Influence of Protein Supplementation on the Feeding Value of Dry Rolled and Steam-Flaked Corn in Diets for Feedlot Cattle

Eighty medium-framed yearling crossbreed heifers (357 kg) were used in a 110-d trial to evaluate the influence of dietary protein level (11 vs 14%) on the feeding value of dry rolled (DRC) and steam-flaked corn (SFC). All diets contained 1% urea, cottonseed meal (CSM) was the source of supplemental UIP. Steam flaking corn reduced DMI (9%, P < .10), and increased (P < .01) feed efficiency (14%), dietary NE (13%) and NE (15%). m g Steam flaking increased the NEm and NEg values of corn by 17 and 19%, respectively. Supplemental CSM decreased (P < .10) feed efficiency (7%), and dietary NE (4%) and NE (6%). There were no m g treatment effects (P > .10) on carcass characteristics. Steam flaking corn increased (P < .05) fecal pH and reduced (P < .01) fecal starch. Supplemental CSM increased (P < .01) fecal pH and reduced (P < .01) fecal starch. Four Holstein steers (413 Kg) were used in a 4 x 4 Latin square experiment to evaluate treatment effects on digestive function. Steam flaking corn increased (P < .05) flow of nonammonia N (11%, P < .05) and microbial N (15%, P < .01) to the duodenum. Supplemental CSM increased the flow of microbial N (6%, P < .01), feed N (21%, P < .10), and nonammonia N (12%, P < .05) to the duodenum. The UIP value of CSM was 28% for the DRC diet and 52% for the SFC diet. Steam flaking corn increased (P < .01) ruminal starch digestion (26%), and total tract digestibility of OM (17%), N (15%), starch (19%), and GE (17%). Steam flaking increased the DE value of corn 21%. Supplemental CSM did not influence (P > .10) postruminal or total tract starch digestion. Supplemental CSM decreased (7%, P < .10) the DE value of the diet. We conclude that increasing postruminal protein supply of a corn-based finishing diet beyond that provided by urea supplementation, alone, will not enhance intestinal starch digestion or the energy value of the diet. Introduction Reduced starch digestibility constitutes the primary basis for the lower feeding value of dry rolled corn (DRC)vs steamflaked corn (SFC)in diets for feedlot cattle. Although both ruminal and postruminal starch digestion are lower for DRC than for SFC, differences in postruminal starch digestion accounts for most of the variation in total tract starch digestion (Zinn et al., 1995). Karr et al. (1966) suggested that variation in postruminal starch digestion may be due to limitations in amylolytic capacity. Because pancreatic amylase secretion and activity is enhanced by increasing protein supply to the small