It is hard to believe how different this crop looks now compared to the end of July.  Although our earliest planted acres and acres planted on ‘thin’ ground have mostly bloomed out of the top, many of our later planted acres and acres on moderate to strong ground have just passed peak bloom.  In 2022, the last effective bloom on many of our earliest planted, cutout acres flowered weeks ago.  In contrast, the last effective bloom of our later planted acres will likely be determined by the number of heat units remaining in the season.  You can easily see these two contrasting scenarios in the above video; the later planted cotton has blooms throughout the canopy and is riding 4-6 NAWF while the earlier planted cotton cutout weeks ago.  In this blog, I tackle the concept of last effective bloom, our predicted last effective bloom date and how we must use these concepts in 2022 to maximize our return on investment.

There is quite a bit packaged up in the concept of the last effective bloom and even more crammed into the concept of last effective bloom date.  To start this discussion, I think it may be best to break these concepts down in the simplest of terms.  First, effective simply means harvestable.  Any bloom which ultimately develops into a harvestable boll is an effective bloom.  The last effective bloom represents the last flower which develops into a harvestable boll.

We understand that numerous factors can impact the harvestability of a fruiting position- insect damage, lack of water, poor pollination, boll rot, nutrient deficiencies.  However, the last effective bloom along the northern edge of the cotton belt is generally determined by one of two factors.  The first factor is the number heat units available from the time of flowering until temperatures no longer support boll development environment.  Many refer to this as seasonal cutout.  I’ll come back to the concept of seasonal cutout and our prediction of the last effective bloom date below.

The second factor which occasionally determines the last effective bloom is plant status/availability of plant resources.  This restriction is often referred to as physiological cutout and represents a point in which terminal growth slows/halts as the plant shifts focus towards developing and maturing reproductive bodies instead of vegetative growth. After physiological cutout, a plant may continue to flower but will likely not have the resources to mature the flower into a harvestable boll.  Subsequently, flowers which open after physiological cutout are often not effective. 

Along the northern edge of the belt, yields are almost always greatest when we extend the effective flowering window as long as possible and then reach physiological cutout as close as possible to seasonal cutout. In other words, our goal is to reach NAWF 3-5 on the last effective bloom date.  Due to the significant drought stress experienced in Tennessee, many of our acres planted early or on thin ground reached NAWF 3-5 several weeks ago; therefore, the last effective bloom on a chunk of our acres opened several weeks ago.  If we continue to manage fruiting positions which flower after the last effective bloom we will decrease our return on investment.  Recall that economic losses from bollworm, budworm, plant bugs and stink bugs are unlikely after 350-450 heat units accrue after the last effective bloom flowers.

Predicting the date of seasonal cutout

Much of our later planted cotton or cotton planted on moderate to strong ground is still maintaining ~5+NAWF.  In these acres, seasonal cutout will likely determine our last effective bloom.  By definition, the last effective bloom date (or the estimated seasonal cutout date) is the day in which the probability of a new flower developing into a boll and making its way into the basket declines to below 50%.  Since it is unlikely (probability less than 50%) that fruiting positions which develop after this date will contribute to yield, end-of-season insecticide termination and defoliation recommendations for our area are based upon protecting/managing  those positions which have already flowered or will be flowering within the next week (August 13-24th, depending on your location).

I’ve calculated the last effective bloom dates for Dyersburg, Jackson and Memphis from 30 year weather averages (1980-2010) and graphed that data below.  The number of heat units required to mature a flower into a boll has generally been reported to fall between 750-850 DD60s (growing degree days with a base of 60).  Using these numbers and location-specific 30 year weather averages, we can calculate the last day on which a flower will historically have enough heat units to mature should 2022 proceed similar to the 30 year average observed from 1980-2010. For a visual, please see the graph below. Note the reference lines at 750 and 850 DD60s.  The colored lines represent the remaining seasonal heat units, graphed by date, for the corresponding locations.

To understand how this works, focus on the Jackson location (green line).  After Monday of this week (8/15), the green line fell below the 750 DD60s reference line.  Therefore, flowers emerging after 8/15 will likely have less than 750 heat units remaining in the growing season and subsequently have less than a 50% chance of making the basket.  We MIGHT have the warm September we dream about, but historical data suggests it is unlikely.  With that said, keep in mind that we have generally observed wetter springs and warmer falls over the past few years.  If this trend holds, our last effective bloom date may actually open later than predicted by the 30 year average from 1980-2010.

Take Home

The drought stress experienced in Tennessee during 2022 followed by above normal rainfall pushed some farms into physiological cutout but has allowed others to move towards a likely seasonal cutout.  In order to figure out the best time to ‘cut acres loose’, you should estimate the last effective bloom for each of your farms.

• On early planted acres or thin ground, determine when the farm reached physiological cutout (I would consider this to be NAWF=~4). This likely represents the last effective bloom and it is possible the flower opened weeks ago.
• On later planted acres or moderate to strong ground which have not yet reached NAWF=4, the last effective bloom will likely be opening in the coming days, assuming 2022 proceeds similarly to the 30 year average.

After identifying your estimated last effective bloom, protect that fruiting position from bollworm, budworm, plant bugs and stink bugs until 350-450 heat units have accrued post-flowering.