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 this year.

Formulation differences

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.

Acid equivalency versus active ingredient

Nearly all crop producers engaged in weed-control practices understand the term “active ingredient.” However, the term “acid equivalency” should be just as, or even more, significant to farmers when they evaluate which glyphosate product to purchase, 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 (see table at the end of the story).

Hager recommends that farmers understand this concept to evaluate price differences between products and the control they offer. “It can help you evaluate products in an apple-to-apple comparison,” he says. “It's good to know how to accurately pencil out the acid equivalency between products so you know exactly what you're getting” (see equations at the end of the story).

Nonionic surfactants versus oils

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. When farmers tell Regehr they achieved good control with an oil-based adjuvant, he believes that higher glyphosate rates simply compensated for the adjuvant's poorer performance.

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 growers to still include adjuvants with all glyphosate formulations.

“Most growers would do well to simply go with formulations that already have the nonionic surfactants included,” he says. “If you consider a quart of glyphosate, applied at 10 gal./acre, the cost of adding a top-notch adjuvant at a rate of 1 qt./100 gal. [0.25% vol./vol.] would be about $0.50/acre. So, if you can buy a full-load product containing adjuvant for about $2 a gallon more, you're getting a bargain, and you don't have the hassle of buying and remembering to add the nonionic surfactant.”

Application timing

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.

Several factors may contribute to the variances. It's possible that heavy dew may contribute to reduced control. Also, some plants may fold their leaves at night, so the weed foliage intercepts less herbicide. Another thought, Peterson says, is that the photosynthetic cycle that naturally fluctuates in plants during the course of night and day may result in reduced uptake, translocation or activity of glyphosate during the nighttime hours.

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 farmers and 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.

Tankmix findings

Peterson reports he has seen minimal benefits to adding tankmix partners to glyphosate in most situations. One of his primary concerns is that “farmers tend to reduce the rates of the individual components used in the tankmix, which can lower the overall weed control provided by the glyphosate. However, tankmixes may be beneficial on some weeds that are hard to control with glyphosate, such as morning glory.”

Early control

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%. However, it is possible that other stress factors also contributed to the loss of yield resulting from delayed postemergence herbicide application.

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.”

Avoiding resistance
Glyphosate works well, farmers know how to use it, and, at about a $10/acre cost in soybeans, it's the practical choice for weed control. Because of its benefits, the chemistry has been used repeatedly on the same acreage for multiple years. Now farmers, university researchers and company representatives acknowledge that weed shifts or resistance problems are cropping up in fields.

“We had some performance inquiries on mare's tail in no-till crops in Maryland and Tennessee,” says Chuck Foresman, Syngenta technical business manager for nonselective herbicides. He adds, “We are working with universities to better understand the extent of the problem and how broad-spread this phenomenon is on other [weed] species.”

Monsanto has addressed similar weed-control issues with its Roundup products. However, Mike Stern, Monsanto's director of technology for agricultural chemistry, emphasizes that “weed resistance is extremely rare, given that Roundup has been on the market now for 25 years.”

Even so, to sustain the viability of the chemistry in general, Syngenta now recommends that farmers use no more than two applications of a glyphosate-based product on any field in a two-year period. One way to accomplish this goal, notes Foresman, would be to choose between growing Roundup Ready corn and Roundup Ready soybeans. “Consider one or the other,” he says, ‘in order to sustain the viability of this tool.”

Another practice to consider is rotating chemistries on row-crop acreage. For no-till scenarios, specifically, Foresman says the company recommends using Gramoxone Max along with a psi (atrazine for corn and metribuzin for soybeans) for burn down prior to crop emergence, followed by an in-crop application of Touchdown.

Formulas to determine acid equivalency Dry formulations:

0.5625-lb. acid equivalent/lb. of product × 1 lb. of product/? lb. acid equivalent/lb. × 16 oz./lb.

Liquid formulations:

0.5625-lb. acid equivalent/acre × 1 gal. of product/? lb. acid equivalent/gal. × 128 fl.oz./gal.

