Dietary manipulation as a tool for mitigating nitrous oxide emissions

Currently, for every 100 units of nitrogen (N) used in agriculture, only about 15 are consumed as crop, dairy or meat products (Steinfeld et al., 2006; Robertson and Vitousek 2009). This points to very low N use efficiency in most agricultural systems. In the dairy industry for example, despite improved genetic potential of cows with high nutrient utilisation, the increased external input of feed concentrates and use of fertiliser N have decreased N use efficiency in these systems (Huhtanen et al., 2008). Increasing N use efficiency is a key strategy by which the increasing food demand might be met without a corresponding increase in N losses and nitrous oxide (N2O) emissions. This chapter focuses on the use of dietary intervention or manipulation by incorporating diverse forages or crops into animal based agriculture as an approach for increasing N use efficiency. Dietary intervention is primarily aimed at either reducing the concentration and amount of N in animal excreta, or changing the partitioning of N in urine and dung, while preferably having no detrimental effect on productivity. This may include, but is not limited to, using low N feed supplements to reduce the amount of N consumed and hence lower the amount of surplus N being excreted, or specific dietary supplementation to encourage the partitioning of N toward dung rather than urine. Additionally, forages may be selected for their potential to inhibit soil N transformations by either the root exudation or urine-excretion of plant secondary metabolites. Dietary intervention may not be feasible in many low input, pasture based agricultural systems, however, in more intensive systems such as the New Zealand dairy industry, particularly with the increasing use of animal feed pads and housing practices, supplements are commonly imported and so dietary intervention is more practical.