Marine Microalgae as a Nutritive Tool to Mitigate Ruminal Greenhouse Gas Production: In Vitro Fermentation Characteristics of Fresh and Ensiled Maize (Zea mays L.) Forage

SIMPLE SUMMARY: Human population growth is expected to increase the demand for livestock-derived food over the next three decades, and consequently, the emission of greenhouse gases, including methane (CH(4)), carbon monoxide, and hydrogen sulfide (H(2)S), will increase. Given this situation, microalgae have received special attention due to their broad nutritional attributes, ecological benefits, and some species’ high digestibility, indicating that they have the potential to be used as a feed supplement in animal nutrition. Interestingly, some studies have revealed that microalgae have the potential to reduce methanogenesis in cattle, and the findings of this study demonstrated that indeed; the use of marine microalgae as a feed additive has positive effects on biogas, CH(4), and H(2)S mitigation in sheep. Furthermore, microalgae can improve the characteristics of ruminal fermentation, and like other microalgae, Dunaliella salina is more effective in reducing the methanogenesis of high fibrous diets. ABSTRACT: The aim of the present study was to evaluate the effects of marine microalgae (Dunaliella salina) as a food additive on biogas (BG), methane (CH(4)), carbon monoxide (CO), and hydrogen sulfide (H(2)S) production kinetics, as well as in in vitro rumen fermentation and the CH(4) conversion efficiency of different genotypes of maize (Zea mays L.) and states of forage. The treatments were characterized by the forage of five maize genotypes (Amarillo, Montesa, Olotillo, Tampiqueño, and Tuxpeño), two states of forage (fresh and ensiled), and the addition of 3% (on DM basis) of microalgae (with and without). The parameters (b = asymptotic production, c = production rate, and Lag = delay phase before gas production) of the production of BG, CH(4), CO, and H(2)S showed an effect (p < 0.05) of the genotype, the state of the forage, the addition of the microalgae, or some of its interactions, except for the time in the CO delay phase (p > 0.05). Moreover, the addition of microalgae decreased (p < 0.05) the production of BG, CH(4), and H(2)S in most of the genotypes and stages of the forage, but the production of CO increased (p < 0.05). In the case of fermentation characteristics, the microalgae increased (p < 0.05) the pH, DMD, SCFA, and ME in most genotypes and forage states. With the addition of the microalgae, the fresh forage from Olotillo obtained the highest pH (p < 0.05), and the ensiled from Amarillo, the highest (p < 0.05) DMD, SCFA, and ME. However, the ensiled forage produced more (p < 0.05) CH(4) per unit of SFCA, ME, and OM, and the microalgae increased it (p < 0.05) even more, and the fresh forage from Amarillo presented the highest (p < 0.05) quantity of CH(4) per unit of product. In conclusion, the D. salina microalga showed a potential to reduce the production of BG, CH(4), and H(2)S in maize forage, but its effect depended on the chemical composition of the genotype and the state of the forage. Despite the above, the energy value of the forage (fresh and ensiled) improved, the DMD increased, and in some cases, SCFA and ME also increased, all without compromising CH(4) conversion efficiency.