New Herbicide-Resistant Weed Confirmed In Ohio

A weed known to many cotton and soybean growers in the South as “pigweed on steroids” has been spotted in Ohio, prompting Ohio State University Extension experts to warn Ohio growers to take measures to prevent its further spread statewide.

Palmer amaranth, which is a glyphosate-resistant weed that has had a substantially negative impact on crop yields and profitability for cotton and soybean growers in Southern states, has been spotted in a large field near Portsmouth in extreme southern Ohio, said Mark Loux, an OSU Extension weed specialist.

The concern about glyphosate-resistant Palmer amaranth, which has caused entire cotton and soybean fields to be mowed down in some Southern states, is that if the weed takes hold in Ohio crop fields, it will be even harder to control than the glyphosate-resistant weeds already present statewide, he said.

“Not only is Palmer amaranth resistant to glyphosate, this weed’s rapid growth, large size, extended duration of emergence, prolific seed production and general tolerance to many herbicides makes it a much more formidable weed to deal with than the pigweed species we already have here in Ohio,” Loux said. “Among the weeds that we already deal with, Palmer amaranth is going to require pre- and post-emergence applications, possibly multiple post-emergence applications.

“It’s already resistant to two of the main types of herbicides we use in soybeans, glyphosate and ALS inhibitors, and the weed has to be less than four inches tall when spraying in order to get control of it.”

Palmer amaranth, which can grow 3 inches a day, can release nearly a half-million seeds per plant. And because the weed is glyphosate-resistant, many growers in Southern states, in addition to spraying, have had to hire workers to go into their fields to chop down the weeds with hoes and pull them by hand.

Losses for growers dealing with this aggressive weed have been in the millions in lost agricultural crops, experts estimate.

The weed, which has typically been prevalent in Southern states, is moving north, with several other suspected cases statewide that OSU Extension experts are investigating. New infestations of Palmer amaranth have also been found farther north, in Michigan and Indiana, he said.

“We suspect Palmer amaranth was introduced in Ohio and other states through contaminated cottonseed shipped from the southern U.S. for use as animal feed here in the Midwest,” Loux said. “Palmer amaranth established from manure spread on fields that was obtained from these animal operations.”

To prevent further spread and growth of the weed in Ohio, growers and producers should avoid spreading manure on crop fields obtained from animal operations where cottonseed was used as feed, he said.

“This manure likely contains Palmer amaranth and should not be spread on crop fields,” Loux said. “To avoid that problem, growers and producers should ask if cottonseed was used as feed before using any manure obtained from an animal operation other than their own.”

The goal is to prevent the spread of Palmer amaranth, considering that Ohio already has healthy populations of glyphosate-resistant marestail (also known as horseweed), giant ragweed, common ragweed and waterhemp, he said.

“Palmer amaranth is a very competitive weed that can reduce soybean yields,” Loux said. “If you let it go, it could have a pretty explosive population increase in a very short period of time.

“It is a very expensive and difficult weed to manage.”

And the record dry, hot weather Ohioans have experienced this summer hasn’t made controlling glyphosate-resistant weeds any easier. Extended hot, dry weather and drought impact the germination, growth, hardiness and competitive ability of weeds and complicate control efforts, he said.

During extended dry conditions, weeds grow more slowly and also develop thicker cuticles on the leaf surfaces, which has the overall effect of reducing herbicide movement into and throughout the plant.

This as Ohioans have suffered through multiple days of record-setting temperatures of over 100° this summer, with scant rainfall that has resulted in most of the state except for some counties near the Kentucky, West Virginia and Pennsylvania borders experiencing moderate drought, with some counties near the Indiana and Michigan borders experiencing severe and extreme drought as of August 7, according to the most recent U.S. Drought Monitor.

And while some Ohio growers already have experienced dealing with glyphosate-resistant waterhemp, which is typically found in western Ohio and is the closest weed that Ohio has that can be compared to Palmer amaranth, the southern weed has more potential to reduce yield if it is not controlled, Loux said.

But with early diagnosis of infested fields and early intervention, the potential for damage from Palmer amaranth in Ohio can be mitigated, he said.

“This is one of those rare occasions where we have enough information to hopefully prevent additional infestations of an extremely aggressive weed, and there could be serious long-term consequences for farm profitability for failing to do so.”


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