The cotton plant’s perennial and indeterminate growth habit is perhaps the most complex of all major row crops. The plant is very responsive to management and changes in the environment. The change in both vegetative and reproductive growth in response to stress or the lack of stress is predictable. However, the ability of the cotton plant to compensate for fruit loss or to recover from stress can be surprising. Producers use PGRs and other cultural practices as a means to manage the balance between vegetative and reproductive growth for efficient cotton production
The use of PGRs encompasses a broad category of compounds that promote, inhibit or otherwise modify plant physiological or morphological processes. Some PGRs are plant hormones or their analogues; others are simply metabolic regulators. These products are classified as organic compounds that alter the growth and development of plants. Unlike plant hormones that are endogenously produced by the plant, PGRs may be considered chemical compounds either produced naturally by the plant or synthetically. They are biologically active at very low concentrations and elicit responses similar to those observed from plant hormones. Since most plant growth and development processes are regulated by natural plant hormones, these processes may be manipulated by either altering the plant hormone level or changing the capacity of the plant to respond to its natural hormones. All PGRs should be considered as management tools in the producer’s arsenal to alter cotton growth and development in an attempt to control growth and improve productivity (Jost et al., 2006)
Commercially available PGRs can often be divided into two basic groups: growth inhibitors and promoters. The growth inhibiting PGRs generally provide consistent height reduction and often enhance earliness. Yield increases may be a bonus if it occurs. However, growth-promoting PGRs offer no direct height reduction and generally provide little advantage, if any, toward enhancing earliness and must provide yield increases to be profitable (Biles and Cothren, 2001).
Excessive vegetative growth can also occur under optimum conditions. The length of the upper five internodes can be a direct measure of the current status of the plant as these are the only internodes on the main stem where elongation is occurring (Bourland et al., 1992). The length of the fourth internode from the terminal or the combined length of the top five internodes can be used to gauge vigor. Plants in which the third internode exceeds 3 to 4 inches or the top five internodes exceed 7 to 9 inches may be experiencing excessive vegetative growth and should be evaluated for using a growth inhibiting PGR. Smaller than expected square size can also be a sign of vigorous growth if this is observed in the first positions of the upper fruiting branches.
Figure 1. Identification of the fourth internode for the purpose of evaluating plant vigor.
Growth inhibiting PGR applications should be well-timed in anticipation of excessive growth rates to more effectively manage vegetative growth and plant height. These compounds will not shrink cotton plants, but will only slow growth of actively growing tissue after application. It is important to monitor the crop for growth, square size and fruit retention when scouting as well as evaluating the current and future potential for stress. The evaluation of stress should also take into account pressure from nematodes and diseases. Growth inhibiting PGRs are generally not recommended for use on stressed cotton. Once the need for this type of product has been established, the application rate should include consideration of the environment and the size of the plant. It is important to read and follow label guidelines for all products.