Effects of repeated application on persistence and downward movement of four herbicides in soil

Abstract

The downward movement and persistence of picloram, terbuthylazine, alachlor, and pendimethalin were studied under tropical field conditions in loamy soil. Picloram and alachlor were applied separately at 0·5 and 2·0 kg a.i./ha, respectively, while terbuthylazine and pendimethalin were each applied at 1 · 0 kg a.i./ha. Each herbicide was subjected to 2 experimental conditions: (i) the required amount of herbicide applied 2 times, and (ii) the required amount applied 6 times. The presence of these herbicides in soil was assessed by bioassay techniques. Rice seed (Oryza sativa) and long bean (Vigna sinensis) were used to assay pendimethalin and picloram, respectively, while cucumber (Cucumis sativus) was used for both alachlor and terbuthylazine. The results showed that both alachlor and picloram moved downward faster than terbuthylazine and pendimethalin. Both alachlor and picloram, but not terbuthylazine or pendimethalin, could be detected in 5–10 cm layer at 1 day after application (DAA). The phytotoxic residues of picloram, pendimethalin, terbuthylazine, and alachlor were detected in the 0–5 cm layer until 84, 56, 49, and 35 DAA (2 applications), respectively. However, after 6 applications, the phytotoxic residue of these herbicides in the top layer was reduced. Our results show that pendimethalin was less mobile than the other herbicides. The half-lives of picloram, pendimethalin, terbuthylazine, and alachlor were 9·7, 6·4, 6·0, and 4·4 days, respectively, after 2 applications, but after 6 applications, the half-lives of these herbicides were shorter.