April 2012
Keeping weeds to a minimum

Potato growers can add a new weapon to their weed- control arsenal for the 2012 season with Reflex, a Syngenta herbicide that has shown a lot of promise in university research trials.

Reflex has been available for several years, but wasn’t labeled for potatoes until 2011.

“Herbicides are usually developed for crops other than potatoes, and Reflex was listed for cotton and soybeans,” said Pamela J.S. Hutchinson, a University of Idaho potato cropping systems specialist. “None of the herbicides were developed specifically for potatoes. They were developed for another crop first, and that is the challenge in getting herbicides for potatoes.”

Reflex is a pre-emergence herbicide that inhibits protox, an enzyme in chlorophyll production. Applied through a ground sprayer or overhead sprinkler irrigation system, it has shown excellent control of pigweed and nightshade weeds, typically at 90 percent over the course of a season. While it is less effective against common lambsquarters, control is increased when Reflex is tank-mixed with Prowl, Metribuzin, Boundary or Dual Magnum.

Older herbicides continue to exhibit success in controlling a variety of weeds, each attacking with a different mode of action.

Matrix, offered by DuPont, inhibits branch chain amino acid production, an important aspect of creating plant cells. It can be applied to the soil as a pre- or post-emergence foliar application. Control is 95 percent or greater against nightshade, with the exception of cutleaf nightshade.
Matrix also works well on redroot pigweed and most grasses, but is less efficient with common lambsquarters. However, a tank mixture of Prowl or Metribuzin with Matrix has shown to significantly increase effectiveness with common lambsquarters.

Outlook, a BASF herbicide, only can be applied as a pre- emergent and has a mode of action that prohibits very long- chain fatty acid production – vital to any plant’s survival. It is effective against pigweed, grasses and nightshade, including 100 percent control of hairy nightshade throughout the season. Hutchinson said Outlook is less useful against common lambsquarters, though effectiveness increases when tank- mixed with Prowl and Metribuzin.

Chateau is a Valent herbicide that suppresses weeds – especially hairy nightshade – and can only be used as a pre-emergent. It has the same mode of action as Reflex and can be applied through a ground application or with overhead irrigation.

“The active ingredient in Chateau, flumioxazin, is used post-emergence in non-crop areas as a burn-down herbicide because it has a lot of activity on the foliage, but if you apply it pre-emergence, the potato is safe and it still has the same activity on the weeds,” Hutchinson said. “The thing about Chateau is you have to have at least two inches of soil covering the sprout coming up from the seed piece.”

Although Reflex and Chateau share the same mode of action, all other herbicides labeled for potatoes have an alternative mode that provides an important tactic in combating herbicide resistance.

A prime example of herbicide resistance is glyphosate, the active ingredient in Roundup. Many growers of Roundup Ready crops, such as cotton and corn, are experiencing resistance in weed populations. While resistance hasn’t presented serious problems for potato herbicides, researchers have discovered a few pockets of herbicide resistance in potato fields.
For example, Hutchinson has located Metribuzin-resistant redroot pigweed populations in southeast Idaho, where growers have relied on the herbicide every other season since the 1970s.

Weed scouting
Many experts promote weed scouting as a method to find resistance before it gets serious.

“Growers have scouted for insects in crops, and the time is now to scout for weeds,” said Les Glasgow, Syngenta technical asset lead for herbicides. Glasgow also helps develop strategies to control herbicide resistance. “They can usually be seen in small sections, and spread out from there, so it’s being out there and seeing how well an herbicide works.”

If there is a question that a weed is exhibiting resistant behavior, labs at ag universities can test the seeds. That typically involves growing the potentially resistant seeds and applying herbicides to which the weeds are suspected of being resistant to assess survivor levels.

Options for growers
Growers who find resistant weed populations in their fields have options. The first is identifying the herbicide that is no longer effective, then finding an alternative with a different mode of action to combat the population. It is also important to tank-mix herbicides with alternative modes of action to reduce herbicide resistance in weed populations.

“Always tank mix two products at the same time that attack different growth sites in a plant’s metabolism, like light process, cell metabolism or cell division,” said Mike Cunnane, DuPont crop production manager for specialty crop herbicides.

Crop rotation allows growers to find new herbicides with modes of action that may not be available for potatoes. Also, early cultivation between rows eliminates weeds, although it isn’t effective the whole season. Residual herbicides have proven effective because they last for weeks until planting before deterioration.

“One major trend being driven by resistance is applying residual herbicides in the fall, when the temperatures are cool and the soil is wet,” Cunnane said. “When things warm up in the spring, the residual herbicide is still in the soil and you start controlling (weeds) in the spring as they start germinating.”

Pre-emergence herbicides have also proven effective at the beginning of the season because they have an impact on weeds shortly after planting, potentially reducing or eliminating the need for post- emergence applications. But the single factor all experts agree on is mixing or alternating herbicides with different modes of action to help prevent or delay the development of herbicide-resistant populations.

-Everett Brazil III, Spudman contributor

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