| Hexazinone, a triazine herbicide used in forest plantations, has a high leaching potential in coarse textured soils. The primary objective of this project was to improve the selectivity of hexazinone for forest seedlings through three cultural methods that included charcoal root dips, controlled release (CR) carriers, and banded, sodium-induced surface crusts. A bioassay and thin layer chromatography (TLC) laboratory study were used to screen five experimental controlled release carriers. Two field studies investigated the effects of the three cultural methods on weed efficacy and seedling performance.; A greenhouse bioassay study was conducted to estimate the dose-response of loblolly pine (Pinus taeda) seedlings and a germinating grass species to soil-applied hexazinone. The dose-response between the two species was relatively small indicating that selectivity is primarily based on soil retention properties. The TLC study determined the effects of five controlled release carriers on hexazinone retention and mobility in a coarse textured soil. Hexazinone retention in the top 10 cm of soil was 0.48 and 0.16 mg ai kg−1 for the anionic, activated charcoal and the standard water carrier, respectively.; The 1997–98' root dip/controlled release carrier field study revealed that the CR carriers did not improve weed control when compared to the water carrier results. The 1997 seedling growth was inversely related to hexazinone treatments without root dips, and quadratically related to the same treatments with the root dip. In 1998 seedling growth was influenced by hexazinone and carrier type, and not by the root dip. Seedling survival was improved by root dips in 1997, but had variable survival among the three carriers in 1998. The sodium carbonate field study revealed that weed control response was quadratically related to both hexazinone and sodium carbonate, without any interactions. There was no seedling growth response to hexazinone, however, there was a positive, linear growth response to sodium carbonate rates. Seedling survival had a linear interaction between hexazinone and sodium carbonate rates. Survival increased from 23 to 89%, as sodium rates increased from 0 to 3,240 kg treated ha−1, at the hexazinone rate of 1.12 kg ai ha −1. |
