Effects of Guanidinoacetic Acid on Ruminal Fermentation and Greenhouse Gas Production Using Fresh Forage and Silage from Different Maize (<i>Zea mays</i> L.) Genotypes

Guanidinoacetic acid (GAA) is a feed additive that promotes growth in animals, while maize (<i>Zea mays</i> L.) is used for the mitigation of ruminal greenhouse gases. However, it is unknown if GAA affects the efficiency of maize in mitigating gases or if there is synergy between them. Therefore, the objective of this study was to evaluate the in vitro production of total gas, methane (CH₄), carbon monoxide (CO), and hydrogen sulfide (H₂S), ruminal fermentation characteristics, and the CH₄ conversion efficiency of fresh forage and silage of different genotypes (Amarillo, Montesa, Olotillo, Tampiqueño, and Tuxpeño) of maize, with and without the addition of GAA. The silage of the Amarillo genotype without AAG had the highest (<i>p</i> = 0.01) total gas production rate and the lowest (<i>p</i> = 0.044) delay time before gas production. In addition, at 48 h, the Amarillo silage with GAA increased the production of total gas (<i>p</i> = 0.0001) and CH₄, as well as the proportion of CH₄ (mL CH₄ 100 mL⁻¹ total gas). The Amarillo and Tuxpeño genotype produced more (<i>p</i> = 0.033) CO in the first 24 h of incubation, while silage and the addition of GAA only increased (<i>p</i> = 0.001) CO at 6 h. The highest (<i>p</i> = 0.02) H₂S production was observed with the ensiled Amarillo genotype with GAA. Regarding fermentation characteristics, the silage of the Amarillo and Montesa genotypes presented the highest degradation of dry matter (DMD), short-chain fatty acids (SCFA), and metabolizable energy (ME), and although there was no effect on CH₄ efficiency, the Amarillo and Olotillo genotypes produced more SCFA, ME, and OM per unit of CH₄. It can be concluded that rumen gas production, fermentation characteristics, and CH₄ conversion efficiency are more influenced by the maize genotype and forage condition than by the addition of guanidinoacetic acid, and of the genotypes evaluated, the forage silage from Amarillo showed the best characteristics and efficiency of CH₄.