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.
The pH of the spray mixture controls the solubility and ionic state of weak acid herbicides and thus influences their uptake and biological activity. When the pH of the spray water is below the pKa of the herbicide, increasing pH can increase solubility and improve activity when herbicide solubility limits uptake. However, raising the pH above the pKa makes the weak acid anionic and thus may make it more difficult to penetrate into the lipophilic cuticle and the negatively charged membrane and cell wall. Decreasing the pH below the pKa converts the weak acid into a neutral or unionized form and thus makes it easier to penetrate these lipophilic and negatively charged barriers. pH also influences other herbicide properties including chemical stability, volatility, and chemical compatibility. Thus, manufacturers need to balance a number of properties when they adjust the pH of their adjuvant and herbicide formulations. These studies show significant differences in the biological activity of several herbicides when the pH is increased and decreased with a range of surfactant types. These results support the concept that the physicochemical properties of the herbicide and adjuvant should be matched for optimum activity.
Urea adducts (urea clathrates) of high molecular weight ethoxylated linear fatty alcohols at low doses are very effective enhancers of sulfonylurea herbicide biological activity. Greenhouse and field trials have identified the preferred alkyl chain length and the degree of ethoxylation for maximum biological efficacy with a broad range of sulfonylurea herbicides at low adjuvant doses. Based on experimental data, the optimum fatty alcohol surfactants include a C16–18 hydrophobe with 25 moles of ethoxylation or a C24 hydrophobe with 13 moles of ethoxylation. However, the latter alcohol ethoxylate is not readily soluble in water while the former dissolves slowly and forms gel phases when mixed into cold water. These characteristics make them generally unsuitable for use as unformulated materials. One readily cold water soluble and commercially available form of this surfactant type which presents very similar performance is based on a C18 linear alcohol ethoxylate with 20 moles of ethoxylation, BRIJ® 78. This ethoxylated linear fatty alcohol remains effective after admixture with related nonionic surfactants and transformation into a readily soluble solid form identified as the urea clathrate of the surfactant. Urea clathrates can contain between 40–60% of nonionic surfactant by weight while remaining free flowing with melting points above 80 °C. As a result, they can be used as tank-mix or combined with the dry sulfonylurea herbicide formulation. Because of the high activity at low doses, a concentrated dry sulfonylurea herbicide formulation can be achieved with a specific urea clathrate ATPLUS® UCL 1007. Such a combination potentially offers the user more convenience, lower chemical volumes, and reduced costs.
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