Environmental fate of terbuthylazine and hexazinone in a planted forest steepland Recent Soil, New Zealand

Abstract

Background: The use of herbicides for weed control in New Zealand planted forestry is important for primary production. The objective of this study was to assess the environmental fate of terbuthylazine and hexazinone in a Recent Soil following an aerial post-plant herbicide application in a New Zealand planted forest. Methods: Soil and forest litter were collected from four plots within a recently harvested and replanted Pinus radiata experimental sub-catchment. Samples were taken before and up to 6 months after the aerial application of terbuthylazine and hexazinone. Findings : There was no drainage below 1-m depth in the first month after spray application. On the following month, there were six drainage events ranging from 12 to 36 mm. Both terbuthylazine and hexazinone were located predominantly in the 0–10 cm soil depth, followed by the litter layer with only small amounts detected below 10 cm soil depth. Following a month of drainage events, hexazinone showed an increase at a soil depth of 10–50 cm. Both terbuthylazine and hexazinone rapidly dissipated with time after spray application, and predicted half-lives for both active ingredients were less than 1 day. Conclusions: The first 2 weeks after spray application were found to pose the greatest potential risk of movement off-site, after which the risks were low. Organic matter management practices that influence the amount and distribution of forest floor, litter and harvest residue have an important role in determining the environmental fate of both terbuthylazine and hexazinone.