Enhancing the Biological Activity of Nicosulfuron with Silicone Adjuvants and pH Adjusters

Abstract

Adjuvants that increased the pH of the spray solution and rapidly solubilized nicosulfuron particles enhanced herbicidal activity with silicone adjuvants under specific conditions. These conditions included high nicosulfuron rates on difficult to control weeds, low spray volumes, and initially acidic spray mixtures. For example, all pH adjusters tested enhanced the activity of nicosulfuron in a spray volume of 140 L/ha with 0.1% w/w silicone surfactant blend on common cocklebur (Xanthium strumarium L.) and large crabgrass [Digitaria sanguinalis (L.) Scop.]. Generally, the most effective pH adjuster was tribasic potassium phosphate followed by triethanolamine. The high pH conditions rapidly dissolved the nicosulfuron particles and usually increased biological activity. However, increasing pH did not always increase biological activity. For example, the silicone-based surfactant and methylated seed oil blend was the most effective silicone adjuvant when applied as the only adjuvant, but the addition of sodium carbonate reduced its activity with on large crabgrass. A possible reason for this difference might be that the silicone surfactant and oil blend would be expected to enhance nicosulfuron uptake through both hydrophilic and lipophilic pathways into the leaf while the increased solubilization caused by the pH adjuster might only increase uptake through hydrophilic pathways. High pH conditions are known to increase silicone surfactant degradation and this could require users to spray silicone adjuvant and pH adjuster mixtures more rapidly than usual. These results generally support the concept that solubilization is necessary but not sufficient for foliarly applied herbicides to express maximum activity.