Modelling environmental impacts of deposition of excreted nitrogen by grazing dairy cows

The soil nitrogen (N) and carbon dynamics model SOILN (which has interactive links to a grass growth model), and the dual-porosity contaminant transport model MACRO, have been used to study environmental pollution arising from grazing dairy cows. The models had been calibrated and tested in previous studies related to livestock agriculture. Information about N contents and other characteristics of urine and faeces excreted by dairy cows was assembled from literature sources. Watercourse pollution by nitrate and ammonium was the main environmental impact considered. Denitrified nitrogen losses were also estimated as an indicator of nitrous oxide pollution of air. Higher levels of nitrate pollution in tile drains (which feed into watercourses) were shown to arise under grazing compared to fields receiving slurry and cut for silage. Much of this raised nitrogenous pollution arises late in the grazing season. High levels of nitrate pollution could be attributed to various factors, including the fact that cows tend to congregate in certain areas of a field at a localised stocking rate much higher than the overall stocking rate, and due to deposition of N at times when grass cannot utilise it as a plant nutrient. The fact that urine and faeces patches are concentrated over a small proportion of the field area did not give an increase in overall loss when this was considered along with field areas receiving no excretions. Rapid transport through soil macropores of ammonium from urine led to high pollution loads during grazing on wet soil. In contrast to leaching, simulated N losses by denitrification were at a low level, and appeared to show little variation with factors which had a large effect on leaching losses. Overall, the forms of pollution most damaging to the environment due to spatially non-uniform excretion by grazing animals, appeared to be leached ammonium from urine transported by macropore flow, and leached nitrate exacerbated both due to cows congregating and due to deposition at times of low plant N uptake.