Herbicides are widely used for weed control in most agronomic crops in Kentucky. A wide array of products are available which vary greatly in their activity and persistence in soil. Carryover or the persistence of some herbicides from one cropping system to another can concern farmers who practice crop rotations.
In Kentucky rotational crop injury caused by herbicide carryover is seldom a problem; however, the potential exists for it to occur. The question is often asked: How can I minimize the risk of injury to subsequent crops? Understanding herbicide persistence and what factors influence persistence helps answer this important question.
Injury due to herbicide carryover is largely a function of:
(1)the chemistry and amount of the herbicide applied,
(2)the sensitivity of the rotational crop,
(3)environmental factors.

For a detailed discussion of specific herbicides as they relate to these factors consult Cooperative Extension publication, Herbicides with Potential to Carry Over and Injure Rotational Crops in Kentucky, (AGR-140).

Persistent Herbicides
Herbicides with a long soil life have the greatest potential to persist and injure rotational crops. Products containing atrazine (AATREX, ATRAZINE), simazine (PRINCEP), chlorimuron ethyl (CANOPY, CLASSIC), imazaquin (SCEPTER), imazethapyr (PURSUIT) and clomazone {COMMAND) are examples of such herbicides. Mis-application by overlapping spray patterns or applying more than the labeled rate of a persistent herbicide increases the chance that a carryover problem will develop.
A herbicide's concentration in soil is measured in parts per million (ppm) or parts per billion (ppb). An acre of soil 6 inches deep has roughly 2 million lb of soil. Therefore, when soil samples are taken at a 0 to 3 inch depth, a detectable concentration of 0.02 ppm or 20 ppb is approximately equal to 0.02 lb/A of the herbicide (active ingredient). If sampled at a 0 to 6 inch depth, 0.02 ppm equals about 0.04 lb/A of the herbicide.

Sensitivity of Rotational Crops
The amount of herbicide that will cause injury is influenced by the rotational crop's sensitivity. For example, soybeans are less sensitive to atrazine than tobacco, so soybeans are less likely to be injured as a rotational crop.
Extremely low amounts of some herbicides can injure certain crops. Imazaquin at a concentration of 0.02 ppm (20 ppb) can cause significant injury to corn. Whereas, atrazine at concentration of 0.05 ppm (50 ppb) or more is required to injure tobacco.
Because environment affects the availability of herbicides to plants, it is difficult to determine exactly how much herbicide a crop can tolerate. There tends to be a broad range in concentration where crop injury may or may not occur. This range for tobacco's sensitivity to atrazine occur,, between 0.05 and 0.20 ppm (50 to 200 ppb).
Environmental Factors
Prolonged periods of low moisture, cool temperatures and extremes in soil pH are examples of soil conditions that may increase herbicide persistence. Other factors (i.e. clay and organic matter content) can also influence the availability and persistence of some herbicides.
The impact that any one of these conditions has on persistence depends on the specific herbicide. For example, high soil pH increases the persistence of atrazine or chlorimuron ethyl.
Figure 1 shows how environment influences the rate of herbicide dissipation. Under ideal conditions dissipation may be rapid enough to decrease the herbicide concentration in the soil to a level safe for the rotational crop. Adverse conditions tend to slow dissipation and may cause too much herbicide to remain in the soil when the following crop is planted.

