Season-Long Grazed Green Manure Systems: Making Biological Nitrogen Fixation Practical, Flexible and Economically Viable on Organic Grain Farms.

Annual legume green manure crops are an important source of biological nitrogen fixation (BNF) in organic grain production systems. However, producing an annual green manure in a short growing season requires taking a year out of cash crop production, which can be an economic barrier to adoption of green manures. Grazing annual green manure legumes with livestock has been shown to maintain the N benefit of the green manure crop to the following crops while also producing livestock products. To make this practice economically feasible and practical for farmers to implement, systems that generate forage supply throughout the season while fixing adequate N to support rotation crops are required. A set of field experiments was conducted in Manitoba, Canada in 2012-2014 to investigate several strategies for extending forage availability throughout the growing season while maintaining high levels of BNF: fall-seeded crops, relay- and double-cropping, intercropping, and staggered seeding dates. We found that each of these strategies can play a role in developing season-long grazed green manure systems and that multiple strategies can potentially be integrated to create customized systems to meet specific goals. Fall-seeded cereals such as rye provide early-season grazing and offer a high level of flexibility for including green manure / forage legumes through relay cropping or double cropping. Staggering annual green manure seeding dates extends forage availability in mid-summer and establishing an annual forage grass such as Italian ryegrass as an intercrop with a spring-seeded green manure provides late-season forage. While long-term testing of these strategies is still required, this research highlights the potential to integrate legume green manure crops and grazing livestock in complementary ways.