Residual effect and nitrate leaching in grass-arable rotations: Effect of grassland proportion, sward type and fertilizer history

Summary

A key point in designing grass-arable rotations is to find the right balance between number of cultivations versus the length of the grass phase. In a field experiment, we investigated the effect of cropping history (grazed grass-clover and ryegrass, proportion of grassland and previous fertilizer use) on crop growth and nitrate leaching in two years following grassland cultivation. In the final year, the effect of perennial ryegrass as a catch crop was investigated. The nitrogen fertiliser replacement value (NFRV) of grassland cultivation was higher at 132 kg N ha-1 in the rotation with 75% grassland compared to on average 111 kg N ha-1 in rotations with 25 and 38% grassland and the NFRV of ryegrass in the rotation was higher than that of grass-clover. Nitrate leaching following cultivation was not affected by grassland proportion in the crop rotation or sward type. However, there was considerable effect of having a ryegrass catch crop following the final barley crop as nitrate leaching was reduced from 60 to 9 kg N ha-1. When summarising results from the crop rotations over a longer period (1997-2005) management in both grassland and arable phases appears to be the primary measures for avoiding nutrient losses from mixed crop rotations irrespective of grass proportion. In the arable phase the huge potential of catch crops has been demonstrated, but it is also important to realise that all parts of the grass-arable crop rotations must be considered potentially leaky.