Effects of guanidinoacetic acid and live yeast cells on cattle rumen fermentation and greenhouse biogas production in vitro of ten shrub species.

Enteric gas production during rumen fermentation, enteric methane (CH4), contributes to approximately 14-18% of global greenhouse gas (GHG) emissions and a reduction of 2% to 15% in consumed energy. Therefore, reducing enteric CH4 production may have environmental (CH4 is a potent greenhouse gas), nutritional (decreased energy losses), and economic benefits (enhanced animal productivity). The trial aimed to evaluate the effects of guanidinoacetic acid (GAA) and live yeast cells (YEA) on methane, carbon monoxide (CO), and hydrogen sulfide (H2S) production, and on ruminal fermentation characteristics of ten shrub species. The ruminal fluid was obtained from four three-year-old Charolais bulls (BW = 500 ± 29 kg; mean ± SD), fed with diets based on 100% free-access forage. The experimental additives were as follows: (1) Control (without feed additives), (2) guanidinoacetic acid (GAA; 0.0015 g), and (3) live yeast (YEA; 0.004 g). Production of CH₄, CO, and H₂S was measured at intervals of 2, 4, 6, 24, 28, 30, and 48 h. After completing the gas measurements, the in vitro dry matter degradability (DMD%) was determined. Asymptotic total gas production (b) was decreased with the administration of live yeast to Diospyros texano (P = 0.010) and GAA to Hilaria mutica (P = 0.0002). Opuntia engelmanni supplemented with live yeast resulted in higher gas production at 24 h (P = 0.0004). Adding feed additives to the shrubs increased the DMD% (P = 0.014). The highest and lowest CH4 production was recorded when GAA was added to Mimosa zygophylla and live yeast to Hilaria mutica, respectively (P = 0.002). The interaction between the feed additive and the forage source increased (P < 0.0001) the concentration of CH4 production. Supplementation of diets with live yeast and GAA significantly influenced asymptotic CO production, rate of CO production, and Lag phase (P < 0.0001). The amount of H2S production was affected by the interaction between GAA and live yeast (P = 0.001). In conclusion, live yeast supplementation reduced CH4 and H2S production in forage-based diets, whereas its effects on CO and total gas production were variable and inconsistently reduced.