In the Mid-South cotton is grown for optimum yield with lesser emphasis placed on fiber quality. Fiber quality usually does not come into the discussion unless discounts are being applied because of poor fiber quality.

 

I would submit that the manner in which we grow and manage cotton is more so to protect against losses from poor quality than to capture premiums for optimum quality.

 

Examination of a loan sheet will reveal that there are many more areas from which deductions are applied than where premiums are applied. To that end, some have expressed concern over potential micronaire values from this year’s crop. When trying to determine if the potential for low or high micronaire exists, it is important to understand fiber formation. Cotton fibers develop in an organized manner.

 

Beginning the day of bloom, cells that will eventually become fibers elongate outward into the boll. Final length is reached in about 16 to 20 days after formation. After fiber length is maximized, the fibers begin to thicken. Cotton fibers thicken from the inside out. Daily growth is added to the inside of that fiber unlike a tree in which annual rings are added to the outside of last year’s growth.

 

Mike Range Diff. 2.5 – 2.6 -1100 2.7 – 2.9 -750 3.0 – 3.2 -325 3.3 – 3.4 -125 3.5 – 3.6 0 3.7 – 4.2 25 4.3 – 4.9 0 5.0 – 5.2 -150 5.3 & Above -275

Cellulose that is added each day is deposited at different angles similar to that of fiberglass which contributes to the strength of the fiber. Generally, when a boll develops into maturity, the layers added to the center of the fiber partially close the center of the fiber 20 days after thickening begins. After fibers thicken, they begin to dry which results in crimping and twisting of the fiber. Crimping and twisting of the fiber are what allows cotton fibers to intertwine and be spun into thread.

 

Causes of low and high micronaire cotton: We all know that cotton bolls located at different fruiting positions on a given plant are at differing levels of maturity. Lower bolls may be fully mature, mid bolls may not be fully mature and higher bolls may be immature depending on when you examine each.

 

These differing levels of maturity can lead to low micronaire. During growing seasons that are cut short for whatever reason, low micronaire can be an issue. Essentially, the number of somewhat immature and fully immature bolls are more in number than fully mature bolls and when harvested and mixed together the result is low micronaire cotton.

 

In addition, when cotton has a large fruit load and inadequate carbohydrates to fill out these bolls low micronaire can occur. There are several factors that can lead to inadequate carbohydrate supply including dense stands, high nitrogen, potassium deficiency, and excess irrigation among others. It is not uncommon in years with high levels of fruit retention to see low micronaire due to carbohydrate demand and supply.

 

However, when more than ample carbohydrates are available, micronaire tends to increase to the point that high micronaire can become an issue.

 

High micronaire can be caused by poor boll set and/or small boll size due to heat and water stress.

 

High temperatures cause boll shed while the same level of carbohydrates are still available. This results in excess carbohydrates being partitioned to remaining bolls and high micronaire. More mature bolls are typically higher in micronaire than younger bolls as they matured during conditions of peak carbohydrate supply compared to younger bolls.

 

First position bolls tend to be higher in micronaire as well. Short fibers can also lead to high micronaire. As above, if fibers are short and the same level of carbohydrates are available, they are going to be used to thicken the fiber. Given equal amounts of carbohydrates, a shorter fiber will be thicker than a longer fiber.

 

So why make such a big deal about micronaire and causes of high and low mike?

 

The way this year is shaping up, the potential for micronaire problems exists. Several folks have reported smaller bolls on plants with reduced seed counts most likely due to weather conditions.

 

With smaller bolls and lower numbers of seeds in each boll, while maintaining adequate carbohydrate supply, the potential exists for high micronaire. Although it is impossible to know without collecting samples and getting a fiber quality analysis, if you suspect high micronaire will be an issue some thought should be given to defoliation timing.

 

Preliminary data from the University of Arkansas using the Hal Lewis method to determine micronaire indicates that selected varieties may have high micronaire values if defoliation applications are delaying until 60-70% open. As a result, some varieties should be defoliated somewhat earlier rather than later.

 

The take home message is this, if the variety you have planted tends to have inherently higher micronaire values than others, you may consider defoliating these at 50% open compared to 60 – 70% open. Defoliating somewhat early will help reduce micronaire due to presence of not fully matured bolls at the time of application.