestion were combined to calculate apparent extent of ruminal digestion (AERD) for diet ingredients. Diets were based on steam flaked milo. The control roughage was chopped alfalfa hay (AH). In 65% concentrate diets, chopped wheat straw (WS), bermudagrass straw (BS) or cottonseed hulls (CSH) replaced 50% of the AH. In 90% concentrate diets, all of the AH was replaced by WS, BS or CSH. Total tract starch digestion was not influenced by source of roughage at either concentrate level. In 65% concentrate diets, total tract digestion of DM and NDF was not adversely affected by substitution of WS for 50% of the AH. Digestion of these fractions was lower (P<.05) for BS and CSH diets, but the severity of depression was greatest (P<.05) for the CSH diet. Reduced digestibility of the CSH diet was attributed to lower digestibility of CSH in comparison with AH. Although WS and BS were also less digestible than AH, their inclusion in the diet improved (P<.05) AERD of DM and NDF from milo and AH. Improved AERD for NDF appeared to be related to the raft-forming properties of WS and BS, and their ability to maintain rumen pH in a more desirable range for NDF digestion. In 90% concentrate diets, effects of roughage source on utilization of milo were minimal. Total tract digestion of DM and NDF was lower (P<.05) for WS, BS and CSH diets than for the AH diet, because of dilution of the more digestible AH by these roughages. Wheat straw had a tendency to moderate ruminal pH and consequently might be more useful in high concentrate diets than roughages such as CSH and BS.