Large quantities of organic manures and soiled water are generated by cattle housing every year. These organic wastes are stored until soil conditions are suitable for landspreading or there is a crop requirement for nutrients. After land application, some nitrogen (N) is lost through the direct emission of nitrous oxide (N2O), a powerful greenhouse gas (GHG) produced by nitrification and partial denitrification of mineral N. The objective of this research was to investigate whether N2O losses could be mitigated after applying cattle slurry pre-mixed with the nitrification inhibitor dicyandiamide (DCD) during anaerobic storage. Contrary to our initial hypothesis, DCD mixed with slurry did not degrade for up to six months post amendment during an incubation study. These results highlight the feasibility of amending cattle slurry with DCD directly into slurry tanks any time before land application. This incubation experiment also showed that a slow release of DCD in slurry could be achieved if the amendment used was beads of a chitosan xerogel impregnated with DCD. A field study revealed that slurry application to grassland plots can cause large N2O emissions under wet and mild conditions when ammonia emissions are expected to be low. Slurry incubated with DCD for six month was effective at significantly (P < 0.01) decreasing N2O net cumulative emissions, which were 88% lower than in the slurry treatment with no DCD. The addition of DCD to slurry also reduced the fraction of N2O in the total GHG net cumulative emissions from 52% down to just 10%. Mixing slurry with DCD during storage could therefore offer farmers a cost-effective, practical, mitigation alternative to DCD broadcast application for the reduction of agricultural N losses.