ALLEVIATING THE ANTAGONISTIC EFFECT OF MOISTURE STRESS ON SMOOTH CRABGRASS (DIGITARIA-ISCHAEMUM) CONTROL WITH FENOXAPROP

The influence of moisture stress intensity and duration, as well as ta nk mixtures with pendimethalin, which mitigated the antagonistic effec ts of drought, on fenoxaprop efficacy were evaluated. In growth chambe r tests, specific soil matric potential (psim) levels of -0.01 to -1 M Pa were imposed and maintained using a polyethylene glycol semipermeab le membrane system. Moisture stress was alleviated by irrigation 48 or 96 h after herbicide treatment. Fenoxaprop efficacy was influenced by moisture stress intensity, post-treatment stress duration, and applic ation rate. Under conditions of mild stress (psim greater-than-or-equa l-to -0.2 MPa), both increased application rate and irrigation 48 h af ter treatment alleviated the effects of drought on fenoxaprop efficacy . Under moderate stress (psim = -0.4 MPa), the antagonistic effects of drought were alleviated by a combination of increased rate and irriga tion 48 h after treatment. However, under more severe stress (psim les s-than-or-equal-to -0.8 MPa), irrigation coupled with increased rate d id not increase crabgrass control above 50%. Where irrigation within 4 8 h of treatment is not feasible, tank mixes of fenoxaprop with pendim ethalin were shown to enhance smooth crabgrass control. Field lysimete r tests indicated that under moisture stress, psim of -0.7 MPa, smooth crabgrass shoot dry weight was reduced 65 to 94% with tank-mix combin ations of greater-than-or-equal-to 0.27 kg ai ha-1 fenoxaprop plus gre ater-than-or-equal-to 2.2 kg ai ha-1 pendimethalin. These rate combina tions were shown to be synergistic and able to alleviate the antagonis tic effect of moisture stress on fenoxaprop efficacy.