The Value of Break Crops for Wheat.

The yield of wheat growing after a broadleaf break crop generally exceeds that of wheat growing after wheat or other cereals. The presumed reasons for the yield benefit vary between break crops. They include reduced root and foliar disease, increased supply of soil water and mineral N, reduced assimilate loss to mycorrhizas, and, after legumes, growth stimulation following hydrogen gas release. To quantify the value of break crops, we compiled data from >600 published experiments on the additional yield of wheat following oilseeds, grain legumes or alternative cereals grown in the previous year. The yield increase was not generally proportional to yield, so the yield contribution of break crops is best expressed in absolute, not percentage terms. For a 4 t/ha wheat crop the additional yield after an oat break crop was 0.47 t/ha, after canola and linseed 0.85 t/ha, and after grain legumes between 1.81 t/ha for lupin and 1.10 t/ha for field pea. These data are used to evaluate the reasons for yield increase. They suggest that control of take-all and residual nitrogen after legumes are the largest benefits of break crops.