Crop selectivity

Cloransulam-methyl and diclosulam are members of the triazolo[ 1,5-c] pyrimidine sulfonanilide family of AHAS-inhibiting herbicides. Both compounds show excellent crop selectivity, broad-spectrum broadleaf weed control and low toxicity. The herbicidal utility of cloransulam-methyl in soybeans was first presented in 1994 [6, 7] and further described in 1995 [8, 9] and 1996 [10-12]. Diclosulam was first described for use in soybeans and peanuts in 1997 [13] with additional description in 1998 [14] and 1999 [15-17]. 

Diclosulam is registered in the United States and in Latin America for use in peanuts and soybeans. Applications can be made preplant surface, preplant incorporated and preemergence at rates of 17.5-26 g-a.i. ha for the control of numerous broadleaf weed species. Diclosulam does not provide control of annual and perennial grass weeds or certain broadleaf weeds such as Solarium spp. 

1 Cloransulam-methyl and Diclosulam Mechanism oTCrop Selectivity 

The metabolism of triazolopyrimidine sulfonanilides (1-4) in plants has been reviewed [23, 24]. It has been shown that diclosulam (3) and doransulam-methyl (4) are rapidly metabolized in soybeans by facile conjugation with homoglutathione which displaces the 7-fluoro substituent (Fig. 2.4.1) [25]. This mechanism was found to only occur in soybeans for 3 and 4. Oxidation at the 4-position of the aniline ring occurs rapidly in maize for 3 and 4. In wheat, 4 undergoes O-dealkylation of 5-ethoxy followed by glucose conjugation and oxidation at the 4-position of the aniline ring occurs for 3 [25]. 

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Nonchemical or traditional practices, such as weed seed removal, optimal crop seeding rates, crop selection, enhanced crop competitiveness, crop rotation, and mechanical weed control are all important components of an effective weed management program (458,459). In the context of modern intensive chemical herbicide appHcation, nonchemical practices may even represent an innovative approach to weed management and should receive careful consideration. 

The implication that "living crops" can allelopathically suppress weeds has been made by Putnam and Duke (28) and Leather (29). These studies demonstrated that the potential for suppression of weeds could be enhanced by crop selection. Putnam and Duke (28) suggested that there is potential for breeding crops to better suppress weeds by utilizing and improving allelopathic characteristics. Fay and Duke (30) evaluated the amount of scopoletin (6-methoxy-7-hydroxy coumarin) exuded from 3,000 accessions of Avena sp. They suggested that the "wild types" of crop varieties could have once possessed allelopathic potential, but this character has been inadvertently selected against over time. 

One fuel alternative involves the more widespread use of biomass produced fuels. More efficient biomass conversion techniques would help make biofuels more cost-competitive. Land availability and crop selection are major issues in biomass fuel usage. Biomass alternatives can be expected to grow to a significantly larger scale for providing fuel. 

The absence of adenosine deaminase in plants is the reason for the crop selectivity of the hydroxypyrimidines. However, the fungicidal... 

The benefits of the triazines in multiple cropping systems range from their application flexibility, effective weed control, soil residual activity, and crop selectivity to their important role in resistance management and conservation tillage. The triazines also have made a major impact on agricultural sustainability and crop yields, as evidenced by the use of atrazine, especially in com. 

Substitution of the chloro atom by alkoxy and alkythio groups, preferably methoxy and methylthio, conserves the high herbicidal activity but leads to a change of the crop selectivity pattern. 

Substitution of the chloro atom by bromine, by fluorine, by nitrilo-, hydrazion-, alkyl-, haloalkyl-, alkoxyalkoxy groups leads very often to remarkable herbicidal but seldom -from the practical point of view - to superior activity. It is thereby obvious to everybody active in this field that the qualification superior activity can never relate to one parameter alone activity against the target organisms is, of course, an absolute prerequisite but this activity can, outside the field of industrial weed control, only be made valuable by a complementary suitable crop selectivity pattern. The following compounds resulting from our project reached the level of practical use ... 

The crop selectivity and season-long effectiveness of the triazines on several weeds were highly useful to farmers, and adoption of the triazine herbicides for use in crop production was rapid. Long-term weed management strategies included ... 

While several of these organic chemicals reached a commercial stage, all had major limitations. Among those limitations were marginal crop selectivity, limited weed spectrum, too short duration of activity, serious failures on some soils or under certain weather conditions, offensive smell or touch, corrosion of spray equipment, drift, secondary adverse effects, etc. At best, many of these organic chemicals had to be used at high rates (e.g., 4-101b/A or 4.5-11.2kg/ha), and were often too costly for their limited benefit. 

Basis of Crop Selectivity and Weed Resistance to Triazine Herbicides... 

Brown, H.M. (1990). Mode of action, crop selectivity and soil relations of the sulfonylurea herbicides. Pesticide Sci., 29 263-281. Brown, H.M., F.T. Lichtner, J.M. Hutchison, and J.A. Saladine (1995). The impact of sulfonylurea herbicides in cereal crops. Brighton Crop Prot. Conf.-Weeds,3 1143-1152. 

As I continue through the beds, pulling weeds, I recall a trip I took with a friend to Oaxaca, Mexico. We traveled through rainforests rich with diversity of tree ferns, cycads, pipers, aroids, bromeliads, and orchids. We also traveled through small villages where farmers practiced subsistence agriculture. They cultivated a diversity of modern corn varieties, as well as traditional landraces—crops selected for their adaptations to specific locations and their culinary characteristics. Often landraces... 

