Effect of irrigation and adjuvant on residual activity of pendimethalin and metazachlor in kohlrabi and soil

Metazachlor and pendimethalin are the most used herbicides in kohlrabi. The main objectives of the present study were to evaluate the residual activity of herbicides pendimethalin and metazachlor in kohlrabi and soil under different irrigation regimes and to evaluate the effect of soil adjuvants on the residual activity of tested herbicides. Pendimethalin dissipation half-life (17.3-38.3 days) was higher than metazachlor dissipation half-life (12.1-16.8 days). The pendimethalin half-life was not affected by an adjuvant, irrigation, and an experimental year. Pendimethalin mobility in the soil was affected more by natural precipitation than by irrigation. The use of adjuvant did not affect pendimethalin leaching in dry weather conditions. In wetter natural conditions, a higher pendimetha-lin leaching was found at early-irrigated plots treated by an adjuvant (9.39% of the applied dose was detected in the soil layer 5-10 cm). Metazachlor dissipation half-life was not affected both by an adjuvant and by irrigation. In the soil layer 5-10 cm, metazachlor was detected only in 2016 on intensively irrigated plots without the use of adjuvant (0.072 µg/g). A concentration of pendimethalin in kohlrabi tubers ranged between 2 and 7 µg/kg. The highest concentration of pendimethalin was detected in tubers, which were intensively irrigated shortly after the application of herbicides without an adjuvant, especially when natural precipitation was high. Metazachlor was not detected in any of the tested kohlrabi samples.