Assessing the Potential of Diverse Forage Mixtures to Reduce Enteric Methane Emissions In Vitro

SIMPLE SUMMARY: Changes in agriculture towards simpler and more intensive systems have contributed to increased environmental problems. In temperate sown grasslands, this has resulted in ryegrass dominance, and forage legume use is limited mainly to three species: red clover, white clover and lucerne. Other dicot forages, such as Lotus pedunculatus and Sanguisorba minor, are of interest as they contain plant specialized metabolites (PSM), especially tannins, potentially reducing methane from ruminants, an important source of agricultural greenhouse gas emissions. In an in vitro study, we compared binary mixtures of perennial ryegrass with one of eight dicot species, including PSM-rich species in different proportions, to assess their potential to reduce methane production. An additional aim was to determine whether moderate additions of these forage species can be sufficient to reduce methane formation or whether the relationship is linearly dose-dependent. Results show that all dicot species studied, including the non-tannin-containing species, reduced methane production. While all plant species rich in PSM reduced methane production, they also decreased digestibility. Additionally, they did not persist in the pasture into the second year. The lowest methane emissions per digestible forage unit were obtained with chicory (Cichorium intybus), a promising forage herb with both agronomic and bioactive potential. ABSTRACT: Methane emissions from ruminants are a major contributor to agricultural greenhouse gas emissions. Thus, eight different forage species were combined in binary mixtures with Lolium perenne in increasing proportions, in vitro, to determine their methane reduction potential in ruminants. Species were sampled in two consecutive years where possible. The aims were: a) to determine if mixtures with specific forages, particularly those rich in plant specialized metabolites (PSM), can reduce methane emissions compared to ryegrass monocultures, b) to identify whether there is a linear-dose effect relationship in methane emissions from the legume or herb addition, and c) whether these effects are maintained across sampling years. Results showed that all dicot species studied, including the non-tannin-containing species, reduced methane production. The tannin-rich species, Sanguisorba minor and Lotus pedunculatus, showed the greatest methane reduction potential of up to 33%. Due to concomitant reductions in the forage digestibility, Cichorium intybus yielded the lowest methane emissions per digestible forage unit. Contrary to total gas production, methane production was less predictable, with a tendency for the lowest methane production being obtained with a 67.5% share of the legume or herb partner species. Thus, linear increments in the partner species share did not result in linear changes in methane concentration. The methane reduction potential differed across sampling years, but the species ranking in methane concentration was stable.