Manage Micronaire With Variety Selection and Defoliation
Micronaire is a measurement of cotton fiber development, which translates into a measurement of fiber fineness. For growers, not hitting the right micronaire measurement can lead to price deductions on their harvested cotton.
To measure micronaire, a known weight of cotton fiber is compressed to a known volume, and air is drawn through it. High micronaire means fibers are more coarse and more air flows through. Low micronaire means fibers are less well developed and less air flows through due to friction.
The micronaire of cotton fiber can affect spinnability at the mill. High-mic cotton is typically coarse, well-developed fiber which does not spin well. It is discounted on the final price of cotton bales as a result. Low-mic cotton is usually less well-developed fiber, which breaks during spinning and is therefore also discounted. Spinners typically prefer a fiber in the mid-range of micronaire measurements for spinning for this reason.
Factors Affecting Micronaire
Variety selection can be a significant influence, particularly in regions with extremes in growing conditions. Micronaire also occurs as a continuum up and down the plants. Lower, more well-developed bolls are typically higher in micronaire. Mid-canopy bolls are about average in micronaire. Uppermost bolls on the plant are typically lower in micronaire.
Harvesting and ginning uniformly blend the cotton from one field together for the crop average micronaire.
Changes in fruiting pattern can also influence micronaire. Cotton bolls in the bottom third of the plant often have reduced micronaire. Bolls in the middle third of the plant tend to have little effect if the top and bottom bolls are also set on the plant. Bolls in the top third of the plant often raise micronaire. Some of the factors can be observed in fruit accumulation patterns and can aid in defoliation decisions.
Defoliation Practices
Micronaire can be influenced by defoliation timing. Typically, high micronaire is associated with high yield, and this should be considered in the defoliation timing decision. Premature termination of crops (early defoliation/frost) can result in lowering micronaire into the discount range. In most cases, micronaire can be lowered by timing defoliation in the 50% to 60% open boll range without greatly, if at all, affecting yield.
Allowing cotton to naturally open bolls to 80% to 90% before applying defoliants is a recipe for high micronaire. Growers should consider this in making defoliation decisions.
We currently have work underway at the Bayer Scott Learning Center (SLC) in Scott, Mississippi, to characterize the effects of defoliation timing on yield and micronaire of various defoliation practices, across several varieties.
ThryvOn™ Technology Observations at SLC
No plots at the Bayer SLC planted to Deltapine® brand Bollgard® 3 ThryvOn™ cotton with XtendFlex® Technology (B3TXF) varieties had to be sprayed for thrips infestation or damage this season. The reductions in thrips feeding also allowed cotton to have a better, healthier start.
“This has at least visually improved the fruiting pattern in the bottom one-third of the plant, and we will be measuring this with plant mapping during the fall,” said Jay Mahaffey, Bayer science fellow and SLC manager. “We observed similar reductions in thrips numbers and damage compared to previous years.”
One complication implied by this improved plant health and growth is the need for growth control earlier in the season. It will happen at similar timing (7 to 9 leaf stage), but that timing will happen somewhat faster than in fields of cotton where thrips are feeding.
“I don’t think we fully understood the negatives associated with even the chronic thrips feeding in non-ThryvOn cotton,” Mahaffey said. “When that damage is removed, the timing of the first plant growth regulator [PGR] becomes even more critical, and the appearance is that it occurs faster. Of course, field history and variety response will also continue to be considerations in the PGR application decisions.”
Lygus Observations
Performance has been consistent with previous growing seasons, but bug pressure this season has been somewhat lighter at the SLC.
“We have observed reductions in lygus reproductive ability in cotton with ThryvOn Technology,” said Mahaffey. “Our cotton systems trials have required two to three applications for adult lygus bugs, which is consistent with past seasons at the SLC. In the last sampling of the plots treated for lygus by threshold, DP 2127 B3XF required treatment due to significant reproduction having occurred. The Deltapine varieties with ThryvOn Technology did not. There are multiple components to this decision, but it appears we have saved one application — similar with past season results.”