Steam-processed corn and sorghum grain flaked at different densities alterruminal, small intestinal, and total tract digestibility of starch by steers

Crossbred steers (n = 7; 400 kg BW) fitted with T-type cannulas in the duod enum and ileum, were used to examine the effects of processing method, dry- rolled (DR) vs steam-flaked (SF) sorghum grain, and degree of processing (f lake density; FD) of SF corn (SFC) and SF sorghum (SFS) grain on site and e xtent of DM, starch, and N digestibilities and to measure extent of microbi al N flow to the duodenum. In Exp. I, diets contained 77% DRS or 77% SFS wi th FD of 437, 360, and 283 g/L (SF34, SF28, and SF22). In Exp. 2, diets con tained 77% SFC with FD of SF34 or SF22. For sorghum and corn diets, respect ive average daily intakes were as follows: DM, 6.7 and 8.1 kg; starch, 3.8 and 4.7 kg; N, 136 and 149 g. Steers fed SFS vs DRS increased (P = .01) sta rch digestibilities (percentage of intake) in the rumen (82 vs 67%) and tot al tract (98.9 vs 96.5%) and decreased digestibilities in the small intesti ne (16 vs 28%; P = .01) and large intestine (.5 vs 1.2%; P = .05). As a per centage of starch entering the segment, digestibility was increased (P = .0 1) within the small intestine (91 vs 85%) but was not altered within the la rge intestine by steers fed SFS vs DRS. Decreasing FD of SFS and of SFC, re spectively, linearly increased starch digestibilities (percentage of intake ) in the rumen (P = .03,.02) and total tract (P = .03,.09) and linearly dim inished starch digestibilities in the small intestine (P = .04,.09). Starch digestibilities (percentage of entry) within the small or large intestine were not changed by FD. The percentage of dietary corn or sorghum starch di gested in the large intestine was very small, less than 2% of intake. Micro bial N flow to the duodenum was not altered by SFS compared to DRS, or by d ecreasing FD of SFS and SFC. Reducing FD of SFS, but not of SFC, tended to decrease (P = .07) microbial efficiency linearly and tended to increase (P = .06) total tract N digestibilities linearly. Steam flaking compared to dr y rolling of sorghum grain and decreasing FD of SFC and SFS grain consisten tly increased starch digestibility in the rumen and total tract of growing steers. The greatest total digestibility of dietary starch occurred when th e proportion digested in the rumen was maximized and the fraction digested in the small intestine was minimized. These changes in sites of digestion a ccount, in part, for the improved N conservation and greater hepatic output of glucose by steers fed lower FD of SFS reported in our companion papers.