Herbicides discussion

YOUNG Degradation Studies of Phenoxy Herbicides Discussion and Conclusions... 

Table 20.8 lists the herbicides discussed here with their annual U.S. Table 20.8 U.S. Consumption of Herbicides...

ALS herbicides. Two classes of ALS-inhibiting herbicides are the sulfonylurea herbicides, discussed in Sections 2.1.2.1 and 2.2.3.1, and the imidazolinone herbicides. A third class of ALS-inhibiting herbicides is the 1,2,4-triazolo [1,5-a]pyrimidine-2-sulfonanilides. The triazolopyrimidine sulfonanilides act by disrupting the biosynthesis of branched chain amino acids in plants. Representatives of this class of herbicides include florasulam (Boxer , Nikos ) [151], initially introduced in Belgium in 1999 and used for the postemergence control of broadleaf weeds in cereals and corn, and flumetsulam (Broadstrike ) [152], used alone or in combination with other herbicides for the control of broadleaf weeds in soybean and corn. 

Chemical weed control technology has progressed greatly during the past 30 years. The discovery of new classes of highly selective, effective, safe and relatively inexpensive herbicides helped revolutionize modern weed control. In the United States herbicides are used on more than 80 million hectares of crop and noncrop lands for control of numerous species of weeds, and more than 50% of the total crop land was treated with herbicides in 1988. Chemical names and structures for most of the herbicides discussed in this paper are given in Table 1. Most weeds are... 

Of the herbicides discussed, paraquat and DCMU are most hazardous to humans. Paraquat generates free radicals that can attack cell components. DCMU poisons the electron transport complex. 

The carbamate herbicides discussed in the foregoing diifer considerably from one another, with respect to both the skeleton of the molecule and the substituent groups. 

Two dithiocarbamates, sodium methyldithiocarbamate (metham, SMDC, 2) and 2-chloroallyl diethyldithiocarbamate (sulfallate, CDEC, 3) have been introduced as herbicides. These compounds were developed before the thiocar-bamate herbicides discussed in the preceding chapter. 

Both herbicides discussed are translocated in the plant. Cacodylic acid is translocated only apoplastically, while DSMA is translocated in the phloem in the assimilation stream (Long and Holt, 1959, Holt et al., 1967 Sachs and Michael,... 

Given their relatively high costs relative to the herbicides discussed so far (this is due to the high cost of pyridine), the rapid expansion in their use is remarkable. Clearly it relates to their novel applications. Their mode of action involves reduction of the cation to a free radical during interference with photosynthesis. Re-oxidation back to the original cation by molecular oxygen then takes place, accompanied by production of hydrogen peroxide which is extremely toxic to plant cells. 

It is not inconceivable that such permeability effects are a result of an initial direct effect of herbicide binding to ATPase or an electron transport chain component and/or a protonophore action. However, most, if not all, the herbicides discussed in this section can be classed as multisite inhibitors as evidenced by their inclusion in previous chapters, particularly Chapter 1, and elsewhere in this chapter. Such a membrane partitioning would not be specific to the inner mitochondrial membrane but would occur in other... 

Other Heterocyclic Nitrogen Derivative Herbicides. The herbicides in this group are heterocycHc nitrogen derivatives that do not readily fall into one of the previously discussed groups. They have a wide range of uses and properties. Most of these herbicides are used for selective, pre-and/or post-emergence weed control. Amitrole is used for post-emergence, nonselective weed control in non-croplands and also as an aquatic herbicide (2,296). 

Extensive use of two more recendy developed classes of herbicides will further dramatically reduce the amount of appHed to control weeds. The sulfonylurea herbicides are extremely active compounds first discovered in the mid-1970s at DuPont they have been discussed extensively (78). Sulfonjiurea herbicides have experienced a rapid and widespread success since thek commercial introduction in 1982 with chlorsulfuron (Table 5). The sulfonylureas are appHed at rates of 2—75 g/ha. The chemistry of the sulfonylurea molecule permits the synthesis of a very large number of useful analogues, consequentiy many new herbicides are anticipated for crop production. As of this writing (1996), over 350 patents have been issued to about 27 agricultural companies covering tens of millions of stmctures known or expected to be herbicidaHy active. 

The efficient recovery of volatile nitrosamines from frankfurters, followed by gc with chemiluminescence detection, has been described (133). Recoveries ranged from 84.3 to 104.8% for samples spiked at the 20 ppb level. Methods for herbicide residues and other contaminants that may also relate to food have been discussed. Inorganic elements in food can be deterrnined by atomic absorption (AA) methods. These methods have been extensively reviewed. Table 8 Hsts methods for the analysis of elements in foods (134). 

Apart from the resistance of insects to insecticides, resistance has been developed by plants to herbicides, fungi to fungicides, and rodents to rodenticides. Rodenticide resistance is discussed in Chapter 11, Section 11.2.5. 

Several aspects of the problem of herbicides being contaminated with nitrosamines, and the resulting inadvertent introduction of nitrosamines into the environment, will be discussed in other papers in this symposium. Unrecognized until less than five years ago, the situation has inspired intense debate and prompted several of the environmental chemistry studies mentioned in this paper. Like the presumed threat from the in vivo nitros-ation of pesticide residues, discussions sometimes lack the type of anticipated dose and effect calculations just mentioned. Unlike the active ingredients, whose benefits can justify residue tolerances and acceptable daily intakes, nitrosamine contaminents afford no known benefits, and the desirability of minimizing their levels is undisputed. 

A number of substituted triazines are used as herbicides, and their biodegradation has been discussed in Chapter 10, Part 1. Treatment of soil contaminated with atrazine (2-chloro-4-(ethylamino)-6-isopropylamino-l,3,5-triazine) illustrated a number of significant features. Although the soil that was used had the potential for degradation, a laboratory experiment with Pseudomonas sp. strain ADP that had an established potential for atrazine degradation revealed important limitations. There was a substantial decline in the numbers of Pseudomonas sp. strain ADP and only limited mineralization. Supplementation with citrate or succinate increased the survival of the strain, and successful mineralization was dependent on the preservation of a carbon/nitrogen ratio >10 (Silva et al. 2004). The last would apply generally to substrates with a low C/N ratio such as triazines. 

Electrospray ionization (ESI) and APCI are the two popular API techniques that will be discussed here. The applications to the analysis of pesticides that will be discussed include imidazolinone herbicides, phenoxy acid herbicides, and A-methyl carbamate insecticides. Matrix effects with respect to quantitation also will be discussed. Eor the... 

The famous herbicide agent orange contained as an additive 2,3,7,8-tetrachlordibenzo-p-dioxin (2,3,7,8-TCDD), the most toxic chemical substance created by mankind. Dioxin makes up about 150-160 kg. in 24,000 tons of herbicide 2,4,5-T. This additive is enough to allow us to continue to discuss the consequences to humans and the environment in Vietnam from not one, but two wars the herbicide and the dioxin [65]. 

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