Herbicide selectivity

It is important that herbicides used for weed control in crops are selective to that crop. Early compounds used the inability of the foliage of upright cereal crops to retain a great deal of spray as the basis of their selectivity and this was successful in many situations although it must be remembered that the alternative to weed control with inadequately selective herbicides was hand weeding or no weed control at all. 

There are a number of mechanisms of selectivity that are found in the herbicides that are used today. Diuron is used as a residual broad-spectrum herbicide in a number of situations such as plantations and forests. It is, however, phytotoxic to most perennial species and the selectivity shown by the established trees is because the compound does not move within the soil profile to a depth where established tree roots will absorb the compound in sufficiently high concentrations to exert an effect. This is selectivity by placement. 

Deamination is another detoxifying metabolic process. Herbicides 

Clearly, if a plant is able to metabolise a herbicide more quickly than the herbicide can accumulate at the site of action within the plant, then that plant will be tolerant of that herbicide. This is detoxification. 

Some herbicides are applied in a form that is inactive as a herbicide and it is the plant s metabolic processes that converts the applied 

while inhibiting GS from plant, microbial, and animal sources, is nontoxic to mammalian cells because it fails to cross the blood-brain barrier and is rapidly cleared by excretion through the kidneys. Inactivation of l-PPT (the D form is not metabolized) by soil bacteria occurs by three routes N-acetylation, transamination, and oxidative deamination, the latter two reactions forming 2-oxo-4-[(hydroxy)(methyl)phosphinoyl]- 

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Breaux. E.I.. Patanella. I.E.. and Sanders. E.F. Chloroacetanilide herbicide selectivity analysis of glntathione and homoglutathione in tolerant, snsceptible and safened seedlings. J. Agric. Food Chem., 35(4) 474-478.1987. 

Dichlorophenoxy)acetic acid, together with the 2-methyl-4-chloro and the 2,4,5-trichloro analogs, 2-(2,4,5-trichlorophenoxy)propionic acid, and many other related compounds, constitute one of the major classes of herbicides, selective primarily to dicotyledonous plants. Monocotyledons,... 

The development of herbicide-resistant weeds has also been an influence on the selection of herbicides used on field corn or soybean. Weed resistance now affects nearly every decision a farmer makes about herbicide selection either a farmer is trying to control resistant weeds or is selecting herbicides that may reduce the possibility of weed populations becoming resistant. The adoption of the imidazolinone- and sulfonylurea-tolerant com hybrids mentioned above was in part a response to the presence of atrazine-tolerant pigweeds or kochia in many fields. However, a recent decrease in die use of imidazolinone and sulfonylurea herbicides can also be attributed to the development of populations of weeds that have become resistant to these herbicides. 

Shimabukuro, R.H. (1967b). Atrazine metabolism and herbicidal selectivity. Plant Physiol., 42 1269-1276. 

Numerous herbicides are currently recommended and used for weed control in agricultural, industrial, and recreational areas. However, the method of formulation and application of such chemicals to plants or soils may markedly affect their biological efficacy and persistence. Chemical additives (primarily surfactants) in both oil and aqueous sprays are already used widely, and when properly understood, promise a virtual revolution in the use of agricultural chemicals. Conceivably, not only weed control performance and herbicidal selectivity can be altered but also distribution, metabolism, and accumulation of chemical residues. With the widespread introduction and increasing use of chemical additives in weed science and technology, we must therefore learn to reflect on what before we knew about the use of herbicides. 

Chemical herbicides have been available for more than a century but major impacts on crop production awaited the development of "truely" selective herbicides or innovations that would permit use of non-selective herbicides in crop situations. We now have some form of selective chemical weed control for most of our major crops. However, continuing problems with herbicide injury to crops as well as poor control of weeds that are botanically similar to crops remind us that further improvements in herbicide selectivity are still needed. Introductions of new selective herbicides will continue but the high costs of these new chemicals are stimulating efforts to make wider use of existing herbicide chemistry. One successful approach has been to genetically improve the tolerance of new crop cultivars to major herbicides... 

In practice, herbicide selection is limited by the number of different products available for use in a particular crop, the weed spectrum, the possible rotational consequences of using residual products, and the treatment cost. In some crop rotations choosing a herbicide from a different mode-of-action group each year may be a viable option, whereas in others the choice of registered products may be limited, and the farmer may have to rely on similar products more frequently. Again, short-term consideration of the important... 

Herbicide selectivity may vary according to the application rate. High rates of selective herbicides usually will injure all plants at the application site. Some nonselective herbicides can be used selectively by applying them at a lower rate. Other factors that affect selectivity include the time and method of application, environmental conditions, and the stage of plant growth. 

These devices are used to apply contact or translocated (systemic) herbicides selectively to weeds in crop areas. Wicks made of rope, rollers made of carpet or other material, or absorbent pads made of sponges or fabric are kept wet with a concentrated mixture of contact herbicide and water and brought into direct contact with weeds. The herbicide is "wiped" onto the weeds but does not come in contact with the crop. Application may be to tall weeds growing above the crop or to lower weeds between rows, depending on the way the wiper elements are designed. Pumps, control devices, and nozzles are minimal or are eliminated altogether, and tanks are quite small because of the small amount of liquid applied. 

Soskic, M. and Sabljic, A. (1993). Herbicidal Selectivity of (E)-3-(2,4 Dichlorophenoxy)Acry-lates QSAR Study with Moleciilar Connectivity Indexes. Pestic.Sci.,39, 245-250. 

Carvalho SJP., Nicolai M., Rodrigues Ferreira R., Oliveira Figueira AV., Christoffoleti PJ. Herbicide selectivity by differential metabolism considerations for reducing crop damages. Sciencia Agricola (Piracicaba, Brazil) 2009 66(1)136-142... 

