Droplet Makeup and the Effect on Phytotoxicity of Glyphosate in Velvetleaf (Abutilon theophrasti)

Velvetleaf was treated at the three and one-half leaf stage with one, three, or nine drops (1 μl each) of glyphosate and an amphoteric surfactant with the concentration of the droplets adjusted so that the total amount of active ingredient applied to each plant was constant. The phytotoxicity of glyphosate to velvetleaf was dependent on both the concentration of glyphosate and surfactant within the droplet A single concentrated droplet of glyphosate and surfactant was more effective in reducing the growth of velvetleaf than were more dilute droplets in greater number but with the same total quantity of herbicide. If the surfactant concentration was increased, the loss in efficacy was negated, provided the glyphosate concentration in the droplet was sufficiently high. Also, loss in efficiency was negated at increased glyphosate concentration and diluted surfactant level per droplet. The phytotoxic response of velvetleaf treated with one or three drops was greater at 45 than 90% relative humidity. This greater response at low relative humidity continued with increased surfactant concentration in each drop. However, the phytotoxic response was greater at 90 than 45% relative humidity when velvetleaf plants were treated with nine drops. When the surfactant concentration in each drop was increased, the plant's response at the two relative humidity levels did not differ.