This article was prepared by Dr. Chris Main, former Cotton Extension Specialist.

Plant growth regulators. Many cotton acres are planted to new varieties in 2012 and producers do not have much experience with PGR applications to these new varieties. Table 1 provides general variety response to PGR’s for research over the past two years. Very responsive varieties do not need much PGR prior to bloom, especially if dry conditions persist. Varieties that are responsive can benefit from pre-bloom applications of PGR when grown under irrigation or on very productive soils. Less responsive varietes are varieties that tend to grow taller and more aggressively. These varieties are often placed on thinner soils to help ‘develop’ a good canopy. While these varieties typically require an aggressive approach to PGR management, be careful to use an appropriate rate for current conditions (see Table 2). On productive soils or under irrigatioon aggressive varietes can benefit from up to 8.0 oz of mepiquat per acre prior to bloom.

Table 1. Cotton variety Response to PGR application.

Table 2 below from the University of Georgia helps determine crop status during different growth stages. Remember that a PGR will NOT shrink the crop, the PGR will only control elongation of cells associated with new growth. As a general rule I like to make an application of PGR (mepiquat product) at 16 oz/ac at first bloom or about 60 days after planting. This application works well in all my variety trials. However, this may be too much PGR on thin-droughty soils and my be ‘to0 little to0 late’ on productive soils or in irrigated conditions.

Helpful Hints:

• Do not make a single application of 0.5 to 1 pint of mepiquat chloride to cotton that is drought stressed.  If using the low rate multiple option, discontinue use until the moisture stress is alleviated.
• Do not apply more than 3 pints of mepiquat chloride or 22 oz of Stance per acre per season.
• Do not apply mepiquat chloride within 30 days of harvest.
• Do not graze or feed cotton foliage to livestock within 30 days of application.
• Do not tank mix with other products other than mentioned on label.
• Do not apply mepiquat chloride through any type of irrigation system.

In trials conducted at 12 locations across the cotton belt during 2009-2010 by myself and colleagues from other Universities we did not see a difference in PGR products. I strongly encourage the use of a rate high enough to control growth with the first application. The most expensive treatment to make is the one that does not work. Applications of high rates at full bloom or ‘cut-out’ is not recommended and is an indication that not enough PGR was applied earlier in the season. What we really want to do with a PGR is to improve earliness and harvest efficiency. Missing with a low rate on that first application reduces the chance of acheiving both of those goals. Table 3 provides some general use guidelines for application rates.

Table 2. Height to node (HNR) ratios for cotton PGR decisions (Jost et al 2005).

Jost, P., S. M. Brown, S. Culpepper, G. Harris, B Kermerait, P. Roberts, D. Shurley, and J. Williams. 2005. 2005 Georgia Cotton production guide p 37-39.

Table 3.  Plant growth regulator application strategies.