Reduced irrigation impact on soil-applied herbicide dissipation and rotational crop response

Soil-applied herbicides are important for controlling weeds in many crops but risk damage to susceptible rotational crops if they persist. Field studies were conducted in Powell, Wyoming from 2015 through 2017 to evaluate the effect of reduced water availability on soil-applied herbicide dissipation. Eight soil-applied herbicides, applied to dry bean or corn, were exposed to three season-long irrigation treatments (100, 85, and 70% of estimated crop evapotranspiration [ETc]) by overhead sprinkler. Soil samples were collected to a depth of 10 cm from 0 to 140 d after application, and soil herbicide concentrations were quantified using gas or liquid chromatography and mass spectrometry. Herbicide concentrations were regressed over time to produce a soil half-life estimate for each herbicide and irrigation treatment. Reduced irrigation decreased dry bean yield by up to 77%, and corn yield by up to 50%. After adjusting for precipitation, the lowest irrigation treatment received 78% as much water as the full irrigation treatment in 2015 and 76% in 2016. This significantly increased the soil half-life of imazethapyr, but did not increase the soil half-life of atrazine, pyroxasulfone, saflufenacil, ethalfluralin, trifluralin, or pendimethalin. Reduced irrigation did not increase carryover injury to rotational crops from these herbicides one year after application. Instead, carryover response was determined by the inherent persistence of individual herbicides. Imazethapyr (0.1 kg ai ha-1) injured rotational sugar beet, and isoxaflutole (0.1 kg ai ha-1) injured rotational dry bean. Pyroxasulfone (0.2 kg ai ha-1), atrazine (2.0 kg ai ha-1), saflufenacil (0.1 kg ai ha-1) + dimethenamid-P (0.6 kg ai ha-1), ethalfluralin (0.8 kg ai ha-1), trifluralin (0.6 kg ai ha-1), and pendimethalin (1.1 kg ai ha-1) did not injure rotational crops regardless of irrigation treatment. Drought stress sufficient to cause up to 77% crop yield loss did not increase soil-applied herbicide carryover.