Using Micronaire to Time Cotton Defoliation

  • Early defoliation can reduce cotton micronaire, quality, and yield potential.
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  • Late defoliation can increase the risk of exposure to poor weather conditions, possibly leading to boll rot or reduced lint quality.
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  • Optimally timed defoliation can help farmers obtain optimum micronaire ratings as well as high yield potential.
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Importance of Micronaire

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Figure 1

Figure 1. Testing of cotton varieties for occurrence of high micronaire. University of Arkansas Cotton Newsletter. 2006.

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Micronaire is the measure of linear density, which is a measure of fineness and maturity of the fiber.1 The micronaire of fiber is quantified by resistance of air flow of a specific sample weight when compressed into a specific volume. Cotton with low mic is not desirable in the textile industry due to dyeability and nepping problems. Conversely, cotton with high mic can be difficult to spin and may only be used for thicker fabrics such as polyester blends and denim. Therefore, cotton with a low mic (3.5 or lower) or a high mic (5.0 or higher) is discounted due to limited range of use. The genetics of a cotton variety can play a large role in the final mic of the crop. Certain varieties may range on the higher end of the mic rating (Figure 1).

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Figure 2

Figure 2. Cotton defoliated and ready for harvest.

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In a study conducted by the University of Arkansas, samples from eight common cotton varieties were tested at three different timings for occurrence of high mic. Study results showed the occurrence of high mic at 50-60% open boll was 42.5% and at 60-70% open boll was 50.11%.

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Environmental and management factors can also increase lint mic. High-mic cotton can be the result of excessive carbohydrates supplied to each boll on a plant. Common causes of high-mic cotton include poor early-season boll set and small boll size due to drought or heat stress.

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Optimum Mic

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Text
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To maintain high quality cotton and avoid discounts due to high mic, producers raising varieties with higher average mic ratings may consider timing their defoliant application according to optimum mic of less than 5.0. One method that can be used to time defoliation according to mic is called the Lewis Method.

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According to the University of Tennessee Extension, this method has become more popular and has shown promise in predicting end-of-season mic rating.3 The Lewis Method uses representative samples of cotton taken from the bottom four first-position bolls and compares the mic to a chart (Table 1) which predicts the whole field mic rating. According to the chart, if the mic is predicted to be 5.0 or higher, a defoliant application is recommended. This method may recommend a defoliant application be made earlier than other methods used, but it may stop mic growth before it reaches discounted levels.

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Table 1. Lewis Method chart for using micronaire samples to predict field micronaire.4
Sample mic30-40% open @ def40-50% open @ def50-60% open @ def60-70% open @ def
2.62.52.72.82.9
2.72.62.82.93.0
2.82.72.83.03.1
2.92.72.93.13.1
3.02.83.03.13.2
3.12.93.03.23.3
3.22.93.13.33.4
3.33.03.23.43.5
3.43.13.33.43.5
3.53.13.33.53.6
3.63.23.43.63.7
3.73.33.53.73.8
3.83.43.53.73.8
3.93.43.63.83.9
4.03.53.73.94.0
4.13.63.84.04.1
4.23.63.84.14.1
4.33.73.94.14.2
4.43.84.04.24.3
4.53.84.14.34.4
4.63.94.14.44.4
4.74.04.24.44.5
4.84.04.34.54.6
4.94.14.34.64.7
5.04.24.44.74.8
5.14.24.54.74.8
5.24.34.64.84.9
5.34.44.64.95.0
5.44.44.75.05.1
5.54.54.85.05.1
5.64.64.85.15.2
5.74.64.95.25.3
Shaded area = desired range, def = defoliation, average maturation weather; maturation weather cool, subtract 0.3 micronaire units; maturation weather hot, add 0.3 micronaire units.
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Increased Income Potential

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The University of Missouri Delta Research Center conducted a study evaluating the four most commonly used methods of timing the application of harvest aid to determine which method provided the highest yield and maximum income per acre.4 Methods evaluated were defoliation at 60% open boll, defoliation at 4 nodes above cracked boll, and the Cotman and Lewis Methods (Table 2). The study, conducted in 1998-2000, evaluated different cotton growing scenarios including irrigated, dryland, and high nitrogen. Results from the study showed while defoliation at 60% open boll generated the highest yield, defoliating according to the Lewis method resulted in the highest income per acre, due to less discount for mic.

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Table 2. Comparison of four common methods used for timing harvest aid applications.5
MethodAverage Yield (all trials, 3 years) (lbs/A)Average Micronaire (all trials, 3 years)Micronaire # of penalty* (over 4.9)
Cotman8204.651
Lewis8364.771
60% Open8774.853
4 NACB8804.914
* number of times lint would receive a penalty in six trials