The suppression of broadleaf weeds by using sorghum as a cover crop was suggested by Einhellig and Leather (1988). However, grassy weeds were not affected. Forage, sorghum-sudangrass hybrid, can suppress weeds in no-till alfalfa (Medicago sativa L.) plantation (Forney and Foy, 1985). Other examples include: suppressive effects of sweet potatoes (Ipomoea batatas [L.] Lam.) on Cyperus esculentus L. (yellow nutsedge) or soybean (Glycine max [L.] Merr.) on barnyardgrass. Environment (e.g. fertility) plays an important role in influencing rye phytotoxicity. Rye grown under low to moderate fertility had high levels of BOA and DIBOA when compared to that grown under high fertility. Hensley and Counselman (1979) studied allelochemical interaction between two biotypes, triazine-resistant and susceptible, of Amaranthus retroflexus L. (redroot pigweed). It was found that resistant biotypes were more influenced by allelochemicals when compared with susceptible biotypes. Resistant redroot pigweed biotypes were less vigorous and less competitive compared to susceptible biotypes. Such resistant weed biotypes can be controlled with allelopathic cover crops or residues. More research, however, is needed on this aspect. Przepiorkowski and Gorski (1994) investigated the effects of rye residues on germination and growth of triazine-resistant and susceptible biotypes of Conyza canadensis [L.] Cronq. (horseweed), Epilobium cilatum Rafin (willowherb) and barnyardgrass. These authors used rye root-incorporated soil to determine rye residue influence on two biotypes of selected weed species. Both biotypes of selected weed species were found to be sensitive to rye residues. Seed germination of both biotypes of willo- wherb and horseweed was adversely affected with increase in rye seeding rate. However, germination by none of the barnyardgrass biotypes was affected. The reduction in weed growth was, however, maximum in both biotypes of barnyardgrass compared to willowherb and horseweed (Table 1).