Large-scale phenotyping and multi-trait selection identify low-methane Napier grass ( Cenchrus purpureus ) accessions under contrasting environmental conditions

Napier grass ( Cenchrus purpureus, NG ) is an important forage in tropical and subtropical livestock systems, but the extent of naturally occurring variation in its methane (CH ₄ ) production and its potential for selecting low-CH ₄ emitting genotypes remains poorly characterised. This study evaluated variation in CH ₄ yield among NG accessions, examined the effects of season and water stress, validated contrasting CH ₄ phenotypes across donor cows, and identified elite accessions combining low CH ₄ yield with favourable digestibility. A total of 750 NG samples representing 84 accessions grown under dry and wet seasons and moderate and severe water-stress were evaluated using a high-throughput in-vitro fermentation system. To further identify forages combining low-CH ₄ production with acceptable digestibility, a multi-trait selection approach was applied using biologically defined thresholds for CH ₄ yield and in vitro organic matter digestibility (IVOMD), followed by a weighted index assigning 60% weight to CH ₄ yield and 40% to IVOMD.

Methane yield varied widely among accessions, ranging from 0.02 to 5.07 mL/g DM and was affected by growing conditions. Methane production was generally lower during the wet season, while water stress was associated with changes in forage quality and fermentation characteristics. Low-CH ₄ producing accessions generally had lower IVOMD, crude protein and metabolizable energy and relatively higher fibre than high-CH ₄ producing accessions. Accessions initially classified as low and high-CH ₄ emitters based solely on CH ₄ yield were subsequently validated using rumen inoculum from three individual donor cows. Although the magnitude of CH ₄ production varied among donors, the contrast between low-and-high-CH ₄ accessions was generally retained, supporting the robustness of accession differences in CH ₄ production. Several accessions also maintained relatively low CH ₄ production across contrasting season and water-stress conditions, indicating that favourable CH ₄ phenotypes were not entirely environment dependent.

A multi-trait selection identified ten elite accessions that produced 0.019–0.810 mL CH ₄ /g DM and had 55.2–69.0% IVOMD, whereas ten poor-performing accessions produced 2.80–3.89 mL CH ₄ /g DM and had 50.1–54.7% IVOMD. Differences in hexose fermentation and CH ₄ produced per unit of fermented substrate further indicated contrasting fermentation efficiency between the groups. These results demonstrate substantial variation in NG and identify accessions with low-CH ₄ production, favourable digestibility and relatively stable performance across growing conditions. In conclusion, incorporating CH ₄ yield and digestibility into forage selection could support the development of lower-emission forage cultivars for sustainable climate-smart livestock production.