Herbicide carryover injury is not a large concern in most years, but it does have the potential to impact a field’s next crop following certain environmental conditions.
Potential risk is variable from field to field and even within each field.
Nutrient deficiencies can sometimes be mistaken for herbicide carryover.
What is Herbicide Carryover?
Herbicide carryover occurs when an active ingredient or metabolites of a herbicide applied the previous crop year remain in the soil at high enough concentrations to cause damage to sensitive crops the following season. Under typical conditions, herbicide rotational planting intervals defined on pesticide labels provide a reasonable amount of time to reduce potential injury, if followed accordingly.
Figure 1. Herbicide carryover matching spray boom width from prior year where boom was chargedHerbicide labels often specify certain geographies, rates, application timing and soil type restrictions. Recommendations can change if specific restrictions are present at time of application. Under extreme conditions, some herbicides may be at risk of showing some crop response (Figure 1). Several factors can contribute to the risk of herbicide carryover:
Herbicide rate
Persistence of the herbicide
Soil characteristics
Annual precipitation and temperature
Interval time between herbicide application and planting the next crop
Susceptibility of the crop to the herbicide
Early-season crop growth rate1
Fate of Herbicides in Soil
Understanding what happens to a herbicide after reaching the soil is important in determining if it will persist and potentially carry over into the next crop. 1 of 3 things typically happen after herbicides are applied:
Herbicides are removed from the soil.
Herbicides can be carried away with surface water or leached through the soil profile. A portion of some herbicides may volatilize from the soil surface if not incorporated. Lastly, a portion of herbicides applied will be taken up and metabolized by plant leaves and roots.
Herbicides adsorb to the soil.
Herbicides can also bind to the surface of soil colloids (small particles) and organic matter.
Herbicides degrade.
Soil microbes, sunlight and chemical reactions will often begin to break herbicides down into an inactive form within the soil.
Types of Herbicide Degradation
Microbial degradation
Soil organisms known as microbes are largely responsible for naturally degrading herbicides in the soil over time. Specific forms of fungi, bacteria and algae commonly use herbicides as a source of food. Environmental conditions that promote microbial development are less likely to experience herbicide carryover. Higher microbial activity often observed in high organic matter soils also helps promote faster breakdown of herbicides. Microbial activity tends to decrease with extended periods of dry conditions, making droughty soils more prone to herbicide carryover. Microbe activity also tends to decrease with colder soil conditions which can be a factor in herbicide degradation in cooler environments.
Chemical degradation
A process in which some very specific herbicides react with soil water in a process called hydrolysis. This form of degradation needs water present to occur. Dry soil conditions reduce degradation activity in general.2 Chemical degradation can decrease in high pH and cooler soils.
Photodecomposition
This can occur with a very limited number of herbicides, such as Treflan™, when not incorporated into the soil. Sunlight will break down the molecules of these specific herbicides and make them inactive if left exposed on the soil surface for periods of time.
Considerations for Carryover Risk
Several factors act together to influence carryover risk. Potential risk can even vary within fields.
Herbicide characteristics influence persistence:
Herbicide interactions in the soil are complex. Characteristics of herbicide active ingredients along with how they interact with the soil and environment determine how much is left at the time of the next crop planting.
The chemical structure of the herbicide impacts water solubility, vulnerability to degradation (microbial and chemical), soil binding and vapor pressure.
Application factors:
Timing: Applying herbicides late in the season decreases the amount of time available for herbicide degradation.3 Rotational restrictions may not be met if applications are made late in the season.
Rate: Higher rates than what is on the label for soil characteristics or the geographical area may lead to injury of rotational crops the following year.
Application uniformity: Sprayer overlap can lead to areas receiving 2 times the herbicide rate, common on turn point rows or at the end of a sprayer pass. Sprayer malfunctions can also lead to application rates exceeding safe levels.
Weather conditions:
Temperature impacts chemical processes and microbial activity. Warm temperatures favor herbicide degradation by both mechanisms.
Rainfall and moist soil conditions favor microbial activity, which increases degradation. Extended dry periods following application greatly reduce degradation, increasing persistence and risk of carryover.
Soil characteristics:
Soil pH plays a role in the persistence of herbicides in the soil since it can influence microbial activity, chemical degradation and herbicide solubility.
Soil texture has an influence on herbicide persistence based on binding potential of the soil colloids. For example, clay and high organic matter soils bind herbicide molecules tightly to the surface, reducing the availability of the molecules to degradation.
Crop rotation:
Reducing Risk of Herbicide Carryover
When planning for the next crop season, there are several things that can be done to reduce potential crop injury.
Review records and labels: Understand which herbicides were applied the previous year and check each product’s labels for any restrictions and rotational intervals to other crops. Labels may list specific conditions, such as seasonal precipitation totals, which may limit some crops from being planted.
Avoid early planting: Herbicides taken up by plants in periods of cold stress can often cause higher levels of injury due to a reduced ability to metabolize herbicides. Delaying planting until warmer conditions promotes rapid growth, helping better tolerate herbicide applications.
Switch crops: Rotating to a less sensitive crop may need to be considered based on risk of carryover.
Consider tillage: Tillage can distribute herbicides evenly throughout the soil often helping dilute herbicide concentrations and helping encourage microbial activity.
Soil testing: Soil testing can be done to measure residue, but it is generally quite expensive and requires a very representative soil sample to have any true value.
Symptoms Confused with Herbicide Carryover
Figure 2. Leaf striping from sulfur deficiency that appears similar to herbicide carryover injuryHerbicide carryover injury is generally difficult to diagnose. Often, carryover injury appears in uniform patterns where herbicide application overlap occurred. Soil type, high or low spots and weather conditions after herbicide application are key factors in potential crop injury symptoms.
Symptoms such as abnormal leaf or growing point development and chlorosis or necrosis of leaves can result from causes other than herbicide carryover injury. Many symptoms, such as disease, nutrient imbalance, frost, excessive moisture, heat stress or drought, can mimic herbicide carryover symptoms (Figure 2). Symptoms observed in early vegetative stages can often disappear quickly with new growth and have little, if any, effect on overall yield potential. A detailed analysis of all cropping information, as well as careful examination of the above- and below-ground symptoms, should be considered when determining the cause of an unhealthy crop.
There is little that can be done to change the amount of herbicide present in the soil at planting, but several options may help reduce injury risk. When herbicides are applied properly under typical growing conditions, there will be little concern of herbicide carryover risk. However, use caution following herbicide applications when conditions are conducive for herbicide persistence, such as a long-term drought.
References
1 Hartzler, B. 2020. Herbicide carryover concerns for 2020. Iowa State University Extension and Outreach.
https://crops.extension.iastate.edu/blog/bob-hartzler/herbicide-carryover-concerns-2020
2 Ikley, J. and B. Johnson. 2018. Factors affecting herbicide carryover in 2018. Purdue University. Extension Entomology. Pest & Crop Newsletter.
https://extension.entm.purdue.edu/newsletters/pestandcrop/article/factors-affecting-herbicide-carryover-in-2018/
3 Sprague, C. 2012. Dry conditions will likely impact herbicide carryover to rotational crops. Michigan State University Extension.
https://www.canr.msu.edu/news/dry_conditions_will_likely_impact_herbicide_carryover_to_rotational_crops
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