Cotton is grown on a wide variety of soil types and environments, even within a farming operation. How a farmer manages these factors is directly related to how compact or extended bolls are set, and subsequently the timing of defoliation in the fall.
Bolls are set over several nodes in an extended fruiting habit in which conventional rates of nitrogen are used and either irrigation or adequate rainfall keeps plants growing over a longer period of time. Or, growers may be in a drought situation, use less irrigation water, use more plant growth regulators, and use less nitrogen, which would result in a more compact fruiting crop.
In both extremes the plant may have the same number of bolls, but the more extended fruiting, in which bolls are spread out over more node zones, compared to a compact crop. In tests by North Carolina State University researchers, in 2004 there was no yield loss from defoliating the more compact crop earlier than fields with more extended fruiting plants.
The North Carolina researchers defoliated cotton at 40, 60 and 80 days, using a modified early bloom strategy to get a more compact crop and left check plots to achieve more extended fruiting habits. In the more extended fruiting habit, the cotton crop was set over 40 days, compared to 18-20 days for more compact cotton.
In 2005, researchers recorded no yield response to growth regulators. Still, there was no yield penalty from defoliating compact crops earlier. The 2006 crop followed the same trends, indicating growers would benefit from defoliating more compactly fruited cotton at 40-60 percent open bolls and would benefit from waiting until 80 percent of bolls are open to defoliate cotton with a more extended fruiting habit.
North Carolina State researcher Guy Collins says, “when we defoliate a compacted crop earlier, harvest should be delayed 2-4 weeks after defoliation to achieve maximum yield. For later defoliation, harvest should be completed shortly after defoliation.
A compact fruited crop is set over 8-10 nodes. “The best indicator we have found is to slice open some of the upper bolls to be harvested, and when you can see a distinct black seed coat, these bolls are likely mature enough to avoid yield loss,” Collins says.
Growers need to understand that there are variations in the percent open boll method and the node to bud crack method. We recommend that growers use all these methods, especially cutting open bolls to determine when to defoliate and to achieve maximum yields, Collins says.
Early defoliation can affect fiber quality. Grades were variable, the major factor was micronaire, which was decreased as harvest was delayed. Staple length followed a definite trend and was not statistically significant across the various tests, according to Collins.
The cotton plant flowers and sets bolls for 4 to 8 weeks. In general, bolls set during the first 5 to 6 weeks of the fruiting cycle are the largest and most mature. Under optimal growth conditions, 95 percent of the crop can be set in 4 weeks.
Typically, the smaller, less mature bolls set later in the season make up only a small portion of the total crop. However, under conditions associated with drought or severe heat these later bolls can make up a significant portion of the crop when earlier bolls are aborted.
The development of the boll can be divided into three phases. The first stage is the fiber elongation phase, which lasts about 3 weeks. Each fiber is a single cell originating from the outer surface of the developing seed.
The second phase is fiber filling, which lasts about 3 weeks. Fiber filling adds weight and thickness to the developing fiber. The thickness of the fiber is ultimately measured and reported as micronaire.
If this process is interrupted by the application of harvest-aids or unfavorable environmental conditions, low micronaire results and the fibers weigh less than if they were allowed to develop fully.
After the filling phase, the boll maturation phase begins. During this phase, boll dehiscence occurs. The capsule walls of the boll dry causing the cells adjacent to the dorsal suture to shrink unevenly. This shrinking then causes the suture between the carpel walls to split, the boll opens and fibers dry.
The three phases of boll development are directly affected by how tightly compacted fruit is set on the plant. When cotton is defoliated early, the plant may be in the second phase, causing both quality and yield reductions.
Row spacing can be another critical factor in fruiting habit and defoliation. Some growers have switched from standard 38-inch row spacings to narrow-row cotton, most recently 15-inch spacings.
How plant growth regulators are used on these narrow-row spacings will directly affect how they are defoliated and how they are harvested.
Ideally growers who use 15-inch row spacings reduce the number of trips across the field, set fruit on lower plant positions and optimize yield potential.
In these narrow-row spacings, timing and application rate of plant growth regulators is a critical factor in maximizing yield and minimizing costs.
Researchers at the Upper Coastal Plain Research Station near Rocky Mount, N.C., tested 2-ounce and 4-ounce rates per acre of Pentia growth regulator at the four leaf, six leaf and eight leaf stage of cotton growth. A 2-ounce rate was applied at the four leaf stage, followed by a 10-ounce rate at full bloom. They also used a 4-ounce Pentia rate at the four leaf stage and 8-ounces at early bloom.
North Carolina State researcher Gary Hamm, says mid-season data indicates later PGR applications gave better height control, but did not get expected stunting from applying plant growth regulators at early stages of cotton growth.
“There is a chance growers can use the same plant growth regulator application timing in 15-inch row spacings as are used in conventional row spacings. We could come in with earlier PGR applications and get some height control and not get yield reductions from it.” Hamms says.
He stresses that yield data at the end of the 2006 season will provide more detailed information on the use of PGRs on narrow-row cotton.
A 3-year study by cotton researchers at the University of Georgia indicated that maximum lint quality was attained when defoliants were applied between 40 and 60 percent open boll. However, maximum lint yields and profitability were not achieved in the Georgia tests until the crop reached 75-90 percent open bolls.
Under the current cotton marketing system, the penalty for reduced yields is much greater than the penalty for reduced quality. Thus, managing harvest-aid application timing to maximize fiber quality does not appear to be profitable if yield is sacrificed in the process.
Regardless of the row spacing or fruiting pattern, Mother Nature and today's economy require 10 mature bolls per foot of row to produce a bale of cotton under good growing conditions. More bolls will be needed if they are higher on the plant and less if they are lower on the plant.
Making the right management decision at harvest time can make the difference in profit and loss in a cotton crop.