COLLEGE STATION – Rain is a good thing, except when it comes to weeds and weed control in cotton.
Dr. Emi Kimura, Texas A&M AgriLife Extension Service agronomist in Vernon, and Dr. Gaylon Morgan, AgriLife Extension statewide cotton specialist in College Station, said this year’s exceptional rainfall in the spring and summer has caused high weed pressure throughout the state and continues to be a challenge in the Rolling Plains cotton crop.
Mid-season weed control is important to prevent yield loss and to reduce the amount of weed seed left in the soil, they said.
Morgan said lint yield can be reduced by 13 to 54 percent when one to 10 Palmer amaranth plants, respectively, are present in every 30 feet of row.
Mid-season is only part of the overall weed control program producers need to have for their crop, which should include cultivation, preplant, pre-emergence and postemergence herbicide applications, he said.
The length of pre-emergence herbicide activity will vary by herbicide, rate, precipitation, soil moisture, soil texture, soil organic matter and pH, Kimura said. Even where the pre-emergence herbicides were applied, scouting for emerging weeds to determine the need for postemergence control is necessary, especially if glyphosate-resistant pigweeds are expected. Glyphosate-resistant pigweeds and other problematic weeds are most effectively and economically controlled as small seedlings.
When it comes to mid-season weed control, the two specialists said there are two major weed management practices, cultivation and herbicides, recommended.
Shallow cultivation is effective in controlling small annual weeds and reducing competition from perennial weeds for a limited time, but caution should be taken to minimize cotton root damage, Kimura said. The tillage equipment should be cleaned thoroughly to avoid spreading weeds to other fields or uninfested areas of the field by weed rhizomes, roots or seeds.
Where glyphosate-resistant weeds are present, hooded or post-directed herbicide applications are one of the only alternatives remaining. As with over-the-top herbicide applications, hooded or post-directed applications should be made in timely manner to control small weed seedlings, she said.
Weed seedlings should be smaller than the cotton height to avoid herbicide injury to the cotton crop. Most post-directed herbicides are the burndown-type, although some provide soil residual activity on the weeds.
Morgan said that multiple applications of post-directed herbicide applications can be made when the cotton seedlings are at 3-8 inches tall and 8-14 inches tall. However, all products differ for timing of applications, targeted weeds and rate of application.
In many cases, a combination of herbicide modes-of-action should be considered to provide postemergence activity, and preferably a herbicide with some soil residual activity, he said.
Excessive rainfall has reduced the efficacy of pre-emergence herbicide controls in cotton this season, which has minimized the cotton-to-weed height differential. Without enough height differentiation between the cotton and weeds, over-the-top herbicide application is the appropriate weed control measure.
Depending on the herbicide, over-the-top application will control or injure weeds and will create the height differential for effective post-directed herbicide applications.
For the detailed information on target weeds, rates and application timing of these products, please refer to the Weed Management in Texas Cotton guide at http://bit.ly/1SK0rMC .