Optimizing Olive (Olea europaea) Leaves as a Sustainable Ruminant Feed: Effects of Chemical Treatments on Nutritional Value and Greenhouse Gas Emissions

The purpose of this study was to investigate the effects of treating olive (<i>Olea europaea</i>) leaves with sodium hydroxide (NaOH) or urea (both at 4% for 40 d) or supplementing with polyethylene glycol (PEG) at 100 mg/g DM on their nutrient composition, gas production, methane (CH₄) and carbon dioxide (CO₂) productions, and fermentation profile. The results showed that NaOH-treated leaves had the lowest organic matter and fiber fractions, while urea-treated olive leaves exhibited the highest crude protein content (<i>p</i> < 0.001). Asymptotic gas production was significantly higher (<i>p</i> = 0.015) in urea-treated olive leaves compared to NaOH-treated leaves. Methane production was highest (<i>p</i> < 0.05) in urea-treated leaves and lowest in NaOH-treated leaves. During incubation, urea-treated leaves produced the highest CO₂ per gram of degradable neutral detergent fiber (<i>p</i> = 0.015). Degradability parameters indicated that PEG supplementation led to the highest DM, neutral detergent fiber, and acid detergent fiber degradability (<i>p</i> < 0.05) compared to the untreated and NaOH-treated leaves. Urea-treated leaves produced the highest acetic acid (<i>p</i> = 0.016) compared to PEG-supplemented and NaOH-treated leaves, showing no significant difference from the control. Butyric acid levels were higher (<i>p</i> = 0.011) in NaOH-treated and PEG-supplemented leaves than in the control and urea-treated leaves. Metabolizable energy (ME) was significantly higher (<i>p</i> = 0.013) in urea-treated leaves than in untreated and NaOH-treated leaves, with the latter producing the lowest ME. PEG supplementation resulted in significantly higher microbial protein (MCP) production (<i>p</i> = 0.023) compared to NaOH-treated and untreated leaves, while the MCP level in the urea-treated group was comparable to all other treatments. In conclusion, PEG supplementation was the most effective treatment for olive leaves' degradability and MCP, with urea treatment being the second option. From an environmental perspective, NaOH treatment was the most effective for reducing CH₄ and CO₂ productions during the fermentation of olive leaves.