Layby herbicides are frequently applied to maintain weed control until the cotton canopy is sufficiently developed to shade the furrow and bed top thereby suppressing small weeds. Layby herbicides are used to complement weed control provided by cultural practices, mechanical weed control and preplant-preemergence and early season postemergence herbicides. Layby herbicides are primarily used to control weeds such as annual morningglory, Palmer amaranth, Wright groundcherry, and other large broadleaf weed species. Problem grass species such as Johnsongrass can be controlled by making spot treatments of selective grass herbicides (refer to the Extension Bulletin "Selective Control of Grass Weeds in Cotton With Postemergence Herbicides"). The herbicides commonly used at layby have both preemergence and postemergence activity. The rate of herbicide used is determined by soil type since more herbicide is required to control weeds through root uptake from the soil than by absorption through leaf surfaces. However, factors related to optimizing postemergence weed control must also be addressed to obtain good results.

Layby Cotton Herbicides

Herbicides that are registered for layby applications in Arizona cotton are listed in TABLE 1 along with the minimum cotton height required before a layby application can be made. Assuming weeds are present, things that should be considered when planning to use a layby herbicide include application timing, herbicide rate, residual herbicide effects on following crops, and optimization of postemergence weed control. Cyanazine, diuron, oxyfluorfen, and prometryn can all be used when cotton exceeds 12" in height, however, the optimum timing is usually when cotton is 15 to 20 inches tall. Preplant/preemergence herbicides used in conjunction with either Buctril (use only on BXN cotton), glyphosate herbicide (use only on Roundup Ready cotton), Staple, or more traditional post-directed, "chemical hoe" herbicides will keep fields weed free until layby. (For more information refer to the Extension Bulletins titled "Control of Broadleaf Weeds in Seedling Cotton With Over-The-Top Herbicide Applications" and "The Use of Postemergence-Directed Herbicide Applications to Control Broadleaf Weeds in Cotton".)

TABLE 1. Selected Herbicides Registered for Layby Applications in Arizona and Minimum Cotton Heights Required for Application.

Herbicide Rates

As with all soil active herbicides, the rate of layby herbicide used must be varied according to soil type or texture. Herbicide labels frequently group soil types into three categories; coarse, medium, and fine as follows:

Alternatively, a herbicide label may list specific soil types along with the appropriate rate. As a guideline, where a range of dosages is given, use the lower rate on coarse textured soils low in clay or organic matter and use the higher rate on fine textured soils high in clay or organic matter. Layby herbicide rates for cyanazine, diuron, oxyfluorfen, and prometryn are shown in TABLE 2.

Layby herbicides must be incorporated into the soil with irrigation water within 3 to 4 days of application in order to obtain optimum weed control. The incorporating irrigation should wet each furrow and thoroughly wet the top of the beds to carry the herbicide into the root zone of germinating weeds. The best weed control system, but the hardest system to manage, involves irrigating near layby time, spraying when field entry is possible (add surfactant to control emerged seedlings), and incorporating the herbicide with the next irrigation. Do not cultivate or disturb soil following layby sprays particularly if oxyfluorfen is used. The next best weed control system is to direct the spray to the base of the cotton plants after the final cultivation and irrigate to incorporate the herbicide.

TABLE 2. Broadcast Layby Herbicide Rates.

*a.i. = active ingredient

NOTES

1. Do not use on sands or loamy sands (soils consisting of more than 70% sand) containing less than 1% organic matter.

2. Layby application may be split into two applications with each application being at 1/2 the indicated rate.

3. Oxyfluorfen forms a chemical barrier at the soil surface so rate does not vary with soil type. Do not exceed 25 psi when spraying.

4. Can be tank mixed with other herbicides (see labels).

5. Cyanazine can be used through December 31, 2002. The maximum use rate is 1.0 lb a.i./acre per year.

 

Crop Rotation Restrictions

The soil residual characteristics of layby herbicides must be considered to avoid injuring sensitive crops following cotton. Cyanazine has a shorter residual period than other layby herbicides which minimizes the risk of carry-over to following crops while diuron has the longest soil residual and the greatest potential to injure following sensitive crops. The soil residual of prometryn is similar to cyanazine but the labels are more restrictive. Oxyfluorfen is mostly deactivated by tillage after harvest which minimizes carry-over risks to most crops but there are restrictions on planting small grains. Check the labels of all herbicides used in a field for crop rotation restrictions.

cyanazine: Any rotational crop may be planted the fall or spring following any of the labeled treatments in cotton providing the soil is plowed or deep-disced prior to planting the rotation crop.

diuron: Cotton, corn or grain sorghums (not forage sorghums nor grass sorghums) can be planted the following spring. Do not replant treated areas to any other crop within one year after last application as injury may result.

oxyfluorfen: Do not rotate to small-grain crops (includes barley, buckwheat, corn, proso millet, pearl millet, oats, popcorn, rice, rye, sorghum, triticale, wheat, wild rice) within 10 months following an oxyfluorfen treatment. Do not direct seed any crops, other than oxyfluorfen-labeled crops, within 60 days following a treatment. Do not transplant seedling crops, other than oxyfluorfen-labeled crops, within 30 days following a treatment. Treated soil must be thoroughly incorporated to a depth of 4 inches after harvest.

prometryn: The following vegetable and cover crops may be planted in the fall when prometyrn was applied by no more than 1 of these methods that year: preplant-incorporated, preemergence, or only 1 chemical hoe treatment. Where layby or multiple applications are made, do not plant rotational crops until the following year as indicated. Vegetables: Cabbage, okra, onions, peas, red beets, sweet corn. Cover Crops (must be plowed down and not used for food or feed): oats, sorghum, winter barley, winter rye, winter wheat. Spring-seeded crops in Arizona should not be planted until after April 1.

Optimizing Postemergence Weed Control

In addition to providing preemergence weed control, layby herbicides will control small emerged weed seedlings (generally less than 2" weeds) if a non-ionic surfactant is added to the spray solution. Add surfactant at a rate of 0.5% (v/v) which is 1 quart/50 gallons of solution. Use a good quality surfactant that contains at least 80% surface-active ingredient. The target weed species for layby applications are usually annual morningglory, Palmer amaranth, and Wright groundcherry, but many other species are controlled (see herbicide labels for additional species). Glyphosate herbicides can be tank mixed with layby herbicides in Roundup Ready cotton.

Appropriate spray placement, carrier volumes, pressures, and nozzles should be used to obtain good coverage of weeds and to avoid herbicide drift and crop injury. Arrange nozzles to cover furrows and seed row keeping spray contact with cotton plants to a minimum while obtaining good coverage of small weeds. Sprayed cotton leaves will show some injury symptoms but this is almost always of no consequence. Use shields if excessive spray contact on larger cotton plants cannot be avoided by directing the spray. Use carrier volumes of at least 20 gal/A to achieve good coverage of emerged weeds. Minimize drift by spraying during calm periods, by using low pressure, 20 to 25 psi (some labels specify higher pressures), and by using speeds below 5 mph. Avoid spraying during mid-day when high temperatures enhance misting and drift due to the rapid evaporation of spray droplets. Use even flat fan, underleaf banding (UB), off-center (OC), double outlet flat spray (TQ) or twin even flat fan (TJ) nozzles but do not use cone nozzles.

Full Disclaimers

Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, James A. Christenson, Director Cooperative Extension, College of Agriculture, The University of Arizona.

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Because labels are subject to frequent change, always consult the label attached to the product before using any pesticide. The user must assume responsibility for proper application and for residues on crops as well as for damage or injury caused by pesticides, whether to crop, person or property.

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