The fall of 2018 is one many growers want to forget. With two hurricanes and other seemingly endless rainfall events, a considerable amount of cotton in South Carolina and Georgia, and to a lesser extent in southeastern North Carolina, was never harvested. That raises questions on the potential for volunteer cotton plants to come up in 2019.
We expect the vast majority of cotton seed lost viability over the winter. However, we know a few seed can survive the winter and come up the next spring. A good example is the volunteers often seen where modules were built and some cotton was left on the ground when the module was moved. Rough math will show that 11 million seed can be produced by a two-bale crop. If only one-tenth of one percent of those seed remain viable and emerge this spring, that would mean 11,000 volunteers per acre.
(Editor's Note: Weed scientists from the University of Georgia and North Carolina State Univeristy prepared this article, and they include Charlie Cahoon and Wes Everman, Weed Science Extension Specialists, NC State University; Alan York, Professor Emeritus, NC State University; David Jordan, Extension Peanut Specialist, NC State University; Eric Prostko and Stanley Culpepper, Weed Science Extension Specialists, University of Georgia.)
The concern with volunteer cotton is two-fold. First is the concern over the impact of volunteers on the current crop. We are not aware of any data documenting the competitive impact of volunteer cotton. However, it is reasonable to expect that higher populations of volunteers can reduce yields of other crops. Even volunteer cotton in cotton could be a problem at higher densities.
The second and greater concern with volunteer cotton relates to maintenance of boll weevil eradication. Pheromone traps are placed in cotton fields to detect any stray weevils that may have been inadvertently moved into the area before an entrenched population is established. Currently, cotton is trapped on a half-mile grid. Soybean or other crops more than one-half mile from a trap need to be trapped if volunteer cotton is present. If such fields exist, the grower can contact Eradication personnel and they will trap the field at no charge to the grower.
Weather in the fall of 2018 also prevented harvesting some soybean fields. Volunteer soybean is less competitive than most common broadleaf weeds. However, depending upon the density, volunteer soybean can adversely impact other crops. Research in Arkansas showed that one soybean plant per six feet of cotton row reduced cotton yield 7%. Research in South Dakota showed that one soybean plant per square yard reduced corn yield 3%, and three soybean per square yard reduced yield 8%.
Volunteer Cotton Control in Cotton
Volunteer cotton in the cotton row will be harvested and thus is of no concern. Volunteers in the row middles, however, are basically weeds. There is no research to guide us in determining when enough volunteers are present in row middles to be of concern. However, our gut feeling is densities of one plant per 12 to 15 feet would be sufficient to adversely affect cotton yield.
The simplest way to deal with volunteer cotton in cotton is to switch traits. Enlist One (choline salt of 2,4-D) or Enlist Duo (2,4-D choline plus glyphosate) applied to Phytogen Enlist cotton would control volunteer XtendFlex cotton or GlyTol/LibertyLink cotton. Dicamba (Engenia or Xtendimax) applied to XtendFlex cotton would control volunteer Enlist or Glytol/LibertyLink cotton.
Volunteer cotton emerged at the time of cotton planting should be killed. This can be accomplished with paraquat. Alternatively, Enlist One or Enlist Duo can be applied when planting an Enlist variety to control XtendFlex or GlyTol/LibertyLink volunteers. Engenia or Xtendimax can be applied at time of planting XtendFlex varieties to control Enlist or GlyTol/LibertyLink volunteers.
Volunteer cotton containing any trait could be controlled by paraquat or Aim applied using a hooded sprayer. If this approach is chosen, the cotton will need to be pretty small (four-leaf or less). Cotton with much more size than four-leaf will be severely burned and defoliated but it will regrow.
Volunteer Cotton Control in Soybean
Research in North Carolina showed that preemergence (PRE) herbicides alone or conventional postemergence (POST) herbicides alone did not consistently control volunteer cotton in soybean. Programs of Canopy PRE followed by Classic, FirstRate, Flexstar, or Resource applied POST to 3-leaf cotton were highly effective. Dicamba could be applied to Xtend soybean to control volunteer Enlist or Glytol/LibertyLink cotton. Enlist One or Enlist Duo could be used in Enlist soybean to control volunteer XtendFlex or Glytol/LibertyLink cotton.
Volunteer Soybean Control in Cotton
Regardless of the soybean variety planted last year, volunteer soybean emerged at the time of cotton planting can be controlled with paraquat. Glufosinate (Liberty, others) would also be an option for Roundup Ready Xtend volunteers although soybean control by glufosinate can be erratic. Obviously, glufosinate would not be an option for volunteer LibertyLink soybeans. If one is planting a Phytogen Enlist cotton variety, Enlist One or Enlist Duo applied at planting is also a good option for both volunteer Roundup Ready Xtend and LibertyLink volunteers. Conversely, if planting XtendFlex cotton, Engenia or Xtendimax applied behind the planter will control LibertyLink or Roundup Ready volunteer soybean.
In-season control of volunteer soybean is obviously dependent on the traits in the soybean. Essentially all cotton varieties currently being planted are tolerant of glufosinate. Hence, glufosinate is an option to control volunteer Roundup Ready, Roundup Ready Xtend, and conventional soybean volunteers. As stated previously, however, glufosinate can be erratic on volunteer soybean. Timely application (soybean two-trifoliate or smaller) will increase the control obtained with glufosinate.
Enlist One or Enlist Duo applied to Phytogen Enlist cotton would effectively control volunteer Roundup Ready, Roundup Ready Xtend, and LibertyLink soybean whereas Engenia or Xtendimax applied to XtendFlex cotton will control volunteer Roundup Ready or LibertyLink soybean.
Envoke (trifloxysulfuron) will control any variety of volunteer soybean, including those with the STS trait. Research in North Carolina showed that Envoke at 0.1 oz/acre controlled 3- to 5-trifoliate non-STS soybean 100% and three-trifoliate STS soybean 98%.
Cotton and Soybean Control in Corn
One might assume that atrazine applied at planting would control volunteer cotton or soybean, but that is not always the case. Do not be surprised if some cotton or soybean survives atrazine applied preemergence.
Mesotrione-containing products will control volunteer cotton and soybean in corn. Depending upon the product (selected products in table below), mesotrione can be applied PRE or POST. Mesotrione should be similarly effective on cotton and soybean containing any herbicide resistance trait.
In corn with the LibertyLink trait, glufosinate can be used to control small Roundup Ready Xtend soybean. A dicamba product, such as Status, can be used to control LibertyLink soybean. Essentially all of the cotton grown in 2018 was glufosinate tolerant, hence glufosinate would not be an option to control volunteer cotton.
Volunteer Peanut Control in Corn, Soybean, and Cotton
Split applications of glyphosate, Liberty, or dicamba should be very effective in controlling volunteer peanuts when used in the appropriate system (RR, Liberty, Xtend, or Enlist). Postemergence applications of Enlist One (2,4-D choline) alone will only provide about 35% control of volunteer peanuts.
In conventional soybeans, preemergence application of metribuzin @ 0.25 lb ai/A will provide approximately 40% control of volunteer peanuts but postemergence control is nearly impossible since peanuts have adequate tolerance to most over-the-top soybean herbicides.
In conventional field corn, split applications of dicamba (EPOST + Lay-by) or dicamba (EPOST) followed by Evik (ametryn) (Lay-By) were good treatments long ago but might not be not very user friendly today.
The primary concern with volunteers in peanuts is that volunteers partially negate the benefits of rotation for disease control. With volunteers hosting disease, you are not really rotating.