Brazilian savanna cotton growers in fear of phytotoxicity have been using the herbicide alachlor below the minimum dosage recommended by the manufacturer, with low residual activity. Two experiments were carried out to study factors related to alachlor selectivity to cotton. The first assay used germination boxes with sand substrate to study two alachlor rates (0 and 96 µg kg-1) at temperatures of 20, 25, 30, and 35 ºC, combined with three levels of substrate moisture (40%, 60%, and 80% of water holding capacity) and evaluated after 10 days. Seedling growth was affected by environmental conditions, but this response was reduced or cancelled when alachlor was present, with better conditions causing proportionally greater growth reductions. The herbicide caused reductions in aerial part traits and, to a greater extent, in root length. The second assay consisted of pots filled with soil substrate and three irrigation treatments (23, 34, and 45 mm) studied after application of two alachlor levels (0 and 2.88 kg ha-1), and evaluated after 21 days. Higher irrigation levels caused reductions in fresh and dry matter of the roots. Alachlor reduced all variables measured in the aerial part of the cotton plants, but this effect, in general, had low intensity and was similar for all the irrigation levels. The independent effect between alachlor and irrigation does not support the assumption that higher water levels would increase herbicide leaching and its phytotoxicity to cotton.
alachlor; cotton; phytotoxicity; injury
