Corn Particle Size and Site and Extent of Digestion by Steers

Abstract

The site and extent of starch digestion by steers as influenced by corn particle size was studied in a 4 × 4 Latin square design. Steers (272 kg) fitted with permanent rumen cannulae were fed approximately 3.5 kg dry matter in eight portions daily (3-hr intervals). Site of digestion was estimated by withdrawing rumen samples and assuming such samples would be equivalent to abomasal samples if a constant flow of digesta was maintained by frequent feeding. Ruminal digestion of dry matter (DMD), organic matter (OMD) and starch was estimated by the ratio of permanganate lignin in feed to lignin in rumen digesta. Corn was fed whole or following grinding through 3.18, 4.76 or 7.94mm screens and comprised 72% of the diets. Geometric mean diameters (micrometers) of the four grains were as follows: whole (5978 $#x03BC;m), 3.18mm (509 μm), 4.76mm (588 μm) and 7.94mm (832 μm). In view of high and variable lignin digestibilities, ruminal digestibility estimates indicate trends rather than absolute values. Ruminal DMD was lower (P<.05) for whole corn (44.9%) than for that ground through 3.18mm (62.3%), 4.76mm (63.6%) and 7.94mm (60.8%) screens. This permitted intestinal DMD to be greater (P<.05) for whole corn and suggests increased quantitative importance of intestinal digestion for whole corn. Whole corn also had a lower (P<.05) ruminal starch digestibility (70.8%) than ground corn treatments (avg 91.7%). Moreover, total tract starch digestion was lower for whole 88.2%) than for 3.18mm (94.5%), 4.76mm (93.7%) and 7.94mm (93.5%) treatments. Intestinal starch digestion was generally low, particularly on ground corn treatments. Alteration of the whole kernel beyond mastication appears necessary to maximize total starch digestion. Rumen pH and liquid outflow rate were not greatly influenced by corn particle size, but tended to increase with particle size. Fecal pH was not related to starch content of feces and ruminal liquid dilution rate was not related to molar percentage acetate, propionate or butyrate.