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. The allelopathic potential of wheat cultivars was positively correlated with their total phenolic content. Crop residues can have allelopathic effects as they decay. Decomposing plant residues might either inhibit or stimulate plant growth, and that inhibition may be confined to a limited period of time.