Reduced soil moisture following a herbicide application, especially in the first two to four weeks, can slow the degradation or breakdown of herbicides, resulting in the potential for carryover. Additionally, lack of soil moisture can also result in increased herbicide adsorption to soil particles and organic matter, reducing herbicide availability for degradation.

Not all herbicides persist in the soil. However, there are several herbicides that we currently use in Michigan that have the potential to persist and cause injury to subsequent crops if rotational restrictions are not followed, especially in a dry season. In fact, the crop rotation restrictions for several products are lengthened if a minimum amount of precipitation is not met. For example, the Laudis label indicates that dry beans and sugarbeets can be planted 10 months after application if 20 inches of precipitation occurs between Laudis application and planting. However, if this minimum amount of precipitation is not met, the rotation restriction is extended to 18 months for these two crops. Similar restrictions stating minimum amounts of precipitation can also be found on Resolve Q, Accent Q and other herbicide labels.

While not all herbicides have minimum amounts of precipitation listed on their respective labels, it is important to follow the stated crop rotation restrictions. For example, several growers are considering planting wheat this fall in fields where the corn crop has failed. Several of the more popular corn herbicides have rotation restrictions to wheat that range anywhere from three to 15 months after application. Following these rotation restrictions are essential to ensuring growth of a healthy wheat crop.

Crop rotation restrictions for the different herbicides can be found in Table 12 of the 2012 MSU Weed Control Guide for Field Crops (E-434) and on the herbicide label. If there are still concerns about the potential for carryover, field or greenhouse bioassays can be conducted to help determine potential carryover problems. These bioassays are generally inexpensive and will help estimate the potential for rotational crop injury from herbicide residues.

Things to consider prior to planting rotational crops

• Know what herbicides and when the herbicides were applied to the field.
• Check the crop rotation restrictions on the label or in Table 12 of the 2012 MSU Weed Control Guide for Field Crops (E-434).
• Are there special factors such as soil pH, soil type or rainfall amounts that may change the length of time for that herbicide to degrade that need to be considered?
• Still concerned? Conduct a bioassay using soil from the field in question planted with the intended rotational crop.

Additional information:

• MSU Extension’s Drought Resources

Dr. Sprague’s work is funded in part by MSU’s AgBioResearch.