In 1987 and 1988, Arkansas farmers in the northeast portion of the state experienced cotton injury due to imazaquin residue carryover. The residue carryover was a result of imazaquin applications made to soybean crops in 1986. Laboratory and greenhouse experiments were conducted at the Altheimer Lab to develop a reliable imazaquin plant bioassay using cotton as the indicator species, to determine the adsorption and desorption of the herbicide in three Arkansas soils where imazaquin carryover had occurred and to determine the relationship between imazaquin concentration in selected soil samples and cotton yield.

The plant bioassay was established using three soils, a clay, sandy loam and a clay, sandy loam soil and a loam soil. Analytical grade imazaquin was applied to the soils to obtain concentrations 0, 0.007, 0.015, 0.03 and 0.06 ug/g in 500 g of soil. Five cotton seeds (var. 'Stoneville 506') were planted into each pot and covered 0.5 cm with the treated soil. The bioassay was conducted in a growth chamber at the following conditions: a 14 h light period, 27/25 C light/dark temperature and 50% humidity. The pots were watered daily to near field capacity moisture levels. Cotton heights were taken at 5, 15, 22 and 29 DAE. The height data were subjected to analysis of variance and the means were separated using LSD tests. The bioassay was found to be sensitive to an imazaquin concentration of 0.007 ug/g; however, the date at which significant differences were found between cotton heights by herbicide concentration differed by soil texture. Significant differences in cotton heights were present at 15 DAE for the clay and 22 DAE for the loam soils.

The difference in cotton height response by soil texture can probably be attributed to the higher moisture holding capacity of the clay soil, allowing increased solubilization of imazaquin per unit-soil solution available for cotton uptake. The exposure of the cotton plants to an elevated soil solution concentration could have resulted in height responses to herbicide concentration at earlier dates in cotton grown in the clay soil than those grown in the loam soils.

The height data was subjected to regression analysis and an equation relating height response to herbicide concentration was derived for each of the three soils. Imazaquin concentrations in soil samples received from Arkansas farmers were determined using the regression equations.

In addition to bioassay determinations of herbicide concentration in the soil samples, chemical extractions were performed on selected soils. The imazaquin concentration determined by the bioassay were 75 and 80% of that found with chemical extractions for the clay and loan soils, respectively.

Adsorption studies were conducted on the same soils as those used to develop as those used to develop the bioassay. The batch equilibrium method using radiolabeled imazaquin at five soil concentrations was used to determine adsorption. The data were subjected to regression analysis to determine the K (intercept) and N (slope) values. A comparison of the regression lines generated from each of the three soils indicated that no significant differences were present among the slopes or intercepts. Therefore, K and N values of 0.134 and 0.959, respectively, were calculated by combining the data from the three soils.

A direct relationship between cotton yield response and soil concentrations of imazaquin was difficult to ascertain due to great variability present in the yield data. However, imazaquin concentrations of 0.008 ug/g resulted in yield reductions of up to 32% on sandy loam soils and a concentration of 0.024 ug/g resulted in reductions of 45 and 52% on a sandy loam and clay soil, respectively.