Weed control used herbicides

Some important weed species ate in the same family as the crop plant, e.g. charlock in oilseed rape or fat hen in sugar beet. Weed control using herbicides is then often difficult as there is not enough selectivity in the herbicide activity to kill the weed without injuring the crop. 

Plants can also be pests that need to be controlled, particulady noxious weeds infesting food crops. Prior to 1900, inorganic compounds such as sulfuric acid, copper nitrate, sodium nitrate, ammonium sulfate, and potassium salts were used to selectively control mustards and other broadleaved weeds in cereal grains. By the early 1900s, Kainite and calcium cyanamid were also used in monocotyledenous crops, as well as iron sulfate, copper sulfate, and sodium arsenate. Prom 1915 to 1925, acid arsenical sprays, carbon bisulfate, sodium chlorate, and others were introduced for weed control use. Total or nonselective herbicides kill all vegetation, whereas selective compounds control weeds without adversely affecting the growth of the crop (see Herbicides). 

The review of triazine benehts studies shows the extensive data collection and analytical effort that is needed to carry out a credible benehts assessment. For the loss of an herbicide used on many crops, and which makes up a large part of the agricultural economy, it is essential to look at effects beyond the farm gate. In this case, losses are substantial because of lower weed control, higher herbicide costs, and indirect costs related to drift damage, sedimentation damage, and losses in reduced tillage com. In 2003, the USEPA estimated an annual value of 1.58 billion in the United States for com (USEPA, 2003). 

On farms practicing no-till com production, herbicides are the main tool for weed control. Without herbicides, notill farmers would have to switch to some other form of tillage operation. They could no longer use no-tillage methods, resulting in much more soil erosion. Triazine and other herbicides in com will continue to represent the main weed control technology for the vast majority of com acreage. 

CgH,3BrN202. A soil-acting herbicide. White crystalline solid, m.p. 158-159" C. It is a non-selective inhibitor of photosynthesis used for weed control In citrus and cane fruit plantations. It is relatively non-toxic to animal life. 

Other Heterocyclic Nitrogen Derivative Herbicides. The herbicides in this group are heterocycHc nitrogen derivatives that do not readily fall into one of the previously discussed groups. They have a wide range of uses and properties. Most of these herbicides are used for selective, pre-and/or post-emergence weed control. Amitrole is used for post-emergence, nonselective weed control in non-croplands and also as an aquatic herbicide (2,296). 

Metal Organics and Inorganics. The metal organic herbicides are arsenicals used for the selective, post-emergence control of grass and broadleaved weeds in cropland and noncroplands. These herbicides are particularly usehil for weed control in cotton and turf crops (2,296,294). CacodyUc acid is a contact herbicide used for nonselective weed control in cropland and noncropland (299). Ammonium sulfamate [7773-06-0] (AMS) is an inorganic herbicide used for control of woody plants and herbaceous perennials (2). 

Miscellaneous Trifluoromethyl Compounds. The herbicides in this group are used for a wide variety of weed-control purposes. Acifluorfen, lactofen [77501-63-4] and oxyfluorfen are used for selective, pre-, and post-emergence weed control in croplands. Fluorochloridone is used for selective, pre-emergence weed control in cropland, and fluridone, fomesafen, and mefluidide [53780-34-0] are used for post-emergence control (296). Fluridone is also used as an aquatic herbicide (2). 

Miscellaneous Other Herbicides. The herbicides in this group are not readily included in any of the preceding groups. Acrolein [107-02-8] (2-propenal) is used as a contact, aquatic herbicide. Sethoxydim, clethodim, and tridiphane are used for selective, post-emergence weed control. 

Between 1979 and 1991, the amounts of herbicide apphed in the United States have remained constant, but the expenditures on herbicides have increased 54%. Agricultural costs accounted for ah. of this increase and more, since herbicide user expenditures in the government/commercial and home sectors combined dropped 3 to 4% during that period. Increased weed control costs related to crop protection have also contributed to the 37% increase, since 1988, in total annual user expenditures for pesticides in general, ie, herbicides, fungicides, and insecticides. In the United States, agricultural uses (ca 1993) account for more than 67% of total pesticide user expenses and 75% of the quantity used annually. Herbicides are now the lea ding type of pesticides in terms of both user expenditures and volumes used (1). 

Chemical, cultural, and mechanical weed control practices have been relatively successful ia reducing yield losses from weeds (448). However, herbicide-resistant weed populations, soil erosion, pesticide persistence ia the environment, and other problems associated with technologies used (ca 1993) to control weeds have raised concerns for the long-term efficacy and sustainability of herbicide-dependent crop production practices (449). These concerns, coupled with ever-increasing demands for food and fiber, contribute to the need for innovative weed management strategies (450). 

Extensive use of two more recendy developed classes of herbicides will further dramatically reduce the amount of appHed to control weeds. The sulfonylurea herbicides are extremely active compounds first discovered in the mid-1970s at DuPont they have been discussed extensively (78). Sulfonjiurea herbicides have experienced a rapid and widespread success since thek commercial introduction in 1982 with chlorsulfuron (Table 5). The sulfonylureas are appHed at rates of 2—75 g/ha. The chemistry of the sulfonylurea molecule permits the synthesis of a very large number of useful analogues, consequentiy many new herbicides are anticipated for crop production. As of this writing (1996), over 350 patents have been issued to about 27 agricultural companies covering tens of millions of stmctures known or expected to be herbicidaHy active. 

Ureides (e.g., diuron, linuron) and triazines (e.g., atrazine, simazine, ametryne) all act as inhibitors of photosynthesis and are applied to soil (see Figure 14.1 for structures). They are toxic to seedling weeds, which they can absorb from the soil. Some of them (e.g., simazine) have very low water solubility and, consequently, are persistent and relatively immobile in soil (see Chapter 4, Section 4.3, which also mentions the question of depth selection when these soil-acting herbicides are used for selective weed control). 

An annual estimated cost of approximately 938,835,000 is required for labor and equipment to apply 204.5 million kilograms of herbicides to 147.6 million hectares of cropland in the United States (Table IV). Mechanical tillage to control weeds between cropping seasons and interrow tillage of crops has been used for centuries as a very effective method of weed control. Approximately 50% of all tillage between crops is done to control weeds. The number of cultivations required for effective weed control within a row crop varies from two to five during a cropping season. 

The estimated cost of equipment, labor, and herbicides for weed control within a specific crop can be very expensive. For example, in 1980, cotton was planted on 5.2 million ha in the United States. The total cost for equipment, labor, and herbicides used to control weeds was estimated to be 547 million (20). Herbicides account for 32% of the total cost while equipment and labor costs were 34% each. 

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