Abstract: Excessive persistence of imazethapyr has been responsible for injury to corn grown after soybean. Factors implicated in corn injury reported from certain parts of the main corn-producing region in South Africa were: leaching of herbicide to deep soil layers during the season of application, followed in the next season by capillary movement to the root zone of corn, and increased bioactivity of herbicide residues following liming of fields. Bioassays were employed to determine to what extent imazethapyr leached in a soil that typically contains less than 10% total clay and 0.1% organic C in the 0- to 300-mm zone and to assess the role of pH and liming in the bioactivity of the herbicide. Undisturbed soil columns were collected in polyvinyl chloride pipe for the leaching experiment. In the greenhouse, the equivalent of 40 g ai/ha imazethapyr was applied on the column surface, followed by leaching with simulated rain of 25 or 50 mm. Leaf area measurements of the test species rapeseed showed that the herbicide was readily leached to at least 30 cm at both water regimes and that it subsequently moved upward in the soil, with evaporation as the driving force. In the other bioassay, the soil was ameliorated with different amounts of Ca(OH)2 or CaCO3 to adjust pH levels to between 5.7 and 7.1 These soil samples were each treated with imazethapyr at rates equivalent to 1.8, 3.75, 7.5, 15, and 30 g ai/ha. The growth response of the test species indicated that where Ca(OH)2 was applied, the bioactivity of imazethapyr in most instances was significantly greater than where CaCO3 was used. At all the imazethapyr rates, the activity of the herbicide increased significantly with an increase in pH from 5.6 to 6.5 where Ca(OH)2 was used, but with CaCO3, activity was significant only at 15 and 30 g ai/ha. Changes in imazethapyr adsorption and in the organic matter in the soil were not monitored, but it is suggested that the increase in herbicide activity caused by Ca(OH)2 may be due to the degradation of organic matter in the soil or to desorption of the herbicide, which would render the herbicide more available for uptake. These effects, if they do occur, are likely to be of significance for herbicide adsorption only in soil with very low organic matter content. Results indicate that imazethapyr could leach easily in coarse-textured soils low in clay and organic matter content and that the type of lime used on those soils may influence the bioactivity of the herbicide.
Nomenclature: Atrazine; imazamethabenz; imazaquin; imazethapyr; corn, Zea mays L.; rapeseed, Brassica rapa L. ‘Salusia’; soybean, Glycine max (L.) Merr.
Additional index words: Adsorption, bioactivity, herbicide mobility, soil pH.
Abbreviations: FC, field capacity; OM, organic matter; PVC, polyvinyl chloride.