North Carolina Cotton: 3 Common Questions About PGRs

Applying a plant growth regulator to cotton. Photo: Alabama Cooperative Extension

We’ve been getting a number of questions this week regarding the need for mepiquat-based PGRs applied to cotton at various growth stages at this point in the season. To be honest, it is somewhat of a joy to write this article, because it means we’re getting badly needed rains.

There are currently many scenarios occurring, but this article highlights the primary calls we’ve received.

Q1. My cotton is at cutout….Ive heard that PGRs applied at cutout can help in many ways. Do I need to apply PGRs to cotton at cutout?

This idea has been around quite a while, and it has been suggested that PGRs applied at cutout could:

  • enhance upper boll maturity by diverting photosynthetic resources away from young terminal leaves, or regrowth, and towards boll development,
  • promote leaf maturity to simplify defoliation, or enhance harvest aid efficacy
  • reduce regrowth and trash in harvested lint
  • promote earlier maturity
  • reduce ginning costs
  • potentially higher yields and improved fiber quality

There has been a lot of discussion around this subject and many of the points above are even suggested on some PGR labels. The idea of applying PGRs at cutout is nothing new and has been thoroughly investigated in NC as well as in GA through a multi-year and multi-environment research project conducted a few years ago.

In this study, we evaluated many of the points above, including defoliation, percent open bolls and nodes above cracked boll as a measure of maturity, number of bolls and boll retention, desiccation, terminal and basal regrowth, yields, and fiber quality. In this study, we found no meaningful benefit to applying PGRs to cotton that had reached cutout, regardless of prior growth or PGR treatment leading up to cutout…period.

There was no evidence to support that any claim listed above nor was yield improved. Earlier applications of PGRs (pre-bloom, early bloom, etc) continue to be recommended in some situations depending on many factors, however, the potential benefit of PGRs appears to erode beyond mid-bloom.

This study also suggested that there is little to no penalty from such late-season PGR applications, other than the added expense of products and application costs, especially if applied alone.

Q2: Now that we’ve received some rain, I need to make a PGR application. My field has both taller and shorter cotton. Will an application hurt the shorter areas of my field?

First, I would be absolutely sure that you need a PGR application. In many fields that we’ve visited, cotton is clearly at or very near a true cutout, in which case you wouldn’t need a PGR application. Only apply PGRs at this point if you have 6-7 or more nodes above white bloom AND your plant height is approaching the point of being too tall.

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For cotton planted in early May, generally speaking, plants can tolerate end-of-season heights somewhere around 45-50 inches, so you want to slow down terminal growth and guide to plant to where it shuts down upward growth at, or just before that point.

For late May planted cotton, final plant height should be somewhere around 35-40 inches tall. If the taller areas of your field meet this criteria, then maybe a PGR application is warranted.

Now, will it hurt the shorter areas of the field? It’s hard to say. If cotton is well into the bloom period, most of the fruiting nodes have already formed, so a PGR application probably won’t hurt anything. In most cases we’ve seen at this point in time, this is the likely outcome.

However, if rains continue (and we hope it does, at least through August), shorter cotton could currently, in theory, be developing a few more upper squares or fruiting sites. Our last effective bloom dates typically range from August 20–25 for northern counties, and August 25–September 1 in southern counties.

The likelihood of additional fruit developing into harvestable bolls of course is dependent on continuous rainfall and warmer weather during the fall which would extend our last effective blooming longer than normal, which has yet to be seen.

If rains continue, allowing for a few more fruit to develop on this shorter cotton, perhaps it could benefit to leave the shorter areas of these fields alone. However, if any meaningful dry spell occurs, or if fall weather is cool or cloudy, it is doubtful that any more squares or fruiting sites will develop into harvestable bolls at this point.

Q3: My cotton is just starting to bloom. Do I need to apply PGRs to hasten maturity because it is late and we finally got some rain?

Well, maybe, but it depends. For cotton that hasn’t yet begun blooming, it better start soon. For cotton that has been blooming for a week or less, we can expect a 3, maybe 4-week bloom period. We can make a decent crop in that time IF we are VERY TIMELY on insect sprays to help retain all bolls made before our last effective bloom date.

Irrigation can help achieve this, but in the absence of irrigation, insect management and PGRs are the next best options. For PGRs however, it depends. Cotton should begin growing now that it has some rain to push it, and Id expect some residual N to further promote growth.

However, keep in mind your current plant height and your target plant height. If it is just now starting to bloom, it is probably June planted cotton, so you need to manage for earliness and guide the plant to a final plant height not to exceed 35-40 inches or so. But if it is currently short, it may need to set fruiting sites for a short while (If our last effective bloom dates are normal this year, any potential fruit that would develop into harvestable bolls would have to begin squaring very soon).

This is why it helps to monitor nodes above white bloom. In many cases this year, cotton began blooming and almost immediately went into a premature cutout due to insufficient stalk height and nodes above white bloom. If your plants a currently shorter statured and have less than 6-7 nodes above white bloom, a PGR application probably wont be of any major benefit.

Be ready to hit it with PGRs right around (or slightly before) our last effective bloom date if rains continue through the next few weeks. At the other end of the spectrum, if your cotton is approaching 30-35 inches tall and has recently begun blooming, AND you have more than 6-7 nodes above white bloom, a PGR application is probably justified in order to achieve your targeted final plant height and manage maturity.

Regardless of the case, keep an eye out for a few things:

  1. Fruit retention – whether due to drought or lygus, if fruit retention is low, growth is more likely to take off now that we have some moisture
  2. Reddening/toughened main stems – this is a sign that the plant is, or has recently been under some sort of stress. For this year, that stress is likely heat/drought. This is an indicator that growth may be slowing although it’s difficult to quantify
  3. Larger (fingernail size) sized squares in the terminal bud – this is also a sign that the plant is, or has recently been under some sort of stress, and that growth is slowing. Consider this before pulling the trigger on a PGR application.



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