Effects of molasses and crude glycerol combined in a liquid supplement on ruminal fermentation in beef steers consuming bermudagrass hay

Two experiments were performed to evaluate the effects of 1) increasing supplementation doses of a 50: 50 (as-fed) liquid supplement of molasses and crude glycerol (M:G) on ruminal fermentation parameters and blood urea nitrogen (BUN) in beef steers consuming Tifton 85 bermudagrass (Cynodon spp.) hay and 2) different proportions of molasses and crude glycerol in a liquid supplement on in vitro fermentation and gas production kinetics. For Exp. 1, 8 ruminally cannulated, Angus-crossbred steers were used in a duplicated 4 x 4 Latin square design, had ad libitum access to Tifton 85 bermudagrass hay, and were randomly assigned to 1 of 4 treatments: 1) CTRL, no supplementation; 2) SUP1, 0.45 kg/d (as fed) of 50: 50 M: G; 3) SUP3, 1.36 kg/d (as fed) of 50: 50 M: G; and 4) SUP5, 2.27 kg/d (as fed) of a 50: 50 M: G. For Exp. 2 in vitro batch cultures were conducted to test the same treatments from Exp. 1 and effects of different proportions of a M: G mixture (100: 0, 75: 25, 50: 50, 25: 75, and 0: 100) when added to a hay substrate simulating the proportions of hay and liquid supplement used in SUP5. In Exp. 1, increasing doses of liquid supplement linearly decreased (P < 0.001) concentrations of NH3-N, BUN, and acetate molar proportions, whereas propionate (P = 0.002) and butyrate (P < 0.001) molar proportions increased linearly. Treatment x time interactions were observed for ruminal pH (P < 0.001), where the greatest decrease was observed at 3 h postfeeding for animals consuming SUP5 (from 6.82 at 0 h to 6.32 at 3 h). In Exp. 2, decreases in acetate molar proportions (P < 0.001) and increases (P < 0.001) in propionate and butyrate molar proportions were also observed for either increasing doses of a 50: 50 mixture or increasing proportions of glycerol in the mixture. Total VFA and in vitro organic matter digestibility were increased linearly (P < 0.001) as doses of a 50: 50 mixture increased. Increasing doses of 50: 50 M: G to growing beef heifers consuming bermudagrass hay caused a shift in VFA profile toward increases in propionate and decreases in acetate molar proportions. This was also confirmed in vitro, as the proportions of crude glycerol increased in a molasses: crude glycerol mix. Thus, molasses and crude glycerol combined seem to be useful to enhance performance in growing cattle consuming forage-based diets.