Many farmers suffered from the drought last year, and now they will continue to with the potential for herbicide carryover in the new planting season. Dry soil mixed with low moisture accumulation created an ideal setting for chemicals to remain in the soil and not breakdown in the off season. The biggest concern with herbicide carryover is the potential for lingering chemicals from last year to cause crop injury to this year’s plants.

The factors that contribute to herbicide carryover include:

Soil Characteristics

The pH level, type of soil and organic matter relate to the soil’s level of potential for herbicide carryover. According to Aaron Hager, in his article Be Aware of the Potential for Herbicide Carryover , soil pH determines the level of herbicide for plant use and for how quickly the herbicide is broken down in the soil. This process is slowed down with a higher pH level.

Soil types can be categorized by the varying amounts of sand, silt and clay that the soil contains. Clay soil, which is noted for its low drainage capabilities, is more susceptible to herbicide carryover than coarse-textured soil.

The amount of organic matter in the soil will also play into the potential for herbicide carryover. Higher levels of organic matter in the soil will slow down the chemical degradation process. Organic matter is more likely to hold higher levels of herbicide in the soil, making it unusable for plants and harder for microorganisms to break the herbicides down.

Precipitation

Both the amount of precipitation and the distribution throughout the year are contributing factors to herbicide carryover. Lower levels of precipitation, such as this past year, decrease soil moisture and limit the soil’s ability to dilute the herbicides. There is less concern for herbicide carryover in areas that received rain in the fall.

Chemicals applied

Depending on what type of chemicals you applied to your fields last year, you may be more at risk for herbicide carryover. The half-life of the chemical used paired with the rate of the application will help you evaluate what your risk for carryover is.

Check out Iowa State University’s list of herbicides that are ranked from slight risk to high risk for herbicide carryover this spring.

Soil Microorganisms

Herbicides are broken down in the soil by microorganisms, and these microorganisms determine how quickly the chemicals are degraded.  Hager also discussed how a microorganism’s species and abundance greatly influences the rate at which herbicide degradation occurs. In soils with adequate moisture, this process occurs much more rapidly than in extremely dry conditions, which explains why herbicide carryover is such a concern following a drought year.

2013 Plan of action

Planting season will be upon us before we know it making it essential to plan ahead with your herbicide management. It is important to take proactive measures to protect this year’s crops from last year’s chemicals.

Collecting a soil sample or conducting a bioassay can help you confirm if there is herbicide carryover. Here is a video from the University of Arkansas on how to properly perform a soil sample.

For more information on proper procedure for a bioassay, check out this article by the University of Nebraska . If you are going to use one of these options for analyzing your soil, you should plan to do so early spring before planting.

If you are concerned about herbicide carryover in your fields, take last year’s application dates and application rates into consideration when planning out planting dates for this year.

According to Clarke McGrath, Iowa State University extension field agronomist, in a Farm World article , “The best management practices from start to finish are good residue and soil management; optimal seedbed conditions; proper seeding depth; optimum plant populations and spacing; sound soil fertility programs; and integrated pest management can minimize the effects of herbicide carryover.”