Comparison of fertilizer strategies for reducing nitrate leaching from grazed grassland, with particular reference to the contribution from urine patches

The effectiveness of alternative fertilizer strategies to control nitrate leaching was investigated in a field experiment at IGER, Aberystwyth using simulated pastures to represent swards grazed by dairy cattle between May and October 1997, following an initial silage cut. Cut-plots with and without applications of artificial urine were used to represent the separate components of grazed pastures managed with the following nitrogen (N) fertilizer strategies: standard (fertilizer applied uniformly to all areas of the sward at a total rate of 180 kg N/ha between May and October), tactical (fertilizer rates adjusted to match the average soil mineral-N content of the pasture to that of ungrazed reference plots receiving the standard rate) and patch-avoidance (fertilizer applied at the standard rate but withheld from areas that had received urine). Calculated stocking rates derived from herbage yields indicated that 12% of the pasture would have been affected by urine up to the end of 'grazing' in October. The presence of urine patches increased the nitrate-N content of the 0-90 cm soil profile in October from 61 kg N/ha for ungrazed pasture to 104 kg/ha for 'grazed' pasture receiving the standard fertilizer rate. Although the patch-avoidance strategy was more effective than the tactical in reducing the accumulation of nitrate in soil under urine patches, they were both of limited effectiveness in reducing the content over the pasture as a whole. Profile contents in October for the simulated pastures managed with the tactical and patch-avoidance strategies were equivalent to 99 and 97 kg nitrate-N/ha, respectively. The tactical strategy achieved a 26% saving in overall fertilizer use. Under the conditions of the experiment this did not significantly reduce herbage yields. The patch-avoidance strategy reduced fertilizer use by only 3%.