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The Art of Cotton Defoliation

Sep 09, 2020

You would expect that cotton defoliation would have become a simpler, more standardized process after 50 years of research, but unfortunately Mother Nature has yet to let us in on the secret. Deciding how and when to remove the leaves and open the bolls remains one of the most challenging tasks faced by cotton producers. 

"Due to a wide range of plant and environmental conditions, what worked last year might not work this year," says WinField® United Cotton Product Manager Robert Cossar. "Any stress impacting the cotton plant can reduce the efficacy of a harvest-aid application, which is why we refer to defoliation as more of an art than a science." 

Using defoliants and desiccants to remove the leaves speeds up the harvest of a mature cotton crop and reduces potential preharvest losses of lint yield and fiber quality. They can also result in earlier harvest, fiber quality preservation and higher seed quality. 

"Timing is the most important element of defoliation," Cossar explains. "Making a harvest-aid application too early can lead to improper defoliation, lower yield and fiber quality, and reduced seed quality. When an application is made too late, longer exposure to the weather and boll rot could reduce yield and quality." 

For many years, growers were advised to begin defoliating once their crop reached 70 to 80 percent open boll. However, the subjectivity of that advice leaves room for inaccuracy, which is why Cossar recommends following these methods described in the Texas A&M AgriLife Extension's High Plains and Northern Rolling Plains Cotton Harvest-Aid Guide.*
 
1.     4 NACB: Four nodes from the uppermost first position cracked boll to the uppermost first position harvestable boll
2.     60-70% open bolls: 60 to 70% of the harvestable bolls on the plant are open.
 
Four NACB (nodes above cracked boll) doesn’t necessarily equate to 60 to 70% open bolls, so the Texas A&M guide advises that a combination of the two methods may be used, and timing of harvest-aid applications should be made on a field-by-field basis.
 
“Both of these measurements can be impacted by boll distribution, variety maturity and management practices,” Cossar notes. “The variability between NACB and the percentage of open bolls in a field can also be high, so I recommend checking at least 50 random plants across a field for consistency. And both measurements should be based on the amount of harvestable bolls on the plants, so only take mature bolls into account.”
 
Boll maturity can be determined by slicing the boll horizontally to expose the developing lint and seeds. A mature boll should be firm and difficult to slice, with mature seeds (fully developed cotyledons with little liquid or “jelly” in the seeds) with a dark seed coat, and the lint stringing-out when the two halves are separated.*

Defoliants, Boll Openers, or Both?
It's a good idea to consult your local GreenPoint AG specialist when selecting a harvest-aid product for your acres, as many options exist. Cossar says most fall in the following three categories:
  • Defoliants like Daze® 4SC cotton defoliant loosen and dissolve the cells on the leaf petiole where it attaches to the stem, working against the indole acetic acid (IAA) that cotton plants synthesize to maintain healthy, productive leaves. 
  • Boll openers like Boll'd® brands are often recommended to be tank-mixed with a range of defoliant materials to increase the percentage of open bolls in preparation for a once-over harvest. They're often used with late-maturing crops when the weather may be too cool to provide enough heat units to open late bolls.
  • Regrowth inhibitors help deter the regrowth of leaves before harvest is completed. There are hormonal and herbicidal products – both of which offer some defoliation activity.
  • Desiccants, such as paraquat, prepare the crop for harvest by killing plant tissue and causing a rapid loss of water from the foliage. This often kills the plants so quickly that defoliation doesn't occur. Leaf stickage drives the lint quality down due to increased leaf content in the fiber.
 
In high-yielding cotton, defoliants are often used to remove leaves for easier harvest, inhibit regrowth and reduce lint staining. Then a desiccant is applied to speed up the drying of stems and remaining leaves. 

“Regardless of what harvest-aid products you use, coverage is absolutely crucial,” Cossar emphasizes. “It begins with selecting the correct water volume and right adjuvant package to maximize performance. Higher water volumes will typically result in greater levels of defoliation and boll opening than lower volumes. Be sure to check your label, as some defoliants require a non-ionic surfactant (NIS), and others a crop oil concentrate (COC).”

Inergy® adjuvant aids fast spreading, uniform distribution and absorption of spray on leaf and stem surfaces through a unique blend of methylated vegetable oil and an organosilicone-surfactant system. It can also reduce the negative effects of adverse environmental conditions for increased spray efficiency. 

Derived from vegetable oil, InterLock® adjuvant reduces the amount of fine spray particles in the spray pattern and increases droplet speed without thickening the spray. By keeping more spray droplets within the most effective size range, it improves coverage, increases canopy penetration and reduces spray drift and evaporation. 

MasterLock® adjuvant combines InterLock and DropTight® additive technology in one crop-based adjuvant to enhance spray coverage and get droplets down in the canopy. It works with fungicides and insecticides and may be used with herbicides and other crop protection products. The combination of increased adhesion, reduction in bounce and increased canopy penetration leads to more fungicide and insecticide where it is needed the most.

"For best product performance, it's also important to select a spray tip and pressure that produces a medium droplet," Cossar adds. "There's more potential for spray drift when the droplets are too small, so refer to your spray tip manual or consult your local agronomist when selecting the right tip and pressure for defoliation." 

Additional application tips from Cossar and the Texas AgriLife Extension Service:
  • Monitor the 5-day forecast and apply defoliants 12 to 24 hours before rainfall, as products become rainfast after 12 hours and harvest-aid products should be applied at least five days before harvest.
  • Cotton plants are more responsive to defoliant chemicals under warm, sunny conditions and evening temperatures above 60 degrees Fahrenheit. 
  • Soil moisture should be low, but sufficient enough to maintain plant activity without drought stress. Low nitrogen levels are also desirable in both the soil and the plant. High soil moisture and nitrogen levels contribute to rank, dense foliage and delayed maturity. 
  • Avoid spraying when plants are severely drought stressed with tough, leathery leaves as the waxy cuticles deter absorption. 
 
It’s important to have a good resource on your team when dealing with the complexity of cotton defoliation, so don’t hesitate to turn to your local GreenPoint AG cotton specialist if you have questions. They’re always eager to provide customized advice for your acres.

High Plains and Northern Rolling Plains Cotton Harvest-Aid Guide, Texas A&M AgriLife Extension, Byrd, Keeling and Morgan. 


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