Micronaire is measured by placing lint in a chamber, compressing it to a set volume, and subjecting it to a set pressure. The reading, when related to a variety, is an approximate guide to fibre thickness, and has been used as a measure of fibre maturity. Other, more accurate, fibre maturity testing methods and devices are now available, but for now the general guidelines below still apply:
The premium range is 3.8 to 4.5 and the base range is 3.5 to 4.9 (G5) and discounts apply for cotton with a micronaire outside the base range. Discounts for low micronaire can be substantial.
Micronaire results are grouped on the schedule for premiums and discounts. Common causes of low micronaire include:
Cool temperatures during fibre wall development.
Dense plant stands.
Favourable fruit set and high boll retention.
Early cut-out due to frost, hail, disease or early defoliation.
Common causes of high micronaire include:
Ginning has little or no effect on micronaire, buckling and entanglement. This can create neps, which can affect preparation, and subsequently grade. Dryland cotton normally falls into the acceptable micronaire range; but under hot, dry conditions some varieties are prone to produce high micronaire. Late planted crops are susceptible to low micronaire, and heavy discounts sometimes apply.
Management practices that open immature bolls, such as pre-mature defoliation, can contribute to the inclusion of immature fibres and an increase in neps. Experiments conducted at the Australian Cotton Research Institute confirmed that defoliating before 60% bolls open lowers micronaire (reduced fibre maturity) and increases neps.