Abstract
This study aimed to evaluate the effects of twenty species of tropical macroalgae on in vitro fermentation parameters, total gas production (TGP) and methane (CH4) production when incubated in rumen fluid from cattle fed a low quality roughage diet. Primary biochemical parameters of macroalgae were characterized and included proximate, elemental, and fatty acid (FAME) analysis. Macroalgae and the control, decorticated cottonseed meal (DCS), were incubated in vitro for 72 h, where gas production was continuously monitored. Post-fermentation parameters, including CH4 production, pH, ammonia, apparent organic matter degradability (OMd), and volatile fatty acid (VFA) concentrations were measured. All species of macroalgae had lower TGP and CH4 production than DCS. Dictyota and Asparagopsis had the strongest effects, inhibiting TGP by 53.2% and 61.8%, and CH4 production by 92.2% and 98.9% after 72 h, respectively. Both species also resulted in the lowest total VFA concentration, and the highest molar concentration of propionate among all species analysed, indicating that anaerobic fermentation was affected. Overall, there were no strong relationships between TGP or CH4 production and the >70 biochemical parameters analysed. However, zinc concentrations >0.10 g.kg−1 may potentially interact with other biochemical components to influence TGP and CH4 production. The lack of relationship between the primary biochemistry of species and gas parameters suggests that significant decreases in TGP and CH4 production are associated with secondary metabolites produced by effective macroalgae. The most effective species, Asparagopsis, offers the most promising alternative for mitigation of enteric CH4 emissions.
Highlights
Methane (CH4) is a greenhouse gas (GHG) produced primarily by methanogenic microbes that are found in natural ecosystems and the gastrointestinal tract of invertebrates and vertebrates, such as termites and ruminants [1]
One of the main sources of anthropogenic CH4 can be attributed to agricultural activities, from ruminant livestock which are responsible for 25% of the total methane emissions in the atmosphere [2]
Total gas and methane production Total gas production (TGP) was lower for all species of macroalgae compared to decorticated cottonseed meal (DCS) (Fig. 2, analyses of variance (ANOVA): 72 h, F20,63 = 14.36, p,0.001)
Summary
Methane (CH4) is a greenhouse gas (GHG) produced primarily by methanogenic microbes that are found in natural ecosystems (e.g. wetlands, oceans and lakes) and the gastrointestinal tract of invertebrates and vertebrates, such as termites and ruminants [1]. Anthropogenic GHG emissions have been increasing rapidly, with the CH4 concentration in the atmosphere more than twofold higher than in the early 1800s [3]. Its accumulation in the atmosphere contributes considerably to climate change. One of the main sources of anthropogenic CH4 can be attributed to agricultural activities, from ruminant livestock which are responsible for 25% of the total methane emissions in the atmosphere [2]. In Australia, ruminants are estimated to contribute ,10% of the total GHG emissions [4,5]
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