Reducing the 2, 4 D+MCPA Antagonism from Hard Spray Waters by Ammonium Sulfate

Water is the main carrier of herbicides (HC) that its quality plays an important role in herbicide performance hard water has a high concentration of Ca and Mg and reviews have shown that calcium, manganese and zinc are the main factors reducing the effectiveness of weak acid herbicides. Weak acid herbicides such as glyphosate, paraquat, clethodim and 2, 4 D are compounds that release the H ions once dissolved in water, but just slightly. Therefore, herbicides that are weak acids partially dissociate. Herbicides not dissociated (the compound remains whole) are more readily absorbed by plant foliage than those that dissociate. Dissociated herbicide molecules have a negative charge. After being dissociated, herbicides might remain as negatively charged molecules, or they might bind with other positively charged cations. Binding to some cations improves herbicide uptake and absorption, binding to others such as Ca and Mg antagonizes herbicide activity by decreasing absorption or activity in the cell. To correct such carriers, the use of adjuvants, such as ammonium sulphate (AMS), is recommended, which can reduce the use of herbicides and cause economic savings. The aim of this study was to investigate the simple effects and interactions between different amounts of AMS and carrier hardness (CH) levels on 2, 4 D MCPA herbicide efficacy in controlling white clover (Trifolium repens L.) in turf grass.
The experiment was laid out in a RCBD with three replications for each treatment during spring-summer 2013 in 10 years old mixed cold season turf grass (Festuca rubra Poa pratensis Poa pratensis) dominated by white clover in Mashhad (Iran). The treatments were the factorial combination of four carrier hardness (CH) rates (Deionized, 45, 90 and 180 ppm of Ca ㈏) and three Ammonium Sulfate (AMS) rates (0, 2, 3 and 4 Kg per100 L of carrier water) were studied. The turf was sprayed with 2, 4 D MCPA (67.5% SL) at 1.5 L-ha applied once on July. The density and dry matter of clover and turf were recorded.
Full performance of 2, 4 D MCPA herbicide to control clover, regardless of the amount of ammonium sulfate used, was obtained in soft water. Adding just 4%, AMS to Carrier water with a hardness of 45 ppm could recover effectiveness of herbicide up to DI water, whereas in 90 ppm of hardness adding only 2 percent ammonium sulfate was enough to increase herbicide efficacy to twice as no ammonium sulfate treatment. The most significant antagonism effect was obtained in 180 ppm hardness level without AMS reducing 84% of 2, 4 D MCPA performance compared to soft water. The highest antagonism effect of the herbicide carrier went to 180 ppm, 90 ppm and 45 ppm of hardness respectively. Overall, the study revealed that only in 45 ppm of CH the addition of 4% of AMS will help to restore the toxicity of 2, 4 D MCPA while in 90 ppm and 180 ppm of CH add more than 2% of AMS to 2, 4 D MCPA carrier water will not benefit the herbicide toxicity. Most reports have considered sufficient two percent of AMS to neutralize the inhibitory effect of CH on the weak acid herbicides. Three weeks after spraying, no phytotoxicity was found in the grass. At the same time interaction between CH and AMS on the lawn dry weight was significant (P