Sulfonylureas chemical degradation

Although studies on the degradation mechanism of SL-950 have just been started, judging from the fiu t that SL-950 has almost same dissociation constant (pKa 4.6) as SL-160, 81 950 is expected to follow a similar chemical degradation pathway. The short-residual properties of SL-160 have been described in the chapter of Haga et al. If 81 950 behaves similarly in soil, it will have a persistence profile that is very attractive for rotational cropping. Such properties may be considered important for future sulfonylurea herbicides. 

The last ten years have seen important developments in this respect in the three most important families of pesticides fungicides of the triazole group applied at about 100 g/ha, insecticides of the synthetic pyrethroid type at 20 g/ha and herbicides of the sulfonylurea type at 30 and even as little as 5 g/ha exert an effect which could be achieved with the pesticides of 15-20 years earlier only at rates of a few kilograms per hectare. These modern highly efficient preparations form only a small part of the selection of pesticides available today, but a rapid increase in their share of the total is to be expected as a result of purposeful research work. Another approach toward diminishing environmental contamination by chemicals is the development of new active substances which are less volatile, are degraded more rapidly or are more readily adsorbed by soil particles. 

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. 

Sulfonylureas are systemic herbicides absorbed by the foliage and roots. They act by inhibiting acetolactate synthase, a key enzyme in the biosynthesis of branched chain aminoacids." This results in stopping cell division and plant growth. The most important degradation pathways of sulfonylureas are chemical hydrolysis and microbial degradation. 

In the soil, there are two major pathways of sulfonylurea degradation [58] (a) chemical hydrolysis and (b) microbial degradation. The breakdown of sulfony-lureas in sterile soils is solely attributable to chemical hydrolysis, whereas breakdown in non-sterile soils is a combination of both microbial degradation and... 

This chemical reactivity is a new mode of degradition, not seen in conventional sulfonylurea herbicides. It explains the short half-life (less than 2 weeks in soil) of SL-160, and provides a new strategy for overcoming the problem of carry-over related to sulfonylurea herbicides in some recropping situations, by means of quick chemical (not enzymatic) degradation. 

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