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Herbicide adjuvants confuse many

State weed science society speaker urges testing with alfalfa Adjuvants may enhance performance of herbicides, but most growers have poor understanding of how the products work and field research on them lags in California, according to Mick Canevari, San Joaquin County farm advisor.

At the 53rd Annual Conference of the California Weed Science Society in Monterey, Canevari said adjuvants modify spray mixtures to enhance product performance or to reduce spray application problems.

He said field recommendations for adjuvants conflict. Part of the reason is inadequate product knowledge, which is not surprising in view of the more than 4,000 types sold in the United States for agricultural and other uses.

"There is even confusion among field representatives for these products, but more importantly, most growers have little understanding about how adjuvants perform with their herbicide programs."

Canevari urged growers to use only products that have been field tested by reliable manufacturers.

He worked with five categories of adjuvants in herbicide trials on alfalfa. Those are only a fraction of the 20 categories in use.

"The five are those I and others have done research on during the last couple of years," he said. Canevari was looking for products to use on alfalfa.

Adjuvant advantages "But more specifically," said Canevari, "adjuvants increase spray deposition, reduce run-off, enhance absorption, correct mixing problems, and minimize evaporation."

The five are COC, or petroleum-based; MSO/ESO, or those derived from vegetable oils; non-ionic surfactants; OS, or organo-silicon, the newest type; and the nitrogen fertilizers having herbicidal activity, UN-32 and ammonium sulfate.

He cited earlier research that showed that herbicides without additives have low rates of absorption by weeds. Recent trials were done in North Dakota with the herbicide Pursuit combined with some 20 adjuvants.

One product, used in the Midwest, is a combination of a non-ionic surfactant, a nitrogen solution, and a methylated seed oil.

In trials with and without the compounds, when adjuvants were added to the Pursuit solution, the levels of control rose from 64 to 99 percent on pigweed, 50 to 95 percent on cocklebur, and 47 to 99 percent on foxtail.

Another study by Steve Orloff, Imperial County farm advisor, showed similar performance from a crop oil concentrate and UN-32 with Pursuit.

In trials in 1998 and 1999, Canevari said he used the two grass herbicides Poast and Prism with various adjuvants on foxtail in established alfalfa during the summer.

He found similar control with Poast with seed oil or crop oil concentrate, and the organo-silicon type with Poast had negative effects. "It did a wonderful job of moving the herbicide over the leaf surface, but it had an evaporation problem," he said.

Pursuit performed well with crop oil concentrate and non-ionic surfactant. In trials with Poast and Prism the oil concentrates were superior, although non-ionic surfactant with fertilizer showed good control.

For use with 2,4-DB, he said he agrees with the recommendation of a non-ionic surfactant for two reasons: economy and the least amount of crop damage.

Roundup Ready system Another speaker on weed control in agronomic crops, Ron Vargas, Madera County farm advisor, said the Roundup Ready system, based on genetically modified cotton tolerant of the herbicide, provides excellent control of grasses, nightshade, annual morningglory, and field bindweed. Season-long control was gained with two to three applications morningglory and bindweed.

Genetically modified cotton varieties were planted on more than 297,000 acres, or about one-third of the cotton acreage in California, in 2000.

That is a dramatic increase from the 600 acres of Roundup Ready and less than 100 acres of BXN Buctril-tolerant cotton only three seasons earlier.

"When genetically modified, herbicide-tolerant cotton was first available, we saw growers selecting varieties having that trait but not agronomically suited for their areas. Many varieties came from other areas of the Cotton Belt," Vargas said.

He said evaluations of surfactants showed that original Roundup and the later Roundup Ultra formulations performed no differently with or without the adjuvants in control of barnyardgrass, pigweed, nightshade, and lambsquarters.

"One big advantage with the Roundup Ready system - at least when it is applied before the fourth-leaf stage - is we have no injury to the cotton," Vargas noted. After the fourth leaf stage, hood sprayers must be used.

Some newer materials, he added, must be applied post-directed, requiring the crop to be more developed.

Experience with Buctril, the broadleaf herbicide, on spring and summer weeds is somewhat less, but Vargas said he and fellow farm advisors found the material quite effective.

In trials tank-mixing Buctril with grass herbicides, he said they achieved good grass control but saw erratic control of pigweed, especially at the lower rates of Buctril. He said a revision of the Buctril labels for California and Arizona to allow a rate of one pound could correct that problem.

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