Herbicidal crop tolerance

Sulfonylurea herbicides are generally applied to crops as an early post-emergent herbicide. Crops that are tolerant to these herbicides quickly metabolize them to innocuous compounds. At maturity, residues of the parent compound in food and feed commodities are nondetectable. Metabolites are not considered to be of concern, and their levels are usually nondetectable also. For this reason, the residue definition only includes the parent compound. Tolerances [or maximum residue limits (MRLs)] are based on the LOQ of the method submitted for enforcement purposes and usually range from 0.01 to 0.05 mg kg (ppm) for food items and up to O.lmgkg" for feed items. There is no practical need for residue methods for animal tissues or animal-derived products such as milk, meat, and eggs. Sulfonylurea herbicides are not found in animal feed items, as mentioned above. Furthermore, sulfonylurea herbicides intentionally dosed to rats and goats are mostly excreted in the urine and feces, and the traces that are absorbed are rapidly metabolized to nontoxic compounds. For this reason, no descriptions of methods for animal-derived matrices are given here. 

Sulfentrazone is a broad-spectrum, pre-emergent herbicide that provides good control over broadleaf weeds, grasses and sedges in crops and turf. The metabolism of sulfentrazone in animals and plants is similar. The major plant metabolite of sulfentrazone is 3-hydroxymethyl sulfentrazone (HMS). The soybean tolerance of 0.05 mg kg includes residues of sulfentrazone plus its major metabolite, HMS. The rotational crop tolerance includes residues of sulfentrazone and its major metabolites, HMS and... 

G-32911, simetryn Simetryn was one of the first methylthiotriazine candidates tested, but was developed slowly until it was confirmed that rice had a higher crop tolerance to simetryn than to prometryn. It found its commercial place in the transplanted rice of Japan and in other countries in the subtropical rice belt. Simetryn is used to control broadleaf weeds in mixtures with other herbicides that are active against grasses. 

On the other hand, the broad weed control spectrum of chloramben, which was introduced in 1959, brought with it widespread acceptance. As an over-the-row band for control of both broadleaf and grass weeds, cost was minimized. With good crop tolerance, chloramben dominated the early soybean herbicide market. During 1972, 85% of the product was applied as the granular formulation (Table 4.2). However, as application practices changed for the soybean crop, banded applications became an inconvenience. Herbicides such as metribuzin (a triazine herbicide), linuron, and bentazon, which could be applied as an affordable broadcast treatment, soon became products of choice, and the marketing of chloramben was eventually discontinued in 1990. 

Application versatility, combined with a high level of crop tolerance, led to atrazine being the most widely used corn herbicide in history. In fact, atrazine led the US com herbicide market within several years of its introduction. Illinois corn farmer surveys show a steady increase in the use of atrazine in corn from its debut in 1960, with 75-85% of com being treated with atrazine since 1975. 

Producers attempt to eliminate perennial weeds in small fruits with soil fumigation and plastic mulches. In caneberry, low rates of simazine can be used in heavier soils. Caneberry has good crop tolerance to other preemergence herbicides such as oryzalin, napropamide, and pendimethalin, though their weed spectrum may be reduced compared to simazine. 

Beginning in the 1950s, when triazines such as simazine, atrazine, prometryn, and ametryn were first synthesized and tested as selective herbicides in the Geigy laboratories in Basel, Switzerland (Gast et al 1955), massive research efforts have focused on the transformation and use of these compounds in the environment. The -triazines represent one of the most widely used and probably the most extensively studied family of herbicides. One of the driving forces for this research was the outstanding performance of triazines with respect to their selective herbicidal effects and crop tolerance. 

The use of antidote chemicals or "anti-herbicides11 on crops to counteract the effect of herbicides and thereby increase crop tolerance is a highly promising procedure. This technique is already being used in one series of compounds and may enjoy greater acceptance as more "anti-herbicides" become available. 

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. 

Use of Subtoxic Herbicide Pretreatments to Improve Crop Tolerance to Herbicides... 

