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Mepiquat chloride, first sold as Pix® brand, was the first PGR successfully used in cotton to make a significant impact on managing plant growth and yield.1Mepiquat pentaborate, marketed as Pentia®, is also a PGR product used by many cotton producers. Both PGR products contain mepiquat which is an anti-gibberellin growth retardant that works to reduce plant cell enlargement and is used to help balance vegetative and reproductive growth. When applied to cotton, it can reduce stem elongation at newly formed internodes and can improve fruit retention. Growth regulators may also be used to help make cotton mature earlier, reducing the crop’s risk of late-season insect damage and boll rots, and can also keep rank growth in check to minimize harvest losses. Mepiquat applications have been linked to increased cotton yield potential when applied at the optimum rate and timing specific to a variety and field conditions.2
It is important to understand the growth habit of a particular variety in order to manage vegetative growth. Certain varieties maintain an aggressive growth pattern when compared to other varieties, and mepiquat rates and timing must be managed accordingly.
The Deltapine® cotton varieties may each respond a little differently to the application of mepiquat. The projected response of each variety is provided in Table 1. DP 1133 B2RF, DP 1212 B2RF, DP 1311 B2RF, DP 1518 B2XF, DP 1612 B2XF, and DP 1614 B2XF have shown the most response to mepiquat, meaning lower rates or fewer applications may be needed. DP 1050 B2RF, DP 1137 B2RF, DP 1219 B2RF, DP 1359 B2RF, DP 1454NR B2RF, DP 1538 B2XF, DP 1549 B2XF, and DP 1558NR B2XF have shown the least response to mepiquat, meaning timely early-season applications at higher rates or multiple applications may be needed to control vegetative growth. Although local soil and management issues may impact PGR applications, consider the following guidelines: for cotton varieties in the moderately and least responsive categories, growers should plan to apply mepiquat at 8 to 10 nodes (match-head square) and aggressively apply mepiquat if there is a history of rank growth, or if the crop received early rainfall or was rotated behind corn or another crop where a high nitrogen rate was applied. Mismanagement of mepiquat or other PGRs can have a negative effect on yield potential, especially when applied too early, when applied to stressed cotton, or when applied to a determinant cotton variety.
When deciding to apply an application of Pix®, the actively-growing top five internodes of the cotton plant should be evaluated.1 Mepiquat chloride rate and timing sticks are available to provide recommendations according to the average internode length of the top five nodes of the plant (Figure 1). Another way to determine application rate is to monitor the height and internode distance between the 4th and 5th node below the terminal node.
For areas with a high average annual rainfall, Figure 2 provides Pix® application rate recommendations based on the internode distance between the 4th and 5th node and the total mainstem node number. The longer the internode length, the higher the recommended rate.
For the more arid regions of the West, a different strategy should be employed. After excessive rainfall across much of Texas, growers should plan a PGR strategy that will provide them the most flexibility. Figure 3 illustrates that if a grower begins applying a mepiquat-based PGR on irrigated cotton at 4 to 12 ounces per acre, they can control plant height by a nominal level of 5 to 8%.
However, if they need more effective growth control, a subsequent application at early bloom is the most effective approach in this 2-year study (14 to 18% height reduction). In other words, don’t wait till early bloom for the first application of a PGR when you expect vigorous growing conditions.
Other plant measurements and field conditions can help assess the need for cotton plant growth control measures. Table 2 provides suggestions for growth control measures.
Mepiquat and other PGRs should be considered management tools used to maintain ideal cotton height and fruiting retention. 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.
Variety response to PGR applications varies greatly among the Deltapine® cotton varieties. Knowing the response of a variety to PGRs can help growers properly manage growth to improve cotton yield potential and harvest efficiencies. For varieties that are less responsive to PGR applications, timely early-season applications at higher rates or more applications may be needed to control growth.