In early September, University of Illinois weed scientist Aaron Hager collected soil samples from the Urbana, Brownstown and Perry test plots.
Hager says his team has detailed records on which herbicides and what quantities were applied at each of these test plots, along with specific weather records. The goal was to see if herbicide carryover is a significant risk for the winter wheat season.
The soil collections were placed in greenhouse pots. Half the pots were sub-irrigated for 7 days prior to planting corn, soybean, or wheat, while the other pots were sub-irrigated only after planting.
Crop species emerged within 7 days after planting. Emerged wheat growing in soil collected at Perry began to demonstrate symptoms of injury from some herbicides within a few days after emergence, suggesting that herbicide residues remain high enough to cause injury to the crop. Injury symptoms have not developed on emerged wheat growing in soil collected from Urbana and Brownstown.
Hager notes bioassays conducted with field-collected soil provide an estimate of whether herbicide residues are high enough to injure rotational crops but do not quantify the amount of herbicide remaining in the soil.
"We recognize that indoor bioassays are not infallible, so we also implemented field bioassays in the same plots from which we extracted the soil collections used in the greenhouse bioassays," Hager adds.
Currently, Hager's team is in the process of testing for herbicide injury in field trials. In late September, he noted that initial observations did not indicate significant herbicide injury.
"We are in uncharted territory here," Hager adds. "There's no way to say for sure if you'll see herbicide carryover on your specific farm."
Carryover risk is dependent on three factors: rainfall, soil temperature and time. Hager says there's no way to predict how much rain it would take to sufficiently break down the herbicides still in the soil.
In nearly all instances, microbial activity is the catalyst for breaking down these chemicals. Microbes thrive in warm, moist soils. But, again, there's really no way to allocate hard numbers in this situation.