Iron Source in 2,4-D Tank-Mixtures Influences Weed Control.

Iron (Fe) is commonly used in the turfgrass industry to correct nutrient deficiencies in high pH soils, for disease suppression, moss suppression, or improved turf aesthetics. Particularly in the lawn care industry, Fe may be tank-mixed with 2,4-D containing herbicides to control weeds in the lawn while simultaneously providing Fe nutrition. Since FeSO₄ is known to antagonize certain herbicides and since other cations are known to antagonize 2,4-D amine formulations, our objective was to evaluate the influence of various sources of iron on 2,4-D efficacy when tank-mixed. Greenhouse trials evaluated four iron sources: none, FeSO₄, Fe diethylenetriaminepentaacetic acid (DTPA), and Fe hydroxyethylenediaminetriacetic acid (HEDTA).  FeSO₄ and FeDTPA are commonly marketed for nutrition, whereas FeHEDTA is marketed for broadleaf weed control. Iron application rates were chosen based on label recommendations or common use rates. These four iron sources were applied to dandelions with or without 2,4-D dimethylamine at 1.6 kg ae ha^-1. Without the inclusion of 2,4-D, FeHEDTA reduced dandelion mass compared to other iron sources and the nontreated plants. This was accomplished by a quick injury to dandelion foliage after application followed by recovery. When 2,4-D was applied with FeHEDTA, all measures indicated better dandelion control than 2,4-D alone. Both FeSO₄ and FeDTPA antagonized 2,4-D efficacy when tank-mixed and these treatments provided less dandelion control than 2,4-D applied alone. Results suggest that applicators should avoid tank-mixing FeSO₄ or FeDTPA with 2,4-D, but that tank-mixing 2,4-D with FeHEDTA improves weed control.