Introduction: An adjuvant has been defined as ‘a material that when added to a spray solution improves or modifies the performance of an herbicide’. There are five categories of adjuvants including drift retardants, extenders, stickers, wetters, and penetrants. The latter can chemically be divided into mineral or vegetable groups. Although some reports have found that vegetable oils are less effective than mineral oils, in view of the environmental concerns, the use of vegetable oils seem to be a suitable alternative to mineral oils because they offer a series of advantages such as being biodegradable and renewable resources. The plants essential oils are extracted from the medicinal plants not only have an adjuvancy activity at low concentrations but also have an herbicidal activity at high concentrations. It is believed that the benefit of vegetable oils as adjuvant can be related to their ability (i) to increase the drying period of the spray droplets during their fly time before impacting the leaves, (ii) to prevent bouncing off after impacting the leaves, (iii) to decrease contact angle of the droplet on the leaf surface and improve the spreading of them on the leaf surface, (iv) to delay crystallization of the active ingredient on the leaf surface, (v) to reduce the volatizing and photo-decomposing period of the herbicide active ingredient, (vi) to act as a penetrant agent on the leaf epicuticular wax in order to enhance the penetrability of the herbicide active ingredient into the plants. A fundamental part of all agrochemical researches is search to detect the natural chemicals offering adjuvancy properties to enhance the biological activity of agrochemicals.Therefore, we conducted a dose-response experiment to determine the potential influence of Blackseed, Fennel, Olibanum, and Rose oils as oily based-adjuvants in enhancing the imazethapyr activity against jimsonweed.Materials and Methods: The jimsonweed seeds were collected from plants in the fields of Qazvin and were stored in the dark at room temperature until further use. Bioassays were conducted in a greenhouse located on the Ferdowsi University of Mashhad, Iran. The seeds were washed every 1 h for 7 days for a total of 168 washings to remove seed germination inhibitors according to Andersen. They were sterilized with 5% (v/v) sodium hypochlorite solution for 10 min and rinsed with distillated water twice. Then, 25 jimsonweed seeds were sown in 2 L plastic pots filled with a mixture of sand, clay loam soil, and peat (1: 1: 1). The pots were irrigated three times a week. At the 1- and 2-leaf stage, the seedlings were thinned to 4 plants pot-1 and fertilized twice with 20 mL of a N: P: K (20: 20: 20) fertilizer with a concentration of 3% (w/v). The pots were irrigated every three days with tap water. Treatments were sprayed at the 4-leaf stage. Experimental design was a factorial arrangement with the 5 doses of imazethapyr (Pursuit® 10% SL) and the four essential oils at concentrations of 0.5 and 1% (v/v) in a completely randomized design with four replications. The emulsifier alkylarylpolyglycol ether was used to emulsify the substances in spray solution (95% oil+5% emulsifier). Treatments were applied at 180 L ha-1 at 200 kPa using a calibrated moving boom sprayer with an 8002 flat-fan nozzle. Three weeks after spraying, biomass from each pot was collected and then weighted after oven-drying at 75°C for 48 h. The data were subjected to a non-linear regression analysis using the following logarithmic logistic dose-response model described. The relative potency that is the horizontal displacement between the two curves was calculated by the ratio of doses producing the same response.Results and Discussion: The slope of dose-response curves for jimsonweed to imazethapyr without or with each adjuvant were significantly similar as indicated by the same slope (b), so that they could be considered to be parallel. This indicates that the adjuvants were biologically inactive if they are used alone (without herbicide) at the tested concentration. Based on shoot fresh and dry weights of jimsonweed, the values of ED10 (6.47 and 7.50 g a.i. ha-1), ED50 (18.08 and 19.01 g a.i. ha-1), and ED90 (50.48 and 43.54 g a.i. ha-1) were obtained. All these values decreased in the presence of all the adjuvants. The relative potency values were significantly different from 1, indicating a significant enhancement in the imazethapyr activity when the adjuvants were added to the spray solution. Based on fresh weight of jimsonweed, the imazethapyr activity was improved up to 4.61-fold with addition of Fennel oil at 1% (v/v). Nonetheless, based on fresh weight of jimsonweed, it was improved up to 4.50-fold with addition of Olibanum at 1% (v/v). Based on dry weight of jimsonweed, the adjuvants were ranked according to their performance in improving imazethapyr activity as follows: Fennel>Olibanum>Blackseed> Rose when they were applied at 0.5% (v/v). But, at 1% (v/v), they were ranked as follows: Olibanum>Blackseed>Rose>Fennel. There are no reports available about the effect of the tested adjuvants as an adjuvant for herbicide, but similar studies were reported by Cabanne who found that the clodinafop-propargyl activity was enhanced up to 5- to 6-fold with adding pine essential oil (Pinus pinaster Ait.) to spray solution at 0.5% (v/v).Vegetable oils have also been found to be effective adjuvants for several post-emergence herbicides. Only in case of Blackseed oil, increasing the concentration had a significant effect to enhance the imazethapyr activity as compared to others. The benefit of concentration effect has formerly observed by researchers who reported that there was a strong concentration effect for rapeseed oil since their influence on phenmedipham and glyphosate uptake and activity was lower at 0.1% than at 1% (v/v).Conclusion: Since the main barrier for diffusing a hydrophilic herbicide such as imazethapyr into the leaf tissue is hydrophobic cuticle, thus oil based-adjuvants have been known to work best with them. On the other hand, a shift from non-environmentally friendly adjuvants to environmentally friendly (namely from mineral oilbased adjuvants to vegetable oil-based adjuvants) in adjuvant trend has been occurred due to being biodegradable and renewable. Thus, it is necessary to look for new essential oils offering adjuvancy properties to improve the efficacy of such foliar-applied herbicides because it is a straightforward approach to reduce the risk of adverse side-effects of herbicide via reducing herbicide dose needed to control suitable weeds. From the present study, it could be concluded that the oils Blackseed, Fennel, Olibanum, and Rose showed a strong adjuvancy properties for imazethapyr against jimsonweed and hence they could be suitable for developing and testing as an oily based-adjuvant. Though it is questioned whether crop selectivity could be threatened when these adjuvants are applied, further work is required under field conditions to answer this question.