The driving force behind the cotton growth and development is temperature. While the temperatures are hotter during this time of the year, they do speed up growth. In the University of Missouri Integrated Pest Management Newsletter, the weather data for the week ending on June 20 shows that the Cardwell weather station averaged 97 degrees for the high temperature and 73 for the low. This will give us 25 heat units for last week. This report also shows that the departure from the long term average is +7 degrees. The rainfall for the month of June is .09 inches which is -2.19 in. from the long term average.
At this stage of the season, it is uncertain of the outcome of this year?s cotton crop. It will depend on the temperatures and the natural rainfall. Irrigation is very important and will boost yields but is no substitute for a good natural rain.
In looking at various weather sources, I was optimistic earlier this week, when I saw that we had 30-40 percent chances of participation on Thursday, Friday and Monday. The probability of rainfall has now dropped to isolated thunderstorms and the National Weather service was mentioning about a tenth of an inch of rain in most areas. While this is not good news, the Weather Channel website is showing high temperatures in the low nineties and lows in the low 70?s and upper 60?s. The break in the temperatures would be welcome.
The bad news is that we still have high temperatures and little rainfall. Non-irrigated cotton does not have the ability to recover at night. If the night temperatures are lower next week, the damage may be limited.
To put this into perspective, we need to look at the pattern in which cotton sets bolls. While the cotton plant will bloom for 10 or more weeks, the first four weeks are the most important. During the first week of bloom, the plant will produce 8.1 percent of its total but will set 94.1 percent of these. This is 16.8 percent of the total bolls set for the season. The second week will show an increase of 23.5 percent of the total blooms, but 77.7 percent will set. This will be 40.8 percent of the total bolls for the season. The third week 29.4 percent of the total blooms are produced and 43.1 percent of these will set. This is 27.2 percent of the total bolls produced during the season. Week four will produce 25.6 percent of the blooms; set 20.7 percent of the bolls which is 10.9 percent of the total. So 95.7 percent of the total bolls are produced during the first four weeks.\
This information shows why this is such a critical time for cotton production. So if we have a prolonged drought, heavy insect pressure, or any other problems during this time, it could delay the crop maturity. If critical boll positions are lost early in the season, the plants are less efficient in setting bolls during the remainder of the season.
The key factors during the fruiting season of cotton are square retention and boll distribution. During this time the square retention should range from 60-80 percent. A minimum of 60 percent of the squares that turn into bolls is the goal. Bolls should be distributed up and down the plant evenly. There should be a minimum of 10 fruiting nodes, but during some seasons, we will have up to 14 productive nodes.
All cotton bolls are not equal. A study conducted by Dr. Johnie Jenkins and co-workers, at the Agriculture Research Service, Mississippi State University, showed that the first position bolls, i.e. those located next to the main stem, produced over 70 percent of the total yield. These bolls were larger because they were closest to the nutrients being translocated within the plant. Second position bolls produced over 20 percent of the total yield with lesser amounts from third, fourth, and vegetative branches.
In southeast Missouri, our last effective bloom date is around the 12th of August. This is the date that a bloom that is set will generally have enough heat units to mature it into a harvestable boll. As I have shown, the boll setting is less efficient later in the season and only a small percentage of bolls will make it.
Cotton producers have been able to harvest the ?top crop? since the Boll Weevil Eradication Program started in the fall of 2001. The ?top crop? accounts for about 10 percent of the total yield and was not harvested during the late season because of boll weevil damage.
Producers will need to monitor their plants during this critical time during the production cycle. If cotton squares are lost during the early season from insects, lack of nutrients, or drought plants tend to have more vegetative branches.
During this hot, dry phase, scouts will need to look for all insects, but spider mites are more common under these conditions. We still have potential for high yields, but there is still a lot that can happen.
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Dr. Michael R. Milam is an agronomy specialist and county program director with University of Missouri Extension in Dunklin County.