Selection of herbicide

The development of herbicide-resistant weeds has also been an influence on the selection of herbicides used on field corn or soybean. Weed resistance now affects nearly every decision a farmer makes about herbicide selection either a farmer is trying to control resistant weeds or is selecting herbicides that may reduce the possibility of weed populations becoming resistant. The adoption of the imidazolinone- and sulfonylurea-tolerant com hybrids mentioned above was in part a response to the presence of atrazine-tolerant pigweeds or kochia in many fields. However, a recent decrease in die use of imidazolinone and sulfonylurea herbicides can also be attributed to the development of populations of weeds that have become resistant to these herbicides. 

A variety of 2,5-diaryl-, 2,5-dialkyl- and 2-alkyl-5-aryl-l,3,4-oxadiazoles show herbicidal activity, particularly against broad-leafed weeds and grasses in crops such as rice and corn (80JAP8027042, 76MIP42301). Several reports on a wide selection of herbicidal uses for oxadiazolinone (124) appear yearly in the recent literature. 

Herbicides are sometimes also used in urban areas for control of weeds on railway lines and roadside verges, and around areas of hard standing. Some of the herbicides used are sufficiently persistent to be able to be washed into drains or soil, and possibly percolate into groundwater. One of the most common of these herbicides is atrazine, which has been found in the groundwater of many countries. Awareness of the potential for contaminating groundwater and careful selection of herbicides, where they are required, has significantly decreased contamination in many of the areas where it was common. 

The cause of the selectivity of herbicides of the triazine type has also been investigated by many authors. Roth (1957) investigated the resistance of maize to simazine, and established that the freshly pressed sap of the maize plant decomposes simazine during 100 hours of incubation, while simazine is not decomposed in the pressed-out sap of wheat, which is sensitive to simazine. However, this detoxifying effect does not occur after heat treatment of the pressed-out sap of the maize plant, indicating that a heat-sensitive substance is responsible for the effect (Roth and Knusli, 1961). The substance that detoxifies simazine has been isolated by Castelfranco er a/. (1961). This product, which is sensitive to heat in the raw state but thermostable after purification, proved to be identical with the... 

The selectivity of herbicides is not absolute and depends on a number of factors. Transport of the herbicide to the site of action in the plant plays a role, as does absorption. One of the most important features is a higher rate of detoxification of the herbicide in the desirable plants when compared with the weeds. Some of this tolerance is genetically determined, and research is being done to enhance it by the techniques of biotechnology that we discussed in Chapter 13. 

The banning of atrazine had agronomic and economic consequences in the agriculture of northern Italy. In the four major regions of the Po Valley, the maize crop represented about 12.5% of the total UAA (utilised agricultural area) and weed control of this crop was mainly with atrazine. There was therefore an urgent need to find substitute chemicals. The selection criteria adopted by the farmers for the choice of herbicides were examined in a recent survey (Infomark, 1993). In Table 2.5 the 13 major factors influencing the selection of herbicide by farmers are listed in order of importance. 

The choice of characteristics was influenced by the results of Infomark (1993) based on an interview survey of 319 maize-cultivating Italian farmers. The survey concerned, inter alia, the farmers criteria for their selection of herbicide treatments. The most important criteria was experience with the product (69% of the interviewed farmers reported this criterion). Other important criteria include low cost (55%), absence ofresidues in soU (54%), selectivity in relation to com (42%), toxicological class (41 %) and wide range of action (38%). The least important criteria turned out to be specific action on dicotyledons (7%) and company s assistance (15%) (see also Sbriscia Eioretti etaZ., 1995, Table 5). 

The limitation in cropping systems, lack of rotation of herbicide chemistry or mode of action, limitation in weed control techniques, reduction of dose rates, etc. are major drivers for the selection of herbicide resistances. Regular country based surveys often make clear that farmers are aware of the problems and their causes. A survey in Germany in 2004 showed that 94% of the farmers are aware that the repeated use of the same herbicide, and 89% that the reduction of dose rates, causes the development of herbicide resistance. However, 86% of the farmers are forced to reduce their costs and they do not have a lot of scope with their weed management techniques [6]. 

An ingenious method for increasing the selectivity of herbicides in the phen-oxyacetic acid series, by )8-degradation of intrinsically harmless homologues, was recounted in Section 3.6 (p. 108) and illustrated in Fig. 3.6. 

Herbicides are usually sold imder a wide range of proprietary names which can be very confusing, but the cortrmon rrame of the active material must always be stated on the container. In the text of this book the cormnon name of the chemical is used when referring to herbicides. The selectivity of herbicides depends on a... 

As indicated in Figure 1, a number of different enzymes participate in the detoxification of herbicides in plants. Some herbicides are acted on by only a single enzyme, whereas for others several modifications occur in sequence. Not surprisingly, the same enzymes that are responsible for selectivity of herbicides in crops are also responsible for herbicide resistance in weeds. 

Early work on aryloxyphenoxypropionates failed to show any metabolic effects except those on acyl lipid synthesis. The general inhibition of labeling of all acyl lipid classes but not that of sterols or terpenoids led to the idea that de novo fatty acid synthesis was being reduced. In fact, there is little effect on fatty acid elongation (e.g.. Table 3.13). Of the two enzyme systems involved in de novo synthesis, it has now been established that the aryloxyphenoxypropionates and cyclohexanediones only affect acetyl-CoA carboxylase. Moreover, this carboxylase from dicotyledons (resistant) appears to be different from that in monocotyledons (sensitive) because the selectivity of herbicides (including stereospecificity) is retained during in vitro measurements (Table 3.14). 

Transgenic plants, resistant to bleaching herbicides, are a future possibility. Integration of a resistant gene into a crop plant would make it simpler to obtain selectivity of herbicides to weeds rather than crops. 

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