SUMMARY Field experiments at Askov and Lundgaard Experimental Stations, Denmark, were carried out in 1987–91. Winter and spring applications of pig slurry at two application rates and an unfertilized treatment were combined with three rates of mineral nitrogen fertilizer; 15 treatment in all. Grain and straw yield, nitrogen content and nitrogen uptake of spring barley were measured at harvest. All main effects were significant, resulting in increasing yield and nitrogen uptake both when increasing the rate of mineral nitrogen fertilizer and slurry, and when slurry was applied in spring rather than in winter. The nitrogen content of the barley was affected by mineral nitrogen fertilizer application, and by the rate of slurry. The nitrogen efficiency of applied ammonia-N in the pig slurry was estimated by a quadratic polynomial yield response function and expressed relative to a reference function based on the application of mineral nitrogen fertilizer. The efficiency was estimated at 10 for spring-applied pig slurry. The value for winter-applied pig slurry was lower (0·6) and more variable. The efficiency of ammonia-N in spring-applied pig slurry decreased by 0·4/100 kg N/ha when supplementary mineral nitrogen fertilizer was applied at the same time.
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