304 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1953 



I do not know who claims first honors for using herbicides for brush 

 control. Probably even the Greeks did not overlook the possibility 

 (as earlier races must have), for it would take no great powers of 

 observation to note that storm floods of salt water would kill plants ; 

 and such an observation might be expected to result in the purposeful 

 use of crystals from evaporated ocean water, either for the destruc- 

 tion of unwanted weeds on one's own land, or of the crops of one's 

 enemies. Modern herbicides have been developed from research in cer- 

 tain hormone and growth-stimulating substances. When it was found 

 that a few of these chemicals applied in relatively concentrated 

 dosages would be lethal, a new industry was born. Compounds re- 

 lated to 2,4-dichlorophenoxy acetic acid, because of their effect on 

 broad-leaved herbs but not on grasses, were quickly adopted for lawn 

 management. 



In the winter of 1945-46, 1 gave my first consideration to the use 

 of herbicides for brush control. At that time I located but one pub- 

 lished reference to effects on a few woody species, and these only 

 on a small scale. At least one manufacturer had been applying 

 herbicides the previous summer on a pilot-plant basis, but no data 

 were available. From then on, the activity has mushroomed like an 

 exploding atom bomb. 



CHEMICALS 



Chemically, the materials now mostly used are derivatives of both 

 2,4-dichlorophenoxy acetic acid (2,4-D) and 2,4,5-trichlorophenoxy 

 acetic acid (2,4,5-T). The latter was found effective against certain 

 species, especially blackberry, for which 2,4-D was useless. Now 

 various mixtures of D and T, or T alone, are promoted. Salts, amines, 

 and the acids themselves have yielded their place to esters. The 

 original esters were methyl, ethyl, butyl, propyl, and other so-called 

 "high-volatility" compounds. Because of extraordinary damage suits, 

 following destruction of such sensitive crop species as tomatoes, cot- 

 ton, and grapes, industry developed the so-called "low-volatility" 

 esters. These have complex organic radicals, one of which is a 

 polyethylene glycol butyl ether group. 



In addition to 2,4-D and 2,4,5-T, ammonium sulfamate is widely 

 advertised and frequently used. This chemical, corrosive to metal 

 and thus presenting problems of its own, is dissolved in water and 

 applied blanketwise as a foliage spray. The cost for one spray is 

 considerably more than for a single foliage D-T spray, but the effects 

 are said to be the equivalent of two or more such D-T sprays. This 

 chemical is relatively unsuccessful in killing the roots of certain root- 

 suckering woody plants, and the results of spraying the herbaceous 

 areas, some becoming predominantly grasses and others f orbs, vary er- 

 ratically. Since botanical analyses have never been made of the pre- 

 sprayed vegetation, it is difficult to estimate the amount of destruction 



