58 Nutritive value, in vitro methane production, and ruminal fermentation parameters of three tropical forage grasses cultivated as monocultures and as a mixture

Abstract Enteric methane from ruminants is one of the most important gases emitted by the livestock supply chain. Effective grazing management can potentially reduce methane intensity emissions and offer a low-cost alternative to sustainable pasture-based systems. The adoption of mixed pastures may generate complementarity in the use of resources, providing several benefits. However, research on the nutritive value and in vitro methane mitigation potential of mixed tropical forage species remains limited in tropical regions. This study aimed to assess whether a mixture of three perennial tropical forage grasses improves herbage nutritive value and its potential to decrease in vitro methane production compared to their individual monocultures. Treatments corresponded to three perennial tropical forage grass species: Andropogon gayanus cv. Planaltina (Gambagrass); Panicum maximum cv. Massai (Guineagrass); and Brachiaria brizantha cv. BRS Piatã (Palisadegrass) cultivated in monoculture or as a mixture composed of the three species. Defoliation management was common to all treatments and corresponded to a pre-cutting height of 35 cm and a post-cutting 17.5 cm. Hand-plucked forage samples were collected every season of the year from December 2020 to March 2022 and analyzed for crude protein (CP), neutral detergent fiber (NDF), and acid detergent fiber (ADF). The same samples were also used for evaluations regarding in vitro incubations for 24 h and were assessed for degradability, total gas production and methane concentration, ammoniacal nitrogen, and acetate:propionate ratio. Differences in chemical composition among treatments throughout seasons of the year shaped differences in vitro incubation variables, such as methane production and ruminal fermentation parameters. The mixture presented intermediate chemical composition values similar to those of monocultures. Gambagrass showed the greatest values of CP content (P = 0.0007). Greater values of ADF and NDF were observed for guineagrass and the lowest for palisadegrass (P < 0.0001). Mixture and palisadegrass showed greater in vitro degradability of NDF (P = 0.0003) and methane produced per unit of dry matter degraded (P < 0.0001). Greater values of ammoniacal nitrogen were observed for the mixture and gambagrass (P = 0.0333). Greater values of acetate:propionate ratio was observed for gambagrass, followed by guineagrass, mixture, and palisadegrass. Methane production showed an inverse correlation with CP content and acetate:propionate ratio. Although the mixture did not present greater nutritional value or reduction in methane emissions, other parameters were superior, such as the concentration of ammonia nitrogen and the in vitro degradability of NDF. In conclusion, the mixture showed similar results to the best monocultures for the studied variables, representing an opportunity to enhance diversity in tropical pastures without compromising their nutritive value.