By: Dave Johnson and Stephen Strachan

• Many areas experienced drought in 2012, and these dry conditions may lead to more potential for carryover of herbicides applied in 2012 to crops planted in 2013...
• Herbicide concentrations remaining in the soil depend on characteristics of the chemical, the site and the weather. Carryover injury depends on the herbicide concentration in the soil and the susceptibility of the intended rotational crop to that herbicide.
• The primary mechanism of degradation of most herbicides is by soil microbial processes, which are reduced by dry conditions.
• While growers cannot do much to change the concentration of herbicides present in the soil, several things can be done to reduce the risk of carryover injury.
• The first step in evaluating carryover potential is to examine spray records and product labels.
  • Compare time intervals between herbicide application and the projected planting date of the rotational crop to time intervals listed on the rotational crop portion of the product label.

Introduction

Because of widespread drought conditions experienced over the past year, many growers are concerned about the potential for carryover injury from herbicides applied in 2012 to the crops they will plant in 2013.

The potential for herbicide carryover injury is driven by 2 main factors:

1. Concentration of available herbicide remaining in the soil at the time of rotational crop planting
   • Depends on herbicide chemical properties, soil characteristics, and weather
2. Susceptibility of the rotational crop to the herbicide
   • Rotational crops differ in their susceptibility to herbicides, with some crops not injured by relatively high concentrations and other crops highly injured by low concentrations.
   • The stresses that the newly planted crop faces during establishment can also affect response. Emerging plants are more likely to show injury to residual levels of herbicide if other stresses such as compaction or cold, wet soils are also present.

Herbicide labels have requirements on how much time should elapse between herbicide application and planting of specific crops (rotational cropping restrictions). Some label requirements are also conditional, and may depend on the rate applied, the geographical region where applied, and the weather conditions experienced since application. Different herbicides have different characteristics and interact with soils and weather in different ways, so broad, sweeping recommendations are not possible.

Understanding how the chemical properties, soil characteristics, weather, and crop susceptibility interact is critical to evaluating the risk of carryover injury. If the risk appears high, the important question is: what can be done now to mitigate carryover injury?

Herbicide Degradation From Soil

Degradation is the transformation of active herbicide molecules to products that no longer have herbicidal activity. Degradation rate is often described by half-life, which is the time required for half of the herbicide molecules to degrade from the soil. Herbicides with longer half-lives tend to be more persistent and have higher potential for carryover.

The primary mode of degradation for many herbicides is by soil microbes, which can use herbicide molecules as an energy and/or nutrient (i.e., nitrogen) source. Nonmicrobial chemical degradation can also be important for some herbicide classes. This can occur in soil water (hydrolysis) or by direct exposure to sunlight on soil surfaces (photodecomposition).

Factors Affecting Herbicide Carryover

Characteristics of the Herbicide

The chemical structure of a herbicide affects its water solubility, vapor pressure, soil binding, and susceptibility to microbial and chemical degradation. These characteristics, and how they interact with soil and weather (described below), determine how much herbicide is left at the time of rotational crop planting the following season. For example, herbicides that are highly bound to soil particles are often less likely to be available for microbial degradation.

Soil Characteristics

Soil characteristics have a large influence on herbicide persistence. Soils that are higher in clay and organic matter tend to bind more herbicide molecules to their surfaces (adsorption). This may reduce their availability for microbial degradation. Soil pH also has an effect, since it can influence herbicide solubility and also microbial activity. Soil microbes (bacteria, fungi, etc.) tend to be most active near neutral soil pH.

Soil pH levels significantly lower or higher than about 6.5 to 7.0 may alter the relative populations of species of microbes growing in the soil and therefore reduce degradation, leading to higher persistence. Soil pH can also affect chemical degradation. Some herbicides, such as sulfonylureas, are more readily degraded by chemical processes at lower soil pH, and therefore may be less likely to cause carryover damage at pH levels below 7 or so. In contrast, imidazolinone herbicides, which are primarily microbially degraded, are more tightly bound to soil colloids in lower pH soils and are more likely to cause carryover injury at lower soil pH due to reduced susceptibility to microbial degradation.

