IMPROVING JOHNSONGRASS (SORGHUM-HALEPENSE) CONTROL IN SOYBEAN AND SUNFLOWER CROPPING SYSTEMS

Overall efficacy of control practices for Johnsongrass was evaluated i n soybean and sunflower cropping systems where timing of cultural and chemical control practices was adjusted to weed phenological stages by using visually estimated indices (stems 15 to 30 cm in height with 4 to 8 leaves) and with a thermal calendar model to forecast the crucial period for Johnsongrass propagation (when the population's rhizome ma ss is at a minimum). When visually estimated indices were used, weed i nfestation at the end of one growing season remained constant for the soybean cropping system and was reduced by about 70% in the sunflower crop. With the thermal calendar, weed infestation was reduced in both cropping systems by more than 90% in one growing season, and Johnsongr ass was eradicated in two growing seasons. Although timing the control procedures to the crucial period severely hampered persistence of the weed in both cropping systems, the crop yield of sunflower was reduce d by 35%. This yield reduction was caused by the delay in sowing date needed in order to adjust control procedures to the crucial period for propagation.