As the acreage treated with herbicides increased in the 1960s and 1970s, ecological shifts occurred and weeds such as prickly sida (Sida spinosa), that had been almost nonexistent in cotton fields became major weed problems. Perennials such as purple and yellow nutsedge (Cyperus rotundus and esculentus), silverleaf nightshade (Solanum eleaegnifolium) and perennial vines became more troublesome because they did not have to compete with other weeds (McWhorter and Abernathy, 1992).