Herbicide Residue Causes and Management PDF

HERBICIDE RESIDUE CAUSES AND THEIR MANAGEMENT The quantity of herbicide that remains in soil after its mission is accomplished is referred as residue. The term residue infers the continued presence of herbicide in the soil in very small quantities even though it may not be present in herbicidal quan...

HERBICIDE RESIDUE CAUSES AND THEIR MANAGEMENT The quantity of herbicide that remains in soil after its mission is accomplished is referred as residue. The term residue infers the continued presence of herbicide in the soil in very small quantities even though it may not be present in herbicidal quantities or available for plant absorption. Herbicide residues in soils are undesirable mainly because they may: 1. Injure sensitive crops in a cropping system, 2. Be absorbed by succeeding crops and residues accumulated in unlawful amounts in the produce, 3. Inhibit in build-up of herbicide residues following repeated treatments that exceed their rate of dissipation, and 4. Inhibit growth of soil microorganisms. FACTORS AFFECTING HERBICIDE PERSISTENCE Factors that influence herbicides persistence are: Soil Factors The soil factors affecting herbicide persistence fit into 3 categories: physical,chemical, and microbial. Soil composition is a physical factor that measures the relative amounts of sand, silt and clay (the soil texture) and the content of the soil. Chemical properties of the soil include pH, organic-matter cation-exchange capacity (CEC), and nutrient status. The microbial aspects of the soil environment include the type and abundance of soil microorganisms present composition affects herbicide phytotoxicity and persistence through adsorption, leaching and volatilization or both have a greater potential for herbicide carry over 1 because there is increase absorption to soil colloids, with a corresponding in leaching and adsorption volatilization. This ‘tie-up results in initial plant uptake and herbicidal activity. Therefore, more herbicide is held in reserve to be released later, potentially injuring susceptible future crops. Some herbicides, principally the triazines (atrazine and simazine), at particularly influenced by soil pH, an important part of the soil chemical maken Lesser amounts of these herbicides are adsorbed or held to soil colloids at higher soil pH, so they remain in the soil solution. Herbicides in the soil solution a available for plant uptake. Chemical breakdown and microbial breakdown-2 major herbicide degradation processes are often slower in soils of higher pit So although decreased adsorption of triazine herbicides occurs in soils having higher pH, there is also less breakdown activity. Therefore, these herbicides are more available for plant uptake for a longer period on soils of higher pH. Certain members of the sulfonylurea group (chlorsulfuron and chlorimuron) can also persist in higher pH soils because rates of chemical breakdown are decreased Low pH affects the persistence of clomazone and the imidazolinones (imazaqun and imazethapyr). Soil pH has little effect on the persistence of other herbicides Research shows that various nutrients and cations in the soil impact both herbicide activity and degradation. The CEC, principally a function of clay type and organic matter content, is directly involved in herbicide adsorption. Some herbicides are more available in the presence of certain cations, whereas others may be tied up and therefore unavailable. The literature indicates that there is much variation in the effect that cations and nutrients can have on herbicide activity and breakdown, depending on soil composition, nutrient type and concentration, and chemistry of the herbicide. Soil microorganisms are partially responsible for the breakdown of matny herbicides. The types of microorganisms and their relative 2 amounts determine how quickly decomposition occurs. Soil microbes require certain environmental conditions for optimal growth and utilization of any pesticide. Factors that affect microbial activity are temperature, pH, oxygen and mineral nutrient supply Usually, a warm, well-aerated, fertile soil with a medium soil pll is most favourable for microorganisms and hence herbicide breakdown. Climatic Conditions The climatic variables involved in herbicide degradation are moisture, temperature and sunlight. Rates of herbicide degradation generally increase with increased temperature and soil moisture because both chemical and microbial decomposition rates increase under conditions of higher temperature and moisture Cool, dry conditions slow degradation, causing greater carryover potential. If winter and spring conditions are wet and mild, herbicide persistence is less likely. Sunlight is another important factor in herbicide