The high value of cotton and the high risk of yield loss from weeds, especially from the continued threat from glyphosate resistant pigweed and marestail, has helped spur the use of Liberty Link cotton varieties in the upper Southeast.

Some farmers have found they can get similar results on weed control by using Phytogen varieties that contain the Widestrike gene for insect control and spraying this cotton with Ignite herbicide.

While it’s legal, it’s high risk and not a practice encouraged by either Bayer, which markets Ignite herbicide as part of their Liberty Link system of cotton varieties or Dow, which markets Phytogen brand cotton seed that contain the Widestrike gene for insect management.

WideStrike Insect Protection is a two-gene, in-plant trait that provides broad spectrum and season-long control of lepidopteran insect pests, such as fall armyworms.

The LibertyLink Cotton System is a new transgenic,postemergence herbicide management system for cotton developed by Bayer CropScience.

LibertyLink cotton varieties marketed by FiberMax have a genetically based resistance to Ignite herbicide (also called Liberty, Finale and Rely).

Glufosinate ammonium is the herbicide used in LibertyLink cotton and other crops. If applied on plants that contain a gene for tolerance, the herbicide does a good job of controlling a wide spectrum of weeds without damaging cotton plants.

As part of the research and development and subsequent manufacturing process for Widestrike cotton, a gene similar to the gene used to provide tolerance to glufosinate is used as a genetic marker. Thus, Widestrike cotton does have some built in genetic tolerance to glufosinate.

In producing Widestrike cotton seed, there is no easy way to turn insects on or off to determine whether the selected gene that provides insect protection is active. By including a gene similar to the LibertyLink gene, researchers can spray cotton plants and if the plant dies, they know the Widestrike gene was not passed on to target seed.

Mostly by accident, and often serendipitously, some cotton growers found they could take a shortcut and use Widestrike-containing Phytogen varieties and spray them with glufosinate.

LibertyLink on Widestrike is a legal application and it usually works. However, the practice is not recommended by university Extension weed specialists and is not supported by either Dow AgriScience or Bayer AgriScience.

Dow AgriScience has made public its stance on using glufosinate or Liberty Link on Phytogen varieties containing the Widestrike gene noting:

• The tolerance provided by the PAT gene in Widestrike cotton is not equivalent to the glufosinate tolerance gene found in Liberty Link-containing varieties.

• Though legal to apply over-the-top applications of ammonium-containing herbicides on Widestrike cotton, this practice may result in crop damage or loss.

Not a recommended practice

• Phytogen and Dow AgriSciences do not recommend or warrant use of glufosinate herbicides on Widestrike cotton and all risk of crop damage or loss associate with this practice is the responsibility of the grower.

Clemson University Cotton Specialist Mike Jones conducted a series of tests in the 2011 cropping season to determine the amount of damage from applying varying rates of glufosinate on Widestrike cotton.

Speaking at a recent field day, Jones stressed that Dow and Phytogen do not recommend this practice and, therefore, neither does Clemson University — or any other university.

In his research project at the PeeDee Agriculture Research and Extension Center near Florence, S.C., Jones looked at one-time applications of 29, 58, 87 and 116 ounces per acre of glufosinate on Widestrike cotton.

He notes that it is legal to apply up to 87 ounces of glufosinate on cotton during one growing season.

LibertyLink 1773, which is tolerant to glufosinate, was planted in side-by-side plots next to Phytogen 375 cotton, which contains the Widestrike gene. The cotton in this test was planted on May 16, in 40-foot long plots.

Glufosinate was sprayed on the plots at the three leaf stage, at various rates, at the nine leaf stage at varying rates and at both the three and nine leaf stage at various rates.

“Even at the lower rates we did see an interaction between varieties,” Jones says. “That’s exactly what Dow and Phytogen predicted would happen,” he adds

The Clemson researchers used a 0-10 rating scale to measure overall plant health in the plots. Across all treatments there was more damage in the Widestrike plots than in the LibertyLink varieties, he points out.

How much the leaf loss and plant discoloration affects yield loss and cotton quality will be determined when the cotton plots are harvested.

In similar tests at the University of Georgia, researchers saw consistent 15-20 percent leaf loss, but there was no corresponding loss of yield.

Easily identified

The cotton plants, which were sprayed with different rates of glufosinate back in June, had grown out of the damage by the time the early August field day was held. Still attendees to the field day had little trouble telling which rows were Widestrike-containing Phytogen 375 and which were LibertyLink 1773, because of the number of dead leaves in the plot rows.

When researchers waited until the nine leaf stage to apply glufosinate, there was less visible damage. “At this growth stage coverage is an issue,” Jones explains. “If you don’t get good coverage, you don’t get good weed control. Likewise, you don’t get as much plant damage,” he notes.

Jones contends any time there is a loss of leaves on a cotton plant, there is a higher risk of reducing boll fill and thus reducing yield. The cotton plant usually compensates for the leaf loss, but any number of factors can prevent the cotton plant from making this compensation, Jones explains.

“To show visitors to the field day what the June damage looked like, we sprayed the two varieties with a high rate of glufosinate in late July, with temperatures at or near 100 degrees F. We actually knocked some bolls off of the Widestrike varieties.”

“If a grower used glufosinate in a late-season salvage treatment on Widestrike cotton, I think there could be significant yield loss, Jones says.

While the practice of using Liberty Link herbicide on Widestrike cotton varieties is legal, the combination of the high value of cotton and the lack of a corporate safety net if something goes wrong, make it a high risk strategy that could cost, rather than save, growers money.