When winter rye is used as winter cover crop and killed with herbicides prior to planting, allelochemical compounds leaching from the rye residues inhibit weed emergence. However, tall stemmy rye decomposes slowly, interfering with seedbed preparation, and its allelopathy may hinder germination of small-seeded crops. Rye also has phytotoxic effects on green foxtail, redroot pigweed, ragweed, common purslane, lambsquarters and eastern black nightshade. Alfalfa, especially older stands, while often beneficial to nonrelated species, inhibits its own new seedlings. Luckily, one year out of alfalfa is enough to nullify its detrimental effects.
Sunflowers are allelopathic to many weeds.
Several years ago there was some speculation about the potential of allelopathic effects of canola to the following crop. Wheat producers reported that yields following canola were reduced - and attributed this to allelopathy. Red clover appears to be allelopathic to corn. Two researchers, Putnam and Tang at Colorado State University, have suggested that allelopathic characteristics are more likely to occur in wild types of crop precursors that have evolved in competition with other species than in our currently popular cultivars. A study by Wu of New South Wales found that some wheat cultivars significantly inhibited both germination and radicle growth of annual ryegrass.