Glyphosate: Back to Basics by Blaine J. Viator
May 2009
Editor's Note: This column is part one of a two-part series. Part two will publish in the June issue.
Those who have dealt with any form of agricultural production in the past three decades will undoubtedly have had some experience with glyphosate. The impact of this herbicide since its introduction in 1972 has been monumental. Its originator, Monsanto, used Roundup Ready technology to launch the first widely grown genetically modified crops worldwide. Although some crops have not yet benefited from Roundup Ready technology commercially, many industries still utilize glyphosate both for weed control and in some cases, as with sugarcane, growth regulation and ripening. We can learn much from other commodities that use glyphosate so extensively.
Understanding Glyphosate
Glyphosate is an acid and is very similar in molecular nature to glyphosine, a natural amino acid. Because an acid is a negative ion (anion), it is formulated as a salt. Originally, glyphosate was often formulated as an isopropylamine salt. The isopropylamine cation is rather "bulky," and limits the amount of glyphosate that can be formulated in a given volume of product.
Lately, we have seen a trend to formulate glyphosate as salts other than the isopropylamine salt so that more glyphosate acid can be formulated per gallon. Simply put, the lower the molecular weight of the cation portion of the salt, the more "room" for glyphosate acid per gallon.
Monsanto and many of its competitors are moving toward a potassium salt so that up to six pounds active ingredient per U.S. gallon can be formulated into the final product. Having more concentrated formulations reduces packaging costs and allows for easier handling on the farm operation.
Some manufacturers have implied that the salt formulation affects the activity of the herbicide. In early research comparing glyphosate formulations in glyphosate tolerant soybeans, some increased injury was detected when the trimesium salt formulation was compared to the isopropylamine salt. The trimesium salt caused some superficial burning of foliage, similar to fertilizer salt injury. But for the most part, the activity of glyphosate within the plant is not affected by the salt.
Calculate Rates Accordingly
With so many generic glyphosate formulations on the worldwide market, it is very important to convert rates of application correctly. Since products contain different glyphosate concentrations per gallon, rates should be adjusted according to acid equivalency, not pounds active ingredient. Pounds active ingredient comprises the weight of both the glyphosate acid AND the salt. Pounds acid only pertains to the weight of the glyphosate acid in the formulation.
In the second part of this series, we'll discuss adjuvants and additives to glyphosate as it relates to water quality.
Glyphosate Basics (part two) by: Blaine J. Viator, Ph.D.
June 2009
In this-two part series refreshing ourselves on the basics of glyphosate, we'll discuss adjuvants and additives to glyphosate as it relates to water quality as well as environmental conditions affecting herbicidal performance.
As the salt formulation is not crucial to the activity of glyphosate, the surfactant is of utmost importance. Many formulations contain no surfactant at all, while others incorporate a potent blend of surfactants in the jug. The latter is generally called a "loaded" formulation. Surfactants aid the herbicide in spreading evenly over the waxy leaf surface and increase absorption into the plant.
With the addition of the proper rate and type of surfactant, the rain-free period after application may be reduced from six hours down to less than one hour. Formulations that do not contain surfactants will in many cases benefit from the addition of a surfactant prior to application. In general, a non-ionic, crop oils and organosilicone components.
With a wide range of surfactant blends, applicators should choose surfactants that contain a predominantly non-ionic component, as some organosilicone and crop oils may actually reduce glyphosate activity. It is always advisable to test locally available surfactants in replicated research trials.
Another important factor affecting glyphosate performance is environmental conditions at application. Because glyphosate is a systemic herbicide, plants need to be actively growing and not under stress at the time of application.
Researchers at the University of Missouri determined that applying glyphosate late in the evening reduced the activity of the herbicide on some weed species; plants begin to shut down certain biochemical pathways at night. Commercial applicators often prefer to apply glyphosate early in the morning or late in the evening when winds are calmer.
When weed control is erratic, the timing of glyphosate application is often the culprit. In addition, sudden thunderstorms often occur shortly after applications are made. Again, these types of adverse conditions are when the proper rate and type of surfactant are most important. In addition, additives that are designed to reduce drift by decreasing the amount of smaller droplet sizes are becoming more and more effective each year as new chemistries are marketed.
Other than a surfactant and drift reducing agent, the only scientifically proven additive that may benefit glyphosate under certain conditions is ammonium sulfate. There are two different theories on the role ammonium sulfate plays in these situations. Some believe the ammonium sulfate, a fertilizer, results in a more actively growing plant and "tricks" the plant into absorbing more glyphosate. However, more research is pointing to hard water aspects.
Water hardness is determined by the amount of dissolved salts, such as calcium and magnesium, in a water supply. These salts in the water bind with glyphosate, making it unable to enter the plant. Ammonium sulfate is thought to prevent the binding of these salts to glyphosate.
Limited research indicates that hard water is only a concern when glyphosate is put out at high volume per acre output-as more water (and thus more salts) are mixed with a given amount of glyphosate than with low sprayer outputs such as those used in aerial applications.
