However, where the harvested material is to be used as a biofuel source, the criteria for suitable material change. Whereas high N concentrations are desirable for livestock feeds, low N concentrations are preferred in material used for biofuels. In direct combustion, N combines with O to produce NOx gas, which contributes significantly to global-warming. Lewandowski and Kicherer (1997) suggested that N concentration in biomass for production of biofuel should be