Specialists Speaking -- June 2001

Improving Weed Control

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 Dale Monks    

Producers in Alabama are now utilizing the Roundup Ready weed control system on most of the acres they are planting this year. This weed control system has been widely adopted on conservation as well as conventional-tilled fields. While there are many weeds that this system will control, there are some species that are not completely susceptible to glyphosate.

Producers should be scouting their fields for various weed species that may not have been controlled by the initial application of glyphosate. These species might include morningglories and prickly sida.

Other weeds that would normally be controlled by the initial glyphosate application can survive if 1) their size was too large for adequate control or 2) if low rates were used in the initial applications. After the initial over-the-top treatments (up to the fourth leaf stage), glyphosate treatments must be directed underneath the crop canopy to lessen the potential for injury in the form of square shed.

As producers scout their fields for escaped weeds, they should carefully take note of the specific weed species that are present. Since glyphosate is more effective on some weed species compared to others, a tankmix partner may be needed at layby to control them adequately.

The tankmix partner should be chosen in an effort to add to the postemergence activity and to provide some degree of residual weed control. Producers also may consider using layby treatments that do not contain glyphosate in order to rotate herbicide families. This will reduce the long-term potential for resistance problems.


Jeff Silvertooth

In most cotton growing areas of Arizona, June is a month that generally offers tremendous potential for developing a strong cotton crop. For most of the state, cotton fields are in early bloom or beyond in June. In the western-most parts of Arizona and the lower elevations we can commonly reach peak bloom by the latter part of June. The weather in June is typically dry and hot, which means we commonly get a wide diurnal (day-night) temperature change, with relatively cool nights and hot, clear days. This is an excellent time and set of weather conditions for cotton plants to set fruit, providing they have strong plant vigor. Therefore, it is in our best interests to set as much fruit on the plant in the early part of the fruiting cycle as possible, whether our objective is to produce a crop with a reduced season, early termination strategy or, whether we plan to go for a top-crop with a long, full season approach. Accordingly, June is often referred to as "prime time" for Arizona cotton production.

To maintain strong plant vigor and a high rate of fruit retention, the most important management factors we must take care of include irrigation and pest control. Of course, many other inputs of course are important, but water and pest management are absolutely critical. Accordingly, it is very important to provide adequate water to the crop on a timely basis. Water stress should be avoided, since it decreases plant vigor and generally increases fruit loss and abortion rates.


Bill Robertson

Scouting is an important component of crop production. Problems related to pests or the environment which add to or relieve stress often are visible but are often difficult to quantify. Plants encounter stress throughout the growing season. Some stress such as filling a heavy boll load is "good stress" while having too much or too little moisture is a "bad stress."

Few tools offer the ability to monitor stress as does COTMAN, a computerized cotton management program which was developed by the University of Arkansas and provides a very sensitive tool to detect stress. The growth of a cotton plant is very predictable. Changes in the predictable patterns of growth such as the development of new nodes can indicate stress if development occurs below that expected.

A target development curve used in COTMAN represents growth and development time lines that attempt to maximize yield and earliness. A crop development curve flatter than the target indicates stress. If growth rates vary (too slow or too fast), corrective measures can be taken to get plants back on course. Square retention is a key component in en-suring a balance between reproductive and vegetative development.

Plants with high retention values will have greater demands as fruit develops than plants with lower retention values. COTMAN offers the ability to predict anticipated needs based on interpretation of crop development curves and fruit retention rates to better time inputs to avoid plant stress. For more information on COTMAN contact your local county Extension office.


Steve Brown

The convenience of Roundup (RR) Ready technology has displaced the precision of traditional weed management programs. The latter requires accuracy in rate selection, timing and placement. The former requires less. Or does it?

Evidence continues to build that sloppiness with glyphosate in RR cotton is costing us.

Glyphosate applied over-the-top after the 4-leaf stage can reduce yields 200 pounds/A. Similarly, post-directed treatments of glyphosate which contact considerable stem and leaf tissue also can result in significant yield losses. The damage is "invisible." There is no leaf discoloration, no stunting, nor typically any recognizable square or obvious boll shed.

Nevertheless, yields can suffer. The remedy is accurate post-directed application of conventional herbicide treatments or glyphosate. The key is precision-directed applications which minimize spray contact with the cotton stalk and foliage. Conventional herbicides are the preferred option. Compared to glyphosate, conventional herbicides provide superior activity on problem weeds such as morningglories and nutsedge.


John Barnett

If you are using a Roundup Ready or BXN weed control system, take maximum advantage of it. Several times last year I observed producers using unnecessary herbicide applications or cultivations instead of taking advantage of what they had. The Roundup management system offers an affordable, easy to manage weed control system for cotton; however, it is critical that you rely on the glyphosate applications and do not cultivate.

Cultivation will increase weed pressure by allowing the germination of weeds that would remain in the soil below the depth for germination. In the case of a BXN system, I found that many times producers will go ahead and apply other herbicides that are labeled to control the same weed spectrum as the Buctril.

This is only costing money and reducing your potential for profit. Select your weed control management system and take full advantage of it. Mixing weed control strategies often will only cost you money or may even cause more weed problems.


Bobby Phipps

When there is a severe weed problem, it is tempting to spray Roundup over the top of Roundup Ready cotton after the five true leaf stage. Cotton will have its first square on the fifth to eighth node. At this stage squares can be present but not be visibly apparent.

The square will be the growing tip on its vegetative branch. Roundup is translocated to the growing tips of the cotton plant. Sometimes when Roundup is translocated to the squares, it will cause the flower not to shed pollen. If the flower does not get enough pollen, it will abort and be shed.

With a reduced amount of pollen the boll will have its tip distorted to one side, which is commonly called "parrot beaked." Other bolls will have to replace these shed bolls which will result in delayed maturity. The manufacturer would certainly like to be able to recommend using Roundup over the top after five true leaves, but the boll shed is an excellent reason for prohibiting use in this manner.