Herbicide concerns after droughtHerbicides may leave residues in the soil after a drought, and it is important for growers to be aware of this and what they can do to prevent carryover to next year’s crop. Herbicides break down in the soil primarily through microbial activity and, for atrazine and some residual SU herbicides, through chemical processes. Microbial activity and herbicide degradation is greatest when soils are moist during the growing season.
By: Christina Rittenbach: NDSU Extension Service, The Jamestown Sun
Herbicides may leave residues in the soil after a drought, and it is important for growers to be aware of this and what they can do to prevent carryover to next year’s crop. Herbicides break down in the soil primarily through microbial activity and, for atrazine and some residual SU herbicides, through chemical processes. Microbial activity and herbicide degradation is greatest when soils are moist during the growing season. Herbicide breakdown may be slowed greatly in drought conditions. If herbicide residues are significant, they may injure rotational crops in the following season. For this reason, growers need to be aware of herbicide residues and take steps to decrease risk of injury.
Check the label of herbicides used during the drought season. It will tell you the normal interval between application and planting for a specific rotational crop. Footnotes frequently show if the risk of carryover is greater under certain conditions (such as soil pH or dry soils).
Use tillage. Tillage will dilute the herbicide, especially if it is concentrated near the surface or in bands over the row.
Look for herbicide tolerance. Select crop varieties or hybrids with greater tolerance to the herbicide used during the drought year. This information is not available for all varieties. Ask your seed supplier for assistance.
Use good management practices. Good seedbeds, proper seeding depth and rate, adequate soil fertility, and insect and disease protection will minimize the effect of herbicide carryover. Many crops can tolerate a single stress relatively well, but two or more stresses can result in significant loss of crop vigor and yield.
Testing for Carryover
If you choose to test for herbicide carryover, the best time to do so is between late October and mid-November. By this time, soil temperatures reach and remain below 50 degrees Fahrenheit, a point at which herbicide breakdown is minimal. Do not take soil samples for residues before this time; they may indicate levels greater than actually present when you plant next year.
A bioassay test may be helpful if doubts remain about planting because of possible herbicide residues. The test will alert you to residue problems by comparing the productivity of your intended crop variety in both affected and unaffected soils. Begin the test at least three weeks prior to planting so that sufficient plant growth is available to assess carryover potential. The herbicide label may also contain suggestions on running a bioassay test, as well as information on crop rotations and carryover potential.
A chemical test for herbicide residues can also be done by private laboratories. These tests are expensive and the results may not be easy to interpret. However, they may be appropriate in cases where bioassays cannot be done or where high value crops are concerned. Page 111 in the “North Dakota Weed Control Guide” contains a list of laboratories that can test for chemical residues, susceptibility of crops to chemical residues, general guidelines for safe levels of chemical residues from laboratory analysis and publications/resources showing herbicide injury symptoms.
Adapted from “Herbicide Concerns after a Drought” by Richard Zollinger, NDSU Weed Specialist.
For more information, contact the Stutsman County Extension Office at 252-9030.