Weed shifts seen in cotton

It’s becoming painfully obvious to many Southeastern cotton producers that weed shifts are occurring, and the dynamics of certain weed populations are changing.

But if you’ve been growing cotton for some time, you know that such occurrences are nothing new. For instance, back in the 1970s, when atrazine came on the market, fall panicum became a major weed pest for growers. Then, in the 1980s, prickly sida became a problem as growers began using Classic and Sceptor.

Another example is the emergence of purple nutsedge as a weed pest along with the use of Dual and Basagran.

But while weed shifts certainly are nothing new, that doesn’t make them any less of a problem, says Stanley Culpepper, University of Georgia Extension weed scientist. "It’s obvious, especially in Georgia, that the dynamics of certain weed populations are changing," he says.

Culpepper sees two major factors in the weed shifts that currently are occurring in Georgia and other parts of the Southeast — the adoption of Roundup Ready technology and an increase in conservation-tillage production.

"We feel that the rapid adoption of Roundup Ready technology, coupled with the increase in reduced-tillage production systems, have together caused some significant weed shifts in Georgia," he says.

Another factor, he adds, contributing to the weed shifts is growers’ heavy dependence on glyphosate and, in some cases, the over-use or abuse of glyphosate.

How a weed might respond differently to conventional and Roundup Ready systems was illustrated in a trial conducted at two Georgia locations and one North Carolina location in 2003. In the test, in which researchers were looking specifically at tropical spiderwort’s response to the two systems. Control of the weed was 26 percent greater in a system using Cotoran plus Prowl, Cotoran plus MSMA and Direx plus MSMA versus a system using Prowl and Roundup sequentially.

A current survey involving 11 states and 12 weed scientists reveals how rapidly Roundup Ready technology is being adopted, says Culpepper. States involved in the survey included Alabama, Delaware, Georgia, Iowa, Illinois, Florida, Missouri, North Carolina, Ohio, Pennsylvania and Texas.

The survey showed that 15 percent of the corn, 89 percent of the cotton and 88 percent of the soybeans planted in the surveyed states was Roundup Ready technology. "Roundup Ready technology obviously is critical to our growers. And if you look at the survey, this adoption of Roundup Ready technology is occurring almost everywhere."

The survey also revealed, says Culpepper, that Roundup Ready corn acreage is expected to increase while Roundup Ready cotton and soybean acreages are expected to remain steady.

Another survey — focusing on cotton production and involving Alabama, Georgia, Florida, Missouri, North Carolina and Texas — asked weed scientists to list the emerging weed problems in their states. That survey showed the top five as amaranthus (pigweed) species, annual grasses, morningglories, commelina (dayflower, tropical spiderwort) and winter annuals, including primrose and horseweed.

There are several reasons, says Culpepper, that these weeds are becoming major issues for cotton producers. "There’s no question that the winter annual weeds survive more effectively in conservation-tillage situations. Also, a common characteristic of these weeds is their continual emergence throughout the growing season. They’ll emerge from day one and continue emerging throughout the year."

Another factor in the emergence of these weed species, he says, is that in the late 1990s and into 2000, there was no or limited residual herbicides in many cotton weed control programs.

"A weed such as morningglory cannot be controlled very effectively with glyphosate only, and morningglory always will be an issue in Georgia. We often need residuals, and we need to adjust our programs if we have a problem with morningglories or other troublesome weeds."

Tropical spiderwort, says Culpepper, has emerged as a major weed pest in Georgia cotton fields.

"Tropical spiderwort is one of the greatest concerns to us, and it currently ranks as Georgia’s most troublesome weed in cotton. In 1999, we didn’t know what this weed was. In 2001, we ranked it as our ninth most troublesome weed. Then, in 2002, we ranked it as our No. 1 most troublesome weed. If possible, I’d rank it as the No. 1, No. 2 and No. 3 most troublesome weed in 2003. We define troublesome as the most difficult and costly weed to control."

Tropical spiderwort, he explains, is native to Asia and Africa. It was introduced into the United States in 1928 and was common throughout Florida by the mid-1930s. It is a noxious weed, he adds.

Tropical spiderwort was present in two or three counties in 1999, says Culpepper, and it was present in 41 counties this past year — a moderate to severe pest in 17 counties. He predicts it’ll be present in about 60 counties by the end of 2004.

Research from the USDA-ARS by Ted Webster and North Carolina State University’s Mike Burton shows that tropical spiderwort’s greatest emergence is when temperatures are between 85 and 96 degrees, with 40 percent of emergence occurring after June 4, he says.

"It may start coming up in May or June, but it will continue to emerge throughout the growing season. We must deal with the spread of this weed. However, at this time we are struggling in an attempt to figure out how it is spreading so quickly. We do know that it can get in a cotton picker or on other equipment and be spread from field to field."

Culpepper advises growers to combat weed shifts by doing several things, including using effective soil-applied herbicides, rotating chemicals and crops, using tank mixtures, and not allowing the weed to produce seed, especially in fallow fields or in fields after a crop has been harvested.

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