Perspectives Online

Cotton versus the Monster Weed - Weed scientist finds strategies to battle resistant Palmer amaranth.

It came up out of the cotton fields, the soybean fields and the cornfields. It grew tall, gobbling up field after field. It was unbeatable; nothing could stop it. No one had ever seen anything like it. It was … glyphosate-resistant Palmer amaranth.

N.C. State University Cooperative Extension weed scientist Dr. Alan York and the state’s cotton growers have been struggling with this resistant monster weed since 2005, when it was first documented in North Carolina cotton. Though it is also common in corn and soybean fields, it is somewhat easier to control in those crops.

From a weed scientist’s perspective, Palmer amaranth is the “ideal weed,” York said. “If you were to design a weed, it would have the characteristics of Palmer amaranth.”

Palmer amaranth is very efficient in carbon fixation and water use. It grows rapidly – an inch or more a day – and it can grow to eight feet tall. And it is extremely prolific – 300,000 to 500,000 seeds per plant.

In one square yard, as many as 300 to 400 Palmer amaranth seeds can come up. And no one knows for sure how long the seeds remain viable. The tall weeds compete strongly with crops, even with other weeds, and its stems are so woody that the weeds can interfere with harvesting equipment.

The weed looms large in the center of this cotton field. York and his colleagues are working to give growers tools to fight the problem.
Courtesy Jack Bacheler
“Once you get it, it’s extremely hard to get rid of,” said York, who will retire in January after 30 years in the Department of Crop Science in the College of Agriculture and Life Sciences.

The weed itself — without resistance — is still fairly new to North Carolina. Though it was common in South Carolina for many years prior to being a problem in North Carolina, York said that it was seldom found here until the mid-1980s. Today, the weed is in nearly every Coastal Plain county, and the resistant biotype is found in 22 eastern North Carolina counties.

Glyphosate resistance in Palmer amaranth and other weeds, including horseweed and common ragweed, came about following the widespread use of Roundup Ready crops. Ninety-eight percent or more of the cotton and soybeans in North Carolina are Roundup Ready, and the percentage of corn planted to Roundup Ready hybrids is approaching 75 percent.

Dr. Alan York holds a specimen of Palmer amaranth, a woody weed that can grow as tall as eight feet and is the bane of the state’s cotton growers.
Communication Services, NC State University
For years, weed scientists and the companies that developed RR varieties believed that the potential for weed resistance to glyphosate was low — much lower than with many other commonly used chemistries. Because of the great efficacy of glyphosate, growers were able to reduce or eliminate use of other herbicides and cultivation. At the time, it seemed to be the logical thing to do, York said. “The technology made weed control easier and more economical. It saved labor and allowed cotton growers to successfully implement no-till systems.” The end result was unprecedented selection pressure placed on glyphosate. And over time, growers selected for the resistant biotype.

York and his colleagues have been actively involved in developing strategies to deal with the resistant weed in cotton and other crops. “Growers continue to plant Roundup Ready crops because the technology still offers many benefits,” York said. “However, we can no longer rely totally on glyphosate.”

Effective management programs for glyphosate-resistant Palmer amaranth have been developed for cotton and other crops. These programs include use of other herbicides in addition to glyphosate.

The most successful strategy for fighting resistant Palmer amaranth has been to apply herbicides to fields before cotton is planted, followed by additional residual herbicides at planting and later in the season. “If we don’t get it up front, we’re out of luck. We can’t salvage a control failure in cotton,” York said.

The strategy is called “early pre-plant” and calls for growers to treat fields two to four weeks before cotton is planted. Herbicides applied at planting can be very effective, but timely rainfall is required. Having some herbicide out early increases the odds of getting rainfall on it before the weeds emerge.

Treating fields this way has not come without costs to growers. York said the intensive sprays can add at least $20 to the cost per acre for producing cotton or as much as $35 an acre, if growers throw all the tools at the problem. Cotton acreage is down in North Carolina, but York attributes that more to the high prices that corn and soybeans have enjoyed the past few years.

In spite of their best efforts, N.C. Department of Agriculture and Consumer Services’ research stations at both Clayton and Rocky Mount have seen resistant Palmer in their fields, and it is now widespread across the coastal plain of North Carolina. The weed is spread by farm equipment shared across fields. More importantly, the resistance trait can be transferred by pollen. Researchers in Georgia have determined that pollen from a resistant male can fertilize susceptible females at least 1,000 feet away.

What the cotton industry really needs to defeat resistant Palmer amaranth is new chemical weapons (new modes of action), but there are few on the horizon. Instead, companies are developing crops stacked with multiple herbicide resistances. Within the next decade, crops resistant to three or more herbicides will be available. These new technologies will give growers more tools to fight problem weeds.