Weedkillers, selective

Trichloroethanoic acid, CCI3COOH. A crystalline solid which rapidly absorbs water vapour m.p. 58°C, b.p. 196-5" C. Manufactured by the action of chlorine on ethanoic acid at 160°C in the presence of red phosphorus, sulphur or iodine. It is decomposed into chloroform and carbon dioxide by boiling water. It is a much stronger acid than either the mono- or the dichloro-acids and has been used to extract alkaloids and ascorbic acid from plant and animal tissues. It is a precipitant for proteins and may be used to test for the presence of albumin in urine. The sodium salt is used as a selective weedkiller. 

MCPA, l-methyl-A-chlorophenoxyacetic acid, Methoxone, CgH ClOj. Made by chlorination of o-cresol followed by reaction with chloroethanoic acid. While crystals, m.p. 118-119 C. As usually obtained, crude MCPA contains both 4- (60%) and 6- (40%) chloro-isomers, and is a light brown solid. Selective weedkiller. 

MCPB, 4-(4-chloro-2-methylphenoxy)-butyric acid, CiiHisClOj. A compound in itself harmless to plants, but when absorbed and translocated in the cells, CnHuClOs is converted to a powerful herbicide, and results in the death of the plant. Acts as a selective weedkiller. Other butyric acid derivatives used commercially are 2,4-Dg and 2,4,5-Tb, the butyric acid analogues of 2,4-D and 2,4,5-T. ... 

Similar in structure to the triazinediones except for the replacement of one nitrogen by carbon, the uracils such as bromacil (5) (B-77MI10702) and lenacil (6) (66USP3 235360) are used as persistent total herbicides at high concentrations, or as selective weedkillers at much lower rates of application. 

Bioactivation can be exploited to obtain selectivity in action. In the field of insecticides, bioactivation in the plant, known as systemic action, is the means used to obtain selectivity in action. Thus, only insects feeding on the sprayed plants will be affected89,111 Similarly, selectivity in the action of weedkillers may be based on selective bioactivation in the target plant90,91> 142> (Fig. 8). 

The reverse situation - selective bioactivation in the uneconomic species — is realized in the selectively acting weedkillers of the phenoxybutyric acid type (Fig. 8). The butyric acid derivatives, as such, are inactive they have to be converted by 0-oxidation to the corresponding phenoxyacetic acid derivatives, which have the required auxin and hence weedkiller action. Since plant species... 

Bromacil is recommended preemergence in asparagus at a rate of 1.8-2.2 kg active ingredient/ha, and in raspberry after planting at 1.1 kg/ha for selective weedkilling. 

Terbacil (Sinbar , Du Pont), introduced in 1966, is also used as selective weedkiller in sugar cane, apple and peach orchards, citrus plantations, established alfalfa and blueberry (Aitken and Arnolds, 1973 Meeklah and McRobb, 1973). 

A number of pyridazines are important as selective plant growth regulators and are used as herbicides, e.g. Pyridate 3-hydroxy-6(l//)-pyrazinone, is used as a lawn weedkiller. 

As herbicides. Complicated chlorine compounds, such as 2,4-D and 2,4,5-T are used as selective weedkillers. They are stable and so persist in the environment. 

Herbicides are used to kill plants. Sodium chlorate, NaC103 and sodium arsenite, Na3As03, were commonly used as weed killers in the first half of the twentieth century, but inorganic arsenic compounds, in particular, are toxic to mammals. Organic herbicides are now used. They are much more toxic to certain types of plants than to others, so they can be used as selective weedkillers. Atrazine, which is a member of a class of herbicides called the triazines, is widely used to kill weeds in cornfields. The triazines contain six-membered rings with alternating carbon and nitrogen atoms. The effects of atrazine on human health are not yet clear. 

A wide range of organophosphorus compounds have some herbicidal activity, but particularly effective as selective weedkillers are compounds (12.112). 

D (2,4-dichlorophenoxyacetic acid) A synthetic auxin used as a potent selective weedkiller. Monocotyledenous species with narrow erect leaves (e.g. cereals and grasses) are generally resistant to 2,4-D while dicotyledenous plants are often very susceptible. The compound is thus used for controlling weeds in cereal crops and lawns. See auxin. 

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