Weather Conditions

Temperature and rainfall have a large effect on herbicide persistence and the potential for carryover injury. Weather patterns that favor microbial activity (warm, moist conditions) favor degradation and lessen carryover potential (Figure 1). Temperature can also influence chemical processes, with warmer conditions favoring degradation.

Figure 1. Illustration* of soil moisture effect on herbicide persistence – herbicides can persist much longer in dry vs. wet soils. Adapted from Colquhoun, 2006. *Does not pertain to any actual herbicide – check labels for rotational restrictions.

Susceptibility of the Rotational Crop
Crop species differ in susceptibility to different herbicides. That is why most herbicides are registered for some crops and not others. Therefore, choice of crop to plant following a specific herbicide application the previous year can greatly influence injury potential. For example, corn is highly tolerant to atrazine but soybeans are relatively susceptible. If atrazine carryover is likely in a field, it may be best to plant corn (or sorghum) that year to avoid potential problems.

All of the factors described above - herbicide characteristics, soil characteristics, weather conditions, and rotational crop planted - interact with each other to cause or avoid carryover injury. These factors also vary from field to field and area to area within individual fields, often leading to uneven carryover response across a field. Figure 2 and Figure 3 show how carryover injury can vary just within a few feet in a field.

Figure 2. Uneven response of corn to soil residues of imazaquin applied to soybeans the previous year.

Figure 3. Uneven response of soybeans to soil residues of atrazine applied to corn the previous year.

Fomesafen: Example of a Broadly Used Herbicide with Rotational Restrictions on Label 

The development of glyphosate-resistant weeds, especially amaranth species such as waterhemp and Palmer amaranth, has led to increased use of several older herbicide products. One active ingredient that has seen high use recently is fomesafen, the active ingredient in herbicides such as Reflex, Flexstar and Prefix. Fomesafen is in the PPO class, which includes herbicides such as flumioxazin (Valor and others), sulfentrazone (Authority and Spartan products), and saflufenacil (Sharpen and others). The average field half-life of fomesafen is reported as about 100 days, meaning it can be fairly persistent. Its primary mode of degradation is by soil microorganisms, so factors that reduce microbial activity, such as dry soils, may increase the half-life and therefore persistence and carryover potential.

Many growers are using fomesafen-containing products in soybeans to control pigweeds and other species, sometimes in combination with other herbicides in the PPO class. Fomesafen product labels specify a 10-month interval between application to soybeans and planting corn. This means that if fomesafen was applied to soybeans in late June 2012, the minimum time until corn planting is late April 2013. Planting corn prior to the 10-month interval increases the chance for carryover injury. The dry conditions experienced in 2012 may increase this potential even more. Symptoms of PPO herbicide carryover injury to corn include buggy whipping (Figure 4), leaf chlorosis and midvein breakage (Figure 5), and necrotic leaf tissue (Figure 6). Corn often rapidly outgrows this injury, but if the injury response remains for an extended time, yield potential may be compromised.

Figure 4. Buggy-whipping symptom from carryover of PPO herbicides to corn.

Figure 5. Leaf chlorosis and midvein breakage symptom from fomesafen carryover to corn.

Figure 6. Leaf necrosis symptom from fomesafen carryover to corn.

What About 2013?

Most areas experienced significant hot and dry weather during the growing season of 2012. The vast majority of herbicide degradation resulting from microbial activity occurs during the summer and early fall after the herbicide is applied. The microbes responsible for herbicide degradation are most active in warm (not hot) and moist soils. Soil conditions most conducive for excellent plant growth are the same conditions for maximum microbiological activity.

Microbial activity is less in hot and dry soils, thus increasing the risk for herbicide carryover potential for those herbicides that degrade mainly by microbial activity. Even where dry conditions have been relieved, some carryover potential may still remain, especially if moisture came primarily in the winter months. Cold soil temperatures decrease microbial activity and moisture during winter may not substantially increase microbial populations to enhance the rate of herbicide degradation. As soils warm this spring, microbes will become more active, but the relatively short time until planting will limit the amount of degradation that occurs.

