Effect of using a catch crop to reduce nitrate leaching losses from winter grazed fodder beet soil

Grazing of winter forage crops such as fodder beet (Beta vulgaris L.) and kale (Brassica oleracea L.) is a common management option for increasing winter feed supply on New Zealand farms. This practice is most often implemented on South Island farms for feeding nonlactating dairy cows prior to calving. These crops produce high yields of 10-16 t DM/ha by the time of winter grazing. However, there is growing concern that the high stock density during grazing leads to large amounts of urinary nitrogen (N) deposited onto bare soil. The soil often remains fallow until early spring and thus could led to large nitrate (NO₃⁻) leaching losses. Catch crops have been proposed as a mitigation strategy for reducing the N leaching losses from these winter forage grazing systems. The objective of this trial was to determine the effectiveness of the use of a catch crop of oats (Avena sativa L. cv. Milton) to reduce N leaching following winter grazing of fodder beet (Beta vulgaris L. cv. Rivage). This trial used the recently developed suction cup and lysimeter array (SCALAR) system to collect field scale measurements NO₃⁻ and ammonium (NH₄⁺) leaching. The results show a catch crop of oats had no significant effect (P>0.05) on N leaching from winter grazed fodder beet. This result is thought to be due to the rapid leaching of N from the root zone prior to the establishment of the oats. This occurred because of the above average rainfall which occurred over the trial period. Total N leaching losses from winter grazed fodder beet soil were nevertheless low at 19 and 23 kg N/ha for the fallow and catch crop treatments respectively. This was attributed to the potentially large quantities of gaseous N loss resulting from denitrification due to the wet anaerobic conditions and soil compaction due to trampling damage.