When Crops Become Weeds: Competitive Potential of Volunteer Roundup Ready Cotton and Soybean

Linda Benedict, Stewart, Sandy, Miller, Donnie K., Lee, Donna R., Lanclos, David Y.  |  8/16/2006 8:24:20 PM

Cotton weeds were planted approximately 2 inches beside soybeans at the Northeast Research Station in St. Joseph. (Photo by Mark Claesgens)

Donna R. Lee, Donnie K. Miller, Steve T. Kelly, David Y. Lanclos and Alexander M. "Sandy" Stewart

Roundup Ready cropping systems, which feature plant varieties genetically enhanced to withstand application of glyphosate herbicide with minimal to no adverse effect on the crop, have proven to be effective and cost efficient for managing weeds in cotton and soybean. Increasing percentages of acreage devoted to Roundup Ready cotton and soybean increase the likelihood of "volunteer" Roundup Ready crop plants that germinate from a previous crop or remain after a crop failure. If left uncontrolled, these volunteer plants can compete with a subsequent crop in the same way traditional weeds do.

In field studies conducted under weed-free conditions in 2005 at the Northeast Research Station in St. Joseph, La., LSU AgCenter researchers identified the competitive potential of volunteer Roundup Ready cotton and soybean as "weeds."

Density Studies
In density studies, soybean weeds were planted approximately 2 inches beside a cotton crop, and cotton weeds were planted approximately 2 inches beside a soybean crop. All weeds were thinned after emergence to densities of zero, 0.05, 0.1, 0.2, 0.4, 0.8 and 1.6 plants per foot of row and allowed to compete season-long.

Significant soybean competition with the cotton crop occurred at low densities. Yield was reduced by 7.6 percent at 0.1 plant per foot of row and 14.6 percent at 0.2 plant per foot of row. Higher densities increased competitiveness of soybean as cotton had yield losses of 44.8 percent at 0.8 plant per foot of row and 53.8 percent at 1.6 plants per foot of row. Analysis indicated an expected cotton yield loss of 32.2 percent at 0.5 plant per foot of row and 50.4 percent at 1 plant per foot of row, if soybeans are allowed to compete with cotton season-long.

Low cotton densities resulted in minimal competition with the soybean crop. Soybean yield was reduced by only 0.6 percent at 0.1 cotton plant per foot of row and 3.7 percent at 0.4 cotton plant per foot of row. Higher densities increased competitiveness of cotton. Soybean yield loss was 10.7 percent at 0.8 cotton plant per foot of row and 34.9 percent at 1.6 cotton plants per foot of row. Analysis indicated an expected soybean yield loss of 5.1 percent at a cotton density of 0.5 plant per row and 15.5 percent at 1 plant per foot of row if cotton plants are allowed to compete with soybean season-long.

Interference Studies
In interference period/removal timing studies, soybean weeds were planted approximately 2 inches beside a cotton crop and cotton weeds were planted approximately 2 inches beside a soybean crop. The weeds were thinned to a density of 1.6 plants per foot of row after emergence. Soybean and cotton weeds were allowed to compete for zero, 1, 2, 3, 4, 5, 6, 7 or 8 weeks and season-long. The weeds were manually removed at appropriate intervals.

Soybean interference with cotton resulted in yield losses of 7.2 percent after 1 week and 10.9 percent after two weeks following emergence. Other yield losses were 18.2 percent after four weeks, 32.7 percent after eight weeks and 61.9 percent for the entire season.

Cotton interference with soybeans for periods of 1 to 5 weeks after emergence resulted in yield losses ranging from 4.5 percent to 5.5 percent. Allowing cotton to compete with soybean for eight weeks after emergence resulted in a yield loss of 10.5 percent, and season-long competition reduced soybean yield by 43.3 percent.

Based on these results, Roundup Ready soybean appears to be a strong competitor with Roundup Ready cotton, necessitating an aggressive and effective management program for this "weed" either before the cotton crop is planted or during the growing season. Roundup Ready cotton does not appear to be a strong competitor with Roundup Ready soybean, indicating a less-aggressive management program may be implemented. Although competitiveness of Roundup Ready cotton as a weed in soybean may be severely reduced or eliminated with minimal management, cotton plants left in a soybean field may provide reproductive sites for boll weevil and negatively affect eradication efforts for this pest. Further research is needed to address the impact of volunteer Roundup Ready cotton and soybean as weeds in other matters, including effects on harvest efficiency, cotton grades, soybean grades and insect and disease infestations.

Acknowledgment
Marcie Mathews, Research Associate, Northeast Research Station, and Louisiana Soybean and Feedgrain Research and Promotion Board for partial funding

(This article was published in the summer 2006 issue of Louisiana Agriculture.)
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