Catch crops to mitigate N loss after winter forage grazing: From plot to paddock

Abstract

Recent research has shown sowing a catch crop can reduce nitrate leaching after simulated winter forage grazing. However, most of this research has been reported from lysimeter and small plot trials where the pragmatic considerations of drilling a crop in winter, and achieving a net economic benefit to the farmer, were not largely explored. We report the first-year results (2018) of a three-year sustainable farming fund project to upscale earlier research into farmers’ paddocks by establishing a series of catch crop trials, post-grazing, on winter forage paddocks in Mid-Canterbury. Two trials were situated at Hororata on a winter forage block on a shallow, stony Lismore soil (Pallic), and a third on a dairy farm at Te Pirita on a moderately-deep Waimakariri soil (Recent). Trials measured dry-matter (DM) yield, nitrogen (N) uptake and soil mineral-N (where possible). The Hororata trials (using oats only) compared differing cultivation and drilling treatments (conventional cultivation/drilling vs direct drill) on ex-kale and ex-fodder beet paddocks whilst the trial at Te Pirita considered differing catch crop species (oats, triticale and Italian ryegrass). Trial establishment from early July to early August was aided by a relatively dry and mild month that provided favourable soil conditions for drilling, with the Hororata and Te Pirita trials being harvested mid and late November, respectively, at a similar time as the surrounding paddock was harvested for green chop silage. DM production and N uptake over both sets of trials ranged from 4-12 tonnes/ha and 100-360 kg N/ha, respectively, significantly reducing soil mineral-N levels. Due to an application of pig effluent in June on the kale paddock, and cultivation before drilling the oats, soil N mineralisation rates were very high with over 400 kg mineral-N/ha measured in the top 15 cm of the soil profile. Soil mineral-N measurements were significantly lower for the fallow and direct-drill treatments suggesting lower N mineralisation rates. For the species trial, significantly more DM was produced by time of harvest in November for the oats, followed by triticale and Italian ryegrass (12, 10 &, 6 t/ha, respectively), and in N uptake (208, 183 & 140 kg N/ha, respectively).