Effects of summer turnip forage cropping and pasture renewal on nitrogen and phosphorus losses in dairy farm drainage waters: A three-year field study

•Dairy farm drainage water TN and TP losses were monitored in a three-year field study.•Summer forage cropping and pasture renewal increased TN and TP losses by 84% and 77%, respectively.•Three-quarters of the increased TN losses occurred immediately following spring cultivation.•Drainage season duration after spring cultivation was a major determinant of elevated TN losses. Spring cultivation of soil under long-term pasture to grow summer forage followed by autumn pasture renewal is a common practice on New Zealand dairy farms. Although this practice is widely used, there is limited research available on the impact that it has on nutrient losses to water. This three-year field study was conducted to quantify the effects of summer forage cropping and autumn re-grassing on nitrogen (N) and phosphorus (P) losses in mole and pipe drainage from a Pallic Soil in the Manawatu region of New Zealand. The two treatments compared were a continuation of long-term grazed pasture, and long-term pasture that was cultivated in spring, sown into turnips as a summer forage for dairy cows, followed by autumn re-grassing. Both treatments where located on artificially drained field plots. Summer forage cropping and autumn re-grassing increased total nitrogen measured in drainage water by 21kgN/ha in total over the three seasons monitored, which was a 84% increase compared to long-term pasture. Approximately three-quarters of this increase happened during the two months following spring cultivation, which was influenced by an atypically wet late spring/early summer period that extended the drainage season. It is estimated that if the spring cultivation had been conducted in a year with a more typical drainage season, then the additional total nitrogen losses are likely to have been about 6.9kgN/ha. Overall losses of total phosphorus in drainage were low for both treatments during the study period, with the forage cropping and autumn re-grassing increasing total phosphorus losses in drainage by 0.36kg P/ha over the duration of the study, which was 77% higher than the long-term pasture treatment.