Cotton lint yield is probably best understood in terms of the components, which make it up. Lint yield in cotton is determined by two major components, i.e., the number of seeds produced per acre and the weight of the fiber produced on the seed. Cotton fibers are elongated epidermal cells of the outer integument of the seed coat. No seed –no fiber.
A relatively small increase in the weight of fiber per seed may have a highly significant impact on lint yield. For example, in the south central and southeastern U.S. cotton belt, the long-term average number of seeds per acre produced is approximately 7 million. Thus, if the weight of fiber per seed were increased by only 5 milligrams, this could result in a yield increase of a little more than 75 pounds of lint per acre.
Historically, yield increases have been highly correlated with increases in the number of seeds produced per acre, or increases in the reproductive potential of the plant.
For example, between 1945 and 1980, the genetic yield potential of U.S. cotton nearly doubled. The record clearly shows that this increase was achieved via a dramatic increase in the number of seeds produced per acre, whereas the weight of fiber produced per seed remained essentially constant.
During the 1980s, further increases in the seeds per acre component were achieved, however, the weight of fiber per seed component was concomitantly lowered from around 75 to 50 milligrams per seed. Many Mid-South varieties still suffer from this genetic blunder, today.
Component analyses of the USDA 2002 acreage yield estimated mentioned above showed that California produced an average of only 5.5 million seeds per acre to achieve a lint yield of 1,222 pounds per acre. The California crop produced an average of a little more that 100 milligrams of fiber per seed.
Meanwhile, the Mid-South crop required approximately 7 million seeds per acre to generate 699 pounds of lint. Further study revealed that this apparent anomaly resulted from the fact that the Mid-South crop produced a paltry 45 milligrams of fiber per seed, which more than accounts for the difference in the yields of the two regions. Apparently, California cottons produce approximately twice as much lint per seed as Mid-South cottons.
During the past 20 years, U.S. cotton yields have become stagnant and highly unstable. In fact, the year-to-year variance in yield almost quadrupled during this era, as compared to the previous 20 years. This problem has been rampant in the Midsouth while California yields have been relatively stable.
It takes about 2.25 times as much energy to make a pound of cottonseed oil as is required to produce a pound of cellulose. Based on this fact, and the data presented above, a study was initiated to examine the energy requirements of seed versus lint production in California as compared to the Mid-South.
The weight of fiber per seed is influenced by both genetic and environmental factors. Nevertheless, the differences between California Acala and commercially produced Mid-South varieties, with respect to yield components, are predominantly controlled by genetics as compared to environmental factors.
Cotton, like all other living things, is a manifestation of the interactions of its genes with the environment in which those genes exist. The fundamental principle controlling all living things on our planet may be reduced to one governing factor – energy.
The photosynthetic machinery of the cotton plant must trap the energy in sunlight and convert this energy into products of value to industry, namely, cellulose, oil and protein. The energy needed to synthesize cellulose and protein is nearly equivalent.
Cottonseed oil is a different story, and this is the reason that a requirement to produce large numbers of seed to gain an economic yield is a serious problem.
Cottonseed contains approximately 20 percent oil. Cotton fiber is nearly pure cellulose and requires about 2.25 times less energy per pound than oil. Essentially, this means that genetic types of cotton which produce more fibers per seed require fewer seeds and thus, less energy to produce a pound of lint than genetic types which produce fewer fibers per seed.
Analysis of the above-referenced USDA yield data on an "energy content" basis shows that nearly 1,500 more calories per pound of lint were required for Mid-South cotton as compared to California cotton.
This translates to about 750,000 more calories per bale. This suggests that we may have – inadvertently – overwhelmed the photosynthetic capacity of the plant by demanding production of large quantities of seed which have little value compared to lint. This may also explain the tremendous escalation in yield variability.
This is a serious problem, which must be addressed and reconciled. Our very survival may depend upon it! From a genetics and breeding perspective, this could be a relatively straight forward undertaking. The genes for improved weight of lint per seed obviously already exist in the current California-Acala cottons. Therefore, expensive, exotic, transgenic breeding should not be required to accomplish the desired results.
Cotton fibers are very small, which suggests "Little things do mean a lot!" Hal Lewis is a scientist, producer and ginner living in Doddridge, Ark.