Selectivity safeners

These chemically diverse safeners all need to be applied to the crop (maize, sorghum) by seed dressing to obtain the selective safener effect. The oldest and best... 

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. 

Research on chemical antidotes or safeners has been summarized in several reviews and published symposia (3.-9). Most of the major developments (Table I) have resulted from impirical screening programs by Industry that may have been stimulated by observations of herbicide antagonism in plants (3, 10). However, some of the research on mode of action of antidotes has been directed at finding new ways to protect crop plants from herbicides (3). The research to be discussed in this text, namely the use of subtoxic herbicide pretreatments to improve crop tolerance to selected herbicides, arises in part from research on the mode of action of R-25788 as a selective antidote for EPIC or butylate in corn. 

Gianessi et al. [7] therefore compared the amount of glyphosate that had been used on herbicide tolerant soybeans in 2001 to the amoimt of mixed herbicides that would be required to achieve the same degree of weed control. For each U.S. state, an alternative mix of herbicides was composed for comparison, based on the outcomes of a survey among experts, e.g. a mix of Boimdary (metribuzin s-metolachlor), Flexstar (fome-safen), and Select (clethodim) in Iowa (Table 1). 

However, ideal levels of crop selectivity are difficult to achieve. Several approaches to supplement natural crop selectivity have been used, including development of chemical safeners that improve herbicide tolerance of corn and sorghum (2), restriction of herbicide use to tolerant cultivars (3), transfer of tolerance to crop cultivars by breeding methods (4), and optimization of the site, timing, and method of herbicide application. 

Figure 3. Dose-response curve highlighting safener efficacy and selectivity margin of the specific pinoxaden plus cloquintocet-mexyl combination (—) in contrast to treatments without safener ( -).

However, the introduction of new herbicides, either from AHAS biochemistry or others such as HPPD inhibitors, ACC-ase inhibitors and others, shows that selective herbicides, sometimes together with safeners, will find their markets when they are competitive with older solutions and when they offer advantages to farmers, such as one application a season. 

Chapter 5, entitled Safeners for Herbicides , demonstrates the progress in this research field, bringing out new compounds that create highly competitive products for the farmers out of only partly selective herbicides having a very broad weed spectrum and very low application rates ( the chemical answer to genetically modified herbicide resistant crops ). 

The safener mefenpyr-diethyl, as with iodosulfuron-methyl-sodium, selectively accelerates the degradation of the active ingredient to non-phytotoxic compounds in cereals but not in weeds. 

The basis of selectivity of foramsulfuron in the presence of the safener isoxadifen-ethyl is a more rapid rate of metabolic detoxification in maize compared with target weeds, in which little or no degradation of the parent sulfonylurea occurs [51]. Three main routes of metabolism have been established in maize - a hydrolytic cleavage of the sulfonylurea bridge, a deformylation of the amino group and oxidative metabolism of the dimethoxypyrimidine ring. 

DE-742 (7) requires addition of a safener to achieve commercial levels of postemergence selectivity in small grain cereal crops, the main target market. Com-... 

Herbicide safeners (also referred to as herbicide antidotes or protectants) fulfill an important role in crop protection. Safeners are chemicals that protect crop plants from unacceptable injury caused by herbicides. Either by placement on the crop seed or by way of a physiological selectivity mechanism, safeners in commercial use do not negatively impact the weed control of the herbicide. Although many herbicides have been developed for use without a safener, some of the strongest and most broad-spectrum herbicides tend towards border-line crop selectivity, which may completely preclude use in a particular crop or at least limit maximum use rates or the crop varieties that can be safely treated. It is for such situations that safeners have been developed. Several books and reviews of safeners have been written over the past 20 years [1-3]. It is not the intention of this chapter to cover in detail older safeners, but rather to focus on more recently developed commercial safeners as well as some of the older compounds still in wide commercial usage. 

Uptake studies were also carried out with the recently developed combinations of the safener mefenpyr-diethyl with the sulfonylurea herbicides mesosulfuron-methyl and iodosulfuron-methyl-sodium, which are used for selective postemergence weed control in wheat crop. In both combinations the safener had no influence on herbicide uptake [29]. 

Metabolism of the proherbicide (i.e., a non-phytotoxic chemical that requires metabolic conversion to yield an active herbicide) FE, is an example of how Phase 1 and II metabolism, and use of a safener imparts selectivity between crop and weed species (Figure 1). FE is used to control many graminaceous weed species such as Avena fatua, Echinochloa crus-galli, Digitaria ischaemum, and Setaria glauca (40) in wheat which is moderately tolerant to this proherbicide. The primary basis for the selectivity of FE in grasses is due to differences in metabolism (40-42). FE is rapidly hydrolyzed to the herbicide fenoxaprop (F) in wheat, and in all the previously mentioned graminaceous weeds (4J). In wheat. 

Herbicide selectivity in crops may involve a three-step process of detoxification, conjugation and transport into the vacuole. These three processes can be induced in a coordinated fashion by safeners. The major detoxifying enzymes occur in large gene families, which results in considerable variation in selectivity patterns. Our understanding of these enzymes and their regulation has been greatly assisted by new molecular techniques. 

Herbicide Safeners Enhance Herbicide Selectivity by Inducing GSTs and Other Enzymes... 

In addition to the use of two- or three-way tank mixtures of selected herbicides, herbicides are also used in combinations with other pesticides (e.g., insecticides, fungicides, nematocides) or with auxiliary chemicals such as synergists, safeners, extenders, and adjuvants. 

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