Increasing the pH of the spray water to solubilize the weak acid herbicide nicosulfuron and then decreasing pH below its pKa so that it converts into a neutral form enhances biological activity under some conditions. The water-dispersible granule formulation of nicosulfuron starts as dispersed particles. Adding 1% wt/wt K3PO4 solubilizes nicosulfuron and increases its activity compared to its dispersion without base. The type of buffer and the surfactant HLB or hydrophilic lipophilic balance, a measure of the molecular balance of the hydrophilic and lipophilic groups, altered the activity of nicosulfuron. Adding 1% wt/wt K3PO4 increases the pH, and the optimum HLB ranged from 13 to 17 on large crabgrass. Adding 1% wt/wt H3PO4 reduces the pH and lowers the optimum HLB range from 10 to 14 on large crabgrass. Adding the acidic buffer converts the solubilized nicosulfuron into its neutral form and increases activity under some surfactant conditions. Thus, neutral nicosulfuron is more active with lipophilic surfactants, while ionic nicosulfuron is more active with hydrophilic surfactants. When tested on other species, low HLB surfactants are the most active at low pH. These results support the concept that the physicochemical properties of the herbicide, adjuvants, and weed species should be matched for optimum activity.

The biological activity of rimsulfuron applied with 33 alcohol ethoxylate surfactants was determined on six economically important weed species. The surfactants had hydrophobes ranging from 6 to 24 carbons and hydrophiles ranging from 3 to 32 ethylene oxides (EOs), and the weeds had different leaf orientations, surface characteristics, and chemical compositions. In general, similar activity was observed among weed species with correlation coefficients ranging from 0.62 to 0.94. A few distinct species-specific differences were identified. For example, increasing surfactant size generally increased rimsulfuron activity on velvetleaf, but activity was reduced on giant foxtail with surfactants having the longest alkyl chain and the highest number of EO units. Increasing surfactant size increased common cocklebur control if the right balance between the hydrophobic and hydrophilic group was maintained. The optimum surfactant type with rimsulfuron across six species from both the empirical and the modeling results ranged from a hydrophobe with 17 carbons and a hydrophilic group with 20 to 30 EOs to 24 carbons and 13 EOs.

Adjuvants that increase the pH of the spray mixture and solubilize nicosulfuron can enhance biological activity under specific conditions. These conditions include high nicosulfuron rates, difficult-to-control weeds, low spray volumes, and initially acidic spray conditions. The most effective pH adjusters are tribasic potassium phosphate, sodium carbonate, and triethanolamine. In low spray volumes, these adjusters make the spray mixture alkaline and often enhance the activity of nicosulfuron on common cocklebur and large crabgrass. Alkaline conditions rapidly dissolve the sulfonylurea particles and enhance activity with crop oil concentrate, modified seed oil, and hydrophilic nonionic surfactants. pH adjusters did not enhance activity with lipophilic surfactants. Ammonium sulfate slightly increases the pH of spray mixtures and increases nicosulfuron activity depending on species, adjuvant type, and pH adjuster. These results generally support the concept that herbicide solubilization is necessary to maximize the foliar activity.

Lack of consistent regulation and marketing of adjuvants and complexity of the interaction among plant, herbicide, environment, water quality, and adjuvant have caused general confusion in adjuvant selection among growers. Choosing the best adjuvant is difficult. Growers must chose from thousands of commercial products and are confused by product descriptions with unfamiliar ingredients and functions. Confusing recommendations, aggressive marketing, and lack of unbiased research and educational information make matters even worse. Manufacturer lists of approved adjuvant products, guidelines that set minimum requirements to qualify adjuvants for use with herbicides, and packaging effective adjuvants with herbicides either in the herbicide formulation or packaged in a different container help reduce grower confusion. University adjuvant research and education have aided grower knowledge and understanding of adjuvants by field testing adjuvants and have influenced herbicide label wording and recommendations.