Sunday, June 23, 2024

Plant Growth Regulators

Mepiquat Provides Crop Management Benefits To Cotton Farmers 

Since the 1980s, plant growth regulators — PGRs — have played an important role in achieving a successful, efficient and more profitable cotton crop. Cotton experts from different areas of the Cotton Belt discuss why PGRs are a tool to consider in cotton production and how to interpret the factors that are involved.

Brian Pieralisi, Mississippi Extension Cotton Specialist

The key purpose of plant growth regulators is to manage cotton internode length for a more efficient plant to improve production. Varietal growth habit and vegetative potential are paramount in making PGR decisions. Vegetative potential is the ability to produce abundant vegetative biomass.

Optimal internode distance of 2–3 inches following a PGR application. Internode distances lower in the canopy exceeded optimal distance before application.
Internode distances greater than 3 inches in cotton without a PGR application.


Cotton varieties differ widely in terms of height and responsiveness to PGRs. The main goal is to monitor the length between the fourth and fifth nodes down from the terminal and strive to maintain an optimal distance of 2–3 inches.

Cotton growth varies greatly across fields and even within fields. Variable-rate PGR applications help improve crop response because you can treat highly vegetative zones more aggressively and vice versa. As cotton enters the reproductive growth stages, a solid PGR strategy will limit plant height, improve air circulation through the canopy, increase defoliation efficiency and enhance harvest efficiency.

Crop growth depends on calendar date, temperatures and other environmental factors that are often unpredictable. In some situations, high fruit retention and boll load may act as a natural plant growth regulator. In this situation, excessive PGR applications can halt growth, greatly reducing plant stature. Using a conservative strategy and closely monitoring the internodal distance four to five nodes down from terminal may help avoid this problem. In fields with extreme vegetative variability, base your decisions on the most productive and predominant zones in the field. For example, if 75% of the field has high vegetation and is typically very productive, base your management primarily on this zone.

PGR management must evolve with both varieties and environmental conditions. Therefore, management strategies may vary drastically from year to year and can change during the growing season. Ultimately, the goal is to use plant growth regulators to manage internodal distances and produce a more efficient plant.

Seth Byrd, Former Oklahoma Extension Cotton Specialist

The need for a plant growth regulator often is based on current visual observations of crop growth or conditions, i.e. if the plants look too tall, then a PGR should be applied. Unfortunately, this method sometimes results in an application occurring too late for optimum growth regulation. Since PGRs only impact new growth, it is critical to base this decision upon the potential for excessive growth to occur. This means that while the size of the plant may dictate use rate (higher rates on larger plants, lower rates on smaller plants to achieve the same level of growth regulation), PGRs have no effect on growth that has already occurred. In addition to taking into consideration the current growth stage and size of plants, the factors shown in Table 1 also should be used to guide PGR decisions.

Because PGRs inhibit gibberellic acid throughout the plant, any areas in which cells are expanding are impacted. This not only includes the main stem but leaves as well. As a result, leaf expansion is reduced following PGR application, producing smaller but thicker leaves. This results in leaves that are darker green following a PGR application when compared to plants that have not received a recent application. This is often referred to as the “Pix effect” but has no impact on crop health.

Steve M. Brown, Alabama Extension Cotton Specialist

Making good decisions about plant growth regulator use involves assessing current and near-term conditions. The questions below help tilt management towards being more aggressive, less so or passive altogether.

Variety – aggressive or not?

Crop growth stage, conditions?

Field conditions – fertility? growth history?

NAWF at early bloom? If 8-9, be aggressive; if 4 or 5, wait.

Current internode lengths between 4-5th leaf down from the terminal? If 3 inches or more, apply or consider being more aggressive.

Calendar date? For late crop, tend towards an aggressive approach if favorable growing conditions exist.

Irrigated? Recent rainfall?

If good-to-excellent rainfall and moderate temperatures (highs <90 degrees F) are forecast, be aggressive unless growth is well in check. If high temperatures, little to no rain is expected and there is minimal or no irrigation, be conservative or passive.

Keith Edmisten and Guy Collins,
North Carolina Extension Cotton Specialists

Mepiquat can help cotton growers manage the development and maturity of their crop. Research conducted in North Carolina, as well as in other areas of the Cotton Belt, has demonstrated that mepiquat treatment can hasten maturity, reduce plant height, facilitate insect management, decrease boll rot and increase yield. These desirable features are caused by the inhibition of cell elongation in the cotton stems. Mepiquat-treated plants are normally smaller and more compact. Internodes along the stem and fruiting branches are shortened. The total number of fruiting branches may also be reduced slightly. Energy is directed toward retention of bolls lower in the canopy and away from vegetative growth, due to increased light penetration to lower subtending leaves.

This field shows cotton plant height reduction associated with PGR application — no PGR (left) and PGR application (right).

Mepiquat can be applied as a broadcast spray or as a banded spray. Research at North Carolina State University has shown that mepiquat also can be applied through a canvas wick applicator. The greatest advantage the wick seems to have over spray applications is that it makes it easier to apply mepiquat to tall cotton and avoid application to shorter, stressed cotton within the same field. More detailed information about using a wick can be found online on the cotton portal under Old Carolina Notes at  and

As with any management tool, the decision to use mepiquat should be based on a consideration of its usefulness in a specific situation. Your decision to apply mepiquat in any given year should be made on a field-by-field (or portion-of-a-field) basis. Certain cotton fields may require treatment every year, whereas others will rarely require treatment. 

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