Nutritional strategies to mitigate enteric methane emissions from dairy cows: state of knowledge and new perspectives.

Like any livestock production system, dairy production faces a major challenge, namely, to be
environmentally sustainable while maintaining and/or enhancing animal productivity to ensure
farms competitiveness and to provide the consumers with safe and high-quality products. The
dairy sector contributes to greenhouse gas (GHG) emissions, mainly through the production of
methane (CH4) gas from enteric fermentation. The global warming potential of CH4 is 28 times
that of carbon dioxide. In addition, enteric CH4 is also a loss of productive energy for lactating
dairy cows (4 to 7% of gross energy intake). Thus, mitigation of enteric CH4 is beneficial from
both nutritional and environmental standpoints. Accordingly, several dietary strategies have been
suggested to mitigate enteric CH4 production. These strategies vary in terms of their effect (i.e.,
direct or indirect) on ruminal methanogenesis and the extent of CH4 inhibition (i.e., low,
moderate, high). Overall, individual dietary interventions have low to moderate (5 to 20%)
mitigation effect with the exception of 3-nitrooxypropanol and red seaweed (e.g., Asparagopsis
taxiformis) for which up to 40% decreases have been reported. Adding lipids (unsaturated) can
also significantly reduce (up to 25%) enteric CH4. However, at high inclusion level (> 4% of diet
dry matter), animal productivity may be impaired, particularly when lipids are added in highstarch diets. It has been suggested that combining mitigation strategies with relatively small
decrease potentials may allow to achieve larger reductions. However, this will be only achieved
if the effects of the combined strategies are additive. Regardless of the type of the dietary
intervention, it is important to ensure that the gain achieved via the reduction in enteric CH4 is
not offset by increased emissions elsewhere in the farming system (e.g., manure). The adoption
of any mitigation strategy by dairy producers would only be possible if it is accompanied by an
increase in milk production. Low-CH4 diets are not usually low-cost and therefore financial incentives are needed to motivate producers to adopt mitigation. Consumers have a negative
perception towards the use of feed antibiotics and chemical additives in dairy cow diets and
therefore, alternatives to these substances (e.g., plant-extracts) are needed. The objective of this
paper is not to discuss all dietary mitigation strategies available to date, but rather focusing on
the potential of specific options not only on enteric CH4 emissions, but also their possible impact
on CH4 emissions from manure and other GHG (e.g. N2O).