The products listed as High Risk have the potential to damage rotational crops under 'normal' conditions. “Most farmers have learned what rates can be used on their soils safely, but this year's drought will result in a high risk even if reduced rates were applied," says Owen. Owen and Hartzler offer the following observations and recommendations.
Both chlorimuron and atrazine are more persistent in high pH soils. Preemergence applications of chlorimuron will have a much higher risk of problems because these rates are much higher than when chlorimuron is applied postemergence (Classic). In certain situations, the best option may be to alter your crop rotation plans to avoid planting a susceptible crop.
Products listed in the 'Moderate to Slight Risk' category have been known to occasionally cause problems or have half-lives that suggest they might cause problems under abnormal conditions. Risks with these products will vary widely from field to field depending on the specific conditions encountered.
Evaluate risk field-by-field for potential damage to sensitive rotational crops
To minimize problems next spring, evaluate this year's herbicide program for compounds that pose a carryover risk. Consider rates, application date, soil characteristics and label restrictions.
Keep in mind that if rainfall returns to normal, this rain will have much less effect on herbicide degradation than had it occurred near the time of herbicide application.
What about tillage? While tillage should dilute herbicide residues within the soil profile, past experience has shown that this practice does not consistently reduce crop injury from herbicide residues. The conditions a crop experiences during establishment greatly influence its ability to tolerate residual concentrations of herbicides. Using practices that minimize additional stresses to the crop seedling (such as planting date, seedbed conditions, etc.) can reduce problems associated with low concentrations of herbicides that carryover in the soil.
For weed management information visit the ISU Extension weed science website.