Can a Winter-sown Catch Crop Reduce Nitrate Leaching Losses after Winter Forage Grazing?

Direct grazing of winter forage crops to feed non-lactating, pregnant dairy cows prior to calving is a common management practice in the New Zealand South Island. However, the high crop yields per hectare grazed, combined with a high stocking density of cows, means this potentially leads to large amounts of urinary nitrogen (N) deposited on bare, wet soil, that in turn, could lead to high nitrate leaching losses. We undertook a study to simulate a winter forage grazing (WFG) event using field lysimeters planted with a kale (Brassica oleracea L.) crop. We report the effect of delaying sowing a “catch crop” of oats (Avena sativa L.) following simulated WFG on nitrate leaching losses from urine applied at different times throughout the winter. A catch crop sown between 1 and 63 days after urine deposition in early winter reduced N leaching losses from urine patches by ~34% on average (range 19-49%) over the winterspring period compared with no catch crop. Generally, the sooner the catch crop was sown following crop harvest, the greater the uptake of N by the catch crop and the greater the reduction in nitrate leaching losses. The results indicate that sowing of a catch crop following winter crop grazing could be an effective management strategy to reduce nitrate leaching as well as increase the N use efficiency of dairy winter forage grazing systems.