Sunflower Associated With Legumes-Based Cover Crop : A Way To Increase Nitrogen Availability For The Following Winter Wheat?

Summary

Sunflower is one of the most important crop of organic crops systems in the South of France. In this region, sunflower is mostly cultivated before soft winter wheat, which is very often deficient in nitrogen because of a lack of nitrogen in the soil when the wheat needs it. To increase the soil nitrogen availability, one way is to introduce a legumes-based cover crop before wheat, which is sown just after the previous crop harvest. Thus, the time between sunflower harvest and wheat sowing is often too short to produce enough biomass. An alternative is to sow the cover crop during the sunflower cultivation, so to be intercropped into it. In a trial repeated over 3 years (from 2015 to 2017) in the southwest of France, Terres Inovia tested this practice, by intercropping 3 kinds of legumes-based cover crops into sunflower: alfalfa, purple vetch and legumes mixture. Over the 3 years, the growth of the cover crops was satisfying, and the average amount of nitrogen returned to soil after cover crops destruction was of 40 kg N/ha for purple vetch, 18 kg N/ha for alfalfa and 19.5 kg N/ha for legumes mixture. Nevertheless, cover crops impact severely sunflower performance because of competition for water and poor weed control due to no hoeing. Sunflower yield was reduced on average by 45% over the 3 years. This economic loss was partially compensated by a benefit on wheat yield, which was observed in 2016 and 2018, but only for wheat following sunflower intercropped with alfalfa.