The are two big ‘Q’s of cotton production – the quantity and quality of the crop that contribute to the gross return.

There are many factors over the duration of a season that contribute to each fo these ‘Q’s – requiring an agronomic juggling act of management, timing and luck.

These past season has seen the scourge of low micronaire – predominantly in the more southern growing regions of Australia. Growers across these valleys saw significant discounts of between $50 and $100 per bale due to low micronaire.

Micronaire (or ‘mix’ as it is commonly referred to by industry) is a measurement adopted by the cotton processing sector to define cotton fibre linear density or ‘fineness’. Based upon the air resistance presented by the fibre being tested when subjected to set pressures, it is a function of both the linear density of the fibre, and its maturity. (Long et al, 2013). While it is a standard measurement for the industry, sometimes confusingly, it no longer appears with units. The cotton trade however, presents these gradings in the following scales.

>= 5.3	G7
5.0 – 5.2	G6
3.5 – 4.9	G5
3.3 – 3.4	G4
3.0 – 3.2	G3
2.7 – 2.9	G2
2.5 – 2.6	G1
<= 2.4	G0

The goal for a grower and their consultant is to deliver bales between the premium range of 3.8 to 4.5, but ideally not outside 3.5 to 4.9.

So, unlike a lot of other fibre parameters, micronaire has a ‘Goldilocks zone’ where you neither want to be too high or too low. A high micronaire fibre (4.5 and above) will produce course yarns with fewer fibres in its cross section. Lacking in tensile strength, the resultant yarn can be weak. Due to its coarseness, this cotton is used to produce our denims and course blends (for added strength).

Low micronaire cotton is prone to knots and makes the ‘process’ slow (and frustrating). The finished product is often not perfect. While the resultant yarn is indeed finer, the knots (knows as neps) formed by tangling and knotting, do not allow for uniform dye uptake.  They result in the white ‘balls’ we see on cheaper textiles that dominate chain stores. Anyone who has ever tried to brush a toddler’s hair in the morning will understand the problems that come with a fine fibre.

Micronaire is not just a southern issue – in previous seasons growers in northern and western regions have suffered equally disappointing discounts for high micronaire. Micronaire is a big issue for the Australian industry. By understanding how day degree accumulation affects cotton micronaire, growers and consultants can best manage their crop to the seasonal conditions experienced each year.

A major factor in determining micronaire is temperature during the mid and latter stages of boll fill. To this extent, some will argue that control of micronaire is beyond the scope of the grower and the consultant. The fact that not all crops in the southern regions were impacted by the problem last year would suggest otherwise. Good agronomic advice may not remove the issue, however it can reduce how much of the crop is affected and to what extent. So how can we as advisors help influence this final figure?

As micronaire is largely a function of boll maturity, this our main control point. Managing the number of immature bolls at crop cutout, is the key factor. Our southern growing valleys are limited by a major environmental factor when it comes to this issue – a shorter growing season with less hot days to finish off the crop. Thus, was the situation last season when we saw an extended period of mild weather and low degree days, when the crop was requiring heat to continue maturing.  If this was the only factor that caused the problem, we would have seen a complete southern downgrade. It is important to ask then why was that not the case?

Obviously, some did things differently and avoided the low mic discounts at the end of the season. At the pre-planting, this may have been changing to a variety like 714B3F when planting in the back half of the planting window.

Post planting however, there is also much the advisor can do, particularly in relation to decision making around timing of crop cut-out.

This lays the foundation for the harvesting of all the fruit which has set on the plant. Due consideration must be given regarding the last effective square on the plant. This is the time in the crop when management decisions must be made, which in turn relates to the last effective flower and harvestable boll. A crop left to grow out too long may feature a high portion of fruit that isn’t physiologically mature at defoliation time. CottASSIST data for Carrathool this year showed the average day degrees per day for May was 2.7, April 5.2 and March 7.2; late maturing bolls were visually observed in the crop to stall in their development. To put it into perspective a flower requires 750 day degrees to become an open mature boll.

The decay in day degrees at the back end of the growing season is marked. Figure 1 and figure 2 are from a crop of 746B3F and 714B3F this year in the Carrathool region. Both figures show bolls that were tagged with the date in February when they became a flower, while the photos were taken when assessing the crops for defoliation at the very end of March. Both figures demonstrate a big difference in boll size and therefore maturity, for only two or three days difference in boll age. Timing of crop cutout is critical, and every day missed can add multiple days onto maturity of the crop at defoliation time.

Figure 3 shows a 748B3F boll, again in the Carrathool region cut open on the 16 April. This boll was a flower on the 13 February and is only just physiologically mature in the middle of April, a time most would agree to be the back end for defoliation to begin. The boll has no jelly or unformed cotyledons in the seed, however the seed coating is only just starting to turn dark, a sure sign of maturity. This particular paddock of 748B3F grew to be 110cm tall and after slow early season growth did not want to stop growing at cutout. 1.2L of pix was applied to this part of the field as part of a variable rate application on the 17 January. The crop then received 2.5L of pix on the 24 January and a further 2L of pix on the 4 February. Aggressive pix management was required to keep crop maturity on track and achieve a final field yield of 11.7 bales per hectare. Out of the 651 bales classed only two bales were G4 micronaire (3.3-3.4). The indeterminate nature of varieties like 748B3F take timely management to induce cutout. Particularly in seasons like the one past, where cool weather delayed squaring, and the assimilate demand of the crop did not act to constrain late January growth.

Overall good management includes timely irrigation scheduling and nutrition supply (including carbohydrate availability) and control of insects when economically viable. For southern growers this will lead to an adequate production higher mic cotton in the middle and lower parts of the plant, that can then blend with the lower micronaire cotton on top of the plant. It is basic maths as the grower attempts to dilute late season bolls that have the potential to bring them a discount. Every season the numbers change.

Growers and consultants continue to strive for the highest yields and quality. Some years in the southern growing region there will be no opportunity to do this late season. Yield will be accumulated within the plant in line with the accumulation of adequate day degrees. Bolls that are grown later in the season will not be as heavy as those created early.

Like all things farming, the factors which will dominate crop production are going to be regionally and even farm specific. It is worthwhile as an industry to share our farming successes, but also reflect on what can be done differently in the future. This is not just a southern problem – this is about our industry’s reputation. Just as importantly, it has the ability to put a few more dollars back into your clients’ pockets.

References

Long, R.L., Bange, M.P, Delhom, C.D., Church, J.S., and Constable, G.A. (2013, April 9). An assessment of alternative cotton fibre quality attributes and their relationship with yarn strength. Retrieved from CSIRO Publishing: https://www.publish.csiro.au/cp/Fulltext/cp12382