Effect of adjuvants on physical properties of glyphosate and PPO-inhibiting herbicide spray mixtures

Adjuvants are known to enhance spray droplet retention on leaf surfaces and penetration of herbicide active ingredients through cuticles due to changes in physical properties such as density, viscosity, surface tension (SFT), and contact angle (CA) increasing leaf wettability. However, previous research has shown that the performance of an adjuvant is dependent on the herbicide with which it is applied, the plant species, and environmental conditions. The objectives of this study were to determine the effect of adjuvants on these physical properties when glyphosate and lactofen are applied alone and in combination and to determine if these changes can be correlated to herbicide efficacy. The impact of the addition of the adjuvants into the treatment solutions was greater on viscosity than on density values. Overall, adjuvants significantly decreased the SFT of treatment solutions when compared to either water or herbicides alone. In addition, reduced CA was observed due to the reduction in surface tension. However, results were adjuvant- and species-dependent. Herbicide efficacy was only partially explained by the changes in these physical properties. Observations from this study highlighted the importance of adjuvants on reducing SFT and CA properties of spray solutions; however, further investigation is needed to better understand the factors influencing herbicide uptake and how they are correlated in order to maximize herbicide efficacy.