Legumes can reduce economic optimum nitrogen rates and increase yields in a wheat-canola cropping sequence in western canada
St. Luce, M., Grant, C.A., Zebarth, B.J., Ziadi, N., O'Donovan, J.T., Blackshaw, R.E., Harker, K.N., Johnson, E.N., Gan, Y., Lafond, G.P., May, W.E., Khakbazan, M., Smith, E.G. (2015). Legumes can reduce economic optimum nitrogen rates and increase yields in a wheat-canola cropping sequence in western canada. Field Crops Research, [online] 179 12-25. http://dx.doi.org/10.1016/j.fcr.2015.04.003
Inclusion of legumes in crop rotations is a sustainable approach to reducing nitrogen (N) fertilizer requirements and increasing subsequent crop yields. However, the magnitude of the benefit will depend on the specific legume, the subsequent crop and site-specific conditions. This study compared the effects of preceding legumes and non-legumes on yields and economic optimum N rates (EONR) in a Hard Red Spring wheat (Triticum aestivum L.)-hybrid canola (Brassica napus L.) cropping sequence. Field pea (Pisum sativum L.), lentil (Lens culinaris Medik), faba bean (Vicia faba L.; faba bean-seed), canola and wheat grown for grain, and faba bean grown as green manure (faba bean-GRM) were the preceding crops and were direct-seeded at six locations in western Canada in 2010. Wheat was seeded in 2011 and canola in 2012, with N fertilizer applied at 0, 30, 60, 90 and 120kgNha<sup>-1</sup> in each year. Averaged across preceding crops, N application increased crop yields at all sites. Wheat grain yield was greater, with a corresponding lower EONR, following legumes than non-legumes due in part to increased N availability. Greater non-N benefits to wheat following legumes than non-legumes were also evident at all but one site, with non-N benefits being lowest for preceding wheat. While preceding legumes had no effect on canola seed yield across sites, EONR for canola were >50% lower for legume-wheat than non-legume-wheat rotations. Among the legumes, growing faba bean-GRM, field pea and lentil were most likely to increase crop yields and reduce EONR as compared to faba bean-seed. Overall, growing legumes for seed before a wheat-canola cropping sequence in conventional cereal cropping systems can increase crop yields, reduce EONR and improve the long-term sustainability of cereal cropping systems.