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Managing cotton at 'cutout'

Last year, an article appeared in Cotton Farming Magazine, written by Jimmy Read that discusses managing "cutout" in cotton. This article gave me several ideas that have been incorporated into this article.

'Cutout' in cotton has been defined as when the nodes above the white bloom reaches N=5. This is also referred to as physiological 'cutout' and is the time of the year when the cotton has a lot of blooms at the top of the plant. This is the stage when the terminal growth ceases due to boll load and not enough nutrients to add bolls of sufficient weight and quality. This of course assumes the normal growth curve for the plant. I well remember one year in the early 1990's that had exceptionally poor yields. That year, we were averaging 5.5 nodes above white bloom early in the season when we would normally expect 8-9 nodes.

In Missouri, we also refer to the last effective bloom date as the date when there will be enough heat units to mature a bloom into a harvestable boll. The average date is August 10. This date is rapidly approaching, but there have been several factors that might change the impact of this date.

The first is the use of boll openers that has changed the defoliation process. We no longer need the required heat units to get the bolls to mature. The chemical can speed up the process making it possible to open bolls much sooner and to get them harvested. However, the defoliation process can cause some of these small bolls to drop off prior to harvest. This is actually good in that it prevents the poor quality fibers from reducing the overall fiber quality.

The second is the Boll Weevil Eradication Program. I can well remember seeing as many as 8-10 weevils per bloom late in the season. While this was after the last effective bloom date, the weevils could still feed on the small bolls. The injury could cause some of these small bolls to abort or in some cases, disease organisms could cause the boll to not open properly or the boll content was damaged too much to harvest. However, since the boll weevil eradication program began in the fall of 2001, we are now able to harvest the "top" crop that is usually lost to boll weevils and other insects. In many years, a ten percent yield increase can be obtained by not having boll weevil damage. This is more than enough to pay for the cost of the program. While we have had better than average climatic factors during the last three years, many believe that Boll Weevil Eradication has had a positive impact on the yield and economics.

Earliness may also impact the cutout period. Several years ago, I went to fields on the last effective bloom date and tagged 100 blooms. I did this in two fields. I went back to those fields about a month later and did not find a boll that had made it. The reason is that both fields had "cutout" early and the fields were under moisture stress. Producers have used growth regulators to not only keep the size under control, but to promote early maturity. Plants that retain squares and bolls early in the season when the plants are more efficient will mature earlier.

Last year is a good example of what happens to a crop that matures later. We were very concerned that the crop might not mature in time and would be harvested under adverse weather conditions. I have seen years where harvest was delayed by frequent rains and I have even seen seeds sprouting in the field. This really has a negative impact on fiber quality and yield. However, we were very fortunate during harvest. The higher temperatures facilitated good defoliation and it helped to mature the top bolls. Last year, we learned the value of boll openers.

Since we have seen a number of fields that have had nitrogen or sulphur deficiencies, I had a question about how long to foliar feed the cotton. I talked with David Dunn, Delta Regional Soils Lab Supervisor. He indicated that their research indicated that once the petiole levels dropped into the deficiency range, then it was impossible to bring that number up to the sufficiency level. However, he indicated that they got yield increases by applying the foliar nutrients till the last effective bloom date.

We like to see the cotton run out of nutrients and water prior to harvest. It can be difficult to defoliate cotton with green vegetation. It is best to terminate irrigation by the middle of August, but we have no control over the natural rainfall.

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Dr. Michael R. Milam is an agronomy specialist and county program director with University of Missouri Extension in Dunklin County.