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Even with herbicide-tolerant cotton: Weed control remains challenging

Roundup-tolerant cotton has unquestionably been a major step forward in controlling tough weed species in the San Joaquin Valley, but it is not the end-all of weed control techniques.

Growers have dramatically reduced hand-hoeing costs, but difficult-to-control weeds like yellow nutsedge and field bindweed continue to offer challenges, according to University of California Cooperative Extension Madera County Farm Advisor Ron Vargas.

Nutsedge continue to be nettlesome for growers of both agronomic and tree and vine crops, Vargas told those at the annual West Side Research and Extension Center Cotton Field day.

Applying Roundup Ultra Max (glyphosate) to yellow nutsedge in herbicide tolerant cotton when the crop is three to six inches tall has been “erratic.”

In his trials, Vargas recorded up to only 50 percent control 35 days after applying one pound active ingredient of Roundup. That increased to 75 percent with two one-pound applications of Roundup Ultra Max. Cultivation jumped that to 90 percent.

Adding 10 to 15 pounds of ammonium sulfate did “kick up the control” slightly.

Touchdown5 (sulfosate) gave similar results, but it also caused significant injury to cotton. “Sulfosate is a different salt than glyphosate and Roundup Ready cotton is not tolerant to Touchdown5,” said Vargas. A one-pound rate caused 50 to 55 percent injury to the plants and the two-pound rate injury rate was as great as 90 percent, he added. This, he said, will result in lower yields.

Avoids injury

However, he added the new Touchdown IQ formulation does not cause the same injury, Vargas added.

Roundup applied three times at 1.5 pounds active ingredient per acre provided up 85 percent control of field bindweed. There was no increase in control when Staple was tank mixed. Goal did increase control to 95 percent, but there was up to 25 percent crop injury.

Shark, a herbicide registered rice and cereal under the name Aim, controlled field bindweed as high as 85 percent, but by 14 days after treatment the field bindweed began to regrow. Shark is not registered for cotton.

“It was a very effective burndown for field bindweed, but regrowth reduced the control to 45 to 50 percent,” said Vargas. “Shark was extremely effective on annual morningglory.”

Tank mixing Shark with MSMA, Staple or Buctril did not increase field bindweed control and Shark, Valor, Liberty and Harvade all caused injury to lower leaves of cotton plants contacted by the herbicides, Vargas added.

The veteran weed control specialists also added that the current generation of Roundup Ready cotton demands that the label be followed and no Roundup be applied after the fourth leaf. A third over the top application in Vargas' trials dropped boll retention from 50 to 70 percent to 40 percent.

Monsanto is developing a new generation of Roundup Ready genes, which will allow producers to make herbicide applications beyond the fourth leaf without injuring the plant.


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