Effect of a catch crop to reduce nitrate leaching loss following simulated winter forage grazing

Abstract

The growing of high yielding forages for dairy winter grazing is a common pastoral farm management practice in the temperate regions of New Zealand. The high stocking rates during winter forage grazing means that once the forage is grazed large volumes of urine are deposited. The aim of this research was to determine the effects of using a catch crop to sequester N following simulated winter forage grazing and to reduce nitrate leaching losses. Two lysimeter trials and one growth chamber experiment were conducted at Lincoln University to measure the effect of catch crops in reducing nitrate leaching loss from a Balmoral stony silt loam (NZ classification: Acidic Orthic Brown soil). In year 1, a comparative lysimeter study was conducted between two potential catch crops, oats (Avena Sativa L.) and Italian (It.) ryegrass (Lolium multiflorum L.), that were sown at the recommended dates. Urine labelled with 15N was applied at 350 and 700 kg N ha-1 in late June and the oats and It. ryegrass were sown 7 and 11 weeks later, respectively. The effect of the nitrification inhibitor, dicyandiamide (DCD), on reducing nitrate leaching after urine application was also investigated for each catch crop at the 350 kg N ha-1 rate only. In year 2, another lysimeter study was conducted to determine the effect of the most effective catch crop in Experiment #1, oats, to capture N and reduce nitrate leaching from urine application at different times over winter (early June, early July and late July). This 2nd lysimeter experiment also determined the effects of increasing the interval between urine applications and sowing date of the oats (1-63 days after application). In year 1, only 3-4% of the urinary-N applied was captured by the catch crops (non-DCD) indicating that both were probably sown too late to make any significant impact on N uptake to reduce nitrate leaching. However, the cool season activity of the oats increased evapotranspiration compared to the It. ryegrass, reducing drainage and thus nitrate leaching, by 22% and 25%, respectively, over the winter-spring period (Jun-Nov). Application of DCD increased N uptake in the oats three-fold (~13% of N applied) and reduced nitrate leaching losses by over 60%. In year 2, nitrate leaching losses from a series of winter urine applications were reduced by around a third (~34%; range 19-49%) after the sowing of an oats catch crop compared to the fallow treatments. Part of this success was likely due to a warmer and drier winter than normal but the size of the decrease suggests that there is significant potential to mitigate nitrate leaching in these low-cost winter feed systems whilst improving N-use efficiency and DM production. Later sowings of oats leached up to 25% more nitrate than the earliest sowing with earlier sowing of the oats increasing N uptake over later sowings by up to a third. Later winter urine applications had lower overall nitrate leaching losses (range 170-270 kg N ha-1). Calculations for paddock N loss indicate a potential reduction in nitrate leaching overall of ~30% from 88 to 62 kg N ha-1. In year 3, a growth chamber experiment was established to investigate in more detail the main factors controlling oats development and N uptake using 15N-labelled urine. Two growth chambers set at 6C (mean winter) and 10C (mean spring), with two lighting levels (mid-winter, 5 MJ m2 day-1 and early-spring, 10 MJ m2 day-1) each were set up to examine the soil temperature-by-light intensity interaction on oats development. Sowing oats in the 10°C and 6°C chambers reduced nitrate leaching losses, on average, by around three-quarters and one-third, respectively. Results indicated a strong interaction between temperature and light intensity at 10°C on oats development but no direct effect of light at 6°C. Nitrogen uptake by the oats treatments in the 10°C chamber was almost complete after 45 days from sowing, with soil mineral-N concentrations reduced close to zero. However, indirectly, increased evaporative loss under the spring lighting and a slower rate of nitrification at 6°C meant drainage losses of nitrate were similar for both 6  and 10°C oats treatments, 90 days after urine application. Where no oats were sown, nitrate leaching losses were large, particularly under the 10°C fallow treatments. This research programme has discovered that oats have considerable potential to reduce winter forage nitrate leaching losses but they need to be established early (sown within 3-4 weeks after urine application). Cool soil temperatures are not an impediment to early establishment of oats