• By Bill Robertson •
Another challenging season is starting to wind down. While our extended planting window resulted in a widely variable crop with regard to stage of development, Mother Nature usually has a way of narrowing the gap as we approach the finish line and this year is no exception.
In Arkansas, our target date for harvest completion is Nov. 1. In central Arkansas, the last date a white flower has the opportunity to accumulate 850 Heat Units 50% of the time is Aug. 15. Back three weeks off this date to get an idea of the last date a new square has a chance to contribute significantly to yield and profit. With this said, we are at the point of the season that we have what we are going to have on the plant right now.
Most have brought vegetative development down as much as possible with mepiquat chloride to help plants concentrate their resources on plant parts that will help make us money. This year, it appears that our root system is the dominant factor in yield potential and ultimately the money making part for our producers.
Compaction, excessive rainfall and poor vigor are among the many factors this year that have negatively impacted our root system. Some producers, consultants, and County Extension Agents are reporting that they are seeing perhaps the best looking cotton they have ever seen while others have fields that will likely not break the 1,000 pound per acre lint mark. The big difference between these situations is the root system.
Maturing plants are showing signs of a good boll load and stress from being near the end of a long hard race. Most plants are doing the best they can. The big problem is the size of their can. Many need a bigger can! This again goes back to the root system. I’m seeing a lot of apparent potash deficiency symptoms such as this image sent to me today.
The upper part of the plant, through our boll load, is writing checks that our root system is having a hard time cashing. This is why we are seeing things like potash deficiency symptoms in fields that tested high for potash or that received recommended applications.
Verticillium wilt is another issue that is becoming more prevalent in areas of the state which historically experiences vert wilt. Wet conditions are resulting in the increased observations. Variety selection coupled with reduced nitrogen rates and irrigation are our primary means of management.
Most producers cut back on plant density and nitrogen rates in 2019. This along with the growth habits of some of our newer varieties have resulted in fields with high yield potential (at or near 4 bales) that are just over waist high and still easy to walk through. I’m also seeing very few parrot-beaked bolls in these good fields.
Most everyone is keeping track of heat unit accumulation beyond cutout. Many of our later maturing fields have yet to reach 350 heat units beyond cutout but are getting close. While our older cotton has open bolls and some producers are starting to roll up the poly-pipe in fields with only one or two irrigations, our younger cotton continues to need irrigation.
Irrigation termination is not as straightforward as insecticide termination. Again, this goes back to rooting depth. Our old standby recommendation is to ensure a good supply of moisture in the profile at first open boll. However, this recommendation can be refined.
Research indicates no additional yield response to an irrigation or good rain beyond 350 to 400 heat units past cutout when good soil moisture and active rooting is occurring all through the top 2 feet of soil at that point.However, we have seen yield responses to irrigation as late as 600 heat units beyond cutout in soils with a restrictive root zone and significant hardpan at 6 inches. Irrigation termination in the range of 600 heat units beyond cutout oftentimes relates to two additional irrigations beyond the 350 irrigation timing in periods with no supplemental rainfall.
Judicious use of inputs and their proper termination help ensure that plants are showing signs of a good boll load and maturity. This level of senescence or maturity helps initiate the natural process of leaf drop and boll opening of cotton to a point that our harvest aids which we use to synchronize this process will work very well.
Dr. Bill Robertson is University of Arkansas cotton agronomist. He may be reached at firstname.lastname@example.org.