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. 

Increasing Crop Tolerance to Herbicides with Herbicide Pretreatments... 

At this point, evidence that similar molecules acted as effective antidotes by inducing needed metabolic pathways for herbicide detoxication was at most very speculative. However, another hypothesis emerged. Could early herbicide pretreatments increase crop tolerance to these herbicides by elevating the substrates and enzymes needed for detoxication While not a new concept in animal systems, such an idea has received little attention in plant systems and it certainly has not been exploited in any practical way. The whole idea has seemed much more credible with the study by Jacetta and Radosevich (19) of photosynthetic recovery in corn after treatment with atrazine. More specifically, they showed that inhibition of photosynthesis was reduced and the rate of recovery enhanced in corn plants treated for the second or third time with atrazine compared to "first exposed" plants (Figure 2). Furthermore, the faster recovery was related to enhanced rates of atrazine metabolism in the previously treated plants (Table III). 

Stimulated by the antidote research, as well as by the work of Jacetta and Radosevich (19). we decided to examine the influence of pretreatment with subtoxic rates of several herbicides on later crop tolerance to these same herbicides. For these growth room studies (25°C/18°C, 16h photoperiod, light intensity 400 uE/mZ/sec, 75% relative humidity), pretreatments at concentrations of 0.1% to 10% of the final herbicidal rate were given as a root drench or seed treatment to seeds planted in moist vermiculite in styrofoam cups. Herbicidal treatments were later applied to the roots. Plants were harvested 8-14 days after herbicide treatment (Table IV). 

For herbicide (glyphosate) tolerant soybean, it is noted that adoption concurs with a background trend towards post-emergence herbicide applications from 1990-1995 [7]. Glyphosate (a.o. sold as Roundup) can be used post emergence over the top of the herbicide tolerant soybean crops. As a broad spectram herbicide it affords protection against a wide variety of aimual and perennial weeds, including the difficult-to-control water-... 

A new synthesis of 2-aryl-1,2,4-triazin-3-ones and 2-aryl-l,2,4-triazepin-3-ones from convenient starting materials has been demonstrated. Of these compounds, the triazin-3-ones were found to have herbicidal properties and with appropriate aromatic substituents, weed control can be obtained at low application rates. However, the weed control/crop tolerance ratio may limit the commercial application of the more active triazinones. 

Crop Selectivity. I values for inhibition of the ALS enzyme from a variety of crop andrweed species have been determined (2). In all cases the highly active herbicides proved to be potent inFibitors of plant ALS enzyme. Crop tolerance results from rapid metabolism of... 

Different to the cases described above, the herbicide propanil is detoxified in rice and weed species by the action of an aryl acylamidase (aryl-acylamine amido-hydrolase). High activity of this enzyme in rice confers crop tolerance. In Colombia, a biotype of Echinochloa colona was found that is resistant to propanil. Enzyme tests with extracts from this biotype revealed an about three-fold higher activity of aryl acylamidase in the resistant than in a susceptible biotype. It was concluded that resistance of the E. colona biotype is based on enhanced propanil detoxification [74]. 

The natural glutamine synthetase inhibitor phosphinothricin as well as its synthetic racemate glufosinate are broad spectrum post-emergent herbicides that will play a role in future agriculture due to the unique mode of action. Because these GS inhibitors fully control weeds that have evolved resistances against other types of herbicides, the use of phosphinothricin-containing herbicides in tolerant crops will remain an important option for future sustainable agriculture. 

The discovery that crop tolerance for a particular herbicide could be enhanced has led to the identification of many compounds that antidote the herbicidal effect of the thiolcarbamate herbicides. iV,A/ -Diallyl-dichloroacetamide was the first material commercialized with thiocarbamate herbicides for use in corn. This same concept also has application for other types of herbicides. Also it was discovered that repeated application of the same thiolcarbamate led to rapid microbial degradation of the material in the soil. Addition of extenders to the thiolcarbamate herbicide counteracted with degrading effect of the microorganisms in the treated soil. 

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