Herbicide efficacy and water volumes

Tom Wolf, owner of AgriMetrix in Saskatoon and long-serving spray applications researcher, says higher water volume doesn’t necessarily increase herbicide performance, but it improves the consistency of performance.

“There are few reasons for this,” he says. In the case of metribuzin (Sencor and others), for example, higher water volumes increase crop tolerance. In the case of Liberty, bromoxynil, carfentrazone, Heat, diquat, Infinity and metribuzin, it increases droplet density that helps create the necrotic lesions that ultimately kill the foliage.

“But the other way to increase droplet density is with finer sprays, so it’s a matter of philosophy or priorities,” he says.

Shifting priorities. “The old school of thought from the ‘70s and ‘80s was that less water makes herbicides work better. That’s why we had lots of low volume sprayers and people cut product rates,” Wolf says. “But then we realized that low volume sprays depend on fine droplets to work, and the drift issue prevented diversification into things like canola. So the current thinking is that higher volumes are better because they permit the use of low-drift sprays which have tremendous advantages. And of course high volumes are needed for late season sprays into deep canopies.”

Glyphosate is the exception. Glyphosate tends to work better at lower water volumes, but with increased tank mixing with contact products, higher water volumes will become necessary.

Contact versus systemic. Clark Brenzil, Saskatchewan weed control specialist, says contact products benefit more from higher water volumes. “With systemic herbicides, one droplet on one leaf is likely going to be enough for a lethal dose of the herbicide to get into the plant – as long as the droplet remains on the leaf. With contact herbicides, the product does not move from where it lands to any degree, meaning that wherever a droplet lands is what part of the plant dies. Therefore to get adequate control, most of the plant’s surface needs to be covered to provide lasting control.”