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Two watersheds were treated with one application of 2,4,5,T + picloram (1:1) at 1.12 kg/ha while one other watershed was treated for 2 consecutive years at the same herbicide rates. Residues of 2,4,5‐T were reduced to trace levels [<10 parts per billion (ppb)] in soils 7 days after application at one location, after 28 days at the second, and after 56 days at the third. Residues of 2,4,5‐T were not detected below 15 cm and usually remained in the surface 2.5 cm of soil. Picloram was reduced to trace levels within 56 to 112 days after application, was not detected deeper than 60 cm, and was usually restricted to the surface 15 cm of soil. There was no evidence of subsurface lateral movement of either herbicide. Surface runoff water contained trace amounts of residue following storms for about 30 days after application. There was no evidence of residue carry over into the second year after treatment in water, soils, or vegetation. Monitoring of cumulative residues in water, soils, and vegetation showed that about 75% of the picloram was dissipated from the ecosystem within 28 days after application and over 90% was lost after 112 days.

The herbicide, 3‐6‐dichloro‐o‐anisic acid (dicamba), dissipated most rapidly from water under non‐sterile, lighted conditions. Pond sediment evidently contained microbial populations capable of decomposing the herbicide. Temperature was crucial in dicamba dissipation, especially in the presence of sediment. Influence of sediment on dissipation rate of dicamba was apparently augmented by light in some cases. Under summer conditions, dicamba at 4.4 kg/ha per surface area of ponds dissipated at about 1.3 ppm/day. Dicamba dissipated as a logarithmic function of concentration with time. Reaction of seedling crops to irrigation water containing dicamba varied among species and cultivars. Relative tolerance from these studies was ranked from most to least tolerant as follows: sorghum [Sorghum bicolor(L.) Moench. ‘RS‐626’ and ‘Pioneer 820’] > cotton (Gossypium hirsutumL. ‘Blightmaster’ > ‘Paymaster’ > ‘Dunn’) > cucumbers (Cucumis sativaL. ‘Straight eight’).