The Effect of Sequence and Time Interval of Adding Adjuvant and Sulfosulfuron to Hard Water in the Control of Winter Wild Oat (Avena sterilis ssp. ludoviciana Durieu.)

In arid regions such as Iran, water hardness, as a result of existence of calcium- and magnesium-containing minerals, is a serious challenge for irrigation practice. The irrigation systems are often applied to spray herbicides. Therefore, using hard water having high cations for herbicide application is inevitable. Such water can adversely affect the activity of some herbicides particularly weak acid ones. A solution for overcoming this problem is the addition of an adjuvant to the tank. It is well established that the addition of (NH4)2SO4 (43) and C6H8O7 (45) will efficiently remove the cations of hard water from the spray solution of weak acid herbicides. However, there is no report on whether the efficacy of sulfosulfuron (a weak acid herbicide, pKa = 3.5) is affected by the cations of hard water. Furthermore, it is observed in literature and practice that the addition of herbicide to the tank is immediately done after adding the adjuvant to the tank. Thus, there is no report on whether the sequence and time interval of adding adjuvant and herbicide to hard water could be affected by the efficacy of weak acid herbicide.

The seeds of winter wild oat (Avena sterilis ssp. ludoviciana Durieu.) were primed in the same method which was already described (3). Then, eight seedlings with a 1-cm radicle and 0.5-cm coleoptile were transplanted at a 1-cm depth within each 3-liter pot filled with a 1:1:4 ratio of sand: animal manure: clay loam soil, respectively. The experiment was carried out as a dose-response study in a completely randomized design with three factorial (6×4×7) and four replications. The 1st factor was the dose of sulfosulfuron(0, 1.25, 2.5, 5, 10, and 20 g a.i. ha-1). The 2nd factor included the type of spray carrier (distilled water, hard water containing sodium chloride (NaCl), calcium chloride (CaCl2) and iron chloride (CaFe3) at 600 mg L-1). The 3rd factor was also the sequence and time interval of adding adjuvant and herbicide to the spray carrier (with and without adding 500 mg citric acid (C6H8O7) L-1 or 20 g ammonium sulfate ((NH4)2SO4) L-1 at 30 min before, simultaneous, and 30 min after adding sulfosulfuron to the spray carrier). Five weeks after spraying, the response of individual dry weight of winter wild oat to treatments was analyzed as a nonlinear regression. This was carried out by using a four parametric logistic model (36) to estimate the values of ED50 and ED90 which are the doses of sulfosulfuroncausing a 50 and 90% reduction in the dry weight as compared to the control, respectively. 

In contrast to ammonium sulfate which was ineffective, the addition of citric acid to distilled water under each sequence and time interval improved the efficacy of sulfosulfuron. Decreasing the pH can permit the weak acidic herbicides to pass through the cuticle and then cell membrane during the ionic trap process. The presence of Na+, Ca++ and Fe+++ cations in the spray carrier increased the amount of sulfosulfuron required for 90% reduction in the dry weight of winter wild oat from 9.80 to 27.60, 47.48, and 50.32 g ha-1, respectively. There was no significant difference between the intensity of the incompatibility of Ca++ and Fe+++ cations. Similar to other weak acid herbicides, sulfosulfuron can also be ionized into anionic (a negatively charged form) and cationic (H+) fragments under alkaline conditions. The anionic part of sulfosulfuron bonds with the cations in hard water. As a result, a crystalline herbicide-cation salt is formed which is not able to pass through the cuticle due to its low solubility in water. The addition of both adjuvants 30 min after the addition of sulfosulfuron to distilled water containing the cations had no effect on removing the adverse effect of cations on the efficacy of sulfosulfuron. Conversely, adding sulfosulfuron 30 min after the addition of both adjuvants to the distilled water containing the cations provided the best efficacy of sulfosulfuron. When the adjuvants are added to hard water before herbicide, the cations of hard water can receive the cation of H+ from citric acid or NH4+ from ammonium sulfate. Then, the formation and sediment of the cation-anion salt turns hard water into soft water, having no problem with the efficacy of sulfosulfuron.

The issue of correct sequence for adding adjuvant and herbicide to hard water is important. Lack of knowledge about this issue can lead to not only the lack of weed control but also the imposition of additional costs on the farmer.