Nutrient Digestibility of Lower-fiber Rations in the Ruminant Animal1,2

Abstract

Digestibility trials with growing steers related chemical composition as defined by detergent analyses to true and apparent digestible components in 14 lower-fiber rations. Linear relationships were significant when the digested fraction of a constituent was regressed on the total constituent consumed in the rations for cell-wall constituents, cellulose, hemicellulose, neutral detergent solubles, and nitrogen fractions, with less nutritive uniformity in the fibrous fractions. Total ration cellulose explained 77% (coefficient of determination) of the variation in digestible cellulose while lignification constituted only 20%. Total hemicellulose explained 59% of the variation in digestible hemicellulose with a small contribution (6%) from lignification. Decreased apparent digestibility of detergent solubles or increased fecal loss of nonfibrous energy were related to increased cell-wall fiber in the ration. Correlation (r = .97, P<.01) between digestible and total N indicated nutritive uniformity among N sources which included urea. Fecal N loss in ruminant animals was controlled by amount of ration N and composition of ration dry matter. A larger metabolic fecal protein excretion was associated with cell-wall N than with detergent soluble N (2.2 and 1.7% of ration DM). Apparent digestible dry matter was related to fecal endogenous loss of detergent solubles and N and to total cell-wall fiber, whether constituted mostly by cellulose or hemicellulose, in the ration. Cellulose, acid-detergent lignin, and cell wall fractions were inversely correlated (P<.01) to digestible dry matter; however, hemicellulose was not. Total N was not related statistically to digestible dry matter, but detergent soluble N (positive, P<.05) and cell wall N (negative, P<.01) were.