Plant Growth Regulators

Mepiquat choloride (Pix) plant growth regulator benefits cotton production in the management of boll rot and excessive vegetative growth. The use of mepiquat chloride may result in one or more of the following: height reduction, shorter limbs, more open canopy, better boll retention, less boll rot, improved defoliation and a darker green leaf color. Mepiquat-type plant growth regulators (PGRs) are products containing mepiquat chloride (such as Pix^®; Mepex^®; or Mepichlor^®), mixtures of mepiquat chloride with other materials (such as Pix Plus^®; Mepex Plus^®; or Pix Ultra^®), or other mepiquat-based compounds ( Stance^® or Pentia^®).

Like any agricultural PGR, applying mepiquat-type PGRs to cotton alters the internal hormone balance of the plant.  Specifically, the mepiquat ion reduces plant synthesis of gibberellic acid by partially inhibiting one of the enzymes involved in its synthesis.  One of the roles of gibberellic acids in plants is to promote cell expansion during growth, which is limited by cell wall development.  Cell walls exposed to mepiquat develop and harden faster, so cells do not expand as much as untreated cells.  The smaller cells in growing shoots of cotton result in shorter internodes in stems and branches.  Therefore shoot growth is more compact if mepiquat-type PGRs are applied. 

For the reduction of cell size to affect plant height, at least 10 parts per million of mepiquat ion is required.  Larger plants require a higher application rate to reach this concentration.  Plant concentration of mepiquat ion may be reduced by growth dilution, which is observed as plants “grow out of” earlier mepiquat applications. Table 9 indicates plant growth vigor at different stages of development.

Table 9. Height to node ratios for cotton PGR decisions. (Jost et al. 2005)

 Jost, P., S. M. Brown, S. Culpepper, G. Harris, B Kermerait, P. Roberts, D. Shurley, and J. Williams. 2005. 2005 Georgia Cotton production guide p 37-39.

Reduction of stem cell size and plant height with mepiquat has many other effects on the plant. Total leaf area is reduced, and a greater proportion of sunlight penetrates to lower leaves, which can reduce the occurrence of boll rot.  Resources such as energy and nutrients that would otherwise go into cell expansion and vegetative growth is available to support reproductive development.  Together, these effects shift a greater proportion of boll production to lower nodal positions than in untreated cotton.  This shift can lead to earlier cutout and maturity of the crop, which is often beneficial to cotton grown in short-season environments like Tennessee. 

The producer has the option of a single, dual, or up to four low-rate multiple applications of mepiquat chloride. When cotton is under stress from dry soil conditions, insect or mite pressure, disease, herbicide injury or fertility stress, the application of mepiquat chloride should be avoided. Wait for rain to reduce plant stress or treat to reduce insects before treating with mepiquat chloride.

Mepiquat chloride can be applied using either water or oil as a diluent. When using water, apply at least three gallons per acre by air or 10 gallons with ground equipment. Thorough coverage of the cotton foliage is required. When using oil as a diluent for ultra low volume (ULV) aerial application be sure to use a nonphytotoxic oil concentrate with either a petroleum or vegetable oil base. Follow the mepiquat chloride label closely for purchasing oil and mixing instructions. The use of a good quality surfactant with mepiquat chloride application can reduce the rain-safe period from eight to four hours. Mepiquat chloride has an aqueous base and is compatible with most insecticides and miticides. Compatibility can be checked by adding a teaspoon of insecticide or miticide to one pint of ready-to-use spray solution of mepiquat chloride.

Restrictions and Limitations

Insect or mite damage before, at or after application of mepiquat chloride can lead to yield decreases.

• Do not make a single application of 0.5 to 1 pint of mepiquat chloride to cotton that is drought stressed.  If using the low rate multiple option, discontinue use until the moisture stress is alleviated.
• Do not apply more than 3 pints (0.135 lbs active ingredient) of mepiquat chloride per acre per season.
• Do not apply more tha 22 fluid ounces of Stance per acre per year.
• Do not apply mepiquat chloride within 30 days of harvest.
• Do not graze or feed cotton foliage to livestock within 30 days of application, or after applying mepiquat chloride in oil as a ULV application by air.
• Do not tank mix with other products other than mentioned on label.
• Do not apply mepiquat chloride through any type of irrigation system.

Under good growing conditions the recommendation would be to get mepiquat into the plant prior to bloom and follow up 10-14 days later with a second application.  As a fruit load increases the plant will naturally regulate growth. If fruit shed occurs for insect or weather related events, be prepared to monitor plant growth closely and apply additional plant growth regulators as dictated by plant growth. The best program is to get plant growth under control going into bloom.  A single application of 12-16 oz based on plant size and weather coditions has shown to be effective at early bloom. At bloom applications should work well in most situations.  For vigorous varieties that get rank in good growth environments, a low rate (2-4 oz broadcast) application at early match-head square with multiple follow up applications based on plant vigor will control height adequately. A good rule of thumb is to make applications based on field history, plant vigor, variety, fruit load and weather forecast.  Remember that PGRs can’t shrink plants, be proactive to maintain growth control. 

Table 10.  Plant growth regulator application strategies.