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Additive may enhance herbicides

One of the interesting things about doing research on agricultural issues — or most other areas of research, for that matter — is that you can never be quite sure of what you might find.

That was the case when Mississippi State University's Dan Reynolds and Joe Massey began looking into the effect of different types of additives on the uptake and efficacy of bispyribac or Regiment, an ALS inhibitor, on barnyardgrass.

“I have never been very high on additives,” said Reynolds, professor of weed science at Mississippi State University. “We've seen a little increase in speed and a little boost in efficacy with the addition of ammonium sulfate to herbicide mixtures, especially when plants were stressed.

“But day in and day out, I've never been that impressed with additives.”

Thus, Reynolds and Massey, an assistant professor in the Plant and Soil Sciences Department at MSU, were surprised when they found that applying a mixture of Carbon 14 Regiment, methylated vegetable oil (Dyne-Amic) and 32 percent UAN resulted in 79.6 percent absorption into barnyardgrass compared to 4 percent for C-14 Regiment alone.

Dyne-Amic is used as a non-ionic surfactant in postemergence spray applications. Tests applying the radioactive isotope of Regiment with a pre-packaged formulation of methylated vegetable oil and 32 percent UAN or urea ammonium nitrate solution (Dyne-A-Pak), produced similar results.

Field tests by Charlie Guy, researcher with G&H Associates in Tillar, Ark., on Regiment, 32 percent UAN and Kinetic HV, another nonionic surfactant, appear to mirror the results of the greenhouse studies by Reynolds and Massey at Mississippi State University.

Valent USA Corp. asked the weed scientists to conduct the studies because of some inconsistencies in the results obtained by growers applying Regiment to control barnyardgrass in rice. Darrin Dodds, research associate and Ph. D. candidate at Mississippi State University, and Cade Smith, a post-doctoral researcher at Mississippi State, also worked on the study.

“They wanted us to try to determine which additives give the best absorption and how the material moved once it was in the plant,” said Reynolds. “They also provided the Carbon 14 isotope of Regiment to help us measure the amount of herbicide absorbed by the plant.”

Ironically, the use of methylated vegetable oil (Dyne-Amic) was a last-minute addition to the greenhouse study. The original protocol required the use of organosilicone adjuvants (Kinetic). “We would expect to get similar results with any organosilicone surfactant,” said Reynolds.

The testing involved an intricate procedure in which a sheet of plastic was placed over the flag leaf before the whole plant was treated in a spray chamber. Researchers then removed the plastic sleeve and “spotted” the herbicide mixtures in parallel rows in the same locations on each plant.

The leaves were removed or harvested at six and 24 hours after the application and measured for radioactivity in the laboratory at Mississippi State.

The Dyne-Amic, 32 percent UAN and C-14 Regiment treatment resulted in 79.6 percent absorption compared to 73.9 percent absorption for Dyne-A-Pak and C-14 Regiment. (The percent absorption is the mean of the percentages from the six-hour and 24-hour treatments from two experiments).

“You could see the Regiment mixture sitting on the leaf after treatment,” said Reynolds. “The surface tension was very high. When you put the drops with Dyne-Amic and the UAN, you could see the material spreading and drying into the cuticle of the plant.”

The percentages for other treatments: Dyne-Amic and C-14 Regiment, 17.6 percent; Kinetic HV and C-14 Regiment, 9.3 percent; Kinetic HV and 32 percent UAN and C-14 Regiment, 46.8 percent; and C-14 Regiment alone, 4 percent.

“We confirmed that the 32 percent UAN is having a tremendous impact on absorption,” said Reynolds. “The next question is whether it really makes a difference. If it only takes 4 or 5 percent of the Regiment to control the plant, is 20 times that amount going to make a difference?”

Guy's research in Arkansas appears to indicate the higher absorption can improve weed control. In his study, Regiment was applied at the rate of 9 grams of active ingredient per acre alone; with Kinetic at 0.125 percent by volume; with Kinetic and 32 percent UAN at 2.5 percent by volume and Kinetic with 23 percent UAN at 2.5 percent by volume.

“I need to point out that this is too low a rate of Regiment — it would normally be 12 grams,” said Guy. “But this is the only way we could get separation in the treatments. At 12 grams there would have been no differences.”

At 21 days after treatment, the Regiment-Kinetic-32 percent UAN application was providing 81 percent control of barnyardgrass compared to 23 percent for Regiment and Kinetic and 65 percent for Regiment, Kinetic and 23 percent UAN.

The herbicide mixtures were applied to four-leaf to six-leaf barnyardgrass, some with tillers, on a clay soil in a rice field near Tillar.

“It's really interesting when you have field studies and lab results that are so dramatically different,” said Guy. “This is one of the more impressive tests that I've seen in a while, especially when you consider the amount of weed pressure in this field — up to 1,500 barnyardgrass plants per square meter.”

He said the biggest problem in adapting the system may be the lack of availability of 32 percent UAN in the more traditional rice and soybean areas of Arkansas. “In some parts of the world, it's not that easy to get 32 percent UAN. Down here in southeast Arkansas where we have a lot of cotton and corn, it's readily available.”

Reynolds said the Mississippi State scientists would like to expand the testing to other herbicides and weed species.

“We can't say if what we've found is true for all sulfonylurea herbicides or for other species of weeds,” he notes. “If it worked with all the sulfonylureas — the Staples, the Envokes, the Classics and Options — it could have a far-reaching impact.”

Such studies aren't cheap. Use of the radioactive isotopes requires extensive permitting and special handling of the materials. Each of the plants is placed in a plastic tub to ensure that all of the wastewater involved in the testing can be contained and properly disposed of.

But the results appear to be worth the investment in paperwork, personnel and greenhouse facilities.

“I've had to eat a little crow on this one because I've never been a believer in additives,” says Reynolds. “But I think this shows that for not much money — the cost of a small amount of UAN — you can get a definite increase in absorption.”

Guy says the research has also helped convert him. “I wouldn't put out Regiment without UAN now,” he says.


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