A simulation modelling study of water pollution caused by outwintering of cows

A simulation modelling study has been carried out of water pollution due to outwintering of beef and dairy cows on sacrifice field areas. Use has been made of the weather-driven MACRO model to simulate rapid transport of components of deposited excreta to field drains through macropores in saturated soil during or after rainfall. Such saturated soil conditions arise around the periphery of field areas which have become poached due to trampling by animal hooves. Simulations were set up to represent outwintering experiments carried out over two winters at two sites. Further simulations were set up as scenario tests over 10 years’ weather at the same two sites. Simulated results show that outwintering is likely to lead to significant levels of water body pollution by ammonium and phosphorus. Results also show no benefit in periodically moving a feeder to a different location in the grassland field, as simulated pollution levels appeared to be similar for a moved feeder to those for a static feeder. The study demonstrates the value of a hydrological model previously calibrated and tested at sites with continuous measurements of outflows, which can be exploited in scenario tests for a field situation which differs slightly from that at the calibration sites.