In this review, conjugation reactions utilized In xenobiotic metabolism In plants will be discussed In reference to functional groups, phase I reactions necessary to produce a functional group suitable for conjugation, relative rates of reactions, competing metabolic pathways, frequency of occurence, plant species, stability of conjugates, and the relationship between metabolism and herbicide selectivity. Pesticides discussed herein are listed In Table I. 

Most of the enzymes that catalyze the formation of xenobiotic conjugates in plants have not been well-studied. Since some conjugation reactions are involved in herbicide selectivity, it is likely that research relating to these enzymes will intensify as a result of efforts to develop herbicide resistant crops through bioengineering. Enzymes that may be useful in bioengineering for herbicide resistance are the GST enzymes, N-glucosyl transferases. 

Before Radosevich and De Villiers found in 1975 that isolated chloroplasts of resistant common groundsel were insensitive to atrazine and simazine (2), it had been erroneously assumed that all living plants would die if the herbicides could reach their target site intact. We now know that mechanisms of selectivity in crops can be due to differences in metabolism rates, uptake, translocation, site of action or avoidance mechanisms. However, the mechanisms of herbicide resistance that have evolved in weeds are usually different from the mechanisms of herbicide selectivity in most crops. This is certainly true with the most prevalent and thoroughly studied cases of herbicide resistance, including the triazines, dinitroanilines, and AHAS inhibitors. 

This approach should be used to enhance herbicide selectivity in crop varieties, to avoid carry-over injury, for specific and limited special problems, and for minor acreage and high value crops. A major objective of developing herbicide resistant crops should be to provide more flexibility in control of resistant weeds. 

However, each new resistance that develops will produce a different genetic situation. The solutions found by weeds to escape herbicide selection pressures may be varied. Due to the high selective value conferred by the resistance genes in herbicide treated areas, mutation events at very low frequencies have high probability to lead to the appearance of resistant plants. In addition, weeds will certainly display after a short delay the bacterial genes transferred to crops for herbicide resistance, and some wild plants could be expected to become new weeds because of resistant genes. 

The previous model kept track of the various influences affecting resistant and susceptible weeds. In particular, a seed bank was presumed to contain fully viable seeds for n years which then died. We neglected the loss of seeds from the seed bank due to germination. In the appendix to our earlier paper (3), an alternative model was developed based on the hypothesis that a constant fraction of seeds in the seed bank perish each year for resistants and for susceptibles). In this alternative model the effects of germination on the seed bank are tallied. The two models agreed and yielded Eq. 1 when the factors / and a of Eq. 1 satisfied fa 1, i.e., when the herbicide selection pressure was sufficiently high. We now know that this will not be met in rotational years with fitness near 0.1 and a < 10. 

Benzoylprop-ethyl (31) and flamprop-isopropyl (32), two anilide herbicides selective in cereals with specific efficiency for the control of wild oat Avena spp.), have been developed in the research latoratories of the Shell Research Ltd. 

Benzoylprop-ethyl is a postemergence wild oat herbicide selective in wheat, while the chemically closely related flamprop-isopropyl, isopropyl N-benzoyl-N-(3-chloro-4-fluorophenyl)-DL-alaninate (WL 29762, 32) is a wild oat herbicide selective in barley. 

Hathway, D. Molecular Mechanisms of Herbicide Selectivity. New York Oxford Univ. Press, 1989. [A short book primarily devoted to the differences in enzyme activity in weeds and desirable plants.]... 

Matsui J, Fujiwara K, Ugata S et al. Solid-Phase extraction with a dibutylmelamine-imprinted polymer as triazine herbicide-selective sorbent. J Chromatogr A 2000 889 25-31. 

The present paper is a discussion of the photosystem II herbicides and their mechanisms of action. Among the topics covered are the green plant photosystems, photochemistry and electron transfers within photosystem II, requirements for herbicidal activity, mechanisms of action, herbicide selectivity and resistance, herbicide-binding proteins, and theoretical studies of herbicidebinding site interactions. 

Figure 9.5 The metabohsm of glutathione and homoglutathione conjugates in higher plants. [Reproduced from G. L. Lamoreux, R. H. Shimabukuro, and D. S. Frear Glutathione and Glucoside Conjugation in Herbicide Selectivity , in J. C. Caseley, G. W. Cussans, and R. K. Atkin, Eds., Herbicide Resistance in Weeds and Crops, Butterworth-Heinemann Ltd., Oxford, pp. 227-262. Copyright 1991, with permission from Elsevier.]...

In many contributions reporting on acidic pesticides in environmental samples ESI applied as ion spray was predominantly performed to analyse these pollutants. APCl, however, was not as effective as ESI as studies with standard solutions of the pesticide mixtures made obvious [325] when phenoxy acid compounds were determined using both types of interface. MSn quantitative results were used for confirmation. Mass detection after CZE-MS interfaced by ESI was successfully apphed to analyse drinking water spiked with chlorinated acid herbicides. Selected-ion elec-... 

As already mentioned, continuous application of a herbicide selects rare genotypes of weeds that are resistant to the herbicide and eventually at the same time already cross-resistant to other herbicides. These genotypes may already exist in a weed population in very low frequency before the introduction of the selecting herbicide. 

Properties Tan powd. sol. in alcohol, benzene si. sol. in water m.w. 211.71 m.p. 67-76 C Toxicology LD50 (oral, rat) 710 mg/kg, (skin, rabbit) 380 mg/kg poison by ing., skin contact, possible other routes mutagen TSCA listed Hazardous Decomp. Prods. Heated to decomp., emits very toxic fumes of Cl" and NOx Uses Herbicide selective weed killer Regulatory SARA reportable Manuf./Distrib. Salor Supelco http //www.sigma-aidrich. com... 

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