Response of paddy rice to fertilisation with pig slurry in northeast Spain: Strategies to optimise nitrogen use efficiency

The increasing pig population in northeast Spain and the need to adequately manage the pig slurry (PS) generated demand the inclusion of PS in crop fertilisation plans. The aim of the present study was to maximise the amount of PS that can be applied to the crops without adverse environmental effects. Thus, a 3-year experiment was established to evaluate the response of flooded rice to different fertilisation strategies. The fertilisation strategies tested were two rates of PS applied before sowing: 170 kg NH4-N ha−1 (PS170 strategy) that would cover the entire rice N requirements and 120 kg NH4-N ha−1 (PS120 strategy) that would theoretically require N as top-dressing to reach maximum yields. These strategies were compared to the rate of mineral fertiliser applied before sowing at 120 kg NH4-N ha−1. Plant density; the presence of weeds, pests and diseases; head rice yield; and rice quality were not affected by the N source (PS or mineral). Maximum rice yields (5567-8235 kg ha−1) varied between years and were attained with the two PS fertilisation strategies. Maximum yields with the PS170 strategy were reached without top-dressing application in the three years, but nitrogen (N) as top-dressing was necessary to reach maximum yields with the PS120 strategy. The nitrogen fertiliser replacement value (NFRV) of the two PS strategies, 87% (PS170) and 96% (PS120) of ammonium N applied, were not significantly different. The high PS NFRV suggests that PS is an excellent N source and rates should be established considering PS ammonium N content. Agronomic and recovery N use efficiencies and unaccounted N were not significantly different between mineral and PS strategies when the same ammonium N rate was applied; however, when the total N was considered, N use efficiencies decreased, indicating that organic N is not taken up by the crop during the crop season.