Vegetable oil soapstocks reduce methane production and modify ruminal fermentation

Four soapstocks from the refinery vegetable oil industry (SSpalm, SSoliv, SSsoyb and SSsunf (RIOSA, Baeza-Linares, Spain), from processing of palm, olive, sunflower, and soybean oils, respectively) and a mixture of lauric and miristic acids (4:1, LMA) added to diets with different forage proportions (50, 150 and 250 g barley straw/kg diet; F5, F15 and F25 diets, respectively) were examined to assess their potential to modify ruminal fermentation in vitro, in particular CH4 production. Serum bottles containing 400 mg of substrate (F5, F15, F25 diets) including 30 or 60 g/kg of the materials and 50 ml of buffered rumen fluid were incubated at 39 °C for 24 h. After incubation, gas and CH4 production, pH and volatile fatty acid (VFA) concentrations in the incubation medium were measured. All materials linearly decreased gas production (P<0.05), and SSoliv, SSsoyb and SSsunf reduced CH4 production (P<0.05) and methane produced per mol of total VFA formed (P<0.05), and LMA also reduced CH4 proportion in the gas (P<0.05), whereas SSpalm tended to decrease it. LMA linearly (P<0.05) decreased acetate production and the acetate to propionate ratio, and raised the propionate molar proportion and pH (P<0.05). SSoliv and SSsoyb caused a linear (P<0.05) decline in acetate, and rise in propionate, molar proportions, thus decreasing the acetate to propionate ratio (P<0.05), whereas SSsunf tended to have the same effects (P<0.10). As the forage proportion increased, CH4 and acetate proportion, acetate to propionate ratio and final pH increased, while gas, VFA (individual and total) and CH4 production, and propionate and butyrate molar proportions, decreased. Soapstocks, especially SSsoyb and SSsunf, are promising dietary alternatives to depress ruminal methanogenesis.