The role of hormones in plant growth can be complex. Plant hormones are produced throughout the plant and have both local and distant effects on a multitude of plant functions. Cotton growth regulators either contain plant hormone related compounds or influence plant hormone production. Plant hormones are chemicals that exist in the plant or can be applied to the plant that influence plant growth with very small amounts of the chemical. Plant growth regulators include various chemicals, most of which are found naturally in the plant, such as auxins, cytokinins, ethylene, gibberellins, and abscisic acid.

The only in-season plant growth regulators that have been found to have consistent effects on cotton growth is Mepiquat chloride and Mepiquat pentaborate. Both compounds inhibit gibberellic acid synthesis and cause very similar responses in cotton growth and development. Gibberellins are involved in cell elongation and the application of these "anti-gibbberellins" results in reduced cell elongation and smaller internode and leaf development following their application. These reductions in growth following application can result in smaller plants that may be easier to manage and harvest and are also less likely to lodge. Timely applications can cause the plant to mature earlier by retaining a higher proportion of the early fruit on the lower part of the plant. It is thought that this increase in fruit retention on the lower portion of the plant is due to increased light penetration into the canopy because of reduced leaf size following Mepiquat applications.

Conditions Favoring Mepiquat Use Mepiquat use is usually warranted when conditions favor rank growth and delayed maturity. Some of these conditions are: . • Late planted cotton. . • Thick stands (more than 4 plants per foot of row). . • High nitrogen rates. . • Excessive rainfall within 7 days of treatment. . • Fields with a history of rank cotton growth. . • Large, indeterminate varieties. . • Fields with delayed maturity. . • Fields that will be defoliated and harvested first.

The more of these conditions that are present, the greater the likelihood of a positive response to Mepiquat treatment. Conversely, if the above conditions are not present, Mepiquat treatment may not be worthwhile.

There are several strategies that are used for Mepiquat use in various areas of the cotton belt. These strategies involve one or more application made sometime from the start of squaring into early bloom. None of the major strategies call for applications to be made during dry weather. The three most common strategies are low rate multiple, early bloom and modified early bloom. The modified early bloom is really a strategy that combines some of the elements of the low rate multiple and early bloom strategies.

Early Bloom Strategy The most commonly used technique is the application of ½ to 1 pint of Mepiquat at early bloom (defined as 5 to 6 white blooms per 25 feet of row) on cotton that is more than 24 inches tall if conditions favor a response to Mepiquat. Cotton that is less than 20 inches tall at early bloom does not receive a treatment. The ½- to 1 pint rate is also applied if the cotton averages 28 inches tall, even if early bloom has not yet occurred.

Applications may be made after early bloom if cotton growth becomes excessive (following early bloom). Treatment rates range between ½ and 1 pint per acre. Note: Treatments applied later than 7 days after early bloom will have less impact on earliness and less potential to increase yield. The idea behind the early bloom strategy is that most of the harvestable cotton should be on the plant at early bloom and any reduction in future growth due to Mepiquat applications should not reduce the number of potential fruiting sites.

Low-Rate Multiple Application Strategy This strategy has been developed to reduce the risks associated with an early bloom Mepiquat treatment that precedes a drought period. This strategy employs the use of low-rate multiple applications (LRMA) of Mepiquat beginning at match-head square (50 percent of plants with one or more squares 1/8 to 1/4 inch in diameter). The first treatment of 1/8 to 1/4 pint occurs at match-head square if conditions favor a response to Mepiquat. Further treatments are made at 7to 14-day intervals when conditions favor a response to Mepiquat.

This approach is logical and should enable you to achieve the benefits of Mepiquat, particularly if you have irrigation capabilities, while reducing the risks associated with the product (early cutout). Instead of running the risk that drought stress may occur immediately after a larger, early bloom treatment, you should be able to mete out smaller doses that enable you to fine-tune the crop’s development. However,this strategy may not be as consistent in areas of the cotton belt where rapid growth is not normally seen in the early squaring stage. This strategy was more popular when Mepiquat applications could be piggybacked with insecticide locations. The reduction in insecticide applications in recent years has made this option less attractive to some producers.

Modified Early Bloom Strategy Many producers have a difficult time treating their entire acreage in a timely manner using the early bloom strategy due to large acreage, lack of equipment, or wet weather. This often results in applications made too late to successfully control plant size and influence earliness. These producers may wish to use the modified early bloom approach on at least a portion of their acreage. This approach involves possible treatments 10 to 14 days before early bloom (10 to 14 days after first square), at early bloom, and 10 to 14 days after early bloom. The last application is seldom necessary if this approach is used successfully.

Applications in the modified early bloom method are triggered by measuring the current growth of the cotton. This is normally done by measuring the top 5 internodes on the plant or the internode below the upper-most fully expanded leaf (fourth mainstem leaf down from the top of the plant). With this method, an application is triggered if the internode below the upper-most fully expanded leaf is 2,5 inches or more. On irrigated cotton or cotton on extremely productive soils, one may want to be less conservative and use 2 or 2.25 inches as the trigger.

No matter what strategy or combination of strategies is used, Mepiquat rates should be based on plant size and environmental conditions. Below are links to State recommendations for the use of the strategies in various areas of the cotton belt.

Georgia: Use of Plant Growth Regulators as a Management Tool in Cotton

Growth Regulator Use in Louisiana

Missouri: Plant Growth Regulators in Cotton

New Mexico: Pix Use

North Carolina: Suggestions for Plant Growth Regulator Use

North Carolina: Pix Ideas in Dry Weather, part 1

North Carolina: Pix Ideas in Dry Weather, part 2

Virginia: Plant Growth Regulator Use