Weed control herbicide choice

The initial federal approval of an atrazine formulation in the United States occurred December 1, 1958, when the USDA registered Geigy Atrazine 50W for preemergence and postemergence control of several broadleaf weeds on corn and for nonselective weed control in noncrop areas. A wettable powder was the most popular choice for herbicide formulations at that time and could easily be sprayed uniformly across a field. 

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. 

The CEEPES (1993) model describes farmer weed control choices with approximately 500 herbicide and tillage options. The expert opinions of Iowa state economists and weed control specialists were used to obtain the yield and cost changes shown in Table 12.3 for the entire country. The description of the CEEPES model does not explain how replacement herbicides were chosen. However, the actual herbicides chosen are given, and they are very different from those in the Pike et al. (1994) and Danielson et al. (1993) studies. The state weed scientists in the Pike et al. (1994) and Danielson et al. (1993) studies estimated that cyanazine, dicamba, 2,4-D, and bromoxynil would replace 80% of the atrazine on com, while the CEEPES experts said that these four herbicides would only replace 36% of... 

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. 

There are a number of different herbicide treatment alternatives available for weed control in maize cultivation. Such treatments, denoted here by h, (i = 1,. . . , y,. . . n), may involve one single herbicide or sometimes a combination of herbicides that is available commercially pre-mixed or is mixed by farmers before application. A farmer s choice of one of the n available treatments depends on agronomic, economic and climatic conditions, soil composition, weed community, etc. The situation where a farmer chooses the/th treatment is denoted by h = 1, h, = 0, V / 7]. 

Weed seed production. Growers know that most weed seeds produced in their fields this year will reinfest the same land and will have to be controlled next year and into the future. This is an even greater concern if the escaped weeds are resistant to the herbicide of choice. 

Resistance. Herbicide resistance is an increasing problem, particularly on mainly cereal farms. Herbicide resistance is the inherited abiUty of a weed to survive rates of chemicals that normally control the weed. Black-grass, wild oats and ryegrass are grass weeds that have developed a problem of resistance to some of the most cotrmionly applied grass weed herbicides. Once herbicide resistance has been diagnosed then chemical choice will be severely affected. 

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