The effects of including wine lees (WL), exhausted olive cake (EOC) and a 1:1 mixture of EOC and tomato pomace (EOCTP) in diets for fattening ruminants on in vitro fermentation parameters and CH4 production were analysed. Ten diets were studied, containing either none of the tested by-products (control), or 6.0, 12.0 or 18.0% of WL, EOC and ECOTP formulated to have similar protein and fiber content. Diets were incubated in vitro with sheep ruminal fluid to measure gas production kinetics and fermentation parameters. Increasing the level of WL, EOC and EOCTP decreased linearly (p ≤ 0.009) the potential gas production, but other gas production parameters were unaffected (p > 0.05), excepting that EOCTP increased the gas production rate. No differences (p ≥ 0.0.05) among diets were observed in total volatile fatty acid (VFA) production at 24 h of incubation for EOC and EOCTP, but NH3-N concentration decreased (p ≤ 0.003). In contrast, WL at 12.0 and 18.0% decreased (p < 0.05) total VFA production and increased the acetate/propionate ratio (p < 0.05). None of the by-products had an effect on CH4 production (p ≥ 0.0.05). Results indicate that EOC and EOCTP could be included up to 18.0% in fattening diets, but lower levels of WL are recommended.

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