Sulfonylurea herbicides application rate

Table 5. Application Rates of Sulfonylurea and Imidazoline Herbicides...

The sulfonylureas described here are new herbicides with unprecedented activity. Activity at extraordinarily low rates of application has been demonstrated by both preemergence and postemergence applications (Ref. 1). These rates are best describee in grams per hectare compared to kilograms per hectare for most herbicides currently marketed. Furthermore, the combination of low application rates, half lives of generally less than two months in the soil (Ref. 2) and excellent safety to mammals [LD5o s of generally >5000 mg/kg for technical material in male rats] (Ref. IC) makes these compounds attractive products from an environmental viewpoint. 

Sales of agricultural products were 600 million in 1984. Du Pont expects to double this amount by 1990. There are many new products in the pipeline. Several of these new products are weed killers based on sulfonylurea chemistry. This family of weed killers is characterized by very low toxicity to mammals and high herbicidal activity at very low application rates in the field—often less than an ounce per acre of cropland. 

LC/MS has emeiged as a sensitive and selective residue methodology for the trace organic analysis of crop protection chemicals. This technology is especially applicable to low application rate herbicides such as sulfonylureas because it requires minimal sample processing and clean-up prior to chromatographic and spectroscopic quantitation. 

Figure 4. Field tests of transformed tobacco. Wild type tobacco (WT) and tobacco transformed with the HRA gene (SUR) were sprayed at 4X the typical field application rate with a commercial sulfonylurea herbicide preparation.

FIGURE 4 Results from a field test of herbicide-resistant tobacco lines, conducted in North Carolina in 1987 in conjunction with Northrup King Co. The row of plants to the right of center contains elite tobacco lines that were transformed with the mutant tobacco ALS gene from the herbicide-resistant Hra line, while the row to the left of center has non-transformed control plants. After treatments with the sulfonylurea herbicide chlorimuron ethyl at 4X normal field application rate, the weeds were killed, the non-transformed controls were severely injured and remained stunted, and the transformed plants remained as vigorous as the unsprayed controls. 

Field dosages often span a very broad range, and the typical application rates , shown in Table 8.1, are for general comparison only. Nevertheless, these numbers indicate how field a.i. rates have decreased historically, as more active molecules have been developed examples include the neo-nicotinoid insecticides, triazole fungicides and sulfonylurea herbicides, which are one to two orders of magnitude more active than the early organochlorine, copper and aryloxyalkanoic acid pesticides, respectively. 

Sulfonylurea herbicides (SUHs) are relatively new herbicides, introduced in the 1980s. Chlorsulfuron was the first sulfonylurea marketed in the United States, in 1982. World-wide, 19 sulfonylureas had been commercialized by 1994, and five more are being developed. This rapid increase is due to their very high and specific herbicidal activity, which results in extremely low application rates of 10 to 40 g/ha. Furthermore, as compared to other herbicides, sulfonylureas are less toxic and degrade more rapidly. Chemical structures of some representative sulfonylureas are presented in Figure 25.2. From a chemical point of view, these herbicides are labile and weakly acidic compounds. The common names, chemical formulas, water solubility, pKa, half-life in soil, and leaching potential through the soil (when available) of the most representative sulfonylureas are reported in Table 25.2. 

The broad-spectrum herbicide sulfometuron methyl Is a member of a class of sulfonylurea analogues being developed by the Du Pont Company ( ). These herbicides are characterized by extraordinarily low application rates of between 5 and 35 grams per hectare, short half lives In many soils and low toxicity towards mammals (LD50 s >... 

The herbicide with the trade name Sanbird is able to control both annual and perennial weeds in paddy fields [5] with application rates of 3-4 kg ha . As a very selective herbicide in rice it was a good innovation for the Japanese rice market. This product reached peak sales in Japan in 1986 with 650000 ha (28.6% market share) [6]. This declined with the introduction of sulfonylurea herbicides such as bensulfuron-methyl in the year 1987. In 2005 Sanbird was only used on 101 200 ha (5.9% market share) in Japan [7]. 

An anecdote illustrates just how much more potent sulfonylureas were compared to the norm for herbicides in the late 1970s. When samples were sent to one university, investigators thought an error must have been made in placing the decimal point for the application rate of Glean, so they used 100 times the recommended rate. Two years later the test plots were still pretty barren. 

The preceding chapters have highlighted some of the diverse substitutions on the sulfonyl fragment of sulfonylurea herbicides that impart an unprecedented level of activity, as well as some of the subtle structural changes that have led to quite different crop selectivities and soil residual properties. This chapter reviews some of the modfications to the heterocyclic amine fragment of sulfbnylureas that have been investigated and found to impart herbicidal activity at low application rates. 

Condensation of cyanoimidate 16 with substituted hydrazines was known (6) to give a mixture of isomeric triazole amines 17 and 18. We found that sulfonylureas derived from 18 exhibited significant herbicidal activity while those derived from 17 were inactive at comparable application rates. The disadvantage with this method of preparing dialkyltriazole sulfonylureas was that the desired aminotriazole intermediate 18 was Ae minor regioisomer formed in the condensation of 16 with alkylhydrazines. 

On the basis of their chemical nature, use, and mode of action, substituted urea herbicides can be divided into two main groups phenylureas and sulfonylureas. Pheny-lurea herbicides were introduced more than 40 years ago and are still widely used, while the sulfonylurea compoimd group is a relatively new class of herbicides first introduced in 1982. Sulfonylurea herbicides have a low toxicity to humans and other animals, although they are nearly 100 times more toxic to target plants than the older compoimds. As a result, they are applied at relatively low application rates. Thus, this class of compound is expected to see continuing research and development activity in the next years. 

In summary, the sulfonylureas are new herbicides which exhibit activity at extremely low rates of application and show very low mammalian toxicity. Exceptionally high activity is shown by compounds containing a benzene ring substituted in the ortho position, an unsubstituted sulfonylurea bridge, and a pyrimidine or triazine heterocycle substituted with methyl or methoxy groups. 

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