For weed control, herbicides play an important role in protecting most agronomic crops. To prevent potential injury to crops, like soybeans, understanding how long herbicides persist, or how long they remain active in the soil, is important. Here’s a look at some of the factors that influence potential herbicide carryover:
The chemical properties of herbicides differentiate them from one another and affect how long a herbicide will persist in the soil. To determine how long a specific herbicide will take to break down in the soil, the most important factor is it’s half-life. Half-life is the amount of time it takes for 50% of the specific herbicide to break down in the soil, whether through microbial or chemical degradation. The longer a herbicide takes to break down, the greater the threat posed to rotational crops. Most herbicides that tend to persist in the soil have re-cropping intervals of 9 to 10 months or longer. Some herbicide families with persistent active ingredients include triazines (atrazine), phenylureas (diuron), sulfonylureas (chlorimuron), imadazolinones (imazaquin), dinitroanilines (trifluralin), isoxazolidinones (clomazone), and diphenyl ethers (fomesafen).
The rate of herbicide application and the seasonal timing both affect herbicide carryover. Misapplication due to applying more than the labeled rate or overlapping spray patterns can increase the potential for carryover. Additionally, the later in the season an herbicide with residual activity is applied, the greater the risk for carryover. The effect of herbicides can also be cumulative in the soil. When using herbicides that target related plant metabolic systems, they may build up in the soil and result in injury to subsequent crops.
Weather is the major factor in herbicide carryover. For soil microbes to break down herbicide residues, it takes adequate soil moisture and warm temperatures. When it’s extremely dry, the rate of herbicide degradation can be slow enough that herbicides can persist into the next season. Cold weather also has a negative effect on microbial activity. This, in addition to winter moisture, may not increase microbial activity enough to increase the rate of herbicide degradation. The worst-case scenario for herbicide carryover problems is late spring or summer herbicide applications followed by dry weather conditions and cool temperatures that extend into early spring.
A soil’s properties also play a role in herbicide persistence. Soils that have higher rates of organic matter (OM) and/or clay, have a greater potential for herbicide carryover. Additionally, finer textured soils with a high percentage of OM and clay slow plant herbicide uptake, the downward movement of herbicides in the soil, and microbial degradation of herbicide residue. This can result in potential injury to rotational crops with the release of herbicide residues in the spring.
Soil pH also affects herbicide persistence and availability, For instance, high soil pH can cause greater carryover in herbicides like triazine and sulfonylurea but shorter persistence in imidazolinone herbicides. However, it’s worth noting that most herbicides are unaffected by soil pH.
The microbes in soil are responsible for most herbicide degradation. So, good soil conditions for microbial growth are important. These conditions include: moisture, temperature, pH, oxygen and minerals. The best conditions for herbicide breakdown are warm temperatures, adequate moisture, good aeration, fertility and a medium pH.
Several herbicides or pre-mixtures used in corn have longer crop rotational intervals. This can be an indicator for longer soil persistence — especially in dry conditions (Table 1). The HPPD inhibitor herbicides are labeled for pre- and post-emergence applications in corn and are part of several pre-mix herbicide products. When conditions are normal, these herbicides have a relatively low risk of carryover. However, dry conditions during the previous season that were then followed by a cold, wet spring have been shown to exacerbate carryover potential of these herbicides and injury to soybeans. Timely degradation of these herbicides relies on soil microbes, but low soil moisture, cool temperatures and changes in soil pH can inhibit microbial degradation. HPPD carryover injury to soybeans has occurred in spray overlap areas where elevated application rates increased the risk for carryover. For more information, please see the Agronomic Spotlight, Soybean Herbicide Injury.
Table 1. Common herbicide active ingredients (group/site of action) with carryover potential.
Soil residual herbicides are an important component of weed management. Under normal conditions, carryover problems generally do not occur. Fall-seeded crops and cover crops may also have limited tolerance to some herbicide residues. In fields where carryover could occur, cropping plans may need to be changed. To minimize the potential carryover to rotational crops, strong weed management planning and record keeping is a must.
Crop rotational intervals can differ between herbicides within the same group, local environmental conditions and use practices (Table 2). Applicators should always consult individual product labels for specific recommendations and precautions prior to application. 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.
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