Conduct Soil Bioassays to Determine Herbicide Carryover Potential
Many factors can influence herbicide degradation, namely precipitation and temperature. But, regardless of the environmental conditions, herbicides require time to break down in the soil. When drought, hail, or other situations change crop rotation plans, it can result in concerns about herbicide injury due to inadequate time for a potentially injurious herbicide to break down. Concerns about herbicide residues can also influence cover cropping decisions. A companion article in this eUpdate discusses cover crop responses to various herbicides.
A bioassay is one of the simplest and most affordable ways to determine the potential for herbicide residues to injure a subsequent crop. A bioassay is an easy at-home test to see how your seeds will grow in the field soil. There are three steps to conducting a bioassay.
- Collect soil. You need to collect both treated soil from the field you are concerned about and similar, nontreated soil. The soil sample should include the top 2 to 4 inches of the soil profile. You’ll want to collect enough soil to fill two to four pots for each field. This 2-4 inch sample depth will be representative of the area of the field where the seed will be planted in the soil profile.
- Plant your crop seeds in the soil you collected. Within a day or two of collecting soil, place soil in pots and plant the seeds. Depending on the pot size and seed quality, consider planting 25 to 50 seeds per pot. Another consideration is to plant in rows or another pattern that will make it easy to determine if seeds have not germinated.
- Monitor crop growth. Put the pots in a sunny place and water them as needed. Monitor seedling emergence over two to three weeks and evaluate seedlings for herbicide injury symptoms. A guide to herbicide injury symptoms is available in the KSRE Bookstore.
Sarah Ganske, Extension Weed Science Specialist
slancaster@ksu.edu
Jeanne Falk Jones, Northwest Area Agronomist
jfalkjones@ksu.edu