The effect of pH and light on the efficacy of spray solution stored of haloxyfop-r-methyl, fluazifop-p-butyl, and sethoxydim against wild barley (Hordeum spontaneum K. Koch)

After preparing the spray solution of herbicide, there are situations where the farmer intentionally or unintentionally has to store the prepared spray solution for some time, even several days, in the sprayer tank. For example, when weather conditions for spraying become unfavorable, when a piece of failure in herbicide application equipment occurs, or when the spray solution at night is prepared to use at dawn. Herbicides either decompose chemically or react with compounds in the water, decreasing the efficacy of controlling weeds. Now, the first question that comes to mind is: how long can the herbicide be kept in the sprayer tank without losing its efficacy? In the water, herbicides inhibiting acetyl coenzyme A carboxylase (ACCase) undergo hydrolysis or photolysis, producing metabolites that do not have herbicidal properties. Therefore, the stored spray solution of ACCase-inhibiting herbicides cannot effectively control weeds. For this reason, it is suggested such a spray solution should be thrown away. The metabolites from ACCase-inhibiting herbicides, while not having herbicidal properties, are leached faster in the soil, have a higher half-life, and are more toxic to non-target organisms. Therefore, the suggestion to throw away the stored spray solution of ACCase-inhibiting herbicides does not seem logical not only from the environmental point of view but also from the economic point of view. Now, the second question that comes to mind is: what is the solution to reduce the speed of hydrolysis or photolysis? This study was carried out to investigate (i) the effect of the storage time of the spray solution of three ACCase-inhibiting herbicides (haloxyfop-r-methyl, fluazifop-p-butyl, and sethoxydim) on their efficacy in controlling wild barley (Hordeum spontaneum) and (ii) the effect of two factors of pH and light on the relationship above.

For each herbicide, an experimental three-factorial arrangement (6 × 2 × 9) as a completely randomized design with 4 replications. The first factor was six doses of herbicide (0, 20, 40, 60, 80, and 100% of the labeled dose); the second factor was two pHs of spray water (5 and 8); and the third factor was nine storage time/conditions (0, 24, 48, 96, and 192 h storage in the dark, 12+12, 24+24, 48+48, and 96+96 h storage in the dark+light). The spray solution corresponding to the zero level of the aforementioned third factor was prepared on the same day of spraying. For other levels of this factor, the relevant spray solution with a volume of one liter was prepared in the previous days (24, 48, 96, and 192 h before the day of spraying) and stored in transparent polyethylene terephthalate plastic bottles. The bottles were kept in two outdoor conditions: 1) kept in complete darkness - for this purpose, the bottles were kept in black plastic packed in a carton; and 2) kept in the darkness of the night and the light of the day - for this purpose, the bottles without any cover were exposed to the darkness of the night as well as to the sunlight. The treatments were applied at the four-leaf stage of wild barley; four weeks later, they were harvested, oven-dried, and weighed. A methodology known as dose-response curves was used to analyze the data to obtain the dose required for 50% control (ED50).

When the solution was sprayed immediately after preparation, reducing the pH from 8 to 5 did not significantly affect the ED50 of haloxyfop-r-methyl and fluazifop-p-butyl. While the ED50 of sethoxydim significantly decreased from 136.64 to 113.35 g a.i. ha-1, indicating that pH reduction can improve the efficacy of sethoxydim in the control of wild barely. The possible reason can be related to the difference in the formulation of the above herbicides. Haloxyfop-r-methyl and fluazifop-p-butyl are formulated as pre-herbicide. Under each condition (in terms of pH and light), when the time of storing the spray solution of herbicides was prolonged, a steady reduction of efficacy was observed. In the case of haloxyfop-R-methyl spray solution, when pH was not changed (pH 8), 24 h storage in the dark (43.78 g a.i. ha-1) and 12+12 h storage in the dark+light (41.44 g a.i. ha-1) significantly increased the ED50 as compared to the control treatment (0 h storage (34.60 g a.i. ha-1)). While when pH was reduced (pH 5), the efficacy of five treatments (including: 24 and 48 h storage in the dark and 12+12, 24+24, and 48+48 h storage in the dark+light) did not differ significantly from the efficacy of the control treatment. In the case of fluazifop-p-butyl spray solution, when pH was not changed (pH 8), all storage treatments significantly increased the ED50 as compared to the control treatment (0 h storage (80.64 g a.i. ha-1)). While when pH was reduced (pH 5), the efficacy of two treatments of 24 and 48 h storage in the dark did not differ significantly from the efficacy of the control treatment. In the sethoxydim spray solution, when pH was not changed (pH 8), all storage treatments significantly increased the ED50 as compared to the control treatment (0 h storage (136.64 g a.i. ha-1)). When pH was reduced (pH 5), the efficacy of all storage treatments still did not differ significantly from the efficacy of the control treatment.

If the pH of the spray solution was reduced, the spray solution of haloxyfop-r-methyl can be stored for at least 48 h in each light condition without losing its efficacy. If the pH of the spray solution was reduced and stored in the dark, the spray solution of fluazifop-p-butyl can be stored for 48 h without losing its efficacy. Sethoxydim is very sensitive to the storage of its spray solution. Changing the storage conditions, especially the pH, not only does not help in maintaining the efficacy but also causes a further loss in its efficacy.