Triazine Susceptibility of Fall Panicum and Small Crabgrass 1

Fall panicum ( Panicum dichotomiflorum Michx.) and small crabgrass [ Digitaria ischaemum (Schreb.) Muhl.] are becoming increasingly important problems for corn ( Zea mays L.) production because of their apparent tolerance to atrazine [2‐chloro‐4‐(ethylamino)‐6‐(isopropylamino)‐ s ‐triazine]. It is important to consider other herbicides for the control of these problem weeds and essential to maintain the control of weeds sensitive to atrazine. Consequently, it seems necessary to determine the value of other triazine herbicides for the control of these problem grasses. Experiments were initiated to assess the relative effectiveness of atrazine, cyanazine {2‐[(4‐chloro‐6‐[ethylamino]‐ s ‐triazine‐2‐methylpropionitrile}, cyprazine [2‐chloro ‐4‐ (cydopropylamino)‐6‐(isopropylamino)‐ s ‐triazine] and simazine [2‐chloro‐4,6‐bis(ethylamino)‐ s ‐triazine] for control of fall panicum and small crabgrass in corn when applied alone and in combination. Before greenhouse trials could begin, procedures for breaking dormancy of weed seeds were needed. Germination studies demonstrated that fall panicum seed required prechilling and small crabgrass seed required H 2 SO 4 scarification in order to germinate under laboratory conditions. Greenhouse studies indicated that the order of effectiveness of the herbicides on fall pauicum was cyanazine = simazine> cyprazine>atrazine, and on small crabgrass was cyanazine>simazine>cyprazine>atrazine. Fall panicum was more susceptible to atrazine, cyprazine, and simazine than was small crabgrass, but small crabgrass was more susceptible to cyanazine. Depth of herbicide placement in the soil greatly affected the weed control obtained under greenhouse conditions, suggesting that leaching of the herbicides under field conditions may reduce their effectiveness. Field studies conducted in 1971 and 1972 at two locations tended to verify the greenhouse observations.