# **Box 8.  Mitigating methane emissions of livestock: the role of diet intensification**

Methane is an important source of greenhouse gases. After CO2, it accounts for 15-23% of global GHG emissions (EPA, 2006). Globally, the most important sources of methane are wetlands, rice paddies, livestock, natural gas, and biomass burning. Domestic ruminants are the most important anthropogenic source of methane emissions (Crutzen et al., 1986; Steinfeld et al., 2006). Their contribution to the methane budget of different countries varies significantly depending on land-use patterns, mining resources, waste management and other factors. In countries whose economies and livelihoods systems are largely based on agriculture and livestock industries, domestic ruminants represent the majority of methane emissions (see Herrero et al., 2008).

Mitigation has been proposed as a key strategy to reduce GHG emissions and it is the subject of considerable international debate and negotiation. This includes strategies for mitigating methane from livestock industries. Improving the diets of ruminants and control of animal herd numbers could be an important mitigation strategy in places, though few studies have tackled this complex problem at a global level (Herrero et al., 2008). To provide some evidence of the biological basis for this strategy, we used a validated dynamic model for predicting feed intake and nutrient supply in ruminants to calculate the methane produced from enteric fermentation. Details of the model can be found in Herrero et al. (2008). We ran the model for a 500 kg cow consuming an _ad libitum_ basal diet of poor-quality _Brachiaria spp_ pastures (8 MJ ME per kg DM), as often found in the humid tropics of Latin America (Holmann et al., 2004). We then simulated improvements in the diet by adding up to 6 kg of high-quality (12 MJ ME per kg DM) grain concentrates to what the cow was consuming.

The results suggest that the poor-quality diet produced very little milk and a high production of methane per kg of milk produced (Figure B8-1). Improving the quality of the diet not only increased milk production but also reduced the amount of methane produced per kg of milk (Figure B8-1). The methane-efficiency of milk production increases as the quality of the diet of animals improves (Baxter and Clapperton, 1965).

A mechanism by which mitigation strategies are effected is through establishing emission targets for GHG (IPCC, 2007). Assume that we have a methane quota of 1 tonne of methane. How much milk can we produce under the emissions target by changing the composition of the diet, and how many animals would we need to have to produce that milk? Figure B8-2 shows that different combinations of diets for cows and numbers of animals can achieve the same methane emission target of 1 tonne of methane. The higher-quality diets using more grain concentrates produce more milk and require fewer animals to produce the milk. Intensifying the diets may be a desirable strategy in places with little land and with high opportunity costs for labor and land (Baltenweck et al., 2003); more extensive systems could also reach the same emissions target, subject to pasture availability and its variability, with more animals and less productivity per animal.

These two concepts form the basis for designing methane mitigation strategies in ruminant livestock systems, as they address the key trade-offs between emissions, livestock production, 66