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Micronaire is a measurement of the thickness of the cell walls of a cotton fiber and is used as in indication of fiber fineness and maturity. When cotton begins to bloom, cells that will eventually form fibers elongate into the boll. Final fiber length is reached about 16 to 20 days after formation. Once fibers are fully elongated, the fibers begin to thicken from the inside out. Carbohydrates produced through photosynthesis allow cellulose to be added to the cotton fiber walls, thus increasing both thickness and strength.1
Cotton buyers and textile manufacturers prefer a micronaire range of 3.8 to 4.5, and a fiber length of 1 1/8 inch or longer. Thicker or thinner fibers cause problems in both spinning and uniform dying of yarn. Buyers discount the value of both high and low micronaire cotton.
When a cotton plant produces more carbohydrates than necessary to support plant development, excess carbohydrates are available for fiber cell walls to thicken. Several factors can lead to the availability of excess carbohydrates.
Cotton with high micronaire ratings can be associated with drought or water stress. Dry hot weather can cause a cotton plant to shed upper bolls early in the season, leaving only the bottom bolls. A cotton plant will produce the same amount of carbohydrates to be distributed among the fewer bolls, the result is higher micronaire.
First position bolls tend to have high micronaire since more leaves are available to feed them and the leaves are less likely to be shaded. Because of this, early crops with high boll retention at the first position tend to have higher micronaire than later crops with bolls farther out on fruiting branches.2
Short fibers can also lead to high micronaire. When fiber elongation is limited micronaire often increases. Cotton varieties that typically produce shorter fiber are coarser and have higher micronaire rating than cotton varieties with longer fiber length.
The best management strategy to avoid high micronaire rating is timely cotton defoliation and harvest. The less mature fibers in the upper bolls, blended with the more mature fibers in the lower bolls will help to reduce the average micronaire for the crop. Consider your risk of high micronaire when selecting a cotton variety as some varieties inherently have higher micronaire. If high micronaire is anticipated consider defoliating at 50% open boll as opposed to 60-70% open boll, after reviewing local university extension recommendations.1
Farmers need to look at multi-year data, and evaluate trends in yields and fiber quality across a number of different locations when selecting cotton varieties. Selection of a cotton variety with an average micronaire rating on the high end of the discount range may increase the likelhood of discounts if growing conditions become unfavorable.