FATE OF FENOXAPROP-ETHYL APPLIED TO MOISTURE-STRESSED SMOOTH CRABGRASS (DIGITARIA-ISCHAEMUM)

Investigations of smooth crabgrass growth and fenoxaprop-ethyl retenti on, foliar penetration, translocation, and metabolism were conducted a t various soil moisture levels using a polyethylene glycol (PEG) semi- permeable membrane system. The activity of fenoxapropethyl was signifi cantly reduced at higher levels of moisture stress and this antagonist ic effect was greater with increased duration of water deficit followi ng herbicide application. Fenoxaprop-ethyl spray retention decreased l inearly (23% total reduction) as soil matric potential (PSIm) decrease d from -0.01 to -0.1 MPa. Foliar penetration and translocation of C-14 -fenoxaprop-ethyl applied on the third true leaf were not affected by level or duration of moisture stress. Only 2% of the absorbed radioact ivity was translocated out of the treated leaf for each moisture stres s level and duration. As the soil PSIm decreased (-0.01 to -1.0 MPa) t he relative levels of fenoxaprop-ethyl increased by 76 and 65% after a 48- and 96-h postapplication moisture stress period, respectively. In contrast, fenoxaprop acid decreased by 59 and 44% after 48 and 96 h o f moisture stress, respectively. The relative level of fenoxaprop acid was linearly correlated to the antagonistic effect on shoot dry weigh t. These results suggest that decreased spray retention and, particula rly, alterations in fenoxapropethyl metabolism contribute to reduced f enoxaprop-ethyl activity observed in moisture-stressed smooth crabgras s.