Field trials were conducted at six sites throughout Ireland in 1992, 1993 and 1994, to assess the efficiency of utilization of N in cattle slurry treated with nitric acid. Slurries were left untreated or were acidified to pH 5·5 with 12 m nitric acid immediately prior to spreading. Slurries were either band-spread or splashplate-spread using an automated tanker system. The efficiency of N (NH+4-N+NO−3-N) in slurry for grass production (Eff-N%) was measured by comparing N offtake values with those from a range of rates of inorganic fertilizer N (ammonium nitrate/calcium carbonate) treatments. Slurry was applied at rates (13–28 m3/ha) which supplied N (NH+4-N+NO−3-N) below the highest inorganic fertilizer treatment. In all experiments, untreated cattle slurry was compared with nitric acid-treated cattle slurry at the same rate of application. The Eff-N% values for unacidified or acidified slurries were highly variable by both spreading methods. The effect of spreading method depended on whether or not the slurry was acidified. On average the Eff-N% value of the band-spread unacidified slurry (59%) was significantly higher than the Eff-N% value of the splashplate-spread slurry (37%). For acidified slurry, the average Eff-N% value of the band-spread slurry (85%) was not significantly different from the Eff-N% value of the splashplate-spread slurry (81%). The overall average Eff-N% value of the acidified slurry was 83% which was significantly less than 100%. Cattle slurry acidified with nitric acid to pH 5·5 was more variable and less efficient than inorganic fertilizer N under the soil and climatic conditions tested.