Late-season catch crops reduce nitrate leaching risk after grazed green manures but release N slower than wheat demand

Late season catch crops could be an effective tool to conserve highly available N after grazing of green manures. An experiment was established to investigate the productivity and N capturing abilities of barley (Hordeum vulgare cv. Cowboy) and oilseed radish (Raphanus sativus L.) crops seeded after a grazed green manure. Green manure was a mix of forage pea (Pisum sativum cv. 40–10), soybean (Gylcine max cv. Prudence) and oat (Avena sativa cv. Legget). The experiment was repeated twice in Carman, Manitoba in 2010 and 2011. Catch crops were seeded in late summer either no-till or after soil cultivation of the grazed plots. Wheat (Triticum aestivum cv. Waskada) was seeded in the second year as a test crop for both experiments. Catch crop productivity and N uptake was influenced by season (greater in the wetter year) and catch crop type (barley and radish produced 1990 and 1490 kg ha−1, respectively) but not tillage system. The catch crops had their greatest overall effect on soil NO3-N content in 2010 under conditions of high autumn precipitation when N leaching was more severe. Here, the catch crops significantly reduced NO3-N at all depths. Under drier conditions in 2011, catch crops only reduced NO3-N in the top 30 cm. There was average 57 and 12 kg ha−1 more soil NO3-N in plots with no catch crops than plots where catch crops were grown in 2010 and 2011, respectively. Wheat N uptake at maturity was reduced around 25% when grown after catch crops. Similarly, wheat grain yield was 12–31% less after catch crops than no catch crops. This study showed that catch crops can be used to capture excess nutrients after grazing, but N release from the selected catch crops in the following year was not in synchrony with the wheat N demand.