Prepared by the CSD Extension and Development Team

When monitoring cotton growth to decide whether to use a plant growth regulator (PGR), several factors must be considered to determine the need for increases or decreases in the intensity of PGR application. These factors include:

• Field history.
• Field health.
• Varietal background/response to PGR.
• Predicted growth conditions.

A combination of cotton plant measurements can provide a more accurate evaluation of crop growth. Some key indicators include:

• Plant height. Plant height is one of the most common cotton measurements, and it is useful in making management decisions when combined with other information.
• Growth rate. A growth rate of about 2.5cm per day is the upper limit of acceptable growth.
• Mainstem node development. Mainstem nodes develop approximately every 29 DD1532 or 2-4 days. Water availability and carbohydrate production are the largest influencers of mainstem node development.
• Squaring nodes. Measurement of nodes above white flower (NAWF) is an excellent indication of vigour. Measurement of NAWF is effective because it directly corresponds to energy absorption by the reproductive and vegetative demands of the plant. This measurement monitors the difference between the rate at which squares reach bloom and the rate that new mainstem nodes are produced.
• Vegetative Growth Rate. Vegetative Growth Rate (VGR) is a key measurement for monitoring cotton plant vigour and guiding plant growth regulator (PGR) application decisions.
  • The process involves measuring the plant height and number of nodes on two separate occasions, approximately a week apart. The VGR is then calculated as the difference in height divided by the difference in the number of nodes. This provides a value in centimetres per node, indicating the rate of internode elongation.
  • Generally, a VGR greater than 5.5 cm/node suggests that a PGR application should be considered. This threshold can vary depending on other factors such as variety, fruit retention, moisture status, and field history.

Here’s a breakdown of the VGR measurement process:

1. Initial Measurement: Measure the height and count the total number of nodes on at least 20 randomly chosen plants.
2. Second Measurement: Approximately a week later, repeat the height and node count measurements on the same plants.
3. VGR Calculation: Calculate the VGR using the formula: (This week’s height – Last week’s height) / (This week’s node number – Last week’s node number).

Implications for Management:

• High VGR (greater than 5.5 cm/node): Indicates excessive vegetative growth. This may require a PGR application to redirect the plant’s energy towards fruit production rather than vegetative growth. The specific PGR rate will depend on the factors mentioned earlier.
• Low VGR (less than 5.5 cm/node): Suggests slow growth. While PGR application is generally not recommended in this case, other management strategies may be necessary to encourage growth, such as addressing nutrient deficiencies or moisture stress.
• The CottonTracka program developed by CSD incorporates VGR measurements to help guide PGR decisions. It plots VGR as a function of accumulated heat units and provides upper limit lines to help assess crop growth. VGR values above the upper limit line suggest the need for PGR application, while those below the lower limit may indicate a need for growth encouragement.

It’s crucial to remember that VGR is just one tool for making PGR decisions. A holistic approach considering all relevant factors is essential for optimizing cotton growth and yield.

PGRs that contain mepiquat, which is an anti-gibberellin growth retardant that reduces plant cell enlargement to help balance vegetative and reproductive growth. When applied to cotton, mepiquat can help control rank growth by reducing stem elongation at newly formed internodes. The application of PGRs can help increase fruit retention and promote earlier maturity.

CSD highlights that misapplication of PGRs can have a negative effect on yield potential, especially when they are applied too early or when the cotton plant is stressed. In-season cotton plant monitoring provides the best information to determine application rate and timing of growth control measures. Every cotton field is under different conditions so growth management strategies should be tailored to each field situation.