The potential impacts of dietary plant natural products on the sustainable mitigation of methane emission from livestock farming

Livestock production is one of the major contributors of greenhouse gases such as methane (CH4) and carbon dioxide (CO2). These gases contribute greatly to global warming, environmental degradation and pollution. Livestock production is responsible for 18% CH4 and 9% CO2 productions of all greenhouse gases emissions. Methane has a greater global warming effect (about 23 times) more than CO2. Currently, livestock production faces a great challenge of increasing production to meet global demand for agricultural products and at the same time reduces environmental impact. Many researchers have reported the effects of substituting phytoconstituents such as tannins, saponins and essential oil as chemical feed additives to modify rumen fermentation. These modifications are aimed at reducing loss of feed energy, improving animal productivity and mitigating CH4 and CO2 emitted during livestock production. This present review is aimed at providing information on the influence of plant natural products or secondary metabolites (PNP) such as tannins, saponins and essential oils on ruminal microflora and their potentials to mitigate biogases during livestock production. This work will also review purported anti-microbial activities of plant secondary metabolites and its ability to improve animal health and enhance productivity. From the findings of this review, PNP have the potential to improve rumen fermentation, reduce loss of feed energy, improve animal health and productivity, increase animal lifetime performance, and reduce greenhouse gases production- CH4 and CO2 during animal production. This review also revealed that supplementation of saponin, tannins or essential oils at low to moderate doses have more potentials and are the promising natural feed additives suitable to manipulate microbial ecosystems, inhibit pathogenic bacteria proliferation in gastrointestinal tract, improve rumen fermentations, mitigate rumen CH4 production and reduce environmental impact of livestock production. However, further research is required to establish the effective daily doses of plant natural products to animals without any detrimental effect.