Get the most out of glyphosate
The many formulations available do about the same job, but the rates required can differ.
While glyphosate still effectively controls many weeds, including annual and perennial grass and broadleaf weeds, many factors play a role in how well it works, says Tim Miller, weed scientist with Washington State University in Mount Vernon.
Since glyphosate came off patent in 2000, many different formulations have been marketed. At least 40 glyphosate products are available in Washington State. Glyphosate is formulated as a salt. About 80 percent of glyphosate formulations contain isopropylamine salt.
Others include potassium, diammonium, trimethylsulfonium, or sesquidodium. The salt makes the glyphosate a little more soluble so the concentration can be higher, and it also stabilizes the product. It enables the glyphosate acid to enter the plant a little more easily, and it moves better throughout the plant.
Although there is little difference in the activity of the different formulations, they do differ in concentration of both the active ingredient (the salt) and the glyphosate acid equivalent. For example, Touchdown HiTech has 6 pounds of active ingredient per gallon, and requires 19 ounces per acre to achieve the 0.75 pounds of acid equivalent, whereas most generics contain 4 pounds of active ingredient per acre and require 32 ounces per acre to achieve the 0.75 pounds of acid equivalent.
Generic formulations usually contain a surfactant, which changes the surface tension of the solution, making it better able to penetrate the cuticle of the leaves. Though there is no need to add a surfactant, it doesn’t harm to do so, Miller said. Normally, moisture beads up on the leaf surface, and adding a surfactant to the mix can increase the surface contact of the moisture on the leaf. It can also slow evaporation of herbicide droplets and increase their rain-fastness.
Because glyphosate is negatively charged, it binds tightly to phosphate sorption sites in soils, and soil activity is rare. Miller said that’s an advantage if you want to plant a crop right after the herbicide treatment, but it means that the herbicide will not provide long-term control because it has no residual effect, and it might need to be applied several times in succession to provide complete weed control.
Negatively charged glyphosate can also bind to cations in the water, such as calcium, sodium, magnesium, and iron. This takes the glyphosate out of solution and prevents it getting into the plant and killing it.
Adding a fertilizer, such as ammonium sulfate or ammonium nitrate, can improve the activity of glyphosate, particularly in hard water. The negatively charged sulfate will preferentially bind to the calcium, sodium, magnesium, and iron in the water and take them out of solution so they don’t bind with the glyphosate, while the positively charged ammonium binds with the glyphosate, making it move through the plant cuticle more easily. More glyphosate in the plant cells results in better translocation through the plant.
Some glyphosate formulations recommend mixing with ammonium sulfate for this reason. Water conditioners have the same effect.
Miller recommended that growers do a compatibility test with the fertilizer and glyphosate before mixing a whole tank. If dry ammonium sulfate is used, nonsoluble materials such as sand and gravel will need to be filtered out. Make it up to a slurry and strain out the solids before adding it to the spray tank to prevent clogging the nozzles. Do this before adding the glyphosate, before the glyphosate has chance to bind to the cations.
Miller said if the water is soft, adding a fertilizer might not be necessary, but he knows of no negative consequences of adding ammonium sulfate, and it does improve control of some weed species, such as spotted knapweed.
At a low pH level, more glyphosate exists as a salt than a free acid. A slightly acidic spray solution—between 4 and 6—results in better glyphosate uptake. If the pH is higher than 7, consider using a buffer. Buffers are commonly used with pesticides and fungicides but not often with herbicides, but Miller said it could make a difference with glyphosate.
Because glyphosate binds with soil molecules, it will also bind to the dust on foliage, making it ineffective. If the foliage is dusty, it might be a good idea to irrigate before applying the herbicide to make sure the leaves are clean and ensure maximum uptake of the product.
Glyphosate seems to work better in lower spray volumes. Miller said it’s not clear exactly why, but it might be because growers use smaller nozzles when using lower volumes, resulting in smaller droplets and better spray coverage. Another possibility is that when there is less volume of water, there are fewer cations to bind with the glyphosate and more active ingredient available to work on the plant.
Some other herbicides make glyphosate less effective against certain weeds when they’re mixed together. Herbicides with this possible effect include Aim (carfentrazone), Spartan (sulfentrazone), Sencor (metribuzin), and certain antidrift adjuvants. These should be applied separately from glyphosate, rather than tank mixed.
Miller said a possible reason for the incompatibility is that the contact herbicides might be killing the plant cells before glyphosate, which tends to be slow acting, has a chance to translocate out into the rest of the plant.
“You want to minimize the amount of damage to the plant to allow the translocation to occur,” he said. “Hit it with glyphosate first, and come back later with the contact herbicide to knock it down quick.”
Temperature and humidity have a big impact on how well glyphosate works, Miller said. The easiest plant to kill with glyphosate is a healthy, rapidly growing plant, because the glyphosate is better able to translocate throughout all the tissues.
It will have much less effect on a plant that’s suffering from cold stress, heat stress, or drought stress and is not growing rapidly. When applied in cold, spring weather, the glyphosate might not work until the weather warms up. “It’s not being detoxified,” Miller said. “It’s just sitting there waiting for the plant to start growing.”
A glyphosate application should be rain fast after six hours, because it should be taken up by the plant within that time.
Glyphosate seems to work better in higher light levels, perhaps because more photosynthesis is occurring, and the plant is translocating glyphosate along with the sugars. For this reason, morning applications are thought to be better than afternoon treatments.
Stage of weed growth
With most perennial weeds, the bud stage is the most vulnerable, either in spring or early summer. Glyphosate can also be applied in late fall as long as the plant still has at least 50 percent green tissue.
Biennial weeds are best treated during the first year. By the second year, they are producing seeds and are more tolerant of the herbicide.
Treat annuals as early as possible, as soon as seed germination is complete for the year, but wait until most of the weeds are up and growing, since glyphosate has no residual activity, Miller advised.
For more information, download
the publication “Glyphosate Stewardship: Keeping an Effective Herbicide Effective” from www.ipm .ucdavis.edu/PDF/ PUBS/miller-glyphosatesteward ship.pdf