Losing families of herbicides in the ongoing battle against Palmer pigweed is tantamount to sending modern-day warriors out to fight battles with a bow and arrow.

And, winning the war against herbicide-resistant weeds is critical to the continued profitability of cotton production in the Southeast.

No-till production systems, combined with nearly exclusive use of glyphosate for weed control in the mid-1990s, dramatically changed the way cotton was grown in the Southeast. The combination allowed growers to use less labor and resulted in a switch from small to large acreage operations, especially cotton, in the Southeast.

The change in cotton production systems didn’t come without costs, and unfortunately many growers in the area have been paying a high price to stay in the game when resistant Palmer amaranth pigweeds occur.

Acetolactate synthate, or ALS-inhibiting herbicides, like Envoke and Staple are frequently used to manage a number of commonly occurring weeds in cotton.

Early on in the battle against Palmer amaranth pigweed, growers basically lost this family of herbicides in some areas of the Southeast due to widespread resistance by a number of weed pests, most prominently Palmer pigweed.

The well-documented resistance by pigweed to glyphosate provided a double-whammy for cotton and soybean growers, leaving them with few options. Or, more correctly, with fewer options that didn’t require lots of money and/or time to use.

The next family of herbicides to enter the fray against herbicide resistant weeds was the family of PPO-based products that includes such popular cotton herbicides as Valor and Reflex. Losing these herbicides to resistance problems would severely hamper efforts by Southeastern cotton farmers to manage any combination of weed problems that includes pigweed.

In the Southeast, so far so good with PPO herbicide use in cotton weed management.

Growers have been quick to recognize the potential future challenges of growing cotton and soybeans without this family of herbicides and most have limited the use of PPOs, especially in areas prone to glyphosate resistant pigweed.

However, a few hundred miles away, the news isn’t so promising. Waterhemp is a close cousin to Palmer amaranth and nearly a decade ago researchers documented resistance to multiple herbicides, the latest being PPO inhibitors, in soybeans in Illinois.

Problem has gotten bigger

The problem has gotten much bigger and more difficult in recent years.

Waterhemp populations have evolved to withstand atrazine, ALS-inhibiting herbicides and PPO-inhibiting herbicides, says University of Missouri Weed Scientist Kevin Bradley.

A similar scenario with Palmer pigweed and resistance to PPO inhibitors would be a worst case situation for most cotton growers in the Southeast.

Alan York, now retired North Carolina State University weed scientist and his former graduate student, Stanley Culpepper, now a University of Georgia weed scientist, have been at the forefront of what is now nearly a decade of finding answers to herbicide resistant pigweed.

Though retired, York continues to lead a group of weed scientists from North Carolina State University in their quest to help farmers in the upper Southeast find new, economic and efficient herbicide combinations to fight the overall problem of weed resistance to herbicides.

Charlie Cahoon is a graduate research assistant at North Carolina State University and one of York's latest protégés to take up the fight against herbicide resistance in North Carolina. Under York's leadership, he and other

North Carolina State weed scientists put in a series of tests to develop some alternative systems to popular multiple applications of PPO inhibitors.

For the past three years we've looked at a number of herbicide combinations that reduce dependence on PPO inhibitors, Cahoon says.

“If we lost PPO herbicides, like they have in some parts of the country, it would be a scary thing for North Carolina cotton growers. About all we would have left is glufosinate and over a period of years of overuse we could lose that to resistance problems, too,” he warns.

Glufosinate, sold by the tradename Ignite, and more recently, Liberty is a broad-spectrum postemergence herbicide that has no soil activity. Ignite inhibits the activity of the glutamine synthetase enzyme that is necessary for the plant to convert ammonia into other nitrogen compounds.

Glufosinate resistant (Liberty Link) crops with an alternative glutamine synthetase enzyme have been developed through genetic engineering.

Over-use a possibility

Over-use of this technology is definitely a possibility in the future, because it can be used on Phytogen Widestrike cotton, and the most widely planted single cotton variety in the Southeast is Phytogen 499 with the Widestrike gene that provides similar protection as does the Liberty Link gene in some Stoneville varieties. 

In no-till cotton throughout North Carolina, the Cadillac program for managing glyphosate-resistant Palmer amaranth includes the sequential application of two PPO herbicides.

Typically, Valor SX goes out about three weeks ahead of planting in a preplant burndown application with glyphosate and 2,4-D. Then at planting, growers usually follow up with another PPO herbicide pre-emergent, Reflex. This is alarming because growers are essentially exposing pigweed to multiple applications of a PPO herbicide, which greatly increases the likelihood of resistance.

The North Carolina researchers looked at Valor SX or no residual herbicide preplant followed by Reflex, Diruon (PS II-inhibitor), or no residual herbicide at planting. 

Additionally, they looked at Diuron preplant followed by Reflex or no residual herbicide at planting. 

Gramoxone was included both preplant and at planting. This allowed the researchers to determine which PPO herbicide growers could go without.  

“Over the two-year period, and at multiple sites, we saw comparable weed control using one of the PPO herbicides in combination with Diuron at either the front or the back end of the system. 

However, in 2010 we did not receive a timely activating rainfall on our residuals at planting and systems with Valor SX preplant outperformed all others.” Cahoon says.

This past year, the researches got similar results with Valor SX preplant followed by either Diuron or Reflex at planting. In contrast to 2010, 2011 brought timely activating rainfall at planting and adequate control of pigweed with Diuron preplant followed by Reflex at planting. 

It appears taking one of the PPO herbicides out of the weed management system did not reduce control of Palmer amaranth and other weeds commonly found in North Carolina cotton fields.” Cahoon says

So we can essentially replace either PPO herbicide with Diuron if we get timely rainfall, but the safer bet is to keep Valor SX preplant. 

Another option

Another option if growers prefer to apply Reflex behind the planter is to include a tank-mix partner, such as Diuron or Cotoran. This gets two modes of action on the pigweed and drives the chances for resistance way down. “ Cahoon says.

The take home message, Cahoon adds, is the value of a residual herbicide as a part of a preplant burndown application in no-till cotton. 

Residual herbicides used preplant protect growers from a failure of pre-emergence herbicides applied at planting, he says.

“Our two primary options preplant here in North Carolina include Valor SX and Diuron. However, when we saw dry weather at planting, Valor SX outperformed Diuron, so it looks like we should keep Valor SX preplant.” Cahoon says.

The North Carolina researchers explored the same treatments using both a Liberty Link and a Roundup system. Cahoon says they saw similar results when substituting Diuron for one of the PPO-inhibitors, regardless of whether glyphosate or glufosinate herbicides were used as a primary weed management tool.

With cotton acreage down this year and expected to drop again in 2013, growers are likely to put more cotton on their most highly productive land. This will put a higher premium on early-season weed management because of its well documented impact on final cotton yields.

York stresses the importance of using multiple herbicide families to improve efficacy of weed control and at the same time slow down the ever-worsening problem with herbicide-resistant weeds, most prominently pigweed.

Several new herbicide products, or more precisely combinations of herbicides and new technology, are likely to be available for cotton growers in the near future.

However, there are no new families of herbicides on the horizon. So it is critical for long-term cotton production in the Southeast to conserve the longevity of herbicide tools we currently have by using them wisely.

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