Potential of catch crops to reduce nitrogen leaching in New Zealand winter grazing systems

Winter grazing of fodder beet and kale is common practice in many regions of New Zealand. However, large quantities of urine-nitrogen (N) is returned by livestock onto bare soil during grazing at a period when the risk of drainage is high. Results from recent field trials in the Canterbury region show that sowing a catch crop directly after winter forage grazing can reduce N leaching losses by up to 49% compared with fallow soil, as well as offer significant gains in feed production, via additional annual production from the catch crop. However, the magnitude of effectiveness varies in response to crop management, catch crop genotype, soil type and seasonal weather conditions. For example, early-sown cereal genotypes adapted to lower temperatures provide the greatest potential to reduce leaching. This summary also highlights three important areas for future research: (i) overcoming the practical challenges of establishing catch crops in unfavourable conditions, (ii) development of biophysical models that can predict outcomes over a wide range of production systems and conditions, and (iii) quantification of other processes in the N cycle causing changes in N leaching, e.g. microbial immobilisation of N.