In Vitro Methane Production of Diets Containing High Concentrations of Crude Glycerin for Feedlot Sheep

Crude glycerin is the major byproduct of the biodiesel industry. Previous studies have indicated this byproduct as an enteric methane (CH₄) mitigating agent. An in vitro rumen batch culture study was conducted to compare the effects of high inclusions of crude glycerin in diets for feedlot sheep on CH₄ production. The technique used in this trial involved the measurement of gaseous species produced during fermentation by a pressure transducer. Isonitrogenous (18.4% CP) and isoenergetic (2.7 Mcal ME/kg DM) diets were composed of corn silage, soybean hulls, soybean meal, mineral premix, corn cracked grain and crude glycerin included at 0 (G0), 10 (G10), 20 (G20), or 30% (G30) of the total diet (DM basis), in a roughage:concentrate ratio of 40:60. Crude glycerin totally replaced corn grain in G30, and contained 83% glycerol, 95% DM, 6% salt, and less than 0.01% methanol. Three adult male rumen-cannulated sheep were used as rumen fluid donors. To carry out this study, 160-mL serum bottles (n=30) containing 0.5 g test feed (placed into F57 Ankom^® filter bags), 50 mL buffer solution, and 25 mL ruminal fluid were used. The bottles were sealed and placed into a forced-air oven (39^oC). Each mixture was incubated in triplicate. The pressure was measured and gas samples were collected at 0, 4, 8, 12 and 24 h post-incubation, and the cumulative gas production was evaluated. After the 24-h incubation period, terminal pH was measured. Filter bags containing feed residues were washed, dried and incinerated to determine organic matter disappearance. Gas samples were evaluated for CH₄ concentration by gas chromatography. Data were analyzed using the MIXED procedure of SAS, and whenever the F-test was significant, contrast analyses (linear, quadratic, 0 × glycerin) were performed. No interaction of treatment × inoculum was observed in this trial (P > 0.10). Total gas production (mL/g organic matter disappeared) was not altered (P = 0.44) by crude glycerin inclusion (average = 54.5 mL). However, CH₄ concentration was linearly increased (P = 0.004) with values from 4.6 (G0) to 7.4 mL/g organic matter disappeared (G30). Terminal pH was also affected by treatments, with values linearly decreasing from 6.7 (G0) to 6.6 (G30; P = 0.003). The inclusion of high concentrations of crude glycerin in feedlot diets for lambs does not alter in vitro gas production, but linearly increases methane concentrations and linearly decreases terminal pH.