The Effect of Black Medic a Self Seeding Cover Crop and Nitrogen on Crop Yield and Soil Residual Nitrogen.

Black medic (Medicago lupulina, cv George) is a weed, however it can also be used as a self-seeding legume cover crop that may improve farm productivity.  Black medic is able to produce a persistent seed bank and self-reseed each year over a wide variety of growing conditions.  Black medic has relatively slow growth in the spring compared to annual crops, this reduces black medic’s competitiveness with annual crops during the early stages of crop development.  Black medic has the potential to increase the nitrogen available to the crop during the growing season; however, once black medic is established it will be hard to eradicate from production field if it is deemed undesirable to have in the field.  The objective of this study is to evaluate the benefits and problems of black medic as a self-seeding legume cover crop.  A winter wheat-oat-flax rotation was established on blocks with and without medic.  Three nitrogen rates, 20, 60 100% of the recommended rate for each crop taking into account the residual N supply in the soil were used.  At Indian Head, the target N rates including soil residual N was 135 kg ha^-1 for wheat, 110 kg ha^-1 for flax and 100 kg ha^-1 for oat.  The medic was seeded in 2002 and the crop rotations were initiated in 2003. Black medic biomass varied greatly depending on the environmental conditions that occur during the growing season.  In cool wet years like 2004 biomass production can be very high.  In a cool dry spring like 2009 growing season biomass production can be very low.  Black medic biomass decreased as the N rate was increased.   As expected seed yield increased as the nitrogen rate increased.  Black medic would increase the grain yield when 20% of the recommended N was applied in some years. At the 60% and 100% N rates this difference between medic and non-medic treatments did not occur.  In the semi-arid region black medic would be a benefit as a cover crop if N applications were limited.