Several reports of volunteer Roundup Ready cotton in Roundup Ready soybeans have surfaced in recent years. Typical populations of volunteer cotton do not limit soybean yield, but a few volunteer cotton plants can seriously jeopardize boll weevil eradication efforts.
Fruit on volunteer cotton provides oviposition sites for weevils. Boll weevils typically produce three to five generations per year. With adequate food resources and oviposition sites, the population can potentially increase 40-fold with each generation. Hence, uncontrolled volunteer cotton can be the foundation for an outbreak of weevils that potentially can infest neighboring cotton fields or produce an overwintering population if they go unchecked.
Volunteer Roundup Ready cotton in Roundup Ready soybeans sometimes results from cotton seed that have overwintered. The potential problem is likely to be greater than normal in 2003 as late-season rains and hurricanes last year left an unusually large amount of seed cotton on the ground.
Volunteer cotton more commonly occurs in fields where a failed cotton crop is replanted to soybeans. In preparation for planting soybeans on old cotton beds, the tops of the rows are commonly dragged off. This moves un-germinated cotton seeds to the row middle and covers them with soil, thus providing a good environment for germination.
When replanting wide-row soybeans on old cotton beds, it may be advisable not to drag off the tops of the rows. This may require planting soybeans a little deeper than normal. Even if the beds are disked down, cotton may still emerge from previously un-germinated seed. However, disking may be the best option as this will allow one to plant narrow-row soybeans which would be more competitive with volunteer cotton.
Growers are advised to closely monitor fields that are rotated out of cotton into soybeans this year. If volunteer cotton survives into the growing season, growers should notify the boll weevil eradication personnel in their states to make them aware of the situation.
If volunteer cotton is known to exist in soybeans, the soybean fields can be trapped and monitored. However, this is expensive and stretches already thin eradication crews even more. Additionally, most volunteer cotton goes unnoticed until late in the season.
Research has been conducted in North Carolina and Louisiana to evaluate a number of potential treatments to control volunteer Roundup Ready cotton in Roundup Ready soybeans. This research has shown that pre-emergence broadleaf herbicides sometimes provide good control of volunteer cotton from seed.
Also, we have noted that postemergence broadleaf herbicides tank-mixed with glyphosate sometimes control volunteer cotton well.
Growers should keep in mind that anything less than 100 percent control of cotton will provide oviposition sites for boll weevils which can seriously jeopardize eradication efforts. In our research, we have had to use both pre-emergence and postemergence herbicides to consistently obtain the desired levels of control.
We evaluated Canopy, Canopy XL, Python, Scepter, and Sencor applied pre-emergence at normal use rates. In trials where greater than one-half inch of rain was received prior to cotton emergence, Canopy, Canopy XL, Python, and Scepter controlled cotton 95 to 100 percent. In other trials, where rainfall was not received very timely, control was sometimes as low as 50 percent. Sencor was generally less effective than the other pre-emergence herbicides.
Canopy would be the material of choice. It was more consistently effective on volunteer cotton than Sencor, and it controls a broader spectrum of weeds than does Sencor. It should be remembered that rotational restrictions following Canopy XL, Python, and Scepter limit planting cotton the following year on some soil types. Read and follow label instructions.
We evaluated the following postemergence herbicides tank-mixed with Roundup: Classic at 0.33 ounce per acre, FirstRate at 0.3 ounce per acre, Reflex at 12 fluid ounces per acre, Resource at 4 fluid ounces per acre, and 2,4-DB at 2 fluid ounces per acre. Classic, Resource, and 2,4-DB mixed with Roundup controlled cotton well in about 40 percent of the trials when applied without a pre-emergence herbicide. Reflex was adequately effective in only 20 percent of the trials. FirstRate was inadequate in all trials.
The greatest and most consistently effective control was achieved with Canopy applied pre-emergence followed by Resource, 2,4-DB, Reflex, or Classic mixed with Roundup. Control of cotton with these treatments was 99 to 100 percent. Combinations of Roundup plus Reflex or Roundup plus Classic reduced cotton fruit production provided 99 percent control, while combinations of Roundup plus Resource or Roundup plus 2,4-DB completely eliminated cotton fruit production.
Although 2,4-DB mixed with Roundup was effective on volunteer cotton, we would discourage its use because of the potential for soybean injury.
Volunteer cotton also can be a problem where a failed cotton crop is replanted to no-till soybeans. Cotton, especially larger plants, can be difficult to kill with burndown herbicides.
On cotton up to 12 inches tall, we have obtained good kill with Gramoxone plus Canopy or Gramoxone plus Valor. Valor is registered for pre-emergence application to soybeans and, although we did not include Valor in the trials discussed above, we would anticipate residual control of newly emerging cotton by Valor.
We have not been able to adequately burn down cotton much larger than 12 inches. Replant decisions would normally be made before cotton is larger than 12 inches. However, if a situation arises where a cotton crop larger than 12 inches will be abandoned and the field replanted to soybeans, we would recommend tillage to destroy the existing cotton.
Aim is reported to kill volunteer cotton. In a limited number of trials, we have obtained poor control of 12-inch cotton with Aim. We do not have data for smaller cotton.
Sandy Stewart and P. Roy Vidrine are with Louisiana State University; Alan York is with North Carolina State University.