Urea herbicides. Phenyl urea chemistry represents one of the earliest classes of herbicides. Replacement of the aromatic chlorines of diuron with a trifluoro-methyl group resulted in improved crop selectivity. Fluometuron (Cotoran ) and parafluron are both selective urea herbicides, fluometuron developed for the control of grass and broadleaf weeds in cotton. It was prepared from the corresponding 3-trifluoromethyl anilines. A number of other phenyl urea herbicides were introduced later, including trimefluor, 3-chloro-4-trifluoromethoxyphenyl-A/,A/-dimethyl urea [19]. 

Dalapon, a chlorinated aliphatic acid, was introduced by Dow Chemical in 1953 (Fig. 3). It is a grass killer, controlling tough perennial grasses such as johnsongrass, bermudagrass, and quackgrass. It possesses almost no crop selectivity. 

It is thereby obvious to everybody active in this field that the qualification "superior activity" can never relate to one parameter alone activity against the target organisms is, of course, an absolute prerequisite but this activity can, outside the field of industrial weed control, only be made valuable by a complementary suitable crop selectivity pattern. 

Hence the opportunity to use metabolism of these herbicides to impart crop selectivity has been exploited successfully. 

Acetolactate synthase (ALS, EC 4.1.3.18) is the first common enzyme in the biosynthetic route to the branched chain amino acids, valine, leucine and isoleucine. It is the primary target site of action for at least three structurally distinct classes of herbicides, the imidazolinones (IM), sulfonylureas (SU), and triazolopyrimidines (TP) (Figure 1). SU and IM were discovered in greenhouse screening programs whereas TP was subsequently targeted as a herbicide. Numerous substitution patterns can be incorporated into the basic structure of all three classes of herbicides to provide crop selectivity as well as broad spectrum weed control. This is amply demonstrated in the seven products based on SU and four based on IM already in the market. A number of others are in various stages of development. The rapid success of ALS inhibitors as herbicidal products has attracted a world-wide research commitment. Not since the photosystem II... 

Since TP, SU and IM are slow to bring about plant death, there are significant opportunities to exploit metabolism of the herbicide to influence crop tolerance. Metabolism has indeed been the overriding parameter determining crop selectivity (5c.16.17). ALS inhibiting herbicides in development and/or full commercialization are known to have selectivities to all the major crops including corn, soybeans, wheat, barley, rice, cotton and canola. 

The factors which Influence the choice of formulation are pesticide physical properties (melting point, solubility, volatility), pesticide chemical properties (hydrolytic stability, thermal stability), soil application vs. foliar application, crop and cultural practice, pesticide biological properties (crop selectivity, transport), and economics. 

When arranging your planting and harvesting schedule, include cultivars that mature earlier or later than your usual crop selections. Pests arrive to find their favorite plant hosts have already been harvested or are not mature enough to produce the fruit on which they feed. Either way, damage to your garden is reduced or eliminated and pest populations dwindle in the absence of the appropriate hosts. 

Thermospray LC/MS has been extensively used for the study of sulfonylurea herbicides (1-2). These compounds are thermally labile and can not be successfully analyzed by conventional GC/MS. Early applications of thermospray LC/MS included metabolite identification and product chemistry studies. We have recently evaluated the use of thermospray LC/MS for multi-sulfonylurea residue analysis in crops and have found the technique to meet the criteria for multiresidue methods. LC/MS offers both chromatographic separation and universal mass selectivity. Our study included optimization of the thermospray ionization and LC conditions to eliminate interferences and maximize sensitivity for trace level analysis. The target detection levels were SO ppb in crops. Selectivity of the LC/MS technique simplified sample extraction and minimized sample clean up, which saved time and optimized recovery. Average recovery for these compounds in crop was above 85%. 

Derivatives containing the s-triazolone ring are among the most active members of this class of compounds showing crop selectivity, whereas the introduction of alkylthio, sulfinyl, and sul-fonyl groups cause total loss of activity. Hydroxy derivatives which are tautomers of urazole are likewise inactive. 

A greenhouse comparison of preemergent grass weed activity of these compounds is given in Table I. In addition, their effect on corn and soybeans at somewhat higher rates is included as a measure of crop selectivity. 

The synthesis and herbicidal activities of various imidazolinthiones, in particular the thiono isosteres of imazapyr, imazethapyr and imazamethabenz are discussed. In the synthesis area it is shown that the imidazolinthione ring functions as an ortho-directing group in aromatic lithiations. In the biological activity area it is shown that replacement of the imidazolinone carbonyl with a thio-carbonyl results in changes in weed toxicity and crop selectivity. 

The thiono counterpart of imazapyr was prepared by treating 2,3-pyridinedicarboxylic anhydride 7 with thioamide 6 (THF, 60°C), affording both possible addition products 8 and 9 as a 60 A0 mixture. These regioisomers were separated by fractional crystallization. Treatment of each intermediate with excess sodium hydroxide followed by acidification afforded thioimidazolinyl acids 10 or 11. Since picolinic acid 11 was not active in the herbicide screen below 500 g/ha preemergence and 250 g/ha postemergence and had no apparent crop selectivity, it will not be discussed further. 

The Imazethapyr Area. As Figure 3 shows, thione 16 is less effective overall than imazethapyr (2) in preemergence weed control on soybeans, particularly in the broadleaves, where 16 is inactive on morning glory and ragweed. Imidazolinthione 16 is also more injurious to soybeans than imazethapyr and shows no other crop selectivity. Thione 16, postemergence at the same rate, is essentially inactive compared to imazethapyr. The methyl ester (19) of thione 16 is essentially inactive both pre- and postemergence at 63 g/ha. 

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