Before you plant, think about herbicide residues

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. Bioassays can be completed by a lab (see Table 4-4. Herbicide Crop Rotation and Soil pH Restrictions - OMAF and MRA Publication 75, Guide to Weed Control) or by anyone, including yourself or your consultant.

There are two types of bioassays, field bioassays and indoor bioassays. In order to conduct a proper 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.

In order to ensure proper planting dates, you may want to conduct an indoor bioassay at an earlier date. In order to conduct a proper indoor bioassay:

  1. Collect soil samples - prior to planting collect representative soil samples, 6 inch (15 cm) depth from the suspect field. In a 15 to 20 acre field take 15 to 20 soil cores and combine them to make a composite sample. For a more precise bioassay, divide each core into 2 separate samples from 0 to 3 inches and 3 to 6 inches. Also, take separate samples from areas where excessive residues are suspected, such as sprayer turn around points and/or field edges.
  2. Check sample - sample an area that is not suspect of residues for a check. This soil may be taken from any nearby untreated area.
  3. Mixing samples - air dry the soil if required. If the soil is cloddy, crush the clods (less than the size of a pea). If the soil contains a high amount of clay, the addition of course sand (50 percent by volume) improves its physical condition. If sand is added mix it thoroughly with the soil.
  4. Fill containers - fill containers (tin cans or milk cartons) with soil from the check, 0 to 3 inches and 0 to 6 inches. Fill at least 2 containers from each soil sample. Make sure you punch a hole in the bottom of the containers to allow water drainage.
  5. Plant sensitive crop - plant the sensitive crop seed in all containers and allow the test plants to grow and develop symptoms of injury from any herbicide residues.

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 lab test or 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) or chlorimuron (Classic) herbicides are suspected, check your soil pH and adjust accordingly.
  5. If imazethapyr (Pursuit) is suspected, check for low soil pH (<5.5). Liming would benefit crop growth and minimize carryover of this herbicide.


Soil bioassays or lab tests 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.

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