The Interaction of Organosilicon Surfactant Type and Spray Volume on Diclofop-methyl Efficacy in Control of Winter Wild Oat

Diclofop-methyl is labeled for use in wheat and barley to control many grassy species, e.g., the genus Avena. Efforts should be made to use diclofop-methyl correctly, allowing the reduced doses to be applied. The response of herbicides to spray volume is different. After determining a suitable spray volume for a foliage-applied herbicide, the next step is to adjust it. The spray volume can be adjusted by two methods the change in application speed or nozzle size. If less spray volume is necessary to apply an herbicide, it is needed to increase application speed. It causes the spray droplets to be more bounced or shattered from the leaf surface, causing the herbicide not to achieve optimal efficacy. Therefore, selecting a smaller orifice nozzle is much more applicable, of course, if the spray drift is controlled. The surface tension of water, which is used to spray herbicides, can be slightly reduced after adding the formulation of herbicides. Therefore, the relatively high surface tension of the spray solution poses three main problems. First, the spray droplets can easily be bounced off the leaf surface. Second, those remaining on the leaf surface after impact have a relatively spherical shape. Third, the crystalline wax in the cuticles, is considered an essential barrier to penetrating herbicides into the leaf tissues. It is well-established that the three main issues mentioned above can be addressed by selecting a suitable surfactant to add to the spray solution. This addition enables optimal efficacy of the herbicide. Consequently, numerous previous studies have highlighted the superiority of trisiloxane surfactants over non-silicone surfactants in enhancing herbicidal activity. This study aims to assess whether the effect of spray volume, adjusted by changing nozzle size, on the herbicidal activity of diclofop-methyl could be influenced by two types of trisiloxane surfactants – one with super wetting properties and the other with non-super wetting properties.

A greenhouse trial was performed as a dose-response relationship at the Bu-Ali Sina University, Hamedan, Iran. The experiment was designed as a four-factor completely randomized design. The first factor was the dose of diclofop-methyl (Illoxan® EC 36%) including 0, 112.5, 225, 450, 900 (labeled dose), and 1350 g ha-1. The second factor was spray volume, including 60, 120, 240, and 480 L ha-1, which were adjusted using 1100075, 110015, 11003, and 11006 flat fan nozzle, respectively. The third factor was two types of trisiloxane surfactants, Break-Thru® S 233 having a non-super wetting property and Break-Thru® S 240 having a super wetting property. Both are non-ionic surfactants and manufactured by Evonik company in Germany. They formed their critical micelle concentration (CMC) at 0.1% v v-1 at which the surface tension of distilled water (72.1 mN m-1) containing Break-Thru® S 233 and Break-Thru® S 240 was measured to be 24.1 and 22.6 mN m-1, respectively. The fourth factor was surfactant concentration, including 0, 0.0125, 0.025, 0.05, 0.1, 0.2, 0.4, and 0.8% v v-1 (a range from ⅛ to 8 CMC, respectively). A compressor sprayer was used to apply the treatments at 300 kPa spray pressure. A nonlinear regression analysis was conducted to analyze the ‘drc’ using the software R.

A 40% increase in the ED50 value occurred with increasing the spray volumes from 60 to 480 L ha-1 (536.4 and 865.1 g ha-1, respectively), indicating a negative relationship between diclofop-methyl activity and spray volume. Adding Break-Thru® S 233 at 0.025% v v-1 to 60, 120, 240, and 480 L ha-1 spray volumes caused a 1.16, 3.31, 2.04, and 2.13-fold decrease in the ED50 value compared with no surfactant at their corresponding spray volumes, respectively. While, adding Break-Thru® S 240 at 0.025% v v-1 to 60, 120, 240, and 480 L ha-1 spray volumes caused a 1.39, 1.32, 1.34, and 1.19-fold decrease in the ED50 value compared with no surfactant at their corresponding spray volumes, respectively. A decrease in the ED50, attributed to the addition of surfactants, signifies an enhanced activity of diclofop-methyl against sterile oat. This improvement may stem from a reduction in the surface tension of the spray solution, resulting in an expanded retention and/or spreading area of the spray droplets on the leaf surface. This, in turn, facilitates increased penetration of the herbicide into the leaf tissue. These findings indicate that Break-Thru® S 233 works better when added at low concentration to a low-volume spray solution, while Break-Thru® S 240 works better when added at high concentration to a low-volume spray solution. It can be attributed to the difference in the wetting property of surfactants. The natural relationship between diclofop-methyl activity and spray volume at higher concentrations of Break-Thru® S 233 may be related to its phytotoxic effect, resulting in an antagonism effect on diclofop-methyl activity against sterile oat. In the case of Break-Thru® S 240, the relationship mode between diclofop-methyl activity and spray volume was not affected by surfactant concentration indicating the lack of phytotoxic effect by this surfactant.

The current study revealed a negative relationship between diclofop-methyl efficacy and spray volume, which was adjusted by nozzle size. Although this finding differs from a previous study in which spray volume has been adjusted by application speed, they showed that the effect of spray volume on the herbicide’s efficacy depends not only on herbicide but also on how it is adjusted. The smaller, more concentrated spray droplets are necessary to get a better action of diclofop-methyl against sterile oat. However, the negative relationship observed between diclofop-methyl efficacy and spray volume could also be observed with two types of trisiloxane when they surfactants, were used at 0.0125 to 0.1 v v-1. While, when they were used at 0.2 to 0.8% v v-1, the relationship mode changed from negative to neutral for Break-Thru® S 233, but it did not change for Break-Thru® S 240. Moreover, Break-Thru® S 240 works better when added at high concentration to a low-volume spray solution due to the danger of spray run-off, while Break-Thru® S 233 works better when added at low concentration to a low-volume spray solution due to its phytotoxic effect.