Constant Glyphosate Pressure
Causes Weed Shift
It is becoming obvious to many Southeastern cotton producers that weed shifts are occurring, 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® (chlorimuron) and Scepter® (imazaquin).
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. He 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. Another factor contributing to the weed shift is grower’s heavy dependence on glyphosate and in some cases, the over-use or abuse of it.
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® (fluometuron)/Prowl® (pendimethalin)/ MSMA/Direx® (diuron) versus a system using Prowl® and Roundup® sequentially.
A survey on cotton production involving Alabama, Georgia, Florida, Missouri, North Carolina, and Texas asked weed scientists to list the emerging weed problems in their states. The top five were reported to be: Amaranthus (pigweed) species, annual grasses, morningglories, commelina (dayflower, tropical spiderwort) and winter annuals, including primrose and horseweed. There are several reasons that these weeds are becoming major issues for cotton producers says Culpepper. "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." Consequently certain plants, such as tropical spiderwort, have emerged as major weed pests 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." says Culpepper. 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, and it was present in 41 counties this past year - a moderate to severe pest in 17 counties.” He predicts it will be present in about 60 counties by the end of 2004. 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. (Southeast Farm Press, 3/22/04).
