When it comes to cotton plant growth regulators, especially with so many varying cotton varieties on the market today, one size doesn’t fit all anymore. What good are PGRs for today’s cotton?

Mepiquat-based PGRs can help control rank growth in cotton, which is a good thing, but a plant still needs to be robust enough to support its optimal boll load in order to shoot for its top yield potential. Knowing what to do and when is tricky.

Five years ago, the PGR decision wasn’t as tough. Then, near 90 percent of Georgia’s cotton crop was planted to Deltapine 555 BR, making PGR decisions easier, if not second-nature routine. Now, a Georgia cotton farmer might have a half dozen or more different varieties on his farm with different maturity and growth characteristics to manage.

According to information provided by University of Georgia cotton agronomists Guy Collins and Jared Whitaker:

“When DP 555 BR was still widely planted, most growers began their PGR applications at the 8- to 10-leaf stage, which was generally followed by applications at or near first bloom and again 2 to 3 weeks later. This program was a more preventative-type prophylactic program that generally worked well for DP 555 BR, especially in irrigated fields, as this variety could consistently result in (and was likely to result in) excessive growth and extremely tall plants.”

Newer varieties tend to make a larger boll load quicker than DP 555 BR did. This can ‘restrain’ terminal growth, so growers “can be more reactive than proactive/preventative with PGR management in some situations, especially with the earlier maturing varieties,” they say. “Some earlier maturing varieties with less growth capacity may not need a pre-bloom PGR to prevent excessive growth, and delaying these decisions until first bloom may allow for better growth-management decisions.”

Some early maturing varieties can have vigorous growth before first-bloom, but the rate may decrease once the plants enter bloom and start rapidly developing boll loads.

“Aggressive, preventative approaches for early maturing varieties may in fact prevent plants from reaching an optimal plant height in some environments, thereby risking yield loss associated with inadequate numbers of fruiting sites, they say. For later maturing varieties with greater growth potential, especially in timely irrigated fields, a more aggressive preventative approach (which may include pre-bloom applications) may be necessary to prevent excessive growth, but a one-size-fits-all approach is no longer suitable.”

What does mepiquat do exactly?

Mepiquat-containing PGRs reduce plant hormones called gibberellins, which help regulate cell expansion. Mepiquat hinders the elongation of the internodes near the terminal of the main stalk or the lateral branches may not elongate to the degree that non-treated plants would. You get a shorter plant with more compact nodes.

“Most mepiquat-containing PGRs, with the exception of Stance, generally have similar effects on plant growth. When applied at similar rates (except for Stance), similar results should be expected. Stance contains a higher concentration of mepiquat than other meqiquat products, and also includes cyclanilide. This product is used at much lower rates than standard mepiquat products,” they say.

Mepiquat does not stimulate flowering and does not create more bolls per plant. At best, mepiquat may improve retention of some bolls. Mepiquat essentially has no effect on yield.

Why use PGRs? Well, these things may or may not happen, but PGRs can:

  • Improve fruit retention on lower nodes and earlier maturity.
  • Improve harvest efficiency.
  • Reduce impedance of insecticides/fungicides/harvest aids.
  • Reduce boll rot and reduce lodging of plants.

“The likelihood of achieving one or more of these positive results greatly increases if the environment is likely to result in (or has historically and consistently resulted in) excessive vegetative growth, but even then, these results may or may not occur,” they say.

Want access to the very latest in agriculture news each day? Subscribe to Southeast Farm Press Daily. It’s free!