Top 10 Factors Affecting Herbicide Carryover/Residue | BotanyLive

There are many factors affecting herbicide carryover or residues in soil and plants like residue itself, some biotic (microbes, plants), abiotic (soil, moisture, light), and chemical (degradation process, half-life) factors. Herbicides destroy plants (herbs) and more specifically off-type plants or weeds. After destroying weed plants or limiting their growth, herbicide residues may persist in the soil and plant for days, weeks, months, or even years. The amount of herbicide that persists in soil or plant after its job is over is called “residue”.

According to WHO, “any impurity, metabolite, byproduct or toxic material in food or fodder that has adverse effects on human and animal health are called residues or carryovers”.  There is a need to read the measures before handling a herbicide pack.

Here is the list of top 10 factors affecting herbicide carryover.

Factors affecting herbicide residues

Herbicides have specific formulations dependent upon their physical and chemical nature. They differ in their water solubility, volatility, and susceptibility to degradation by biotic (weeds, microbes) and physical (water, sunlight (UV), soil) agents.

Some herbicides like 2,4-D and trifluralin are more volatile.

Some chemicals follow a simple chemical process and dilute to the intensity they pose no residual threat. However, a few follow a complex degradation process (microbial degradation).

1.   The half-life of herbicide

The persistence of residues is also described in terms of their half-life. Half-life is actually the time required to degrade 50% of the active ingredient (a.i.) of the herbicide. It is the measure of how long it takes for 50% of a chemical to degrade. E.g. Half-life of pyrazosulfuron in various soil ranges from 42.9 to 85.5 days [1].

2.   Microbial Decomposition

Micro-organisms e.g. bacteria and fungi can readily degrade residues of herbicides in soil. This degradation is favored under the conditions those favor growth of these microbes.

3.   Degradation by Chemicals

Some herbicides react with chemicals present in soil and even with water. This reaction may change or reduce their chemistry and activity to work as lethal residue. For example hydrolysis of sulfonylurea breakdown it chemically and makes it biologically inactive.

4. Degradation by light

Ultraviolet light coming from the sun is among the potential factors affecting herbicides. Trifluralin, cyclohexanones, and ethalfluralin can easily degrade when exposed to sunlight especially UV light (photo-degradation).

5.   Plant uptake

Plants uptake some herbicides effectively and thus reduce their carryover ratio.

3.   Soil Factors affecting herbicide carryover in soil and plant

1. Soil pH determines the stability of herbicide residues in the soil. Organic acids released by the decomposition of organic compounds reduce the pH of the soil and also help in the degradation of residues of herbicides [2]. Organic matter also enhances the activity of micro-organisms and thus is among leading factors affecting herbicide degradation in soil.
2. Soil texture favors other soil features like aeration, water holding capacity, compaction, etc. These features also help in the degradation of residues of herbicides effectively. Soil texture is mong factors affecting herbicide residue degradation.
3. Soil moisture also helps in the leaching and volatilization of chemicals. An ample amount of water applied particularly in sandy soil leads to the dissolution of residues in water and then leaching along with [3]. In dry soil, water moves through the soil pore upward carrying residue contents. In dry conditions, the herbicide is bound to soil particles more tightly and is not available for breakdown.
4. Soil organic matter content also provides a binding site for herbicide

4.   Management factors affecting herbicide residues

1. A higher application rate or exceeding the dose prescribed by the manufacturer may be one of the biggest reasons for the buildup of residues in the soil.
2. Application time also addresses the management of herbicide residues. A long period between herbicide application and the time of maturity of the crop will reduce the chances of herbicide carryover. Less injury is reported in such cases. Fall application generally involves fewer risks of crop damage than do the spring application. For example, 2,4-D application in spring results in greater damage to canola, lentils, and peas than in fall application
3. Non-uniform application and incorporation usually result in hot spots in the field carrying more herbicide contents. Double dose of spray while moving in the field with a sprayer or not switching sprayers at the proper time results in such situations. These hot spots and patches can be observed on corners, headlands, or strips.
4. Zero or minimum tillage reduces the mixing of herbicide to the greater soil proportion. The topsoil layer (up to 2 inches) in case of banding will be containing a higher content of chemicals. Conventional tillage accelerates microbial degradation and diluting of chemical residues.
5. Soil fertility is also directly linked to the degradation of chemicals. Fertile soil supports more and more microorganisms. These microbes help in the speedy degradation of residues.
6. Consecutive use of herbicide year after year results in increased risks of herbicide residue carryover. Even using a different formulation of the same herbicide group may result in an additive effect.

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Reference

1. Sing SB, Sharma R and Singh N. 2012. Persistence of pyrazosulfuron in rice-field and laboratory soil under Indian tropical conditions. Pest Management Science 68(6): 828-833.
2. Singh S and Kulsherestha G. 2006. Soil persistence of triasulfuron herbicide is affected by biotic and abiotic factors. Journal of Environmental Science and Health, Part B: Pesticides, Food Contaminants, and Agricultural Wastes 41(5): 635-645.
3. Sondhia S and Yaduraju NT. 2005. Evaluation of leaching of atrazine and metribuzin in tropical soil. Indian Journal of Weed Science 37: 298-300.