Roundup Ready cotton contains a gene not normally found in cotton plants. The gene allows the plants to withstand Roundup herbicide. That means that cotton growers can spray their fields with Roundup herbicide to control weeds without damaging the cotton plants. And that makes controlling weeds much less tedious and time consuming and less expensive, explains Dr. Keith Edmisten, an associate professor and North Carolina Cooperative Extension cotton specialist in the College of Agriculture and Life Sciences at N.C. State University.

For the most part, Roundup Ready cotton has performed as promised, Edmisten says, but in certain cases, growers have found that Roundup Ready cotton isn’t always ready for Roundup. And now, thanks to the work of a graduate student in the College, they know why.

Since the introduction of Roundup Ready cotton, there have been sporadic problems with the genetically engineered crop, Edmisten says. In some fields, cotton bolls didn’t develop as they should, particularly on the lower parts and sometimes on the middle portion of the plant. The plants appeared to have suffered herbicide damage, but it wasn’t clear how and why the damage occurred.

While working toward her Ph.D. in crop science at N.C. State, Dr. Wendy Pline, who received her doctorate in the spring of 2002, studied the sporadic damage that cotton growers throughout the South have noticed on Roundup Ready cotton. Pline found that even though Roundup Ready cotton is able to withstand Roundup, when and how growers apply the herbicide is critical to avoiding damage.

Controlling weeds in conventional cotton — cotton that is not genetically engineered to withstand herbicide — is a difficult, time-consuming process, says Edmisten. Edmisten and Dr. John Wilcut, associate professor of weed science, directed Pline’s doctoral work.

Because herbicides will damage conventional cotton plants, growers must carefully spray herbicide around the cotton plants, a particularly delicate task when the plants are only a few inches tall. Up to a point, Roundup herbicide may be sprayed over the top of Roundup Ready cotton. Spraying over the top of the plants is faster and less expensive than spraying around the plant, which is known as a post-directed spray.

Pline’s work showed that over-the-top sprays can damage even Roundup Ready plants if the timing isn’t right. It seems Roundup Ready cotton is truly ready for Roundup only in the early stages of the plant’s life. Indeed, Monsanto, the company that makes Roundup herbicide and licenses the Roundup Ready cotton technology to several seed companies that produce Roundup Ready cotton, advises growers to use over-the-top sprays only during the early part of growing season, through what is known as the four-leaf stage of cotton plant development.

Pline’s work shows how cotton plants may be damaged if growers don’t follow this recommendation and take care to make sure that any herbicide applications later than the four-leaf stage are post-directed and that herbicide does not get on the cotton plants.

Pline began her Ph.D. work in 1999 by trying to replicate the damage cotton growers were seeing. She grew Roundup Ready cotton in fields and in enclosed growth chambers, applying Roundup in different amounts and at different times during the growing season. Pline saw the same kinds of damage growers were seeing. In some cases cotton bolls would abort; the bolls would die but remain on the plant. In other cases, the bolls were misshapen. And some bolls suffered a condition called cavitation; the bolls die but remain on the plant.

“The damage is usually on the lower branches of the plant, early in the season,” she says. Cotton plants usually continue to grow, and the bolls formed higher on the plant are usually intact, so growers rarely lose entire crops. But the bolls on the lower portion of the plant are often the largest, so yields can be decreased significantly.

Pline then tried to determine where glyphosate, the herbicidal ingredient in Roundup, accumulates in cotton plants. She applied a radioactive version of the herbicide to plants. The radioactive herbicide can be detected in plant tissue. Pline found that when Roundup is sprayed on the leaves later than the four-leaf stage of plant development, glyphosate tends to accumulate in the reproductive portions of the plant. Glyphosate does not accumulate in the reproductive parts of the plant when it is applied prior to the four-leaf stage.

The reproductive machinery of the plant is, of course, what eventually forms cotton bolls.

Pline also found that pollen viability is affected by the timing of Roundup applications. She found that even when the Roundup label directions, which specify over-the-top sprays only through the four-leaf stage and post-directed sprays thereafter, are followed, pollen viability is reduced by 30 to 60 percent.

When Pline looked closely at cotton flowers on plants that had been sprayed with Roundup and those that had not been sprayed, she found a striking difference. Cotton is self pollinating. Each flower contains a central structure called a stigma, which is surrounded by pollen-carrying anthers. The flower is pollinated when pollen moves from the anthers to the stigma.

As would be expected, the flowers on plants that had not been treated with Roundup were normal. The flowers of plants treated with herbicide were quite different. The anthers surrounded only a portion of the stigma and did not extend to the tip of the stigma, where pollination is most effective. Such flowers clearly could not be pollinated as effectively.

“We measured pollen grains on the stigmas of treated and untreated plants,” Pline says. “There was less pollen on the stigmas of the treated plants.” But that’s not all. The pollen grains themselves were less robust on the herbicide-treated plants. The pollen grains on the plants treated with Roundup were shrunken and collapsed, not full and round as they should be. Pline cut the deformed pollen grains open and found that their development had been inhibited. They hadn’t fully matured.

Pline found yet another factor that contributes to yield losses when she looked at where the protein that gives Roundup Ready cotton the ability to withstand herbicide ends up in the plant. She found less of the protein in the reproductive parts of the plant. There was 32 percent less protein in the reproductive parts of the plant than in other parts.

“We know Roundup accumulates in the reproductive tissue. We need more protein in the reproductive tissue, not less,” says Pline.

Pline’s research showed that Roundup Ready cotton sprayed with Roundup beyond the four-leaf stage can be hit with something of a triple whammy. Not only do the plants produce less pollen, the pollen may be less potent, and the plants have less protection in their reproductive tissues.

Pline’s work has shown growers what’s going on in their fields, Edmisten says, that “the timing of over-the-top applications of Roundup is really critical. Growers also need to be careful with post-directed sprays not to get any herbicide on the plant.”

In some cases, particularly when Roundup Ready cotton first became available, growers may not have been as careful as they should have been with their Roundup applications, Edmisten adds. Now they know, and Edmisten says he’s seen less damage each year as word has spread.

 

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