Due to the continued expansion of the world population, the emergence of an emerging middle class, rising incomes, and urbanization, it is estimated that by 2050, the global demand for meat and milk is expected to increase by 73% and 58%, respectively, compared to 2010 (Gerber et al., 2013 ). The expansion of the livestock industry, especially the rearing of ruminants, is a matter of concern as it contributes to rising atmospheric concentrations of greenhouse gases, particularly methane (CH4), and consequent climate change. On the other hand, the massive emission of CH4 from the gastrointestinal tract has further contributed to the increase of energy consumption and environmental costs in the process of animal feeding. Therefore, how to reduce the generation of CH4 in the gastrointestinal tract of ruminants has been highly concerned by the global animal husbandry and environmental science community for decades, because it is not only a livestock nutrition issue, but also an environmental issue.
Total global GHG emissions from livestock (animals, manure, feed production and land expansion into forested areas) are estimated to account for 14.5% of total anthropogenic emissions. Among them, CH4 from the gastrointestinal tract of ruminants accounts for about 6% of global anthropogenic greenhouse gas emissions (accounting for 40% of all livestock emissions; Gerber et al., 2013). Compared with another greenhouse gas, carbon dioxide (CO2), the global warming potential of CH4 is nearly 28 times higher (Grossi et al., 2019). Fortunately, however, the lifetime of methane (half-life 8.6 years; Muller and Muller, 2017) is much shorter than that of atmospheric CO2, thus making methane reduction strategies an attractive short-term profitable goal for ameliorating global warming. In addition, reducing CH4 emissions from the gastrointestinal tract can also reduce huge energy losses for ruminant livestock; the dietary energy loss caused by CH4 emissions is about 2% to 12% of digestible energy and 6.5% of metabolizable energy. %～18.7% (ppuhamy et al., 2016), because the methane reduction strategies for livestock, especially ruminants, have been widely concerned by governments and researchers in various countries for a long time.
2. Methods to reduce methane emissions (1) Improving animal productivity Through the application of methods such as genetic breeding and feeding management, animal performance can be improved, and the number of animals and total feed consumption required to produce a fixed quantity of products can be reduced, thereby To achieve the purpose of slowing down CH4 emission intensity (g CH4/kg product). Genetic selection though can permanently reduce an individual animal's methane production, which can be passed on to future generations. However, incorporating CH4 production into genetic selection is a major challenge because it is difficult to measure CH4 in a phenotypic way that reflects long-term CH4 production in animals. And because the production of CH4 is mainly affected by the dry matter intake and the fermentation characteristics of feed materials, the long-term fluctuation of CH4 output depends on the influence of animal physiology, diet composition and other environmental factors, and it is difficult to ensure the complex relationship between genotype and environment. interactive relationship. In addition, the economic indicators developed for the selection of commercial breeding males are weighted based on the economic value of multiple traits. Since the economic value of CH4 emission reduction traits is low, the weight in the multi-trait index is relatively low. The effect of selecting breeding males with excellent CH4 emission reduction traits is limited. Another major limitation is that this approach is technically demanding and time-consuming, and it is not well understood whether selecting for low CH4 emissions will affect the performance of economically important traits (such as feed efficiency) in animals. Therefore, this strategy has not had convincing results so far.
(2) Change the composition of the diet
1. Adjustment of concentrate and roughage diets The efficiency of CH4 mitigation strategies for a specific diet mainly depends on its effect on H2 flow and concentration in the rumen, microbial communities, fermentation pathways, residence time of feed in the rumen, and the relationship between these factors. Interaction effects. Cereal concentrate diets usually contain a large amount of starch, which is conducive to the production of propionic acid in the rumen, increases the consumption of H2, and reduces the production of methane. In addition, the intake of high-starch diets will cause the pH value in the rumen to drop rapidly, the activity of methanogens will be inhibited, and the synthesis of methane will also decrease. However, increased grain feeding, on a small scale, may come at the expense of reduced fiber digestibility and increased risk of acidosis in livestock, and on a large scale, may result in a rapid increase in concentrate production and large changes in land use, which will not contribute to overall greenhouse gas emissions. improve.
