Balancing Risks and Benefits: Anti-Nutrient Content versus Methane-Suppressing Potential of Medicinal Plants in Maize Stover-Based Ruminant Diets

In Ethiopia, greenhouse gas emissions from the agricultural sector are rising rapidly, with enteric fermentation and manure deposited on pastures contributing the majority of emissions. A study was carried out to determine the anti-nutrient profile and methane-suppressing potential of extracts derived from widely used Ethiopian medicinal plants, using maize stover as the basal substrate in an in vitro rumen simulation system.The total phenolic, flavonoid, tannin, and essential oil contents of the plant extracts were quantified using standard analytical methods. Leaf extracts from Acacia nilotica, Azadirachta indica, three accessions of Cymbopogon citratus (C. citratus-I, C. citratus java, and C. citratus upper Awash), Leucaena leucocephala, Moringa stenopetala, three accessions of Rosmarinus officinalis (R. officinalis I, II, and III), and Thymus schimperi; seeds of three coriander varieties (Coriandrum sativum Batu, Tulu, and Waltai); and roots of Echinops kebericho were tested at different inclusion levels. Their effects on total gas production, in vitro dry matter digestibility, and methane yield from maize stover were evaluated following established in vitro protocols. Results revealed that Acacia nilotica leaf extract exhibited the highest (P < 0.001) concentrations of total phenols and total tannins. All Cymbopogon citratus accessions contained significantly higher (P < 0.001) levels of flavonoids compared to the other species. Rosmarinus officinalis II showed the greatest (P < 0.001) essential oil content among all tested plants and rosemary accessions. At an inclusion level of 50 mg/kg dry matter (DM), Cymbopogon citratus java and Thymus schimperi extracts achieved the most pronounced methane suppression, reducing methane production by 22.5 % and 16.7 %, respectively, relative to the unsupplemented control (P < 0.001). A significant (P < 0.001) plant species × dose interaction was observed at this inclusion rate. Importantly, these reductions in methane were not accompanied by any adverse effect on substrate digestibility. It is concluded that supplementing maize stover with 50 mg/kg DM of Cymbopogon citratus java or Thymus schimperi extract offers a promising strategy for lowering enteric methane emissions in ruminants fed low-quality roughages. Additional research is recommended to assess the stability of these extracts during storage and to validate their efficacy and safety through in vivo feeding trials.