abstract

Introduction: Adjuvant application is one of the most important ways to increase herbicide efficacy and decrease environmental damaging effects of herbicides. In general, It has displayed that a very few of the spray droplets retained on the surface of leaf plants and the majority of them bounce off the leaf surface. Therefore, in spraying processes, adjuvant designed to enhance the absorbing, emulsifying, dispersing, spreading, sticking, wetting, or penetrating properties of pesticides. Adjuvant are most often used with herbicides to help a pesticide spread over a leaf surface and penetrate the waxy cuticle of a leaf or to penetrate through the small hairs present on a leaf surface. Surfactants and crop oils are two types of adjuvant that are used for increasing efficacy of herbicides. In many cases, significant increases have been observed in biological activity with the addition of surfactants or crop oils. For example, the performance of specific graminicides and some sulfonylureas is usually increased by the addition of tank-mix oils. It is generally accepted that the benefit of oils is related to their ability to increase the drying period of droplets during their fly time before their impact on the plants, to improve the spreading of the deposit on difficult-to-wet targets (mainly Graminaceae), to act as solubilizing agents, and above all to enhance the penetration of herbicides into the plants. Among commercially available adjuvants, emulsified vegetable oils have been shown to increase droplet retention and spreading, and enhance absorption and translocation of active ingredients. It has been reported that efficacy of atrazine, bentazone, phenmedipham and rimsulfuron on various weeds were increased by the addition of rapeseed oils to solution spray. Materials and Methods: In order to evaluate the effect of adjuvant concentrations on surface tension of aqueous solutions, an experiment was conducted as completely randomized design with 4 replications at 8 levels of adjuvants (0, 0.01, 0.05, 0.1, 0.15, 0.2, 0.25 and 0.3 (% v/v)). Moreover, the effects of surfactant and vegetable oil on the efficacy of mesosulfuron + iodosulfuron commercial mixture on littleseed canarygrass (Phalaris minor Retz.) were investigated under greenhouse conditions in Agriculture Faculty of Ferdowsi University of Mashhad. Greenhouse study was conducted in 6 concentration levels of mesosulfuron-methyl+idiosulfuron (0, 5.62, 11.25, 22.5, 33.75, and 45 g ai ha-1). This herbicide was applied alone and with these adjuvants including: (i) citogate (a nonionic surfactant), (ii) castor oil and (iii) rapeseed oil each one of them at two levels 0.1 and 0.2 %, the experiments were arranged in a completely randomize design with a factorial arrangement of treatments with four replications. Results and Discussion: When adjuvants alone were applied against littleseed canarygrass, none of them had not phytotoxic effect so that fresh weight and dry weight of plants did not decrease significantly as compared to the control. The results of lab experiment showed the lowest and highest surface tension belonged to Citogate and rapeseed oil, respectively. Also the results of greenhouse experiment indicated all adjuvant were able to increase the efficacy of mesosulfuron + iodosulfuron herbicide. Relatively potency (R) of dry weight increased with usage of Citogate surfactant, castor and rapeseed oils to 1.4, 1.35 and 1.13 respectively. The Citogate surfactant leads to the greatest enhancement of herbicide efficacy. The foliar activity of the tested herbicide enhanced with increasing adjuvant concentration from 0.1 to 0.2 (% v/v). According to the accessible information, surfactants (nonionic Citogate) are suitable for reduction surface tension. Previous studies showed that nonionic surfactant was too effective in decreasing surface tension of spray solution. Despite, it has been observed that vegetable-derived oils could not greatly decrease surface tension. For example, castor oil decreased surface tension of water from 73 to 71.5 mN/m. Whereas, nonionic surfactants can lower the surface tension of the spray solution to 33 to 34 mN/m. It seems that drop in surface tension by the citogate surfactant is considered sufficient to decrease the contact angle of spray solution proplets and makes an extension of droplet on leaf plant. Ultimately, spray droplet spread can be very important in obtaining the desired foliar coverage for contact pesticides and appears to play a role in moving the active ingredient to more absorption sites on the plant. Conclusion: Based on the results of this study; the following conclusions can be made: (1) the ranking of adjuvants to enhance the tested herbicides efficacy was alike this order: citogate > castor oil> rapeseed oil.