Implications of grazing by sows for nitrate leaching from grassland and the succeeding cereal crop

Outdoor pig production may have benefits in terms of animal welfare but could be undesirable in terms of nutrient losses. In paddocks on sandy soil grazed by lactating sows, nitrate leaching was determined using the suction cup technique from the start of grazing in October to after the following barley/pea wholecrop with perennial ryegrass. The inorganic N concentration in soil was uneven after sow grazing, with the highest values found near the feeding area. In contrast to low nitrate leaching in the year of grazing (25–30 kg of N ha–1), losses were considerable in the following autumn and winter. Throughout the measuring period of 18 months, nitrate leaching inside paddocks was on average 320 kg of N ha–1 with the major part near the feeding area. Ten metres from the feeding area, leaching losses were 500 kg of N ha–1 and, at 16 m distance, losses were 330 kg of N ha–1. At 22 and 28 m distance from the feeding area, about 200 kg of N ha–1 was leached and outside paddocks about 100 kg of N ha–1. The combination of a sandy soil with high winter rainfall leads to a large proportion of the N surplus being lost by nitrate leaching near the feeding area. To reduce nitrate leaching, stocking density and the level of dietary nutrient input must be adjusted, and more uniform distribution might be achieved by manipulating the excretory behaviour of the sows.