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TECH BULLETIN: Insights on using plant growth regulators to manage cotton in West Texas

Plant growth regulators are used to manage a perennial-natured plant, such as cotton, in an annual system.

For maximum benefit from a plant growth regulator (PGR) program, the first step is to have a good understanding of how PGRs work and what they can do to maximize yield and make harvest operations more manageable.

This chart classifies the general plant growth regulator (PGR) needs for PhytoGen brand varieties.

Managing growth in a perennial-natured plant

In cotton, mepiquat chloride and mepiquat pentaborate are used to limit vegetative growth. These mepiquat compounds inhibit the hormone (gibberellic acid) that causes cell elongation. Shorter cells result in a more compact plant, potentially making the plant more manageable and able to efficiently produce cotton.

Remember, cotton growers are managing a perennial-natured plant as an annual crop. In many areas of the country, adoption of a successful PGR program aids in reaching the ultimate goal of consistently producing higher yield. Growers can harvest plenty of cotton without PGRs, but PGR use often makes the crop more manageable, which is why they are used in most successful modern cotton production systems.

There is no single best method or prescription for successful PGR use. Many approaches are adequate. Understanding the factors that affect vegetative growth helps you make better decisions on when and how much PGR to use.

Know what to expect from PGRs

Growers can count on shorter internodes and a darker green appearance just about every time with mepiquat-based PGRs. Higher yield and improved quality also are possible but not guaranteed. If the crop is more manageable and/or harvestable, producers often are able to harvest more of the crop and may see some quality improvements due to crop uniformity and ease of harvest preparation. However, don’t expect to shrink a cotton plant; start early enough for the selected dose to affect new growth.

What is the right rate and timing for a PGR application?

Mepiquat-based PGRs inhibit vegetative hormone production, and the plant is in constant production of these hormones through mid- to late bloom. Rate and timing can be discussed in the same relationship as a gas/brake pedal. For example, when the plant is small, it takes much less braking power (rate) to slow it down. When the cotton plant is in the second week of bloom, it is more like a loaded 18-wheeler, requiring significant braking power to get a noticeable response.

In the West Texas region, PhytoGen® brand varieties with “moderate” or “aggressive” PGR needs have generally benefited from applications made prior to bloom. Rates will be highly dependent on irrigation capacity, field history and fertility levels (read on below for more details). Once an initial application is made, monitor growth rate closely every 7 to 10 days by observing plant height, as well as the length of the fourth internode from the terminal. Fourth internode lengths exceeding 3 to 3.5 inches during bloom will require heavier rates of follow-up applications of PGR to control growth and maturity.

PhytoGen brand varieties with “conservative” PGR needs may or may not need PGR applications season-long, and will be less likely to benefit from an application prior to bloom. These particular varieties are determinate, shorter-statured varieties that tend to have rapid growth during squaring with slower growth rates during bloom as they mature quickly.

Consult your local PhytoGen cotton development specialist or university Extension cotton specialist for more specific recommendations for your field.

Factors to consider when developing a PGR program

  1. Variety response to PGRs. Determinate varieties tend to be less aggressive and need less PGR than indeterminate ones. The chart above offers a general key to understand the relative response of PhytoGen® brand varieties to PGRs.
  2. Soil moisture and rain forecast. Follow above average rainfall with aggressive PGR rates. Avoid applications during drought or if hot, dry conditions are predicted in the five-to-seven-day forecast.
  3. Irrigation capacity. Fields with high irrigation capacity require a more aggressive PGR program. Fields with moderate or limited irrigation capacities favor a more conservative-to-moderate approach.
  4. Field history. Nothing beats grower experience. Current plant height compared with a field’s historical height is a key indicator. Consider past and current fertility programs, irrigation capacity and soil type.
  5. Fruit load. Lower rates may be needed if retention and fruit load are high. Higher rates may be needed if significant shed has occurred. Nodes above white flower (NAWF) should be 7 to 10 during early to peak bloom for maximum yield potential.
  6. Pest stress. All mepiquat product labels indicate avoiding applications during stress periods. Insects, weeds, nematodes and diseases can affect growth rate.
  7. Herbicide injury. Avoid PGR applications if the crop is stressed by injury from an unintended herbicide.
  8. Harvest machinery. Managing plant height for cotton intended for stripper harvest is more critical to avoid excessive leaf and stem trash that can lead to higher leaf grades, as well as bark discounts. Use a combination of variety selection and PGR management to achieve a final plant height of no more than 30 inches for optimum stripper harvest.
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