Avoiding Herbicide Carryover Problems
A fundamental approach for avoiding carryover problems is not to use persistent herbicides in situations where carryover problems are likely to develop. Examples where precautions may be needed are (a) in soils with pH above 6.8; (b) when rotating with sensitive crops; (c) when production practices result in short recrop intervals; or (d} in fields prone to flooding or other problems, which often requires replanting to a different crop.
In many instances persistent herbicides can be used in Kentucky without a serious risk of a carryover problem. Consider the following suggestions as a means of preventing carryover problems:
(1)Read the herbicide label. Be aware of label restrictions concerning soil pH and recrop intervals for the crops in your rotation system.
(2)Use the recommended rate for your soil type. Avoid using more than is needed to provide adequate weed control.
(3)Apply herbicides accurately. Be sure all nozzles are calibrated and operating properly. Turn the sprayer off at the end of the field to avoid overlaps at end rows. Overlapping of point rows, and around electrical poles or similar structures can cause carryover.
(4)Incorporate uniformly. Depending on the herbicide and the tillage equipment, a 2-pass incorporation may be needed to get the herbicide evenly distributed in the soil. "Hot spots" can result when herbicides are applied to a rough surface with just one pass with a disc.
(5)Treat early in the season. Atrazine used during early spring is less likely to cause a carryover problem than if a late season post-emergence treatment is applied in June.
(6)Maintain soil pH for optimum crop production. The persistence of some herbicides is not likely to increase if soil pH is maintained between pH 6.0 and 6.8. Adding lime to soils already having a high pH may increase the potential for carryover of chlorimuron ethyl (CANOPY, CLASSIC) or triazine (ATRAZINE, PRINCEP) herbicides. Lowering soil pH to reduce the risk of carryover may be impractical.

Unexpected changes in the environment, like drought, increase the likelihood of a carryover problem in Kentucky. The following suggestions may help manage fields suspected of having carryover problems:
(1)Determine if a potential carryover problem exists. Collect a soil sample and conduct a chemical analysis or plant bioassay. Results can be highly variable depending on the time and methods used for collecting soil, and on the procedures used in analysis. Also consider other factors, such as amount of herbicide applied, time of treatment and method of application.
(2)Evaluate cropping options:
a.Plant the same crop instead of rotating. Doing so avoids the herbicide carryover problem. However, continuously growing the crop in the same field could cause a 5 to 10% reduction in yield and increased problems with diseases, insects, weeds, nematodes, etc.
b.Determine the recrop interval for rotational crops by following the directions on the herbicide label. Guidelines for replanting and rotating crops after corn and soybean herbicides are summarized in Cooperative Extension publication, Chemical Control of Weeds in Kentucky Farm Crops, (AGR-6). Always consult herbicide label(s) for specific instructions. Under extreme drought conditions herbicides may persist longer, so consider using a longer time interval between crops than indicated on the label.
c.Use suspect fields for set-aside acres. Depending on the herbicide, establishing a cover crop may be difficult due to herbicide carryover.
(3)Delay planting suspect fields. Doing so allows the herbicide more time to dissipate.
(4)Plant under ideal conditions. A vigorously growing crop has a greater chance to overcome adverse effects caused by herbicide carryover. Cold soil temperatures, excess moisture and compaction are conditions that may increase rotational crop injury from persistent herbicides.
(5)Do not follow a persistent herbicide with other herbicides that may cause the crop more stress. Using metribuzin (SENCOR, LEXONE) in a field previously treated with a high rate of atrazine or simazine may lead to triazine injury to soybeans.
(6)Moldboard plow and disc before planting sensitive crops. Doing so helps dilute the herbicide. Moldboard plowing to invert the soil may be more effective in diluting the herbicide than is chisel plowing. However, tillage may not be feasible in fields where soil erosion is a potential problem.

The following Cooperative Extension publications give more information on specific herbicides with carryover potential and on recrop intervals:
Herbicides with Potential to Carry Over and Injure Rotational Crops in Kentucky, (AGR-140).
Chemical Control of Weeds in Kentucky Farm Crops, (AGR-6) (revised each year).

About the Authors
J.R. Martin has worked in Kentucky since 1980 as an Extension Weed Control Specialist and held a similar position for a year at Virginia Tech. Jim is an advisor to the board of the Kentucky Fertilizer and Agricultural Chemicals Association and to the board of the Kentucky Vegetative Management Association. His primary emphasis is weed control in agronomic crops.
J.D. Green has been an Extension Weed Control Specialist at the University of Kentucky since 1986. A native Kentuckian, he spent 3 years in Oklahoma where he received his Ph.D. degree in Weed Science at Oklahoma State University. He works with weed control recommendations for various crop production systems.