An understanding of fiber quality parameters — such as length, uniformity, micronaire and strength — will go a long way in helping cotton producers address any quality issues that might arise, says Phil Jost, University of Georgia Extension agronomist.
“When we talk about fiber quality, we're basically talking about cotton classification — the set of standards that has been developed by USDA to evaluate physical measurements of raw cotton that ultimately will affect the spinning process and the finished product,” said Jost, speaking at the recent Georgia Cotton Production Workshop in Tifton.
Many things are measured when cotton is classed, including length, uniformity, micronaire, strength, color, preparation, leaf and extraneous matter, he says. But as far as production quality goes, length, uniformity, micronaire, strength and color probably are most important.
“There was a mill bias against Georgia cotton, and when this all started, the comment was that it ‘ran rough.’ That doesn't tell us a lot. But it probably will be the uniformity and short fiber issue, along with staple,” says Jost.
“Those probably will be the things that matter the most. But to address those issues, we have to know what the quality parameters are, what influences them, and what we know and do not know about them,” he adds.
Turning to fiber length, Jost says many different things are measured in terms of the length of fiber, and the first one is staple. “That is the upper one-half mean length. So it's the average length of the longest half of the fiber in a sample. If we take 10 fibers, line them up from shortest to tallest, and measure the average length of the longest five, that's the upper half mean length or staple, and that's what is reported on your classing document,” he says.
The mean length isn't reported on the classing document, but it's an important factor, he says.
“We also can talk about short fibers, those having a length of less than one half inch. It's a very important parameter. Although it's not on our classing document, we need to be aware of it. The mills are concerned with fiber length overall. They want the longest fibers they can get so the mills can run at their optimum.”
The elongation of a fiber, says Jost, is driven by the internal water pressure of the elongating cells. This occurs roughly three weeks after pollination. There are several things that can influence fiber length, he says.
“One is variety. There are huge differences in fiber length in terms of variety, temperature, fertility, or water stress. Temperatures that are too hot or too cold, inadequate fertility and water stress all reduce fiber length. And variety sets the bar. Once you select the variety, you have set the genetic potential for the fiber length that you can produce in that field. After that, temperature, fertility and water are going to reduce the length from that point.”
Fiber length in Georgia averaged about 35 or 35.5 from 1994 through 1996, and then began to fall in the late 1990s. We had a significant drop into the late 1990s, and we may be making a climb back out in the last couple of years. During this time, growers began planting mostly transgenic varieties, and they experienced a severe, prolonged drought.
“I think it's a function of both,” says Jost. “From 2000 to 2004, we're starting to make a climb out of it. Environment probably has a lot to do with it. But I think variety improvement is on the way in terms of fiber length.”
The big issue in Georgia now is uniformity, says Jost. Uniformity is the ratio between the upper one-half mean length and the overall mean length of the fiber in a sample. It also is an indication of short fiber content. Short staple is cotton that has an upper half mean length of less than 34. If you get a classing document back showing a staple of 33, it indicates short staple.
“Short fiber is individual fibers of less than one half inch. This would appear to be the issue with Georgia cotton. It severely detracts from mill efficiency. They have a difficult time running cotton when it has more than a set amount of short fibers in a sample.”
Short fibers, short staple and uniformity all are related, says Jost. “We can take 10 fibers and line them up from shortest to longest. If I take the mean length, divided by the upper half mean length, that gives me the uniformity, and that number generally ranges from 79 to 83 or 84. What's more important than that number is what it implies about short fibers. A low uniformity number implies that more short fibers are in that sample.”
The statement has been made, says Jost, that improved staple can overcome some uniformity issues. “That's why we don't see an issue with Arizona cotton. If you look at data comparing Georgia cotton to Arizona cotton, our uniformity basically is the same, but their staple is much greater than ours, and therefore we don't hear a lot about the short fiber content issue with Arizona cotton. If you increase staple, you can overcome some of the uniformity issues.”
Variety selection, harvest timing, pest management all are potential causes of short fiber content, he adds‥
Uniformity in the Southeast, up until about 2002, was about 81 but dropped significantly in 2003 and 2004, says Jost.
Looking at micronaire, from 17 to roughly 53 days after flowering, fiber will thicken, he says. Cellulose will start to be deposited inside the fiber, developing its thickness. This fiber thickness will determine fiber or micronaire, says Jost.
Air is blown through a 10-gram fiber sample to determine micronaire, he explains. The more air that comes out the back end, the higher the micronaire — the less air, the lower the micronaire. “High-mike cotton, which is relatively thicker fiber, will weigh more. Thus, it will take fewer of those individual fibers to make up that 10-gram sample. You'll have a lot less surface area in low-mike cotton and very thin cell walls. It'll take more of those fibers to make up a 10-gram sample. That's the relationship between fiber thickness and micronaire.”
Mills are concerned about micronaire because it affects dye uptake, says Jost. A thicker fiber will take up dye differently from a thinner, more immature fiber.
“Unlike other parameters that we talk about in fiber quality, micronaire isn't one of those things you necessarily want to increase or decrease — you want to stay in a certain range, and 3.5 to 5 is where you want to be. Mills really like 3.7 to 4.2.”
The breaking strength of the cotton fiber is considered to be the most important factor in determining strength, says Jost. “Fiber strength varies along the length of the fiber. Strength is not the same at different positions along the fiber.”
Research has shown that as much as 60 to 70 percent of the fiber strength has been developed well before fiber thickening has occurred, says Jost.
“So, while there's somewhat of a relationship between micronaire and strength, it's not as hard and fast as you might think.”
Fiber strength trends in the Southeast have been influenced by the environment, he says. “We saw much higher fiber strength this year than we've seen in the past several years, which makes no sense, considering the number of washer and dryer cycles the cotton went through this past year.”
Fiber color, says Jost, is most directly linked to the environment. “Weathering leads to fiber darkening and loss of brightness. Color also is correlated with overall fiber quality. The whiter and brighter the fiber, the higher the quality probably will be. Color trends in the Southeast are very much influenced by the environment.”
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