Marestail, or horseweed, has been confirmed to be resistant to glyphosate in California.
Have you read your glyphosate herbicide label lately?
Today’s glyphosate herbicide is not the same as it was when first introduced as Roundup by Monsanto more than 35 years ago. Generic glyphosate is made in many countries around the world, and, as a result, formulations and inert ingredients vary and rates have changed.
“A lot of the problems in the field with glyphosate are not from the herbicide itself but from the rates and application,” said Don Waddle, certified crop consultant for Bleyhl Farm Service in Grandview, Washington. Originally, the Roundup rate was 1 percent per 100 gallons of water, and it was effective killing weeds, he said. But as Roundup came off patent, there was a shift in rates to be based on acreage.
With so many different glyphosate brands out there, Waddle said that it’s important to read the label carefully. Formulations of glyphosate brands that Bleyhl Farm Service carries range from 4 to 5.5 pounds of product. Most have additives to enhance activity, and some may need additional surfactant.
“I can’t say that one brand is better than another, but there are differences depending where it’s made,” Waddle said, noting that on some labels, inert ingredients are counted as active ingredients. “You need to read the label closely and understand it. If you don’t know what you’re getting, then check with your field representative or where you’re buying it.”
Waddle shared these tips to help growers make their glyphosate applications more effective:
Calibrate your spray rig. Understand the difference between sprayed acres and total acres. Determine the amount that is actually being sprayed. For example, in a vineyard with nine feet between rows, sprayed acreage in 10 acres is actually a third or 3.3 acres, not 10 acres.
Know your water quality. Water quality can make a big difference in glyphos-ate effectiveness. Take water samples and learn your pH numbers. The pH of irrigation water changes from the beginning to the end of the season. Well water is often too hard for herbicides unless something is added. For glyphosate, the optimum pH is 2.5, but Waddle believes that 4.5 to 5.5 pH works well. Some glyphosate labels recommend adding ammonium sulfate to the tank before mixing.
Higher concentration is better. Generally, the higher the concentration, the more effective glyphosate will be. He suggested using the lowest possible water volume that still provides adequate coverage. Only 17 to 20 percent of what’s sprayed actually gets into the plant, according to research Waddle has read.
Soil moisture. Most labels state that weeds must be actively growing for effective kill. Stress due to heat or drought or poor soil moisture can affect glyphosate kills. “You want to put it on ground that has been properly irrigated and has moisture—not bare and dry,” he said. “Then, you need moisture to set the herbicide.” Some labels recommend that moisture be applied within 30 days of application.
Grasses may require extra effort. Grasses are always difficult to control because it’s hard to get enough chemical into the rhizome to kill it. Bermuda grass may take extra effort, involving chemicals and cultivation.
Waddle shared his information during a weed control session at the Washington State Grape Society’s annual meeting last fall.