Before You Plant, Think About Herbicide Residues

Kristen Obeid, M.Sc. – OMAFRA Weed Management Specialist – Horticulture

As you get ready to plant, stop and think about which field you should be planting and what its herbicide history is.  If you have a mixed field and horticulture crop operation you need to be extremely careful with your crop rotation.  Herbicide residues and carry over can be real issues, maybe not every year but they can come up unexpectedly.  For example, in excessively wet or dry years or if soil pH is high or low, different types of herbicide carry-over may be more pronounced.  The amount of herbicide residue in the soil is directly related to herbicidal properties, soil and climatic factors. 

If you are unsure, it is always best to test for residues.  The easiest and cheapest method is to conduct a soil bioassay.  This is a simple way to determine if it is safe to seed or plant into areas previously treated with herbicides or into soil with an unknown history of herbicide use.  A bioassay uses susceptible plants to identify if the herbicide is present in concentrations high enough to inhibit germination and/or alter plant growth. Bioassays can be completed by a lab or by anyone, including yourself or your consultant.

There are two types of bioassays, field bioassays and indoor bioassays. 

Field bioassay:

1. Plant one or more strips of a species sensitive to the suspect herbicide in several locations in the suspect field, as close to planting as possible for more accuracy.  Choose an area that is the most suspect of residues as well as an area that can be used as a check.
2. Before planting the crop allow the test plants to grow and develop symptoms of injury from any herbicide residues.

Indoor bioassay:

Neal (2021)

1. Collect representative soil samples.  Sample from areas suspected of having herbicide residues as well as areas which are known to be free of herbicides. You will use the herbicide-free soil for comparison.  Keep only the upper two inches of soil as most residual herbicides will be bound in this zone. On sandy soils sample to four inches.  Take several samples from an area and combine them. You need enough soil to fill several pots in which you will grow the bioassay plants.
2. Select the bioassay species. The best bioassay species is the one you intend to grow or select a particular species known to perform well in bioassays such as ryegrass, oats, beans, peas, and tomato. Table 1 provides a list of recommended bioassay species for different herbicide families or modes of action. 
3. Seed and grow for about three weeks.  Seed or transplant the bioassay species in “clean” and “contaminated” soil. Place the pots in a greenhouse or on a sunny windowsill and keep them watered. Examine the overall growth, leaves, and roots. Look for stunting, yellowing (or other discoloration), abnormal leaf or stem growth, and root swelling.

Bioassays can be completed for any type of herbicide if the mode of action of the herbicide is known. 

Correcting for Herbicide Residues

If the bioassay indicated a potential herbicide-residue problem, several steps can be taken:

1. First select a tolerant crop or variety.  This depends on the herbicide of concern.  Check the label.
2. Till the field.  Tillage can help dilute the herbicide.
3. Plant the field of concern last.  Delaying planting allows more time for the herbicide to dissipate.
4. If triazine (Treflan – Group 3) or chlorimuron (Classic – Group 27) herbicides are suspected, check your soil pH and adjust accordingly.
5. If imazethapyr (Pursuit – Group 2) is suspected, check for low soil pH (<5.5).  Liming would benefit crop growth and minimize carryover of this herbicide.

Conclusion

Soil bioassays are another tool, they are not 100 percent accurate in predicting herbicide-residue problems.  However, a bioassay will allow you to make better decisions about crop rotation, herbicide selection, planting date and other cultural practices.

Table 1.  Bioassay species for residual herbicides and the expected injury symptoms

Reference

Neal, J. (2021, August 4). Conducting a Bioassay for Herbicide Residues. NC State Extension Publications. https://content.ces.ncsu.edu/conducting-a-bioassay-for-herbicide-residues