In the early days of July, I received a phone call from John, a farmer with 3,000 acres of wheat and canola near Wadena, Sask.
John had sprayed his five canola fields with glyphosate in early June, but within two weeks he noticed that some weeds were still thriving and many were beginning to grow back. “It doesn’t appear that my herbicide worked, but I have no idea why,” John explained to me.
At John’s farm, I could see the weeds were indeed coming back to life — in some instances, the older leaves of the weeds had been killed off but the plants were still doing well. Apparently, the glyphosate had not translocated within the weeds.
After eliminating tank mixing error, incorrect water usage, sprayer malfunction and chemical failure as the cause of the problem, we checked the quality of the water John uses for pesticide application. Test results indicated John’s water was hard!
The calcium and magnesium salts in hard water bind tightly to the glyphosate molecules, stopping those molecules from binding to the active sites within the plant, hindering the ability of the glyphosate to kill the weeds.
By the time we determined the source of the problem, it was too late in the season to re-spray the canola fields. The weeds, no doubt, would outcompete the canola plants and have a negative impact on yield.
However, determining that poor quality water was to blame for the herbicide failure wasn’t all bad news because this problem is easily managed. Adding ammonium sulphate to the tank mix, for example, will ensure herbicide efficacy because it preferentially binds to the glyphosate molecules before the calcium and magnesium. The ammonium-glyphosate complex then binds to the active sites within the plant, eventually killing it. There are several other products on the market that work the same way as ammonium sulphate. However, with ammomium sulphate, much lower volumes of product are required, making them a more convenient option.
Ammonium sulphate has been shown to boost glyphosate’s efficacy in targeting larger weeds as well as increasing its rate of killing those weeds when used on summerfallow.
Water quality is an important aspect of herbicide application and should not be overlooked. In addition, there are other factors affecting water quality such as pH levels and the presence of organic matter that should be monitored. Farmers would be wise to evaluate water quality regularly and, if necessary, consider adding a treatment to avoid tying up herbicides. †