Winter cereal yields as affected by animal manure and green manure in organic arable farming

The effect of nitrogen (N) supply through animal and green manures on grain yield of winter wheat and winter rye was investigated from 1997 to 2004 in an organic farming crop rotation experiment in Denmark on three different soil types varying from coarse sand to sandy loam. Two experimental factors were included in the experiment in a factorial design: (1) catch crop (with and without), and (2) manure (with and without). The four-course crop rotation was spring barley undersown with grass/clover - grass/clover - winter wheat or wheat rye - pulse crop. All cuttings of the grass-clover were left on the soil as mulch. Animal manure was applied as slurry to the cereal crops in the rotation in rates corresponding to 40% of the N demand of the cereal crops. Application of 50kg NH₄-Nha⁻¹ in manure increased average wheat grain yield by 0.4-0.9MgDMha⁻¹, whereas the use of catch crops did not significantly affect yield. The use of catch crops interacts with other management factors, including row spacing and weed control, and this may have contributed to the negligible effects of catch crops. There was considerable variation in the amount of N (100-600kgNha⁻¹ year⁻¹) accumulated in the mulched grass-clover cuttings prior to ploughing and sowing of the winter wheat. This was reflected in grain yield and grain N uptake. Manure application to the cereals in the rotation reduced N accumulation in grass-clover at two of the locations, and this was estimated to have reduced grain yields by 0.1-0.2MgDMha⁻¹ depending on site. Model estimations showed that the average yield reduction from weeds varied from 0.1 to 0.2MgDMha⁻¹. The weed infestation was larger in the manure treatments, and this was estimated to have reduced the yield benefit of manure application by up to 0.1MgDMha⁻¹. Adjusting for these model-estimated side-effects resulted in wheat grain yields gains from manure application of 0.7-1.1MgDMha⁻¹. The apparent recovery efficiency of N in grains (N use efficiency, NUE) from NH₄-N in applied manure varied from 23% to 44%. The NUE in the winter cereals of N accumulated in grass-clover cuttings varied from 14% to 39% with the lowest value on the coarse sandy soil, most likely due to high rates of N leaching at this location. Both NUE and grain yield benefit in the winter cereals declined with increasing amounts of N accumulated in the grass-clover cuttings. The model-estimated benefit of increasing N input in grass-clover from 100 to 500kgNha⁻¹ varied from 0.8 to 2.0MgDMha⁻¹ between locations. This is a considerably smaller yield increase than obtained for manure application, and it suggests that the productivity in this system may be improved by removing the cuttings and applying the material to the cereals in the rotation, possibly after digestion in a biogas reactor. Cereal grain protein content was increased more by the N in the grass-clover than from manure application, probably due to different timing of N availability. Green-manure crops or manures with a relatively wide C:N ratio may therefore be critical for ensuring sufficiently high protein contents in high yielding winter wheat for bread making.