⋅ BY SHELLY PATE KERNS ⋅
For many cotton growers, Hurricane/Tropical Storm Francine has thrown a wrench in cotton defoliation and harvest plans. Prior to Francine, there were many reports of cotton lodging, or “laying over”. This was most likely due to the fruit shed event Louisiana experienced in August.
After shedding fruit in the lower canopy, cotton plants became heavier further up the canopy as bolls continued to develop and mature. As the bolls grew, the plant became heavier. The rains that we received in the last week of August and first week of September added moisture and weight to the lint.
As we approached the first week of September, many farmers were ready to start making defoliation decisions. However, Francine delayed the application of these chemicals and further contributed to cotton lodging.
Additionally, prolonged wet periods in combination with lodged cotton can play a role in making defoliation decisions.
The longer that cotton remains lodged in contact with moist soil or neighboring plant material, the more at risk the plant is to develop boll rot and regrowth from buds that were previously shaded by neighboring plants.
Good News In Louisiana
The good news is now that Francine has passed, Louisiana is forecasted to receive some windows during the third week of September of dry weather with temperatures higher than they were before the storm.
This slight boost in daytime temperatures will be good for defoliant absorption by the plant. Overall, if the state has good weather, and therefore gets good activity out of defoliants, it should somewhat help with the lodging growers are seeing.
This is because cotton harvest aids remove leaves, reduce moisture in the lint and seed, and facilitate drying by increasing air movement within the canopy. There could be a good opportunity to get most of the cotton defoliated this month.
However, for those that had later planted cotton due to the heavy rainfalls in May and June, it is imperative to consider how the weather shift into fall to be experienced later this month/into October will impact the efficacy of your chosen defoliant. Weather and defoliant/desiccant information can be found in the 2024 Mid-South Cotton Defoliation Guide: https://www.mississippi-crops.com/2024/08/21/2024-mid-south-cotton-defoliation-guide/ (Table 1).
As for making decisions on when to defoliate, there are several methods to determine plant maturity:
Nodes Above White Flower (NAWF) + Degree Day (DD) 60’s
DD60’s are used to calculate heat unit accumulation of the cotton plant. Once the plant reaches cutout (NAWF=5), the plant has ended its boll loading period.
Many growers often only use NAWF5 as a measurement of plant maturity, this can lead to a premature defoliation of the plant which can prevent some bolls from maturing (impacting fiber quality and yield).
Therefore, when using the NAWF or DD60 method, both should be used together to determine plant maturity. In general, the field should have reached NAWF=5 and have accumulated 850-degree days before defoliation.
DD60’s can be calculated using the following equation: DD60 = ((Degrees F Maximum + Degrees F Minimum) / 2) – 60.
Percent Open Boll
Visually inspecting plants for % open boll is another “classic” method of determining when to defoliate. Defoliating plants at 60% open boll is broadly used as the recommendation.
However, there are other decisions that factor into this method. For example, defoliating at 60% open boll may lead to premature defoliation if the crop has set its fruit at varying time periods due to insect damage and stress (leading to varying maturity rates of bolls).
To ensure that you are getting an accurate percentage of open bolls, it is best to measure three feet of row in 10 places in the field, then count open and closed bolls to calculate what percent are open.
Node Above Cracked Boll (NACB)
Cotton can be harvested around 4 NACB. To determine NACB, find the uppermost cracked boll and count upwards to the uppermost harvestable boll.
Once the uppermost harvestable boll is found, it should be cut into a knife with a knife to determine if the boll is mature. If it is not mature, defoliation should be held off until the plant reaches NACB3.
Sharp Knife Method
Choose the uppermost boll that you think is harvestable (generally 4-5 nodes below the terminal) and cut a cross section. If the boll is difficult to cut into, lacks “jelly,” and has darkened seed coats, then that boll is mature.
A Combination of All Methods (Most Recommended)
It is best to use a combination of all techniques to determine plant maturity and make defoliation decisions. For example, a field is ready to defoliate when plants have accumulated 850 DD60’s, is at NAWF=5, has approximately >60% bolls open, NACB=4 or 3 and the uppermost harvestable boll is determined to be mature using the sharp knife technique.
Hopefully the application of defoliants will help with lodging in the field. However, if plants are still laying over, it is important to use caution when picking.
It is recommended to slow the picker down and accurately adjust the height of the header if you are having trouble picking cleanly.
Overall, be mindful of changing weather patterns and field conditions to help you determine when it is best to defoliate and harvest your crop.