Mepiquat chloride-based plant growth regulators (PGRs) are a critical management tool for cotton production since the first introduction by BASF in the 1980s.

Research conducted in Arizona has shown consistent results in plant height control, but lint yield response has proven much more variable. Increased yields may be possible under conditions of high crop vigor and low fruit retention. However, in cases of low crop vigor, decreased yields are also possible. In most cases, no yield response is observed.

PGRs excel in the control of plant height without inducing crop stress resulting in a more balanced vegetative and reproductive growth pattern, more open canopy, better spray penetration for pesticides, and improved defoliation.

Current cotton PGRs in Arizona include products containing mepiquat chloride (MC), MC mixtures, and other mepiquat-based compounds. Some products are MC mixtures with a bacterium, Bacillus cereus, with a cell division hormone, kinetin, or with cyclanilide. Mepiquat pentaborate contains mepiquat with the chloride ion replaced by the boron ion.

A Beltwide test of these PGRs indicate that all control plant height effectively and to a similar degree, but with yield benefits only about 10 percent of the time.

Determining when a PGR is needed can be a difficult decision. The Arizona PGR use guide gives decision-making guidance.

PGR labeled rates are 0.022 to 0.044 pounds (lbs) of active ingredient (AI) of MC per acre depending on the growth pattern of the crop. Near cut-out, up to 0.066 lbs/AI per acre may be used.

Do not apply more than 0.132 lb/AI of MC or 0.31 lb/AI mepiquat pentaborate per acre during the growing season.

For the table and photos associated with this article, click on this link: http://ag.arizona.edu/crops/cotton/files/PGRsvF.pdf.