Effects of supplemental corn grain on forage OM intake (FOMI), digesta kinetics, ruminal fermentation patterns, in vitro OM digestibility (IVOMD), and in situ OM digestion were examined in steers grazing summer blue grama rangeland in northeastern New Mexico during July and August 1988. Sixteen ruminally cannulated steers (average BW 507 kg) were allotted to four treatments and individually fed whole-shelled corn at 0, .2, .4, and .6% of BW in a complete random design with repeated measurements over time. Forage OMI decreased linearly (P = .02) with increasing levels of supplemental corn; however, a tendency toward greater FOMI, as well as faster particulate and fluid passage, was observed when corn was fed at .2% of BW compared with 0, .4, or .6% of BW. Molar proportions of butyrate increased (P less than .10) but molar proportions of acetate and propionate, ruminal pH, and total VFA concentration did not change (P greater than .10) with added corn. Added corn linearly decreased (P less than .10) ruminal ammonia N concentrations in July, but patterns were inconsistent in August. A cubic response (P less than .05) for in situ OM disappearance with added corn was noted after 24, 72, and 96 h of incubation. Supplemental whole corn fed at .2% of BW had no detrimental effects and tended to increase FOMI. However, supplemental corn fed at .4 or .6% of BW decreased FOMI compared with 0 or .2% of BW.

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Two trials were conducted to evaluate the effects of ionophore rotation programs on performance and digestion by feedlot cattle. A 90% concentrate diet was fed with treatments of no ionophore (C), 33 mg lasalocid/kg diet daily (L), 29 mg monensin plus 11 mg tylosin/kg diet daily (MT), and daily (D) and weekly (W) rotation of L and MT. In Trial 1, feedlot performance of 200 crossbred steers (average initial BW 296 kg) was evaluated during a 133-d period. In Trial 2, four crossbred steers (average initial BW 376 kg) fitted with ruminal, duodenal and ileal cannulas were used in a 4 x 4 Latin square design to evaluate treatment effects (excluding W) on ruminal fermentation and site and extent of digestion. In Trial 1, daily rotation of L and MT improved (P less than .10) feed:gain ratio compared with other treatment groups, but daily feed intake did not differ (P greater than .10) among treatments. Daily gain was greater (P less than .10) for steers fed D than for those fed C or MT, but not different from that of steers fed L or W. Carcass measurements did not differ (P greater than .10) among treatments. In Trial 2, ruminal molar proportions of butyrate and valerate were decreased (P less than .07) by MT and D compared with C and L. Proportions of other VFA, ammonia concentrations and ruminal pH did not differ among treatments. Ionophore treatments did not affect site or extent of digestion of OM, starch or N; no differences among treatments were observed for efficiency of microbial protein synthesis. Although daily rotation of L and MT improved performance of growing-finishing feedlot steers, this improvement was not attributable to alterations in ruminal fermentation, or in site or extent of nutrient digestion.

Twelve ruminally cannulated steers (average initial BW 357 kg) were allotted to four treatments (three steers per treatment) in a replicated 4 x 4 Latin square design with 21-d periods (12 d for adaptation and 9 d for collection) to compare the effects of protein supplements that differed in percentage of CP and feeding level on low-quality forage utilization. Treatments were 1) control (C), ad libitum access to 5.6% CP prairie hay, 2) C +600 g of DM.steer-1.d-1 of a 43% CP supplement based on cottonseed meal (PS), 3) C + 1,200 g of DM.steer-1.d-1 of a 22% CP supplement based on corn grain and cottonseed meal (GS), and 4) C + 600 g of DM.steer-1.d-1 of a 22% CP supplement based on corn grain and cottonseed meal (LS). Ruminal total VFA concentrations were increased 8% (P less than .07) by PS vs GS 1 h after supplementation. Among supplemented steers, ruminal acetate (mol/100 mol) was decreased 1.2 mol/100 (P less than .03) by GS vs PS and LS; however, supplementation did not affect (P greater than .10) acetate proportions compared with C. Neither propionate nor butyrate was affected (P greater than .10) by supplementation, but among supplemented steers, butyrate proportions were 8% greater (P less than .03) for GS than for PS and 5% less (P less than .10) for LS than for the average of GS and PS. Ruminal pH did not differ (P greater than .10) among treatments. Ruminal ammonia concentrations were increased 1.4 to 4.8 mg/100 mL (P less than .07) by supplementation and typically were less for LS than for PS and GS at most sampling times. Prairie hay DMI (average = 16.3 g/kg BW) was not affected (P greater than .10) by supplementation. Fluid dilution rate was 8% faster (P less than .01) when steers were supplemented than when they were not fed supplement, and fluid dilution rate was increased 4% (P less than .04) by GS compared with PS. Particulate digesta passage rate was not affected (P greater than .10) by treatment, but total tract retention time was decreased (P less than .01) 10% by supplementation. Extent and rate of prairie hay NDF digestion in situ were not greatly affected by supplementation, but in situ disappearance of supplement N was 6 to 10 percentage units less (P less than .06) for GS than for PS and 2 to 6 percentage units less for LS than for the average of PS and GS supplements.(ABSTRACT TRUNCATED AT 400 WORDS)

Two trials were conducted to determine the effects of steam-flaked sorghum grain bulk density on animal performance, cost of production, and propensity to induce ruminal acidosis in feedlot steers. In Trial 1, 336 yearling steers (343 kg; SEM = .346) were fed diets for 125 d that contained sorghum grain (82.5%, DM basis) flaked to .283 (L), .322 (M), or .361 (H) kg/L (i.e., 22, 25, and 28 lb/bu). Steers fed L consumed 3.2% less DM than those fed H (linear, P < .05), resulting in 6.9% lower ADG (linear, P = .02) and 3.6% lower gain efficiency (linear, P < .15). Sorghum grain flaked to M and L had 16 and 46% greater starch gelatinization than H (measured using differential scanning calorimetry; linear, P = .002). Dressing percentage increased linearly (P < .05) with increasing flake density, but no other carcass measurements were affected by treatment. Increasing flake density increased mill production rate linearly (P < .01), resulting in the lowest energy usage per unit of flaked grain for the H treatment. Trial 2 was an acidosis challenge study that incorporated six ruminally cannulated steers (422 kg; SEM = .129) into a replicated 3 x 3 Latin square experiment. Reducing flake density resulted in linear reductions in ruminal pH following intake challenge at 3, 33, and 36 h after the d-12 challenge (P < .05). There was a linear increase in the area between the pH vs time curve and a line at pH 5.5 (P < .01) and 5.0 (P = .09) with decreasing flake density (28.0, 25.2, and 18.2 pH-hours below 5.5 and 9.6, 7.3, and 3.9 pH-hours below 5.0 for L, M, and H, respectively). Cattle consuming L also tended to have higher VFA concentrations (mM) at 36 h after challenge (P = .12). There was no significant treatment effect on ruminal lactate. Flaking sorghum grain to .283 and .322 kg/L resulted in reduced intake and poorer animal performance compared with .361 kg/L (58.7% starch gelatinization), higher susceptibility to subacute acidosis, and higher costs of production.