North Carolina Cotton: Pre-Bloom PGR Decisions

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

The 2020 crop is currently variable with some fields still struggling and lagging behind due to early season stress (4-6 leaves) while other fields are starting to grow vigorously (9-11 leaves with easily visible squares). Very few fields will see a bloom during the first week of July this year.

Rainfall has been variable but many areas have received some over the last week or so. With top-dress fertilizer having been recently applied in many cases, the fields that have received rain are now taking off (assuming they aren’t still trying to recover from early season stress).

Growth stages vary but it is safe to say that the 2020 crop is 2 weeks behind for most folks, and 3 weeks behind for the fields that encountered significant early season stress. Due to this delay in maturity, there will be an incentive to accelerate maturity with PGRs. This is logical, however there are other considerations to take into account.

Pre-bloom PGR applications are most likely to reduce yield compared to early bloom or mid-bloom applications. Most commonly, this is because applications are made when current conditions vary vigorous growth, but the situation quickly changes are drought ensues.

In most areas of NC, it is important to remember that we are never more than 4-5 days away from a significant drought at any given time, regardless of recent rainfall amounts. This is especially the case in our more sandier soils.

Pre-bloom PGR applications are also more likely to reduce yield in early planted cotton, as they can limit the formation of potential fruiting sites. Therefore, we generally see the most positive responses to pre-bloom PGR applications to:

  • Late-planted cotton.
  • In years where soil moisture is not limited (i.e. timely rains resulting in little to no plant stress for most of the season).

Both of these conditions are currently met for many fields, but the moisture situation could change at any time.

Before making a pre-bloom PGR application, below are some considerations you must take into account:  THERE IS NO ONE-SIZE-FITS-ALL APPROACH, nor should all fields be treated as such.

Field History / Soil Type

Field history is often underestimated or not considered when making PGR decisions. Fields that have a history of rank growth, or where soils retain water longer than other fields (but not saturated to the point of plant stress), tend to result in rank growth again.

If these fields take a while to dry out enough to allow for sprayer applications, or if they are significant distance from your farm shop, then pre-bloom PGR applications may be warranted, if other criteria are met.

Variety growth characteristics

The prevailing environment drives the need for PGRs far more than genetics, however it is important to know the variety’s genetic tendencies, and target pre-bloom PGR applications to later maturing, more growth varieties, especially when they are planted in fields with a history of rank growth.

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During the pre-bloom period, some early maturing varieties may be currently growing vigorously, however many of them tend to drastically reduce their rate of growth once blooming begins and a boll load accumulates.

Varieties of this nature can generally be managed with a PGR application during bloom fairly easily, and are more likely to negatively respond to pre-bloom PGR applications, especially if dry weather prevails (even a short-lived dry spells). The chart and table below illustrate growth characteristics of some of our newer varieties to help you understand how they compare to known varieties in terms of growth potential.

The PGR recommendations table is a very general guide that applies to situations where rainfall is not limited and therefore plants are not stressed.

Click Image to Enlarge

Click Image to Enlarge

Harvest Schedule

Cotton that is to be harvested first should be managed for earliness. This is probably more critical this year than most. If other criteria that justify a pre-bloom PGR application are met that have been previously discussed, it may be a good idea to focus efforts on fields that are to be harvested first, or earlier during the harvest period.

However, we need to capture any advantages to earliness with timely defoliation and harvest. In typical cases, PGR-treated cotton could be defoliated or harvested a week or so earlier than non-treated cotton, however, if we wait a week or so on the non-treated cotton to mature, then there is no difference in maturity or yields.

For later-planted cotton, especially June-planted cotton, the differences in maturity between PGR-treated and nontreated cotton tend to be greater than a week, which is important to keep in mind as well.

Signs of Stress

Cotton is currently behind schedule essentially everywhere in the state. Weather during July and August, and even into the Fall will dictate whether maturity will be accelerated (dry weather, rapid heat unit accumulation) or prolonged even further (cooler, cloudy, wet weather).

Regardless, we know that PGRs can hasten maturity but applications may only be justified in certain situations when specific aforementioned criteria are met. Just because it is behind schedule may not mean it needs a PGR applications.

If there are any signs of stress, such as severe stunting compared to other fields, reddening or toughening of the main stalk, formation of large squares in the terminal, discoloration, poor root development, etc., it’s best to leave it alone and maybe top-dress with realistic rates of fertilizer and let it grow for now.

PGR applications will only add to the existing stress and prolong its recovery.

General thoughts

Focus pre-bloom applications of 6 to 8 oz/A for standard mepiquat products (use 2-3 oz/A for Stance) at the 9-10 leaf stage on fields that have the following three criteria:

  1. Heavier soils with good moisture retention and a history of rank growth,
  2. varieties with the greatest growth potential, and
  3. later planted cotton.

Additionally, focus these applications to fields that are more difficult to treat in a timely manner (greater distance from your shop, or fields that tend to stay wet for a while following a rain). Timeliness is much more important than rate.

Many growers use excessive rates thinking it buys them more time, when in fact, it may only add stress and doesn’t really control growth any more than the rates listed above.




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