Factors Contributing to Carryover Potential
The potential for carryover injury to rotation crops is influenced by the amount of herbicide present in the soil, the susceptibility of the rotational crop, and conditions that occur after application.1 Factors influencing herbicide carryover potential are:
- Herbicide chemistry and chemical half-life
- Herbicide rate and application frequency
- Herbicide application date
- Soil characteristics (organic matter, pH, texture)
- Total amount and distribution of rainfall
- Rotational crop sensitivity to herbicide
- Environmental conditions
The rate of application of herbicides and the half-life (time it takes for 50% of the herbicide to break down) have the most impact on the persistence of phytotoxic residues in the soil. Herbicides vary in their potential persistence and carryover. Herbicide families with persistent active ingredients include triazines (atrazine), phenylureas (diuron), sulfonylureas (chlorimuron, cloransulam), imadazolinones (imazethapyr, imazaquin), dinitroanilines (trifluralin), and certain plant growth regulators belonging to the pyridine family (clopyralid).2 The later a herbicide with residual activity is applied in a growing season, the higher the potential risk of carryover. Herbicides with a similar site of action or herbicides that affect related metabolic systems may interact to cause crop injury, particularly under stress conditions (soil characteristics, temperature, moisture, or herbicide management practices).3
Degradation by soil microbes is the primary method of dissipation for many herbicides.2,4 Herbicide degradation by microbes is usually rapid when there is adequate soil moisture and warm temperatures. The majority of herbicide degradation resulting from microbial activity occurs during the summer and early fall after the herbicide is applied. Under extremely dry conditions the rate of herbicide degradation by soil microbes can be slow enough to allow herbicides to persist into the next season. Cold soil temperatures decrease microbial activity, and moisture during the winter may not increase microbial activity enough to enhance the rate of herbicide degradation. Microbes become more active as soils warm in the spring, but a short time until planting can limit the amount of herbicide degradation that occurs.
Chemical hydrolysis is the most common process for detoxifying herbicide residues. Soil pH can influence the degradation of some herbicides. Hydrolysis is more rapid under acidic soil conditions. The rate of hydrolysis is slowed during dry and cold soil conditions.4 Triazine (Group 5) and sulfonylurea (Group 2) herbicides are degraded by hydrolysis.
Soils with higher amounts of organic matter (OM) and clay have a higher potential for herbicide persistence or carryover. Certain herbicides are adsorbed to the surface of organic matter and clay, making them temporarily unavailable for plant uptake, downward movement in the soil, or microbial degradation. A lack of soil moisture causes herbicides to be adsorbed or bound more tightly to soil colloids. Potential injury to rotational crops can be caused by the release of herbicide residues if soil moisture is replenished the following spring.1
Several herbicides or pre-mixtures used in corn have longer crop rotational intervals, which can indicate the potential for longer persistence in the soil, particularly during dry conditions (Table 1). The HPPD inhibitor (Group 27) herbicides are labeled for pre- and post- emergence applications in corn and are part of several pre-mix herbicide products. Under normal conditions these herbicides have a low risk of carryover, but dry conditions during the previous season followed by a cold, wet spring has been shown to contribute to the carryover potential of these herbicides and injury to soybeans. These herbicides are degraded primarily by soil microbes. Low soil moisture, cool temperatures, and changes in soil pH can inhibit microbial degradation. HPPD carryover injury to soybean has occurred in spray overlap areas where elevated rates of application increased the risk for carryover.1,4,5
Table 1. Some Herbicide Active Ingredients (Group/Site of Action) with Carryover Potential.
|Moderate to Slight
|atrazine (5)||fomesafen (14)||trifluralin (3)|
|simazine (5)||clopyralid (4)||isoxaflutole (27)|
|chlorimuron (2)||cloransulam (2)||mesotrione (27)|
|imazaquin (2)||imazethapyr (2)||tembotrione (27)|
|pendimethalin (3)||topramezone (27)|
Potential carryover from fomesafen products is a concern in corn-cotton rotations. Problems may occur when microbial activity is reduced after fomesafen was used preplant and at layby in cotton.6
Soil residual herbicides are an important component of weed management, and carryover problems generally do not occur under normal conditions. Fall-seeded crops and cover crops may also have limited tolerance to some herbicide residues.7 Cropping plans may need to be changed in fields where carryover could occur. Good weed management planning and recordkeeping is necessary to help minimize potential carryover to rotational crops.
Crop rotational intervals can vary between herbicides within the same group, local environmental conditions, and use practices (Table 2). Product users should consult individual product labels for specific recommendations and precautions. Information on herbicide characteristics, crop tolerance, and performance under local conditions should be obtained from local experts.
Table 2. Crop rotational guidelines for some common herbicide active ingredients.*
|Site of Action
|Herbicide||Months (unless otherwise specified). NS – next season|
|ACCase Inhibitor (1)||clethodim||1||None||None|
|ALS Inhibitor (2)||chlorimuron||9||9||None|
|Microtubule Inhibitor (3)||pendimethalin||None||None||None|
|Auxin Inhibitor (4)||2,4-D||None||3||15 days|
|dicamba||None||21 days||15 days|
|clopyralid||None||18||10 to 18|
|PSII Inhibitor (5)||atrazine||None||NS||NS|
|PSII Inhibitor (6)||bentazon||None||None||None|
|PSII Inhibitor (7)||diuron||None||None||6 to 12|
|EPSP Inhibitor (9)||glyphosate||None||None||None|
|GS Inhibitor (10)||glufosinate||None||None||None|
|saflufenacil||None||3||1 to 2|
|Fatty Acid Inhibitor (15)||acetochlor||None||NS||NS|
|PSI Inhibitor (22)||paraquat||None||None||None|
|HPPD Inhibitor (27)||mesotrione||None||10||10|
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