Influence of grain source on ruminal characteristics and rate, site, and extent of digestion in beef steers

Six cannulated Salers steers (305 +/- 17 kg initial BW) were used in a doub le 3 x 3 Latin square design to compare the effects of the nature of the ce real (wheat vs corn) and the corn genotype (dent vs flint) on rate, site, a nd extent of digestion of high-concentrate diets. The cereals were coarsely cracked, and the diets were balanced to have the same percentage of starch (47.7 +/- 2.3%) and CP (14.6 +/- .7%). Differences in ruminal starch diges tion were observed between wheat- and corn-based diets (86.6 vs 47.8%; P < .001) and between corn genotypes (60.8 vs 34.8% for dent and flint corns; P < .001). For flint corn, more than half the starch was digested in the hin dgut. Total tract digestion of starch was greater (P < .001) by steers fed wheat than by those fed corn and did not differ (P > .1) between the two co rn genotypes. Ruminal mean pH (P < .01) was lower and total VFA concentrati on (P < .1) was higher for wheat- than for corn-based diets. Ruminal acetat e:propionate tended to increase with the decrease in the amount of starch d egraded in the rumen, but differences were not significant (P > .1). When w heat replaced corn, nonammonia, nonmicrobial N duodenal flow decreased (P < .01), and microbial duodenal flow increased (P < .05), so there were no di fferences in the duodenal flow of nonammonia N duodenal flaw (P > .1). The lower nonammonia N duodenal flow for the dent corn- than for the flint corn -based diet (P < .05) was related to a lower passage of nonammonia, nonmicr obial N into the duodenum. Efficiency of microbial protein synthesis was in versely correlated with the amount of starch degraded in the rumen. Nature of the cereal, wheat vs corn, and genotype of the corn, dent vs flint, alte r the site and extent of starch digestion.