I. CHEMICALS USED FOR WEED CONTROL 



Many herbicides are being used for weed control, and many others are 

 being evaluated experimentally to determine their usefulness. Only those 

 of current general interest and usefulness are described in this report. 



Available information on the degree of toxicity of herbicides is listed 

 in the descriptions of chemicals used for weed control. The symbol LD50 

 (lethal dosage that kills 50% of the experimental animals) precedes each 

 number that indicates relative toxicity. For example, the single acute 

 oral dose for calcium cyanamide, LD50 = 1,400 mg./kg., indicates a rela- 

 tively low oral toxicity. The larger the LD50 number, the less poisonous 

 the herbicide. 



All LD values listed in this guide are based on a single dose of material 

 orally administered to animals, followed by observation of the treated 

 animals for a definite period of time. However, these findings do not indi- 

 cate the possible hazards that may arise from skin contact or inhalation 

 of the substance or substances indicated. Likewise, these data do not ac- 

 curately predict the toxicity of a formulation which may differ according 

 to the solvent or diluent employed. 



PHENOXY COMPOUNDS 



Several compounds in this group, including 2,4-dichlorophenoxyacetic 

 acid (2,4-D), 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), 2-methyl, 4- 

 chlorophenoxyacetic acid (MCPA), and 2,4,5-trichlorophenoxypropionic 

 acid (silvex) are used as post-emergence selective herbicides to control 

 broadleaved weeds in corn, small grains, sorghum, rice, flax, lawns, 

 and to control brush and weeds in pastures, along roadsides, rights-of-way, 

 and drainage and irrigation ditches. Some of the phenoxy compounds also 

 may be applied to the surface of the soil as a pre-emergence treatment to 

 control grasses and broadleaved weeds in corn and other crops. 



Phenoxy compounds usually are formulated and marketed as two basic 

 types. They are of low to intermediate oral toxicity (LD^q = 375 to 1,200 

 mg./kg.) for the various formulations. 



1. Salts 



The most widely used salts of 2,4-D, MCPA, 2,4,5-T, and other phenoxy 

 acids include such organic amine salts as diethanolamine, triethanolamine, 

 alkanolamine, dimethylamine, triethylamine, isopropylamine, and others. 

 These organic amine salt formulations are available chiefly as water- 

 soluble liquids. The amine salt formulations are more phytotoxic per pound 

 of acid equivalent than the other salt forms, and are more effective in con- 

 trolling a wider range of weeds. 



Some of the phenoxy compounds also are commercially available as 

 sodium and ammonium salt formulations. These compounds are available 

 chiefly as water-soluble powders, but some of these herbicides also are 

 available as water-soluble liquids. These salt formulations are satisfac- 

 tory to use on easy-to-kill weeds, such as mustard, pigweed, and lambs- 

 quarters, but they are less phytotoxic per pound of acid equivalent than 

 the amine salts and are not as effective in controlling as wide a range of 

 weeds . 



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