Effects of source and level of dietary energy supplementation on in vitro digestibility and methane production from tall fescue-based diets

There is a lack of information about the effect of different sources, levels, and the mixtures of energy supplements commonly fed to cattle grazing tall fescue. Therefore, the objective of this study was to evaluate different common energy sources for beef cattle grazing tall fescue using an in vitro fermentation system. Four ruminally cannulated Holstein steers (571 ± 16.1 kg BW) were fed tall fescue hay once daily and served as donors of rumen contents for an in vitro fermentation experiment. Treatments were tall fescue hay (CON), soybean hulls (SBH), corn gluten feed (CGF), corn (CORN), or combinations (SBH + CGF, SBH + CORN, CGF + CORN, SBH + CGF + CORN) of these. Flasks were incubated for 48 h and then assessed for digestion by measuring true DM digestibility, VFA and ammonia concentrations, and total gas and methane production. Supplemental energy sources did not affect total VFA concentration, ammonia concentration, valerate proportion, isobutyrate proportion, isovalerate proportion, or acetate:propionate ratio. True digestibility and digestible energy were generally greater than CON (Table 3; P < 0.001) in all CORN containing treatments. The CORN (P < 0.001) and CGF + CORN (P = 0.023) treatments had a lower acetate proportion than CON. Propionate proportion was reduced by CORN (P < 0.001). Corn grain supplementation increased butyrate proportion in all corn-containing treatments (P < 0.035). All treatments containing energy supplements increased methane production and CH4-E (P < 0.027). The DE loss from CH4-E was increased in CORN, SBH + CGF, SBH + CORN, CGF + CORN, and SBH + CGF + CORN (P < 0.01). Corn grain supplementation at low levels (≤0.20 diet DM) can improve diet utilization by increasing digestibility and energetic efficiency to improve beef cattle production.