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Plant growth regulator (PGR) application is an important cotton management tool. Application of PGRs can help balance vegetative and reproductive growth, improve square and boll retention, and control plant height to improve harvest efficiency. Cotton varietal growth habit and response to PGRs is an important factor in developing a growth management strategy.
Figure 1. Cotton canopy prior to bloom.
PGR applications are used to reduce cotton growth until the fruit load naturally reduces vegetative growth. Applications should not be made prior to 50% matchhead square or when the crop is under any stress. There are several factors to consider when deciding to apply a PGR, including current internode length, boll retention, current and forecasted moisture and heat conditions, soil nitrogen content, and cotton variety planted. PGR applications are recommended in fields with high levels of nitrogen, optimal growing conditions, low boll set, and fields that are planted with vigorous cotton varieties.
PGR application decisions in West Texas can be difficult. In fields that are non-irrigated or minimally-irrigated, or environments with high winds and low humidity, growth can be reduced making it especially important to follow weather conditions and forecasts.1
Mepiquat chloride reduces cell expansion by decreasing gibberellic acid production in plant cells. Reduced cell expansion helps to shorten internode length and control growth. With optimal growing conditions, mepiquat chloride may help increase boll retention.2 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.3 PGRs should not be applied when the crop is under stress.
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 1212 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 1639 B2XF, DP 1646 B2XF, and DP1219 B2RF have shown moderate 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.
For the more arid regions of the West, a different strategy may need to be employed. After excessive rainfall across much of Texas, growers should plan a PGR strategy that will provide them the most flexibility. Figure 2 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%.
Figure 2. For the arid West Region, Pentia® plant growth regulator application rates and timings effect on cotton plant height. *2009, 2010 trials in Lorenzo, TX. Plant height response averaged over all varieties.
However, if more effective growth control is needed, 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.
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.
1 Morgan, G. and Kelley, M. 2015. Plant growth regulators as a tool for 2015’s challenges. Texas A&M AgriLife Extension. http://agrilife.org/. 2 FOCUS on South Plains Agriculture. 2013. Texas cotton: plant growth regulator considerations. AGFAX. http://agfax.com/. 3 Oosterhuis, D. and Robertson, W. The use of plant growth regulators and other additives in cotton production. University of Arkansas. AAES Special Report 198. Proceeding of the 2000 Cotton Research Meeting. http://www.uark.edu/. 4 Jost, P., Whitaker, J., Brown, S., and Bednarz, C. Use of plant growth regulators as a management tool in cotton. The University of Georgia Cooperative Extension. http://www.caes.uga.edu/. Web sources verified 06/21/16. 160621195020