The Persistence and Carryover of Imidazolinone Herbicides in Flooded vs Non-Flooded Soils.

Imazethapyr was selected to be the primary herbicide used with Imidazolinone-Resistant (IR) rice due to crop tolerance and red rice efficacy. Imazethapyr has been shown to be relatively persistent in the soil and may cause injury to rotational crops. Studies in non-flooded conditions have indicated that soil persistence of imazamox is less than that of imazethapyr. Imazamox therefore has been viewed as a possible alternative to imazethapyr when carryover injury is a concern. These herbicides have been studied to a great extent in dryland conditions. Little research has been conducted as to how they dissipate from a flooded soil environment such as would be found in production rice fields.  Our objective was to determine the relative persistence of imazethapyr and imazamox on two rice soils under flooded and non-flooded conditions. Imazamox and Imazethapyr were applied to bare plots of a Dewitt silt loam, and Portageville clay. Soil was sampled at 0, 3, 6, 12, 24, and 52 weeks and frozen to stop herbicide degradation. Additional soil was treated with known herbicide concentrations. Standard curves were developed from the rice response in the freshly-treated soil and then compared with the response of rice grown in samples from the field. Equations derived from the standard curves were then used to estimate the quantity of herbicide remaining the field samples. The herbicide concentrations versus time were fit to a first–order decay curve and half lives were determined from the resulting equations. Based on the half-lives, imazethapyr was more persistent than imazamox. Available herbicide concentrations decreased more rapidly in flooded soils as compared to the corresponding non-flooded soil. Plant available residues also decreased more rapidly in the clay soil as compared to the silt loam soil.