Cotton growing conditions have varied across Georgia this year, ranging from very wet and cool weather at planting to dry hot conditions, with a return to heavy downpours in some fields. A common question during these conditions includes the rainfastness of plant growth regulators, or PGRs.

In the August 2023 UGA Cotton Team Newsletter, Wade Parker, University of Georgia Extension Southeast District agronomist, provides good information on basic plant growth regulator products relative to rainfastness.

He said most cotton acres are treated with mepiquat chloride products under several trade names. Other products contain other chemical compounds and require different application rates. The Compact 6x is a mepiquat chloride standalone product but has approximately 6x the amount of active ingredient, therefore one ounce of Compact is equivalent to six ounces of a mepiquat chloride product. Subtle differences such as these make a big difference when giving a recommendation to a client.

Previous research conducted at UGA indicated mepiquat chloride applications that are washed from the surface of the leaf two hours after application still reduce plant height, but reapplication intervals may need to be sooner than originally planned. In the same study, four-hour rainfree periods were as effective as eight hours in reducing plant height.

Fields should always be evaluated to ensure the PGR is working as intended and adjustments in reapplication interval or rate should be made accordingly.

Below is a description of basic growth regulator products relative to rainfastness, according to the product label. Read the label carefully to fully understand the effects of precipitation.

Mepiquat Chloride (MC) products (0.35 lbs a.i./gal – several tradenames)

Gin Out Plant Growth Regulator (4.2% Mepiquat chloride – 0.0025% Kinetin)

Pentia Plant Growth Regulator (9.2% Mepiquat Pentaborate)

Stance Plant Regulator (2.1% Cyclanilide + 8.4% Mepiquat Chloride)

Potenza (4.2% Mepiquat Choride + 0.065% Kinetin)

Compact 6x (23% Mepiquat Chloride)

Additional resources for cotton PGR management, include: