by John S. Kruse, Ph.D.
Cotton producers integrated the practice of applying Mepiquat chloride or related plant growth regulators many years ago. Despite its widespread use and long history in modern cotton production, the use of plant growth regulators (PGRs) remains as much of an art as it is science. The main reason for this is that plant response to an application varies widely depending on the condition of the plants at the time of application – particularly as it relates to drought stress. It also depends on the current and upcoming growth conditions. Every cotton grower has his own story or one he heard from a neighbor of how the weather turned to drought immediately following an application of Mepiquat and the plant seemed to shut down.
Mepiquat chloride acts on the cotton plant by inhibiting gibberellic acid synthesis. Gibberellic acid (GA) is a naturally occurring plant growth hormone that influences cell growth and elongation. Since GA synthesis is promoted by warm temperatures and adequate moisture, cotton plants that are well watered and rapidly accumulating growing degree day units (DD60s) tend to extend internodes and favor vegetative growth. An application of Mepiquat reduces the presence of GA, resulting in plants with shorter internode length and better fruit set. In fact, the main reason to use Mepiquat is to control plant height and fruit retention, not necessarily to try to increase yield. Higher yields can result from effective PGR use, usually because the treated plant maintained a more compact canopy, limiting boll rot in the lower portions of the plant. However, too many scientific trials have been conducted over the years with mixed yield results to state that Mepiquat use increases yields consistently.
Cotton plant growth monitoring should begin at match-head square, and the grower should be prepared to make an initial application at nine to ten nodes. The rule of thumb is that, in general, the height to number of nodes ratio should be 2 or greater and that the cotton be generally stress free with adequate fertilization. Consider that plant stress can be induced by arthropod attacks (anything from mites to aphids to plant bugs, etc.), disease, nematodes, cool temperatures, herbicide injury, and weed competition, as well as drought. You will need to take soil type into consideration, as those soils that favor aggressive growth will require a higher dose of growth regulator than the weaker soils.
Once PGR applications are initiated, future application decisions should be based on direct observation of plant growth, not based on a calendar. Continue to follow the height to node ratio rule or measure the internode length of the longest of the third or fourth internode from the terminal – particularly in larger cotton that is closer to bloom. Once the internode length reaches 2.5 inches or greater (many consultants measure the width of three fingers for practical purposes), an application may be due. In general, lower rates are safer prior to bloom, especially in dryland cotton that is subject to drought stress. Irrigated cotton that is an aggressive variety may require a higher dose. As cotton begins to flower, higher rates are generally needed. Irrigated cotton, cotton behind corn, and cotton with too much nitrogen applied will especially need higher rates. Be sure to integrate pest control in your rate and timing decisions, since heavy plant bug pressure will cause stress and reduce overall growth. The key – and the challenge – is to always be ahead of the growth curve and not behind it. If a PGR application was due, but then it rained 2 inches on strong ground before the application could be made, the grower is behind the curve and will struggle to catch up the rest of the season. On the other hand, it is usually wise to delay an application on drought stressed cotton (no one said this was easy). Applications of Mepiquat are not recommended after cutout.
Major seed manufacturers are communicating what they can to assist producers in formulating their overall PGR program. The following varieties are loosely categorized by their general response to an application of growth regulator – given that the soil is well watered and well fertilized.
Aggressive: Bayer – ST 4145LLB2. Phytogen – PHY 499WRF, PHY 565WRF. Deltapine – DP 1137B2RF, DP 1050B2RF. These varieties will require more aggressive PGR management, which would involve applying earlier and perhaps in higher rates. They do not respond as well as others to PGR application.
Moderately Aggressive: Bayer – ST 5288B2F, ST 5458B2RF, FM 1845LLB2, ST 5445LLB2. Phytogen – PHY 375WRF, PHY 485WRF. Deltapine – DP 0912B2RF, DP 1044B2RF, DP 1133B2RF. These varieties respond well to PGR applications. They will require PGR management, but field growth conditions will play more of a factor on these varieties than the more aggressive varieties. Under high growth conditions these varieties have potential to be just as aggressive as those in the Aggressive category, but normally respond better to PGR applications.
Moderate Growing Varieties: Bayer – FM 1740B2F, FM 1944GLB2, ST 4288B2F. Phytogen – PHY 367. Deltapine – DP 0920B2RF, DP 1212B2RF.
These varieties normally require a less aggressive PGR management. They respond well to PGR applications. Under High growth conditions they will require PGR management, but have shown to be easier to manage than more aggressive varieties.
The cotton varieties listed above represent the dominant varieties grown in Louisiana and are categorized based on industry input as well as University researcher, grower and consultant feedback. It is important to remember that factors other than variety such as soil type, irrigation, fertility, and insect pressure can play a significant role in PGR management decisions.