If you're running cost comparisons or just flipping a coin to decide which glyphosate-based product best fits the bill, consider what these university extension weed specialists have to say.

For most soybean growers, the question isn't whether to use a glyphosate-based product, but which one to use and how. Both parts of that equation still interest researchers today, even though growers have used the product for 25 years. University extension weed specialists say the new generic product formulations have generated much of the recent interest.

“On the surface, these glyphosate programs appear simple to use but they aren't really,” says Mike Owen, Iowa State University extension weed specialist. “They require astute management.”

To that end, both university and company researchers continue to fine-tune their glyphosate recommendations to help farmers achieve the best possible weed control results. Here are some of the highlights of their work.

The majority of the extension weed specialists we interviewed say there are few to no differences among the 10 or so brands of glyphosate products available. Overall results from a study at Kansas State University (KSU) show that these products are equally effective when applied at equivalent rates and with proper adjuvants.

Dallas Peterson, KSU extension weed specialist, says the researchers evaluated about six different glyphosate products. Weed control with each of the products tested was better than 90% on large crabgrass, palmer amaranth and velvetleaf. In addition, the soybeans showed no injury symptoms from any of the products.

Nearly all crop producers and custom applicators engaged in weed control practices understand the term “active ingredient.” However, the term “acid equivalency” should be just as, or even more, significant to them when they evaluate glyphosate products, according to Aaron Hager, University of Illinois extension weed specialist. He explains, “The active ingredient and acid equivalency are identical for many glyphosate formulations, but others vary in the amount of acid they contain.”

Acid in the glyphosate formulation is what actually kills weeds. The parent acid glyphosate is formulated with a salt, and various salts have different weights, which influence the amount of acid contained in a gallon or pound of formulated product. Hager says, for instance, that Roundup Ultra 4L and Glyphos 4L both have 3 lbs. of acid and 4 lbs. of active ingredient and therefore are equivalent. However, other products are equivalent in terms of their active ingredient but not in terms of their acid content.

Some glyphosate products already include surfactants, whereas others require that surfactants be added. Formulations without surfactants are generally cheaper, and some dealers prefer to sell adjuvants separately, says Dave Regehr, extension weed specialist at KSU. “Last spring we had questions regarding the use of methylated, soybean-based oil concentrates with glyphosate in place of nonionic surfactants,” he says. “Farmers like the idea of using inputs derived from their own products.”

KSU scientists designed a test to evaluate glyphosate adjuvants, including soybean-based and petroleum-based crop oil concentrates, and a variety of nonionic surfactants. They applied a glyphosate product formulated without surfactant at 3 fl. oz./acre to a uniform and sensitive target. The carrier was water, conditioned with ammonium sulfate.

The research showed clear differences between nonionic surfactants and the oil-based adjuvants for glyphosate, with about 20% better performance by the nonionic surfactants. Glyphosate performance was still fairly good using the oil-based adjuvants.

Perhaps the biggest surprise of the test was that glyphosate applied with no adjuvant produced almost no injury symptoms at all on the target. Because this is just one test, Regehr encourages applicators to still include adjuvants with all glyphosate formulations.

Research confirms that the time of day that glyphosate is applied does affect efficacy. Peterson says farmers may achieve less weed control when they spray in late evening or early morning. Peterson's research showed less than 50% velvetleaf control when glyphosate was sprayed at 9 p.m. and 6 a.m. More than 95% velvetleaf control was achieved when spraying was done at 10 a.m., 1:30 p.m. and 5 p.m. Similar results have been shown on both grass and broadleaf weeds, although the magnitude of difference varies greatly due to rates, weed size, weed species and environmental conditions.

This creates a dilemma for applicators who have a lot of acres to cover and may not be able to spray during the midday hours because of high winds and the risk of spray drift. The bottom line: Be aware of the potential differences.

Peterson reports he has seen minimal benefits to adding tankmix partners to glyphosate in most situations.

Because glyphosate is so effective, many farmers wait too long to apply it. Many soybean fields across Iowa in past years likely sustained yield losses of between 5 and 10 bu./acre from late control measures. In some severe cases where applications were extremely delayed, Iowa State's Owen suspects yield losses may have approached 50%. Owen says farmers need a new perspective on their weed management program. “The objective should not be to focus on killing weeds, but to protect potential crop yield,” he explains.

He emphasizes that farmers must evaluate weed populations early, consider weather conditions and be prepared to take action. Hager agrees. He advises farmers to evaluate weed growth and plan to spray between three and five weeks after crop emergence.

“You generally won't see much yield depression before three weeks, and you typically won't want to wait longer than five weeks to spray fields,” he says. “If you pull the trigger too late, then weeds are large, more difficult to get good coverage on and more costly to control.”

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