At this point, the confirmation of a new glyphosate-resistant weed is unlikely to raise eyebrows. However, nonchalance is hardly the suitable reaction when considering the implications of what USDA-ARS researchers have uncovered in a Mississippi cotton field in Lafayette County.

It turns out a weed that is commonly found in pastures, spiny amaranth, has made the leap into a row crop environment and is finding the new surroundings comfortable. It has done this through hybridization with the ubiquitous resistant Palmer pigweed found throughout the South.

In mid-March, Stoneville, Miss.-based researchers Bill Molin and V.K. Nandula spoke with Delta Farm Press about the field circumstances, the science behind the hybridization, and management options. Among their comments:

On the hybridization…

Nandula: “Everyone is likely familiar with (herbicide-resistant) Palmer amaranth and the havoc it has caused across the Southeast. The mechanism of resistance — the way by which the weed can withstand a herbicide application — is through ‘gene amplification’, i.e., multiple copies of the target enzyme of glyphosate, 5-enolpyruvylshikimate-3-phosphate synthase, are functional. Thus, glyphosate phytotoxicity is diluted, so to speak.

“In this case, we’re talking about a second weed, also an amaranth, commonly called ‘spiny amaranth.’ It’s actually not a very widespread weed, typically limited to pastures and other non-row crop areas.

“Back in 2011, (Mississippi State University weed researcher) Jason Bond brought us some plants that were suspected of being resistant and had traits like spiny amaranth. They came from a cotton field in Lafayette County. These plants were taken to seed. Plants generated from this seed turned out to be resistant to glyphosate at a 1X rate.

“There are several things to keep in mind. Palmer amaranth is a dioecious plant — that means male and female flowers appear on different plants. Spiny amaranth, meanwhile, is monoecious — the male and female flowers are not only on the same plant but also on the same flower stalk.

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“Both spiny and Palmer amaranths produce extensive amounts of pollen and so it is likely that if these species were growing close to each other there may be some cross pollination. We have evidence that indicates it was a resistant Palmer female that was pollinated from a spiny plant. That’s probably how the spiny amaranth plants got the (herbicide) resistant gene.

“When glyphosate resistance is transferred to a hybrid, glyphosate now becomes the agent that selects for resistant plants. The seed we got from the 1X rate-surviving plants were replanted and sprayed again. We wanted to make sure the resistance gene had been passed on to another generation, which turned out to be the case. That would point to a real problem.

“Amaranthus weed species — and there are four or five others, like waterhemp — are all known to cross with each other. The most prevalent crosser is Palmer amaranth.

“When you have so many crosses, so many hybrids, it could lead to a buildup of resistance and a bad management problem.”

On moving from pasture to cotton field…

Molin: “Spiny is normally a relatively short pasture weed. Now, the hybrid that we’re studying is being found in row crops. So, the hybrid now has the ability to enter into new environments.

“It also has Palmer genes that have changed its look. It’s actually grows taller, like Palmer, so it’ll be more competitive on top of the resistance. It’s just one more weed that producers must be aware of. It could mean greater scouting, greater expense, more problems in keeping it out of row-crop fields.”