In west Texas, no-till planting cotton into rye or wheat cover crops reduced growth and lint yield of cotton compared with no cover crop. Production of allelopathic compounds, a known phenomenon in small grains, was suspected. Field and greenhouse trials verified cotton plant suppression by these small grain residues and by direct application of allelopathic chemicals. Grazing the cover crop by cattle may help alleviate these negative effects. Cover crops have environmental benefits but negative effects need further investigation.