Relay-Intercropping of Forage Legume with Winter Wheat Improves Weed Control and Nitrogen Nutrition of the Following Crop in Organic Conditions.

In organic grain production, weed infestation and nitrogen deficiency highly limit performance and increase variability. Relay-intercropping of forage legume may provide functional and ecological values thanks to nitrogen fixation, improving soil N availability and weed infestation mitigation without impairing cash crop performance. Farmers generally used off-farm nitrogen fertilizer to insure cash crop production but it potentially limits ecological services from legume cover crop. This work aims at understanding the incidence of off-farm N input on ecological value of legume and performance of wheat-legume association.

Two species of forage legumes (Trifolium pretense L. and Trifolium repens L.) were undersown with soft winter wheat (Triticum aestivum L.) in ten organic fields. The cover crop was maintained after the wheat harvest until the sowing of following maize.

Intercropped forage legume had no effect on wheat yield but might decreased protein content by 0.4% when red clover growth is high under nitrogen deficiency. At wheat harvest, weed density was decreased by 52% compared to wheat sole crop. At cover destruction, weed biomass was decreased by 90%, showing high interest of relay intercropped forage legume before and after wheat harvest on weed infestation. Forage legume accumulated, on average, about 90 kg N ha^-1 in aerial parts. After tissues senescence, better nitrogen nutrition was observed on following maize and yield was increased by 30% (+0.27 t ha^-1). Organic nitrogen fertilizer increased both wheat yield (+40%) and proteins content (+7%) but reduced forage legume biomass by 40% and nitrogen accumulation at cover destruction for white clover by 20%.

Relay intercropping seems to be an interesting way to mitigate weed infestation and increase subsequent crop yield. Tradeoff between these services and wheat production may be modified by legume species and fertilization, future research are needed to improve wheat performance by nitrogen fertilization while maintaining ecological services.