When it comes to managing weeds with herbicides, the worst thing a grower can do is encourage herbicide resistance through overuse or improper management.

Years of mismanaging glyphosate in Ohio cropfields has resulted in widespread herbicide resistance of horseweed and a few cases of resistance of some problematic weeds such as common lambsquarters and giant ragweed. However, initiating proper glyphosate management now can help reduce further damage, said Jeff Stachler, an Ohio State University Extension weed specialist.

“Glyphosate controls nearly all weeds. That’s why it is such a great product,” Stachler said. “But because of its effectiveness, growers are overusing glyphosate, which is actually having a detrimental effect on controlling weeds. Our expectations of glyphosate are too high and we should no longer count on glyphosate to consistently control weeds that are too large, greater than 12 inches.”

Reducing the frequency of glyphosate applications is the most simple and effective approach to reducing glyphosate-resistant weeds, but it may not always be practical for growers based on their field situations. As an alternative, one approach growers can take is properly managing glyphosate, Stachler said.

“If you don’t properly manage glyphosate, you are at a greater risk for poorer weed control, which can result in lower yields, and you are at a greater risk for increased pressure of glyphosate-resistant weeds,” he said. “Proper glyphosate management will not solve the problems associated with glyphosate overuse, but it will greatly improve its negative impacts.”

Improper management practices with glyphosate that growers tend to fall victim to include not using the correct rate, applying at the wrong weed size and weed age, applying during the wrong time of the day, using improper boom height, not maximizing rain fastness after an application, not choosing the best additives, paying little attention to environmental conditions and applying during dusty conditions.

Stachler offered the following recommendations to growers when using glyphosate:

• Use the correct rate—For most weed species, the traditional use rate is 0.75 pounds of acid equivalent per acre (lbs ae/A). However, research has shown that to more consistently control such species as giant ragweed, common lambsquarters, velvetleaf, common pokeweed, marestail, morningglory, and perennial nightshade species requires a minimum rate of 1.1-1.5 lb ae/A.

• Apply at the right weed size—Annual weed species should be less than 6 inches tall for the best control, with the exception of giant ragweed, which may be up to 10 inches tall. For perennials, however, plants should be sprayed when they are larger.

“You want perennials in the bud-to-early flowering stage of development for maximum control during the growing season,” Stachler said.

Most Ohio fields have a mix of different species, so it is very difficult to correctly apply glyphosate for all species with a single application. The use of a pre-emergence herbicide can allow for a more timely glyphosate application for a broader spectrum of weeds.

• Apply at the right weed age for annual species—A younger plant can be controlled more effectively than an older plant. Growers should not allow annual weeds to get much older than 21-30 days.

• Consider making a second glyphosate application—“A second application can and usually does improve control of weed species even if the first application was not completely effective,” Stachler said. “However, use this only as a short-term strategy. With numerous applications, weed populations will change by developing glyphosate resistance.”

A second application of glyphosate is most effective when using the correct rate during the first application.

• Apply at the right time of day—The best time to apply glyphosate is between 9 a.m. and 6 p.m. for certain species. Nutrient movement in plants is at its maximum during the middle of a sunny day. Hence, glyphosate movement will also be increased. Studies have shown that the worst time of the day to apply glyphosate is between 9 p.m. and 6 a.m. for species such as giant ragweed and velvetleaf.

• Maximize rain fastness—Rain fastness is the rain-free period required after application to obtain maximum control. “The rain fastness for some glyphosate products is only 30 minutes. But 30 minutes is not enough time to effectively control species such as common lambsquarters,” Stachler said. “The longer the rain-free period, the greater the effectiveness.”

• Be mindful of additives and tank-mix partners—Control can be reduced for some weed species when adding certain drift-control agents when also using a drift-reducing nozzle.

“You create a large droplet with the nozzle and even a larger droplet with the drift-reducing agent,” Stachler said. “Because of the increased volume and the heavier weight, a droplet may miss the target and hit the soil.”

Adding a high quality nonionic surfactant can improve control, but Stachler noted the technique would not be as beneficial as increasing the glyphosate rate. Not all glyphosate formulations are the same. The largest difference in products is the amount and type of surfactant that is added. The addition of manganese fertilizers usually decreases glyphosate activity. The EDTA form of manganese causes the fewest weed control problems when mixed with glyphosate. Clay-based herbicides such as atrazine and Sencor can tie up glyphosate and make it less effective. Other herbicides, especially contact herbicides, can also decrease glyphosate activity.

• Always add the correct amount of ammonium sulfate (AMS) with glyphosate mixtures—When applying glyphosate only, the minimum rate of AMS, 8.5 pounds per 100 gallons of spray mixture (lbs/100 gallons), can be effective during ideal growing conditions. The maximum use rate of AMS, 17 lb/100 gallons, should always be used when mixing a manganese fertilizer with glyphosate during less than ideal growing conditions, when tank-mixing with antagonistic herbicides, and when using hard water.

The biggest reason for adding AMS to glyphosate mixtures is to reduce cations from attaching to the glyphosate molecule. When large cations attach to the glyphosate molecule, weed control is almost always reduced.

• Check the boom height—Weed size and boom height go hand-in-hand, Stachler said. Spraying smaller weeds means more uniform plant height and an easier time to adjust the boom than spraying when weeds are much larger.

“Many growers lower the boom height to reduce drift. With larger weeds, they tend to overcompensate, which results in poorer herbicide coverage,” Stachler said. “With smaller weeds, it’s easier to achieve proper boom height.”

Since perennial weed species are best controlled with glyphosate when they are larger, carefully adjust boom height. The minimum distance from the nozzle on the boom to the tallest weed in the field always should be 15 inches, but may need to be higher depending upon nozzle angle and spacing.

• Be aware of environmental conditions—Growers should pay closer attention to environmental conditions that could stress the weed and make it more difficult to control. Factors such as moisture, temperature, humidity, wind speed, pests and soil nutrient levels all play roles in the overall health of a plant.

• Spray in low-dust conditions—Clay particles and organic matter are negatively charged, while glyphosate molecules are positively charged. When the two come into contact with each other, the glyphosate is rendered ineffective.

“Using a higher rate can reduce the problem, but not always,” Stachler said.

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