The plant growth regulator mepiquat has been labeled for use in cotton since the 1980s. Mepiquat-containing products have been widely used as a means of restricting plant height. Other potential benefits of mepiquat can be less susceptibility to boll rot, increased early fruit retention, enhanced earliness, less lodging of plants and increased harvest efficiency. Potential risks associated with its use are a reduced ability to withstand season-long stress, slower canopy closure and inducement of early cutout.
Methods and philosophies for the use of mepiquat in cotton vary widely. This publication explains the effect of mepiquat-containing products on cotton growth and development and suggests some guidelines for its use that will increase the likelihood of realizing the benefits and minimizing the risks.
What is Mepiquat?
Plant growth is promoted by naturally occurring plant hormones, one of which is gibberellic acid. Mepiquat is an anti-gibberellin that inhibits cell expansion but not cell division. Cotton is a perennial plant, continuing to grow vegetatively while producing fruit. Mepiquat can restrict vegetative growth, inducing the plant to direct more carbohydrates into reproductive organs. This effect on cotton can be either positive or negative. If redirection of carbohydrates into fruiting structures occurs early in plant development, early cutout can be induced and overall plant growth may be overly restricted. Conversely, if vegetative growth is restricted well into the bloom period, increased fruit retention can be realized while also obtaining the benefits associated with less rank growth.
Mepiquat applications usually result in shorter plants; however, mepiquat will not shrink cotton, nor will it place squares and bolls that have already abscised back on the plant. Mepiquat is not thought to be readily translocated through the plant.
Mepiquat-containing products are sold under a variety of trade names and formulations: Pix Plus, Pix Ultra, Pentia, Mepex, Mepex Ginout, Mepichlor and Mepiquat Chloride.
Why is Mepiquat Used?
Rank growth of cotton plants can be problematic because of an increase in boll rot, a decrease inharvest efficiency, less penetration of insecticides into the canopy and difficulty in scouting. Therefore, keeping plants to a manageable size in a sub-tropical climate such as Louisiana has some benefits. Mepiquat also can enhance earliness because it redirects carbohydrates away from vegetative growth to reproductive growth. Yield enhancements directly related to mepiquat are rare. Therefore, mepiquat should be used as a management tool only.
What are the Expected Results?
The overall effect of mepiquat on plant growth and development depends on growing conditions after it is applied. Many years of research in Louisiana and other states show that the most consistent effect is a reduction in plant height compared to untreated cotton. Replicated research in Louisiana has indicated that positive yield responses directly related to mepiquat applications are rare. Therefore, its use and utility should be viewed as a management tool to reduce the likelihood of rank growth and boll rot incidence, while increasing picker efficiency, insecticide deposition and scouting efficiency. These responses can have an overall positive effect on the manageability of the crop. Conditions favoring growth (adequate fertility, rainfall, etc.) typically increase the likelihood of realizing positive plant responses and benefits. Conversely, growing conditions that are abnormally dry, excessively wet, lack adequate fertility, etc. increase the likelihood that early cutout will be induced, vegetative growth will be overly restricted and yield will be reduced.
Mepiquat responses depend on rate. Increasing rates usually result in greater suppression of growth, although the actual response is also related to plant size and growth stage. Pre-bloom cotton is more sensitive to mepiquat than flowering cotton. The later in the bloom period, the less sensitive cotton is to mepiquat. Excessive applications to pre-bloom cotton are the most likely to induce early cutout. Likewise, low rates applied to cotton in mid- or late-bloom are unlikely to be effective.
Cotton plants that have recently received an application of mepiquat are often greener than nontreated plants. This is the result of thickening of the leaves and concentration of chlorophyll and is a purely aesthetic response.