The parasitic weed Striga hermonthica is a major problem in African millet fields. International Agricultural Development (Jan/Feb 1994) reports that dense intercropping of cowpea in millet stands can reduce Striga emergence. Farmers in Mali commonly plant the two crops together, but the cowpea is only sparsely planted out of concern for reduced millet yields. A denser planting of cowpea cools and shades the soil and increases the relative humidity of the soil surface. Researchers were unsure which effect of intercropping reduced weed emergence.
Cowpea roots are known to stimulate Striga germination underground, but the close intercropping reduced the weed’s emergence from the soil. Researchers achieved best yield results by high-density intercropping of cowpea with local short-cycle millet. Long cycle (120-day) millet had reduced yield in dense intercropping.