As for the high-fiber coarse material basal diet, generally speaking, it is conducive to the production of acetic acid in the rumen, resulting in the appearance of more H2, which in turn increases the production of methane. However, it must be noted that differences in forage quality may not always change absolute CH4 emissions (g/day) in one direction. Therefore, high-quality pastures help to promote the degradation of organic matter in the rumen due to their lower proportion of middle-washing fiber and higher proportion of non-fibrous carbohydrates/ middle-washing fiber, so more H2 will be produced. The increase in the absolute production of CH4 is also due to the increase in the amount of dry matter ingested and digested in the rumen. But on the other hand, high-quality forages may increase dry matter intake, rumen passage, and overall productivity of animals, which may lead to lower CH4 production per gram of dry matter intake. Thus, the net effect of forage quality on daily CH4 emissions may be variable, but overall improvements in forage quality generally reduce CH4 emission intensity due to increased animal productivity (Beauchemin et al., 2020 ).
2. Application of macroalgae Macroalgae can be divided into three types: brown algae, red algae and green algae according to their pigmentation. Due to the richness of halogenated compounds (aliphatic compounds containing one or two carbon atoms covalently linked to one or more halogen atoms - fluorine, bromine, chlorine or iodine, such as bromoform, chloroform and bromochloromethane, etc.) and polyphenols Bioactive compounds such as secondary metabolites can inhibit the production of methane during feed fermentation in the rumen of ruminants, so more and more attention has been paid to it. In in vitro studies, algae that have been shown to have significant mitigation potential (more than 50% reduction) include Asparagus chinensis (red algae), Cladoides expanses (green algae), Cytoseira trinodis (brown algae), Phytophthora fluorescens Algae (brown algae), Cyrella sp. (red algae), Padonia indigo (brown algae) and Ulva sp. (green algae), etc., but further confirmation of in vivo studies is needed (Abbott et al., 2020).
Pandey et al. (2021) pointed out that macroalgae can directly and indirectly affect methane production in ruminants: (1) direct impact: red macroalgae seem to directly affect rumen methane production through the following two mechanisms: ① Utilize the antimicrobial activity of halogenated compounds to directly reduce the abundance of rumen methanogens; ②The halogenated compounds are structural analogs of methane and other methanogenic intermediates, which have higher affinity for methanases and competitively inhibit The combination of intermediates or methane substrates with methanogenic enzymes achieves the purpose of slowing down methane production. (2) Indirect effects: By reducing substrate availability or changing the rumen environment, the composition of the rumen microbiota is unfavorable to methanogens, which indirectly affects methane production. Methanogenic red macroalgae species, such as Asparagus taxus and Ascophyllum nodosum, have been found to reduce the abundance of rumen microorganisms, including rumen protozoa, alter the rumen methanogenic environment, and may also transform the rumen into Fermentation patterns with more propionate formation, lower acetate/propionate ratios, lower methane production in the context of altered protozoa abundance and activity, and reduced H2 production in the rumen.
However, when feeding certain macroalgae, due to the higher content of ash and complex carbohydrates with low rumen degradability, rumen fermentation patterns and total gut digestibility may be negatively affected, resulting in reduced overall animal performance . Another halogenated compound such as bromoform is a known carcinogen and can even negatively impact the ozone layer. At the same time, it is also unknown whether the inhibitory effect of bromoform on methanogens is only temporary and how long it can last. Overall, macroalgal species may be an important component of ruminant feed in the future, but further in vivo animal studies are needed to determine any potential adverse effects.
3. Lipid addition
Patra (2013) pointed out that low levels of dietary lipid supplementation (<4% of dietary dry matter intake) can replace rumen fermentable organic matter in the diet, reduce the number of rumen protozoa and methanogens, and increase propionate. production, and the inhibition of methanogenesis through the biohydrogenation of unsaturated fatty acids to achieve the purpose of reducing CH4 production. However, the ability of lipids with different fatty acid compositions to inhibit methane synthesis is different, and the effect may not last for a long time, and may also vary with ruminant species, physiological state, supplementary amount and feed composition. What's more, lipid supplements are often costly, and may affect animal feed intake, reduce fiber digestibility, inhibit rumen fermentation, inhibit milk fat synthesis, and change the fatty acid composition of the product, so they are not completely suitable for actual use on the farm.
Source: China Animal Husbandry Chen Hongzhi