Weeds fighting back in battle to inhabit crop fields

ORIGINALLY WRITTEN JERRY ENGLER
Along with worries about what heat and dry weather are doing to crops, some producers have begun wondering about some of the stands of weeds that keep growing in their fields-despite treatment with herbicide.

The persistence of weeds, especially pigweed, in Marion County fields that were sprayed with herbicide has some producers concerned that they may be seeing the development of strains resistant to most frequently used chemicals.

Researchers at Kansas State University list pigweed as one of the plants most able to adapt to climate swings in this area. They say it also has ability to change its genetics at a fairly rapid rate when faced with threats that might wipe it out. It can even cross among its several varieties in this region to create new pigweed hybrids.

K-State literature identifies several pigweed varieties in this area that can contribute genetics for change, including redroot pigweed, smooth pigweed, Powell amaranth, spiny amaranth, tumble pigweed, prostrate pigweed, common water hemp, tall water hemp and Palmer amaranth.

The only comfort all of this genetic variety offers is if farmers ever decide to grow pigweed as a forage crop-a periodically contemplated possibility.

Kevin Suderman, a certified crop advisor and agronomist for Cooperative Grain & Supply based in Hillsboro, said pigweed as a forage crop will remain unlikely because although pigweed can have good feed value for livestock, it can also be very “stemmy or woody” with lower digestibility than other crops available.

Suderman and Jeff Mayfield, agronomist with Ag Service in Hillsboro, agreed farmers are probably seeing strong weed stands in fields because of high heat and lack of moisture. The entire warmer winter and ensuing dry weather may have been contributing factors.

Mayfield said if producers are seeing “tremendous pigweed pressure that you didn’t see in past years,” it probably isn’t because they brought in unwanted pigweed seed from another location. It’s because the seed has been waiting in the ground for a long time.

“Pigweed seed can lay in the ground up to 30 years before germinating,” Mayfield said. “Then the right conditions come along, and it’s ready to go.”

Suderman said those conditions definitely were right this year because there is more pigweed in the area, along with other weeds such as mare’s tail.

“This is the second season in a row that shatter cane has seemed more of an issue,” Jost said.

Much of the concern about this resurgence of weeds, he added, revolves around the extensive use of Roundup herbicide, its generic counterparts, or the glycophosphate class of chemicals they represent in general.

Soybeans and corn have been genetically altered for resistance to glycophosphates.

It is becoming increasingly important, he said, especially with the development of no-till practices, for farmers to be able to spray a field after both crop and weed seeds have germinated to kill the weeds, and leave the crop healthy.

It would be damaging to be confronted with classes of weeds, like pigweed, that had developed resistance to glycophosphates.

“It is a real concern,” Suderman said, adding that pigweed drops part of its leaves, reducing the surface area that can absorb chemical, and develops a waxy “cuticle” covering to protect itself during drought that also helps repel spray or “make it hard to penetrate with chemical.”

Suderman said a farmer may have to apply more chemical during dry weather to ensure that weeds are killed.

If spraying a plant such pigweed injures it, but doesn’t kill it, resistance may be increased as a result.

“Any time you stress a plant, but don’t kill it, you are moving it closer to developing genetic tolerance,” Suderman said. “You’re helping the evolution of the weed. I hate to call it that, but that’s what it is.”

Mayfield said pigweed has developed resistance to 2-4D, a herbicide that was used for nearly a half-century.

Both men said using tank mixes of two or three herbicide, or applying different herbicide at different times, may be part of the solution.

Suderman said adding a herbicide that formerly was of more importance for use with soybeans, like Pursuit, may be needed for added weed-kill punch.

Added to all of this, Suderman said, farmer need to consider costs such as like the amount of herbicide and the number of times over a field, versus the likely short and long-term gains in crop production.

As always, the profit margin has to be taken into account, he said.

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