Effects of Onion Peel Inclusion on In Vitro Fermentation, Methane and Carbon Dioxide Emissions, and Nutrient Degradability in Dairy Cow Diets

Enteric greenhouse gas (GHG) emissions represent a major challenge in livestock production, contributing significantly to global methane output. Various strategies have been explored to mitigate these emissions, including dietary modifications, feed additives, and genetic improvements. In the present study, the focus was on onion peel (OP), a byproduct of the onion processing industry that has shown promise as a natural feed supplement with potential methane-reducing properties. We evaluated the effect of different inclusion levels of OP at 2.5% (OP2.5), 5% (OP5), 7.5% (OP7.5), and 10% (OP10) on the in vitro fermentation of two diets: a total mixed ration referred to as high concentrate (HC), and corn silage referred to as high forage (HF). A 48 h batch culture experiment using a 2 × 3 × 5 factorial arrangement was conducted to assess total gas production (GP), methane (CH₄), carbon dioxide (CO₂), ammonia (NH₃), and hydrogen sulfide (H₂S) concentrations, and nutrient degradability. Measurements were taken at 6 h, 24 h, and 48 h of incubation. Significant additive × diet interactions were observed for most of the parameters. The HC diet produced more gas but less CH₄, CO₂, NH₃, and H₂S compared to the HF diet (<i>p</i> < 0.05). At 24 h of incubation, the OP at all levels increased CH₄, CO₂, NH₃, and H₂S concentrations in the HF diet (<i>p</i> < 0.05). The OP2.5 treatment had the lowest (quadratic effect, <i>p</i> < 0.05) degradable dry matter (<i>d</i>DM) in the HC diet, while the OP linearly (<i>p</i> < 0.001) increased degradable acid detergent fiber (<i>d</i>ADF) in both diets. The lowest total volatile fatty acids (VFA) and acetate (quadratic effect, <i>p</i> = 0.027) were observed with the OP5 treatment in the HC diet, while OP5, OP7.5, and OP10 had lower total VFA concentration in the HC diet. At 48 h of incubation, the OP7.5 treatment increased (<i>p</i> < 0.05) GP and CH₄ and CO₂ production in the HC diet. However, the OP5 treatment had the lowest CH₄ production (quadratic effect, <i>p</i> = 0.027) in the HF diet. The highest <i>d</i>DM was observed with OP7.5 treatment (quadratic effect, <i>p</i> = 0.038) in the HC diet with lower values noted at different inclusion levels in the HF diet. Inclusion of OP had no effect on total VFA, and individual VFA in both diets. In conclusion, OP supplementation is more suitable for HF diets than HC diets. A 5% inclusion level is recommended to decrease ruminal CH₄production and improve nutrient degradability.