Step-By-Step Guide for Conducting a Bioassay to Assess Herbicide Carryover Risk

I recently attended wheat variety plot tours near Rearden and Almira to speak about good stewardship practices for CoAXium wheat. Growers at both locations asked me about how long they needed to wait to plant a non-Clearfield wheat variety after several years of using Beyond® herbicide for the control of jointed goatgrass, downy brome, or feral rye. The simple answer can be found on the Beyond label.

In Washington and selected counties in Idaho and Oregon (as of 07/2023):
>16 inches of rainfall + irrigation and soil pH >6.2 = a minimum rotational interval of 9 months
<16 inches of rainfall + irrigation or soil pH <6.2 = a minimum rotational interval of 28 months

Unfortunately, the latter situation is the most common for most of Eastern Washington. However, a rotational interval of 28 months may not be sufficient in drier regions of the state where Beyond has been used for multiple years, resulting in elevated residual levels compared to where Beyond was only used once or maybe twice in the last ten years.

Growers who want to move away from using Beyond in Clearfield wheat after multiple years of using the technology may want to consider testing for soil residues before planting a non-Clearfield wheat variety. One option to test for herbicide residues is to send a soil sample to a lab for analysis. However, lab analysis can be costly and time consuming, and predicting the amount of injury caused by a given level of herbicide residue is difficult. A better option is to conduct a bioassay.

In a bioassay, representative soil samples are collected from several locations in the field. The bioassay is only as good as the soil sampling procedure.

To conduct a bioassay yourself:

1. Collect samples from high spots, low spots, and different soils. If you have areas in the field where you suspect soil pH may be low, sample these areas separately.
   
   Soil cores should be taken from the top 2 to 3 inches of soil, unless the field has been cultivated to a certain depth, in which case samples should be taken to the depth of cultivation. Sandy soils should be sampled to a depth of 4 to 6 inches or the to the depth of cultivation. Remove crop residues if present. You will need enough soil to fill several 3- to 4-inch pots. Label the pots so you know where the soil in each came from.

2. Take several samples from an area and combine them.
3. Also collect a soil sample from an area known to be free of herbicide residues. This herbicide-free soil will be used for comparison (control).
4. Fill several 3- to 4-inch pots for each sample area and the control area (area free of herbicide residues). Label the pots so you know where the soil in each came from.
5. Plant 3 to 6 non-Clearfield wheat seeds per pot. After plant emergence, thin back to 1 or 2 plants per pot. All pots should have the same number of plants.
6. Place the pots in a greenhouse or a sunny spot in the house. Keep the pots watered but not waterlogged. Consider adding a small amount of fertilizer to ensure active plant growth.
7. Watch the plants for about three weeks after germination. Compare the growth from the herbicide-free pots with the other pots to determine if you see any potential injury.

You may also want to consider planting a pot or two from each location with a test species that is particularly sensitive to the herbicide you are concerned about. For imazamox and other imidazolinone and sulfonylurea herbicides, tomato, cucumber, and spinach make good test species.

If you have exceeded the rotational interval listed on the Beyond herbicide label and your bioassay indicates that herbicide residues are insufficient to cause injury to a non-Clearfield wheat variety, you can feel comfortable planting a non-Clearfield wheat variety in those fields.