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Cotton Fruit Retention and Plant Growth Regulators
Author: Tom Barber, Extension Weed Scientist

Several questions have come in this week regarding a sharp decline in square retention. Most of these observations have been made by finding the third fruiting position down from the terminal and noting whether the squares are present or absent. This is a common way that consultants figure square retention when fields are scouted (every five days or so). Based on these counts, the square retention has decreased significantly this week because many plants produced a vegetative branch above one or more fruiting branches. What causes this to happen? Minute physical damage to the cotton terminal, coupled with the extremely high temperatures in early June are likely to blame. Terminal damage could be the result of blowing sand, small hail, thrips injury etc. This usually occurs 2 to 3 weeks prior to visual observations of the node or position on the main branch. Four-bract squares are another abnormal observation that could be the result of this physical injury. There is nothing that can be done, but vegetative growth and internode elongation should be monitored for timely plant growth regulator applications. If internodes are elongating, a 6-8oz/A application of generic mepiquat chloride may help to settle plants and encourage fruiting.

Growth regulation is extremely important for maintaining earliness and preventing late season issues. Stresses from dry conditions, high winds, hail, and sand damage have injured some of the younger cotton, and it is still recovering. I would be extremely cautious in making growth regulator applications right now on any cotton that is catching up from physical or chemical injury. Previous data from many years has shown that applications made at the pin-head square stage are more likely to stunt the cotton and may result in premature cutout.

Fourth internode down from terminal

Moisture supply, high nitrogen availability, and heat generally result in vigorous growth conditions early in the season (plant on right). What does all this mean? Applications need to be made on a field by field basis. On cotton that is 10 nodes, take a look at the internodes.  The fourth internode down from the terminal will give you a good indication of development, vigor and horsepower. If the 4th internode is 3 fingers long, then the cotton will most likely need an application of a plant growth regulator. Monitoring fruit retention is going to be very important this season. 80% or higher square retention is optimal going into bloom. If fruit retention decreases, growth regulator applications will be necessary. Variety, history of vigorous growth, the current moisture, and crop condition are the major factors in helping to select the proper growth regulator program, or if it is needed at all.

In Arkansas, the early maturing varieties such as DP 0912 B2RF, DP 0920 B2RF, FM 1740 B2F, PHY 375 WRF ST 4554 B2RF, and ST 4288 B2F respond fairly well to plant growth regulators. Out of these listed, the FM 1740 B2RF probably responds the best. In most cases, the low-multiple rate strategy works very well on these varieties under irrigated conditions. Applications should be delayed until bloom if you have planted these varieties on dryland fields. Later maturing varieties such as PHY 499 WRF, ST 5458 B2F, ST 5288 B2F and any Deltapine classes of 2010, will probably require more plant growth regulator to maintain a manageable canopy height during the season. Irrigated fields containing these varieties should be monitored closely, and most likely initial applications will be needed at the 10 node stage. The low-multiple strategy will work well in this case as well. Keep in mind as plant heights and biomass increases, plant growth regulator rates will need to be increased to manage growth. Early applications should not exceed 8 ounces of mepiquat chloride product such as Mepex and Pentia or 2 ounces of Stance. Mepiquat products at 6-8 ounces/A or Stance at 2 ounces/A is the most common recommendation for 10 node cotton. Initial applications in the low-multiple rate scenario should be followed by a second application 10 to 14 days after the initial application. Rates on the second application should be 8 to 10 oz/A of a mepiquat product or 2-3 oz/A of Stance.

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