of 2,4,5-T is no longer permitted in the United States following a series of Environmental Protection Agency directives in the 1970s.

Which of these four major chemicals (2,4-D, 2,4,5-T, picloram, or cacodylic acid) was chosen for a specific application depended on the desired effects. 2,4-D and 2,4,5-T are chlorinated phenoxy acids, and each is effective against a wide array of broadleaf plant species (Irish et al., 1969). They persist in soil only a few weeks (Buckingham, 1982). Picloram, like 2,4-D and 2,4,5-T, regulates plant growth. Compared to 2,4-D, picloram is more mobile and therefore better able to penetrate the plant's roots and be transported throughout the plant's tissues. Unlike the phenoxy herbicides, picloram is extremely persistent in soils. The fourth compound, cacodylic acid, contains an organic form of arsenic. Cacodylic acid is a desiccant, causing a plant's tissues to lose their moisture and eventually killing the plant.

The different types of herbicide used by U.S. forces in Vietnam were identified by a code name referring to the color of the band around the 55-gallon drum that contained the chemical. These included Agents Orange, White, Blue, Purple, Pink, and Green (see Table 5-1). From 1962 to 1965, small quantities of Agents Purple, Blue, Pink, and Green were used. From 1965 to 1970, Agents Orange, White, and Blue were employed; from 1970 to 1971, only Agents White and Blue were used in the defoliation program (Young and Reggiani, 1988).

Agent Purple was a 5:3:2 mixture of the n-butyl ester of 2,4-D and the n-butyl and isobutyl esters of 2,4,5-T that was used on broadleaf plants. Because of its volatility, Agent Purple was replaced by Agent Orange in 1965. Blue was the code designation for a liquid formulation of cacodylic acid and its sodium salt. The term Blue was first applied to cacodylic acid in a powder form that was mixed in the field with water. It was later replaced by the liquid formulation