The responses of sorghum and sunflowers to 2,4-D and dicamba residues in clay soils in central Queensland

Abstract

The residual phytotoxicity of 2,4-D mine and dicamba to sorghum (Sorghum bicolor L.) and sunflowers (Helianthus annuus L.) was investigated under central Queensland conditions of variable rainfall and high temperatures. Effects of soil water content, sowing depth, and leaching in clay soils were determined. Phytotoxicity, as measured by decreases in shoot dry matter in pots, increased with herbicide rate and decreased rapidly in moist soil (34% w/w) but was maintained for at least 14 days in dry soil (114% w/w). 2,4-D and dicamba were phytotoxic when leached into the soil but not when they remained on the soil surface. Seedling growth and yield were not affected by 2 kg 2,4-Dha applied at 7 or more days pre-sowing, or by 0.5 kg 2,4-D/ha and 0.14 kg dicamba/ha applied at 1 day pre-sowing, in 5 field environments. When either herbicide was applied at higher rates 1 day pre-sowing, seedling growth was reduced if 25 mm of irrigation was received within 4 days of sowing, and yields were reduced in the field environment that received 144 mm of rainfall within 14 days of sowing.