Hopefully the Final Update: Defoliation Timing and Chasing This Top Crop (Collins & Edmisten)
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Hopefully, this will be the last update on weather and heat unit forecasts for this year. In an effort to keep this article brief, see our previous newsletters on this subject for detailed recommendations on defoliation strategies/chemistries for the 2020 crop. This is an update to the newsletter article we released last Monday, October 5, which now includes heat unit accumulation since that date and a new forecast through October 26. Much needed heat units were experienced in some parts of the state last week, which was noticeably greater than what was previously forecasted. Some progress in boll maturity and boll opening occurred in places last week, albeit small progress. As seen in the tables below, there is a reasonable chance at some heat unit accumulation between now and Thursday, but not enough to drastically move boll maturity very much. Unfortunately, this will be the last opportunity for any heat unit accumulation, but that doesn’t mean we need to wait for further boll maturity. In fact, a noticeable prolonged cooling spell is forecasted beginning on Friday or Saturday. In most parts of the state, daily highs are forecasted to be in the 60s (or low 70s at best) with several nighttime lows in the 40s beyond Friday. Needless to say, if the forecast is accurate, there will be no meaningful heat unit accumulation that could contribute to further boll maturity beginning on Friday or Saturday in most areas.
Hopefully, growers have been defoliating a few acres all along for the past 2-3 weeks, saving the least mature fields to acquire the needed heat units that occurred last week. Most folks probably won’t be able to resume defoliating until Tuesday or Wednesday at the earliest, depending on the amount of rain that occurred over this past weekend. With that said, growers are encouraged to take action on their last remaining acres and defoliate the remainder of their crop this week. Simply put, our boll opening products work better when applied with a few days of decent sunshine and heat. When temperatures (day and night) decline into the 60’s/40’s or less, our boll openers are slower to act and efficacy may be reduced. Beginning on Friday or Saturday, it will likely become much more difficult to open these remaining bolls, nor is there any advantage to waiting any longer for bolls to mature. For folks that have been defoliating all along, only a fraction of their acreage (latest-maturing fields) will need to be defoliated this week, which is the ideal situation. For folks that waited for bolls to mature on the majority of their acreage, they could find themselves in a situation where they can not effectively defoliate the remainder of this crop within the next few days, and harvest progress will be slow. This is due to a combination of lower temperatures and the associated slower boll opening, inability for fields to dry out quickly to allow for harvest, and shortening of daylight hours for effective and timely harvest.
Our hopes of opening bolls are essentially gone at this point for bolls that are high in moisture with seed that are not well-formed. Bolls that are tough to slice, have a well-formed seed with distinct cotyledons inside them, even if they don’t have a dark seed coat yet, may have a chance, but this will likely be the last chance. It’ll be critical to defoliate during somewhat warmer temperatures and sunshine (before Friday) in order to achieve the best activity from our boll openers which is our only hope at opening any bolls now.
As mentioned in earlier newsletters, it is generally accepted that bolls require 50 to 60 days to mature, from the day of blooming to cracked boll. The 10-day difference in this range exists because upper bolls often encounter lower temperatures during the latter part of their development, thereby requiring more time to develop. As seen in the tables below, we’re very close to the 60 days required for bolls that bloomed on or before our earliest Last Effective Bloom Date (August 15) and by the end of this week, bolls that bloomed on or before August 20 will be very close to the 60-day benchmark. However, we remain behind on required heat units for bolls to mature for most parts of the state. Although it’s not an exact science, nor an excellent predictor of when bolls might reach maturity and open, it is generally accepted that bolls require somewhere between 800 to 1200 DD-60s from the day of bloom to cracked boll. The wide range is due to other factors that may affect boll development in addition to heat unit accumulation, such as soil moisture or lack thereof, cloudy, damp conditions, etc. For the sake of optimism and argument, let’s assume that we can achieve boll maturity with 800 heat units. When observing these tables, you can see that only the southern-most regions of the state have reached the bare minimum required heat units for the earliest couple of Last Effective Bloom Dates. Greenville and Clinton are forecasted to reach the 800 DD-60s benchmark by October 26, with the very large majority of those heat units acquired during this week. Points north of Greenville, or in the Piedmont, will not likely reach this point, even for the earliest Last Effective Bloom Dates. Regardless of location, beyond this week, very few or no more heat units are predicted in nearly all regions for quite a while, therefore there is no reason to delay defoliation any longer. We need to defoliate while there are a few days of reasonable temperatures, before any cooling spell sets in for the long haul.