Although plant growth regulators have been used for many years in Georgia, there is still much confusion about what they actually do. Contrary to what many may think, mepiquat does not cause the plant to produce more flowers or bolls. What is can do is limit the amount of vegetative growth which may enhance fruit retention in lower nodes. This early node fruit retention will help with “earliness” or a quicker maturation of the crop, which tends to benefit later planted cotton more so than early planted cotton.
Excessive vegetative growth can also decrease harvest efficiency, decrease coverage of insecticides and fungicides below the canopy, decrease below canopy airflow (which may increase risk of foliar diseases and boll rot), and excessive shading of the lower canopy which can impact the fiber quality of bolls of the lower portion of the plant.
There are several key factors that play a role in a crop’s need for PGR applications. Current crop condition should always be the most important factor considered as mepiquat applications to stressed or stunted cotton may have negative impacts on the crop. Checking internode lengths between the 4th and 5th nodes from the top of the plant is a great way to determine the crop’s current condition and growth potential. If this internode measures 3” or greater mepiquat application is warranted.
Other factors should also be considered. What is the vegetative growth history of the particular field? If a field is known for growing rank cotton, higher rates and/or more applications may be necessary.
Considerable thought should also be given to the crop’s fertility program. If nitrogen is excessive (include all N forms: poultry litter, legume cover crops, standard fertilizer) the vegetative growth potential may also warrant additional mepiquat. Planting date should also be considered, as later planted cotton will often realize a greater benefit from mepiquat applications compared to earlier planted cotton.
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Additionally, in 2019, special attention should be paid to cotton that was planted in mid to late May. Although not technically “late” this cotton most likely received excessive stress from intense heat, dry weather, and thrips and maturity was most likely delayed. This cotton should be managed as if it were planted late and a more aggressive PGR program should be applied.
One final consideration is cotton variety. Each variety’s vegetative growth potential and response to PGR applications is unique. Below is a chart that is to be used as a guide for PGR needs by variety. It is important to understand that although variety plays a large role in the overall PGR program, the other factors must always be considered. For example, a variety in the lower growth potential group will become rank with excessive nitrogen and irrigation/rainfall occur.
For more information on rates and timings please contact your local UGA Extension Agent.