Methane production from locally available ruminant feedstuffs in Ethiopia – An in vitro study

Abstract

Achieving optimal nutrient composition in locally sourced ruminant feeds is important, but can be challenging in resource-limited production systems. For example, improving the composition of available local feed resources is a key obstacle to efficiently mitigating enteric methane (CH₄) emissions in ruminants. This study characterized the nutritional content and in vitro methane (CH₄) yield of ruminant feedstuffs accessible in Ethiopia. A survey of 60 experienced farmers in two representative districts in Amhara region, Ethiopia, provided 33 feed samples, which were classified into four ruminant feed categories: Grasses (n=10); indigenous plants (trees, shrubs, herbaceous plants) (n=13); crop residues (n=5); and agro-industrial by-products (n=5). Nutritional composition was assessed by proximate and detergent methods. Methane yield (g CH₄/kg feed dry matter (DM)) and total gas yield (L/kg DM) were evaluated using a fully automated in vitro gas production system. A colorimetric assay was conducted to measure condensed tannin content (CT, mg/g) in relevant feeds. Lower crude protein (CP) values were observed for the grass (mean 65.2 g/kg DM) and crop residues (mean 54.5 g/kg DM) categories. Agro-industrial by-products had the highest CP (mean 260 g/kg DM), while indigenous plants exhibited intermediate levels (163 g/kg DM). There was significant variation in CH₄ yield (P<0.01) between grasses (12.4–24.7 g/kg DM) indigenous plants (1.8–19.3 g/kg DM), and agro-industrial by-products (8.1–26.9 g/kg DM). The indigenous plant Trifolium acaule gave the lowest in vitro CH₄ yield (1.8 g/kg DM). A positive relationship was observed between in vitro dry matter digestibility (IVDMD), CH₄, and total gas yield. Percentage of CH₄ in total gas production varied with feed category (grasses 14.5–19.6%; indigenous plants 3.1–16.9%; crop residues 15.8–20.6%; agro-industrial by-products 12.8–18.7%), and within category, e.g., Trifolium acaule (3.1%), Acacia nilotica L. (7.1%), Ziziphus spina-christi (9.9%), brewer’s spent grains (BSG) (12.8%), local liquor (areki) residues (14.1%), and local beer (tella) residues (15.1%). A negative relationship was observed between CT content and in vitro CH₄ yield, with a stronger (P<0.05) correlation for soluble CTs (R² = 0.46) than cell-bound CTs (R² = 0.25) and total CTs (R² = 0.29). Based on methanogenic properties and effects of CTs on in vitro CH₄ yield, indigenous plants should be prioritized in ruminant rations in Ethiopia. Making nutritional composition and CH₄ data publicly available could help develop environmentally sound, cost-effective rations for ruminant livestock, benefiting local farmers and leading to more sustainable and efficient livestock production in Ethiopia.