Andrew I. Hsiao

Effect of drought and formulation on wild oat (Avena fatua) control with imazamethabenz and fenoxaprop

Experiments were conducted in the greenhouse to determine the effects of drought, rehydration and herbicide formulation on the phytotoxicity to wild oat of imazamethabenz and fenoxaprop. Under both well-watered and drought conditions, the activity of imazamethabenz with the liquid concentrate formulation tended to be higher than the suspension concentrate formulation, and the activity of the soluble powder formulation was enhanced by the surfactant Agral 90. Long-term drought did not reduce imazamethabenz phytotoxicity to wild oat with any formulations examined, whereas the phytotoxicity of three fenoxaprop formulations, fenoxaprop-ethyl, fenoxaprop-p-ethyl and fenoxaprop-p-ethyl plus safener, was decreased by long-term drought. Fenoxaprop-ethyl activity was reduced by drought stress imposed before spraying, after spraying or, most significantly, both before and after spraying. The adverse effect of drought on fenoxaprop-ethyl activity was still evident even when daily watering was resumed 8 to 48 h before spraying, although plants rehydrated in this way were more susceptible to fenoxaprop-ethyl than plants where rehydration was deferred until after spraying.

Effect of decapitation on absorption, translocation, and phytotoxicity of imazamethabenz in wild oat (Avena fatua L.)

The release of apical dominance by the physical destruction in situ of the apical meristem and associated leaf primordia (decapitation) promoted the growth of tillers in non-herbicide-treated wild oat plants, as indicated by increased tiller lengths and fresh weights. At 96 h after [14C] herbicide treatment following decapitation, the absorption of [14C]imazamethabenz and total translocation of radioactivity were respectively increased by 28% and 49%. By 96 h after [14C]imazamethabenz application, the radioactivity detected in the roots of decapitated plants was 45% higher than that in the roots of nondecapitated plants while the radioactivity in tillers of decapitated plants was 2.6-fold that in tillers of intact plants. Decapitation together with foliar spraying of imazamethabenz at 200 g ha−1 further reduced tiller fresh weight, greatly decreased the total tiller number, and thereafter significantly increased overall phytotoxicity by 32% as measured by total shoot fresh weight. The results of this study support the hypothesis that main shoot apical dominance limits translocation of applied imazamethabenz to lateral shoots, rendering tillers less susceptible to growth inhibition by the herbicide.

Impact of temperature on the phytotoxicity of imazamethabenz and fenoxaprop to wild oat (Avena fatua)

Abstract Controlled environmental experiments were conducted to determine the effect of short- and long-term temperature stresses on the phytotoxicity of imazamethabenz and fenoxaprop to wild oat. Short-term (16 h) temperature stresses, imposed either before or after herbicide spraying, had no influence on imazamethabenz phytotoxicity. However, short-term temperature stresses affected the phytotoxicity of fenoxaprop, with pretreatment high temperature (35°C) reducing the phytotoxicity and post-treatment high temperatures (25–35°C) enhancing the phytotoxicity. Imazamethabenz phytotoxicity was similar at long-term temperatures between 20/15°C and 30/20°C. Constant low temperature (10/5°C) reduced imazamethabenz phytotoxicity, whereas such temperature had no effect if only imposed before or after spraying. Among three constant temperature regimes, fenoxaprop was more effective at 20/15°C, followed by 10/5°C. Constant high temperature (30/20°C) greatly decreased fenoxaprop phytotoxicity. The adverse effect of long-term low temperature on fenoxaprop phytotoxicity was due mainly to the influence of post-treatment low temperature, whereas long-term high temperature imposed either before or after spraying reduced the phytotoxicity of fenoxaprop.

Influence of nutrient supply and plant growth regulators on phytotoxicity of imazamethabenz in wild oat (Avena fatua L.)

The influences of nutrient supply and plant growth regulators on the phytotoxicity of imazamethabenz in wild oat (Avena fatua L.) were evaluated in the greenhouse. Wild oat plants supplied with half-strength rather than one-eighth-strength Hoagland solution were more susceptible to imazamethabenz, showing greater growth reduction in main shoot and tillers. The improved herbicide efficacy at higher nutrient levels appeared related to increased herbicide interception by the greater leaf surface available. Leaves developing at either nutrient level did not differ significantly in epicuticular wax, so differential absorption appeared unlikely. Wild oat plants supplemented with nutrient, switching from low to high levels at the time of herbicide application, were as susceptible to imazamethabenz or even more so than plants growing with a constant high level of nutrition. The wild oat pure-line Montana 73, a strongly tillering line, was more susceptible to imazamethabenz than the limited-tillering line, Crop Science 40. Both 2,4-D and GA3 reduced imazamethabenz-induced tillering. Imazamethabenz efficacy was increased by GA3 but not by 2,4-D. These results support the hypothesis that lowering apical dominance of wild oat increases imazamethabenz activity in tillers, and that increased tillering following sublethal doses of imazamethabenz treatment is associated with the release of apical dominance.

Interaction between imazamethabenz and fenoxaprop in wild oat control and crop tolerance

Greenhouse experiments investigated wild oat (Avena fatua L.) control, and crop tolerance of spring and winter wheat (Triticum aestivum L.), and durum wheat (T. turgidum L.) to applications of imazamethabenz and fenoxaprop. Both regression technique and Colbys analysis showed the interaction between imazamethabenz, either liquid concentrate (LC) or suspension concentrate (SC) formulation, and fenoxaprop to be antagonistic. The antagonism was stronger with imazamethabenz-LC than with imazamethabenz-SC. The effect was greater with higher rates of fenoxaprop or imazamethabenz-SC. A 2 h interval between imazamethabenz-LC application followed by fenoxaprop application overcame the antagonism. The sequence of the split application was important in determining the interaction of these two herbicides. Norstar winter wheat was tolerant to imazamethabenz alone at all rates up to 600 g ha−1 active ingredient (a.i.) but a rate of 600 g ha1 reduced shoot dry weight of Kyle durum wheat and Katepwa spring wheat. All crop varieties tested were injured by fenoxaprop alone at 100 g a.i. ha−1. Combinations of imazamethabenz and fenoxaprop reduced crop phytotoxicity compared with fenoxaprop alone.