Glyphosate formulations and product equivalentsa
Trade name Company Active ingredient (ai) and acid equivalent (ae) per gallon or pound Product rate equivalent to 0.375 lb. ae Product rate equivalent to 0.5625 lb. ae Product rate equivalent to 0.75 lb. ae Cropb
Roundup UltraMax 5L Monsanto 5 lbs. ai; 3.68 lbs. ae 13 fl. oz. 19.6 fl. oz. 26 fl. oz. C&S
Roundup Ultra 4L Monsanto 4 lbs. ai; 3 lbs. ae 16 fl. oz. 24 fl. oz. 32 fl. oz. C&S
Roundup UltraDry 71.4WDG Monsanto 0.714 lb. ai; 0.649 lb. ae 9.24 oz. 13.9 oz. 18.5 oz. C&S
Roundup Original 4L Monsanto 4 lbs. ai; 3 lbs. ae 16 fl. oz. 24 fl. oz. 32 fl. oz. C&S
Roundup Custom 5.4L Monsanto 5.4 lbs. ai; 4 lbs. ae 12 fl. oz. 18 fl. oz. 24 fl. oz. C&S
Roundup D-Pak 6.42L Monsanto 6.42 lbs. ai; 4.75 lbs. ae 10 fl. oz. 15 fl. oz. 20 fl. oz. S
Touchdown 5 5L Syngenta 5 lbs. ai; 3.426 lbs. ae 14 fl. oz. 21 fl. oz. 28 fl. oz. S
Touchdown 3AE Syngenta 3.7 lbs. ai; 3 lbs. ae 16 fl. oz. 24 fl. oz. 32 fl. oz. C&S
Glyfos 4L Cheminova 4 lbs. ai; 3 lbs. ae 16 fl. oz. 24 fl. oz. 32 fl. oz. C&S
Glyfos X-Tra 4L Cheminova 4 lbs. ai; 3 lbs. ae 16 fl. oz. 24 fl. oz. 32 fl. oz. C&S
Glyphomax 4L Dow Agro 4 lbs. ai; 3 lbs. ae 16 fl. oz. 24 fl. oz. 32 fl. oz. C&S
Glyphomax Plus 4L Dow Agro 4 lbs. ai; 3 lbs. ae 16 fl. oz. 24 fl. oz. 32 fl. oz. C&S
Credit 4L Nufarm 4 lbs. ai; 3 lbs. ae 16 fl. oz. 24 fl. oz. 32 fl. oz. S
Gly-Flo 4L Micro Flo 4 lbs. ai; 3 lbs. ae 16 fl. oz. 24 fl. oz. 32 fl. oz. S
Glyphosate Original 4L Griffin 4 lbs. ai; 3 lbs. ae 16 fl. oz. 24 fl. oz. 32 fl. oz. S
Acquire 4L BASF 4 lbs. ai; 3 lbs. ae 16 fl. oz. 24 fl. oz. 32 fl. oz. S
Silhouette 4L Cenex/Land O'Lakes 4 lbs. ai; 3 lbs. ae 16 fl. oz. 24 fl. oz. 32 fl. oz. C&S
Rattler 4L Helena 4 lbs. ai; 3 lbs. ae 16 fl. oz. 24 fl. oz. 32 fl. oz. C&S
Mirage 4L UAP 4 lbs. ai; 3 lbs. ae 16 fl. oz. 24 fl. oz. 32 fl. oz. C&S
Buccaneer 4L Several 4 lbs. ai; 3 lbs. ae 16 fl. oz. 24 fl. oz. 32 fl. oz. C&S
Rascal 4L Several 4 lbs. ai; 3 lbs. ae 16 fl. oz. 24 fl. oz. 32 fl. oz. C&S
Honcho 4L Several 4 lbs. ai; 3 lbs. ae 16 fl. oz. 24 fl. oz. 32 fl. oz. C&S
a Used with the permission of the University of Illinois.
b Labeled for use in Roundup Ready (glyphosate-resistant) designated corn (C) and/or soybean (S).