Sediment and nutrient losses under winter cropping on two Manawatu hill country soils

Abstract

Abstract Aerial spraying and surface seeding of winter crops in uncultivable hill country areas is rapidly being adopted as a method of increasing winter feed supply and as a precursor to regrassing. However, there is little research on the sediment and nutrient losses that may result from this practice. In the current study, winter swede crops were established on an imperfectly and on a welldrained soil and these crops were grazed by beef cattle. Soil damage caused by the winter grazing of the swedes generated sediment losses that were 5.5 times greater on the imperfectly drained soil than the well-drained soil. Surface runoff over 3 months (which included crop grazing and the non-grazed crop stubble period) resulted in losses of 1.1 t/ha of sediment, 0.85 kg of phosphorus (P)/ha and 5.4 kg of nitrogen (N)/ha from the poorly drained soil. This key risk period contributed between 88 and 99% of the total annual sediment and total N and P losses, compared to the pre-crop (pasture) and crop establishment phase. A simple comparison with two other sediment and nutrient loss studies located on the same farm as the current study, suggested that the losses associated with winter cropping in this landscape may be extreme. The current study highlights the need for targeted mitigation strategies and/or strategic grazing management to reduce soil and nutrient losses and to minimise the impacts on waterways of winter grazing of hill country crops. Keywords: winter cropping, hill country, surface runoff, sediment losses, nutrient losses