For a producer, the importance of cotton fiber quality is demonstrated in the bottom line, in the form of deducts whenever a crop doesn't meet recognized standards. But despite the obvious importance of quality, most cotton research to date has been done to improve yields, with limited investigation into improving quality.
“Processors still want the highest quality at the lowest price, so we're trying to address this issue,” says Phil Jost, University of Georgia Extension agronomist.
A basic understanding of how cotton fiber develops can help growers attain the highest quality possible for their growing conditions, he says.
“Fiber development begins in the plant once pollination of the flower has occurred,” says Jost. “And we can talk about this development in three stages — elongation, secondary wall thickening or maturation and then drying.”
Fibers originate, he says, from the outer seed coat of the developing seed. “The growth of these fibers occurs quickly, and it is driven by the internal water pressure in the fiber cells. Basically, we can think of these fiber cells as a hollow tube or straw. As water is pumped into these cells, they elongate,” he says.
This occurs, says Jost, during the first 18 to 21 days after pollination. From zero to 21 days, fiber length increases. “After that point, it levels off. So, we're elongating these fibers in the first three weeks after pollination.”
The length of the fiber is determined by the variety grown, he says. “The variety sets the bar, and it determines the uppermost potential. After that, the environment will do only one thing for us, and that is to reduce the fiber length from its top potential. So we can set the bar by variety selection, and the environment will bring it down from that point.”
Temperature also can affect fiber length, says the agronomist. “We can't do much about it, but if the temperature is too hot or too cool, it'll reduce the fiber length by impacting the growth of the plant.”
Plant nutrition can be another factor in fiber length, explains Jost. “We know potassium is essential in other parts of the plant for maintaining internal water pressure in cells. It stands to reason that potassium also may play a role in fiber development. We don't know enough to say that we need to be fertilizing more with potassium, or that we need to be putting it on at a different time, but we know it does play a role.”
Water stress, he continues, can severely reduce cotton yields, and it also can reduce fiber length. By correlating rainfall with classing data from specific regions across the state, significant relationships have been observed with fiber length, where length decreases with decreasing rainfall, he adds.
Fiber length in Georgia has changed in the past 10 years, from 1993 to 2003, says Jost. “If we look at the mid-1990s, we averaged more than 34 1/2 in staple length in Georgia. From about 1998 until last year, we averaged about 34 and even less in some years. What happened during those 10 years?”
One factor has been a major change in the varieties grown by Georgia cotton producers, he says. “From the late 1990s to 2000, we made a huge shift in Georgia in the percentage of acres that are planted to transgenic varieties. We could make the argument that the change in fiber length is due to variety selection.”
The other factor, says Jost, is that Georgia entered into a long-term drought during the same time period. It is difficult to say that the change is due to variety alone or environment alone, he adds.
In 2003, which was a non-drought year, fiber length still averaged barely more than 34 across the state. “It's probably going to be both environment and genetics.”
The number that indicates fiber length, he says, does not tell the whole story. There is much variation in the plant, all the way down to the seed, he says.
“There are variations down to where fibers are produced on the seed. Fiber length is influenced by the position of that seed in the boll. As we move from the tip of that boll to the basal end of the boll, we also get variation. There's a lot of variability in that fiber we're producing, just on a genetic or anatomical level within the plant.”
The number reported as the fiber length is the upper one-half mean length, explains Jost. “This means that if we have 10 fibers, and we rank them from shortest to longest, we would get an average length of all those fibers. But what's given on our classing report is the upper one-half mean length, which would be the average length of the longest five fibers. That takes out some of the variability.”
The other issue, he says, is uniformity, and that's where the shorter fibers come into play. “We have the upper half mean length of the longest five fibers, and we have the mean length, which would be the average of all 10 fibers. The uniformity ratio would be the mean length divided by the upper one-half mean length. The more short fibers we produce, the more we reduce the uniformity ratio.”
The second stage of fiber development is fiber thickening, says Jost, and this occurs from 17 to 35 days after pollination.
“We elongate these fibers for the first three weeks, and then we start pumping cellulose inside of those fiber cells. Cellulose is deposited at slightly different angles, and that ultimately will play a role in giving strength to that fiber. This is one factor that determines strength.”
This fiber thickening is measured as fineness or micronaire, he says, and there is high micronaire cotton and low micronaire cotton. High micronaire cotton has a thick cell wall containing a good amount of cellulose. Low micronaire cotton will have a thin cell wall with a smaller amount of cellulose in the fiber cell.
Micronaire is determined, says Jost, via an instrument that measures the air permeability of a constant mass of fibers.
Several factors affect micronaire, he says. “Low micronaire would be due to an insufficient carbohydrate supply, and that's caused by several things, including nutrient deficiency, excessive vegetative growth causing shading so that bolls don't fill out as well, or a heavy boll set on the plant which causes each boll to fill out less.”
High micronaire cotton occurs whenever there is an ample or over-abundant supply of carbohydrates. This can be caused by poor boll set with very few bolls on the plant.
“Also, varieties that produce significantly shorter fiber than others may play a role in influencing high micronaire cotton. We've seen this in variety trials. But the main factor that determines micronaire is environment.”
In 2002 — which was a very hot and dry year in Georgia — the state's cotton averaged a 49 in micronaire, with half of the crop being docked for high micronaire.
“This past year, with much cooler and wetter temperatures during the summer, we averaged a much lower micronaire across the state. Very few bales were docked for high micronaire.”
Micronaire also is influenced by boll distribution on the plant, says Jost. Bolls positioned lower in the plant canopy set earlier in the season and have the opportunity to fill out more. Those probably will be higher micronaire bolls. The low micronaire bolls may set later in the season, when the nutrients have been depleted.
“The ideal situation is to harvest everything at one time, blending your high micronaire cotton with your low micronaire cotton, and hopefully reaching the desired range.”
The final stage of fiber development is drying, says Jost. Once the fiber dries, he explains, the boll shrinks in diameter, causing the fibers to twist and crimp. Ultimately, the boll opens.
“At this time, color grade is influenced, and color grade is determined by reflectance and yellowness, according to the HVI color chart. The day a boll opens, it is at its premium color and quality. When the crop sits in the field, the color starts to degrade and the strength also will degrade with weathering and microbial meltdown breakdown.”