degradation. Photo- degradation, or decomposition by light, has been reported for many herbicides. The dimitroanilines (trifluralin and pendimethalin) are sensitive to light degradation. May be lore degradation applied if they remain for Mahal Therefore, degradation is accelerated on very sunny days. This sensithour light and loss by volatility are primary reasons for soil incorporationer Herbicidal Properties 3 Finaly, the chemical properties of a herbicide affect its persistence. Important factors are: water solubility, soil adsorption, vapour pressure and susceptibility to chemical and microbial alteration or degradation. The water solubility of a herticide helps determine its leaching potential. Leaching occurs when a herbicide dissolved in water and moves down through the soil profile. Herbicide that readily leaches may be carried away or carried to rooting zones of susceptible plants. Herbicide leaching is determined not only by water solubility of herbicide. But also by its ability to adsorb to soil particles. Additionally, soil texture and available soil water affect herbicide leaching. Herbicides that are low in water solubility, are strongly adsorbed to soil colloids, and exist in dry soils are less likely to leach and have a greater potential to persist. The vapour pressure of a herbicide”determines its volatility, the process of changing from a liquid or a solid to a gas. Volatility increases with temperature. Volatile herbicides such as the thiocarbamates (EPTC, butylate) must be incorporated immediately to avoid gaseous losses. These herbicides are less likely to persist than herbicides with low vapour pressures. Herbicides may be rapidly decomposed by microorganisms in the soil if the numbers of microorganisms are present and if soil conditions are favourable for their growth. However, herbicides vary greatly in their susceptibility to microbial decomposition. For example, microbial degradation of 2,4-D occurs very quickly in the soil, whereas microbial degradation of atrazine is slow. Chemical decomposition is dependent not only on the chemistry of the herbicide (how susceptible it is to chemical breakdown) but also on soil and climatic factors. Chemical breakdown of a herbicide involves reactions such as hydrolysis, oxidation and reduction. The occurrence of these reactions and the rates at which they take place vary with soil type and climatic conditions. These reactions, along with microbial degradation, are important processes in the decomposition of herbicides. 4 Management of Herbicide Residues in Soils Various management techniques have been developed to minimize residue hazards in soils: Use of optimum dose of herbicides Trifluralin @ 0.72 and 0.96 kg/ha to soybean leaves exhibits no adverse residual effect on succeeding wheat, but @ 1.2 kg/ha the wheat will be affected. Application of farmyard manure Residues of atrazine @ 0.5 kg/ha applied to pearlmillet [Pennisetum glaucum (L.) R. Br.] affect subsequent greengram and cowpea. However, an application of FYM @ 12 t/ha to pearlmillet can mitigate the residual toxicity of atrazine. Cultural practices These include ploughing and intercultivation. Residual toxicity of propyzamide, trifluralin and atrazine can be considerably brought down by ploughing. Crop rotation Residues of herbicides applied to preceding crop should not be toxic to the succeeding crop in rotation. Application of atrazine to maize, diuron to cotton and chloramben to soybean does not exhibit direct or residual effect on succeedingcrops in cotton-maize-soybean rotation. Application of diuron @ 1.0 kghate cotton will not injure oat and wheat grown in a rotation. 5 Use of non-phytotoxic oil and activated carbon Atrazine residual hazard can be decreased by mixing it with non- phytotoxs oil. Activated carbon shows high adsorptive capacity due to its tremendous surface area. Use of herbicide safeners or antidotes Safeners can protect the crop from possible damage by a herbicide. R- 25788 marketed as a mixture with EPTC and butylate for soil application and naphthalic anhydride (NA) is used for seed dressing on rice to protect the crop against molinate and alachlor. Another safener cyometrinil is used along with metolachlor in crop of grain sorghum. Leaching of soil Herbicide residues can be removed by leaching. Trap-crop approach One obvious answer to herbicide-residue problems in sul ist deliberately include crop plants in rotation that are resistant to a particular Berbicide. This permits the land to stay in production and at the same time, it hastens the dissipation of the herbicide. Such crops are called herbicide-trap crops eg sugarcane, sorghum and maize, are good trap crops for eliminating residues of arazine and simazine from soil. Crop breeding for selecting crop 6 7