Effect of precipitation on the dissipation, efficacy and selectivity of three chloroacetamide herbicides in sunflower

Most of the herbicides used on sunflower are registered for pre-emergent application. The re- sidual activity of these herbicides ensures control of weed emergence, but the environmental and crop safety risks are higher. The efficacy of pre- emergent herbicides is significantly affected by soil moisture; under dry conditions, their efficacy usually decreases. However, intense precipitation after the application of these herbicides can cause ABSTRACT The aim was to compare the efficacy, selectivity to sunflower and dissipation of three chloracetamide herbicides (acetochlor, metolachlor and pethoxamid) in dependence on precipitation after application. A small plot field trial was carried out with sunflower in Central Bohemia in 2010 and 2011. The remaining concentration of active ingre - dient in the soil of all studied herbicides was lower in the year with a higher temperature and a low level of total natural precipitation at the beginning of the growing season. Higher leaching of metolachlor and pethoxamid was recorded in irrigated plots. No leaching was found in the case of acetochlor. The highest leaching was found for metolachlor (9.2-25.5% in soil layer 5-10 cm). The highest phytotoxicity was found for acetochlor (9.8%) > pethox- amid (4.6%) > metolachlor (1.8%). The mean phytotoxicity in the irrigated plots was 6.9%, compared with 3.9% in the non-irrigated plots. The efficacy of the tested herbicide was affected by wet conditions. The highest efficacy on Chenopodium album, Amaranthus retroflexus , Echinochloa crus-galli and Solanum physalifolium was recorded af- ter application of acetochlor and irrigation.