Mixtures of wheat and high-moisture corn in finishing diets: feedlot performance and in situ rate of starch digestion in steers

Wheat and high-moisture corn (HMC) were fed singly and in three combinations using dry-rolled wheat (DRW) (ratios of 75:25, 50:50, and 25:75, respectively, Trial 1), or singly and in two combinations using steam-rolled wheat (SRW) (ratios of 67:33 and 33:67, respectively, Trial 2) to finishing beef cattle fed a high-concentrate diet. In situ rate of starch digestion Trial 3) was measured on grains used in Trial 1 (excluding the 25 HMC: 75 DRW mixture) and ground dry corn. In Trial 1 (132 d), gain/feed did not differ (P > .10); however, final weight, hot carcass weight, and ADG decreased linearly (P < .05) and DMI exhibited a cubic response (P < .05) as the percentage of wheat in the diet increased. Carcass characteristics were not different. In Trial 2 (113 d), there were no differences attributable to treatment for ADG, DMI, gain/feed, or carcass characteristics. Positive associative responses for gain efficiency (gain/feed) were greatest for the first increment of wheat addition (25% DRW and 33% SRW in Trials 1 and 2, respectively) and for the early portion of the feeding period (57 and 28 d for Trials 1 and 2, respectively), indicating a more rapid diet adaptation and (or) less propensity for subacute acidosis. In Trial 3, the treatment grains or mixtures did not differ in rate of starch digestion. Although the differences were not statistically significant, starch in the 100% wheat diet was digested twice as fast as the 100 or 75% HMC mixtures. Starch in ground corn disappeared in a curvilinear manner (quadratic, P = .12; linear, P = .06) across treatment; the slowest rate of disappearance was observed in animals fed the 75% HMC diet and the fastest rate in animals fed the 100% DRW diet. The rate of starch digestion of grain mixtures was inversely related to DMI (r = -.78, P = .21), ADG (r = -.83, P = .17), and gain/feed (r = -.85, P = .15) in Trial 1. When feeding combinations of wheat and HMC, 65 to 75% HMC seems to be beneficial. Increased performance may be associated with a decreased rate of ruminal starch digestion.