At this point, there is not much growers can do to affect the amount of residual herbicide present in their fields. However, there are a few things that can be done to reduce the risk of crop injury:

• Review spray records for each field and review product labels to see what restrictions are indicated.

  Many labels specify the time required between herbicide application and planting of a rotational crop (see Table 1). Planting sooner than the specified time increases the risk of injury. In addition to a time interval, labels may also list conditions under which a particular crop may or may not be planted, so scrutinizing the "fine print" is key. After a drought year it is probably best to err on the conservative side regarding plant-back times.

• Ensure seedling stresses are minimized to give the young crop plants their best chance of surviving herbicide residues with little damage.

  This can include making sure soil pH and fertility levels are optimum for the crop, reducing compaction, and avoiding planting into cold, wet soils. Other stresses that the seedling experiences can exacerbate response to herbicide residues (and to the herbicide applied in the current year).

• Change planned crop.

  In some cases it may be best to plant the same crop as the previous year, or at least a crop for which last year’s herbicides are also labeled. This significant step has the most potential to reduce the risk of crop injury, and is worthy of consideration in high-risk fields.

• Delay planting.

  In 2012, herbicide degradation rates were probably slower than normal, so more than the normal amount of time may be required for sufficient degradation. With spring moisture and warming soil temperatures, the microbes will start to act again to degrade herbicide residues. However, it is unlikely that this will have a significant impact in early spring, and the yield potential gained with earlier planting could be lost to herbicide injury. Growers could plan to plant suspect fields last to give more time for degradation to occur. Seedlings in later-planted fields often experience lower stresses and faster development than those in early planted fields, which could help the crop outgrow putative carryover injury more quickly.

• Tillage

  The jury is out on whether tillage impacts carryover potential. Tillage may dilute the herbicide in the soil profile and provide aeration and faster soil warming to stimulate microbes, but results are mixed on whether this will provide a significant benefit. Growers in long-term no-till who try to reduce carryover potential by tilling will sacrifice many of the soil quality benefits accrued from no-till over the years, possibly without a major impact on crop response this year.

• Conduct a bioassay or chemical analysis.

  Some growers plan to sample fields and plant their intended crop in greenhouse pots to see if any symptoms appear (Figure 7). However, to be valuable this must be done with care, and interpretation of the results can be difficult or misleading. Laboratory analyses, while fairly accurate, are costly and only tell you the concentration of herbicide present. As discussed previously, carryover injury is impacted by many factors besides just how much herbicide is present. Differences in the inherent susceptibility of different crops to each herbicide affect how concentration results should be used.

Figure 7. Bioassay showing response of alfalfa (left and middle) to fomesafen applied to soybeans the previous season. The pot on the right shows alfalfa growth in soil from an untreated part of the field.

Conclusion

In 2013 we could see higher than normal carryover response to herbicides applied in 2012. Although there is not much a grower can do to change the amount of herbicide present at planting, several options are available to reduce risk, including:

• understanding which herbicides were applied the previous year and what the label requires for rotational crop planting intervals
• working to reduce the other stresses the seedling crop faces as it germinates, emerges and grows
• rethinking the intended crop to plant, and
• in some cases, delaying planting to extend the time for herbicide degradation and reduce cold stress that exacerbates crop injury from carryover.

Table 1.Carryover risk to corn, soybeans, cotton, and sugar beets for several commonly used herbicides. Risk may be higher in drought conditions.

¹ See product labels for details.
² Label states planting interval depends on amount of rainfall received after application and/or soil organic matter content.
³ Label requirements differ for regions.
⁴ Label prohibits planting the year following use.
⁵ Low at pH < 7-7.5, moderate at pH > 7-7.5. See label for details.
⁶ Varies with region, use rate, and soil characteristics. See label for details.
⁷ Depends on use rate.
⁸ Label requires 12 month planting interval.
⁹ Label restrictions in place if mesotrione applied twice to corn the previous year.