Some Roundup Ready problems linked to herbicide applications As the leaves from cotton plants fall to the ground in anticipation of harvest, some Mid-South cotton growers are finding fewer bolls than they anticipated.

The fruiting loss problems coming to light in some Roundup Ready cotton fields may be the result of late applications or sloppy post-directed applications of the herbicide Roundup Ultra to Roundup Ready cotton, according to Steve Kelly, LSU AgCenter weed scientist at the Scott Center in Winnsboro, La.

"Overall," Kelly says, "early-season weed control was acceptable using the Roundup Ready system this year. However, the environmental conditions experienced in our area this summer magnified any poorly timed or sloppy Roundup applications."

"With more favorable rainfall and cooler temperatures, Roundup Ready cotton may have been a little more forgiving. In years with cooperative weather patterns, growers can get away with less-than-perfect applications of Roundup to Roundup Ready cotton. That wasn't the case this year," he says.

According to Louisiana warehouses and Extension agents, the state's Roundup Ready cotton acreage may top the 65 percent level this year, and will certainly surpass that mark in some localized areas of the state. A large number of Louisiana cotton growers planting Roundup Ready cotton in 2000 had little to no Roundup Ready cotton in 1999.

Kelly says his initial concern with such a large increase in Roundup Ready cotton acreage was that producers may not be entirely familiar with the importance of timing in this system. "I was afraid we could run into some situations where early-season weed competition robbed growers new to the Roundup Ready system of high yields."

To maximize yields, Kelly says, cotton requires a 10- to 12-week weed-free period. "We began this crop season with very clean fields and seemed to get through the early-season period with good success, although we did run into some problems with a few late-season weeds," he says. "However, with the beginning of defoliation, fruiting positions absent of bolls were observed, and in some isolated cases, very little fruit at all was present."

While Kelly attributes some of the fruiting loss he's seen to poorly timed irrigation and extremely hot, dry weather, some, he says, is due to late over-the-top applications of Roundup Ultra.

In some cases where Roundup was applied over-the-top to cotton with a fully expanded fifth leaf or where the herbicide was post-directed as high as 10 inches up the plant, no visible injury was noticeable. However, Kelly says, even though these applications didn't cause visible injury to the plants, poor or little pollination was realized until the plants grew and diluted the Roundup in the plants. After this, fruiting resumed in a normal manner.

"Late applications or sloppy post-directed applications can often be traced back to the time they were made by plant mapping and determining which fruiting positions are vacant," he says.

Harold Hurst, weed scientist at the Delta Research and Extension Center in Stoneville, Miss., says the fruiting loss experienced by some cotton growers this year is similar to the problems reported by growers when Roundup Ready cotton was first released commercially.

"Since 1998, there have been several attempts by researchers to mimic the fruiting loss, without success," Hurst says. "The reason it is so inconsistent and hard to replicate is because so many factors, including environmental conditions, come into play."

The theory behind the boll loss, according to Hurst, is that cotton is less resistant to the herbicide Roundup than soybeans, which are completely resistant.

"The enzyme that detoxifies the Roundup in the plant works very quickly in soybeans, but in cotton, it is much slower acting," he says. "The time it takes to detoxify the herbicide depends on the growth of the cotton plant. If the plant is growing rapidly, then the enzyme can detoxify the chemistry in about the time it takes the cotton plant to grow two leaves. If the cotton plant is growing slower, though, there apparently is the opportunity for more Roundup to be absorbed into the plant."

Hurst says, in his observational experience, growers can lose about two positions of fruiting after making a Roundup application to slow-growing Roundup Ready cotton. "The more sloppy the Roundup application, the more opportunity you have to get it absorbed in the plant," he says.

Although it's too late to recover the lost yields of those growers who experienced fruiting loss in their Roundup Ready cotton this year, Kelly says there are some guidelines growers can follow so they aren't in the same boat next year.

The first, he says, is to make over-the-top applications of Roundup before the cotton plant's fifth leaf is the size of a quarter. Applications made during this window will not interfere with pollination.

Also, Kelly says, post-direct rigs should be set so that the spray solution doesn't reach above the cotyledon area. If set properly, then the usual bouncing that the rig does should not get the solution too high on the plant.

A grower's choice of spray rigs doesn't appear to affect fruiting as long as the equipment is set properly, he says. "Many producers get very good results using layby rigs for directed Roundup applications."