Defoliation Timing and Chasing This Top Crop (Collins & Edmisten)

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The 2020 season doesn’t seem to want to let up on us. Our last article on August 28 discussed decisions on whether or not to chase the bottom versus top crop. This year, most growers are focused on making the top crop that developed since early August when rains returned to drought-stricken areas. Nearly everyone was hoping for Fall weather that we experienced in 2019, when drier and noticeably warmer temperatures prevailed through mid-October, in order to capture the most yield from this top crop. So far, that hasn’t happened, nor is it likely to. As suggested in our last article, hopefully, some growers flagged a few upper nodes sometime around our normal Last Effective Bloom Date (LEBD) which could help you know which bolls have a reasonable chance at becoming harvestable bolls. Unfortunately, given the September weather that has already occurred, and the current predicted forecast through October 9, our last effective bloom date may be shifting to a noticeably earlier date than normal. So far, we have been fortunate that no hurricanes have hit us directly, so we should at least be thankful for that. However, the hopes of making this top crop mature are fading.

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. The 50-60 day range is based on normal average heat unit accumulation. 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-60’s 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. For the sake of optimism and argument, lets assume that we can achieve boll maturity with 800 heat units, which would be a best-case scenario for us. The tables below illustrate heat unit accumulation to date, and forecasted until October 9, as well as days elapsed for several last effective bloom dates, for several locations within our cotton-growing regions in North Carolina. When observing these tables, you can see that we are currently behind on days required for boll maturity, but we will be within the normal range by October 9th for a last effective bloom date of either August 15 or 20. However, we are noticeably behind on heat unit accumulation, as we’ve experienced a cooler-than-normal September with several periods of multi-day rainy spells. This weekend and the first few days of next week will bring the last foreseeable chance at any meaningful heat unit accumulation. Beyond October 1, the current forecast does not look promising for any additional nor meaningful heat unit accumulation. Using this information, only our southern-most areas are likely to reach the minimum required DD-60’s accumulation by October 9 for the earliest last effective bloom date of August 15, and for most other regions of the state, there is little to no (even negative in places) heat unit accumulation predicted between October 1 and the 9.

Roanoke Rapids heat unit accumulation chart Greenville heat unit accumulation chart Laurinburg heat unit accumulation chart Albemarle heat unit accumulation chart

So what does this mean? First, there are a couple of disclaimers we must make. This information is based on heat unit accumulation up until this point in time, AND the predicted heat unit accumulation for the next two weeks. The key word here is “predicted.”  If the weather forecast for the next two weeks from today changes for the better over the next few days, this could influence some decisions regarding defoliation timing. Keep in mind that a daily high/low-temperature range of 70/50 provides no DD-60s. For any meaningful progress to occur in boll maturation, the daily high/lows need to be 75/55 or above, but ideally 80/60 or above to observe much progress. Secondly, we can only see the forecast through October 9, so we have no idea what conditions will follow, which could be better, but could also be worse than what is currently predicted for the next two weeks. We have a chance of decent temperatures over the next few days and until October 1, assuming the current forecast is correct. Beyond that date, waiting longer for bolls to mature is likely futile and risky.  Therefore, assuming the forecasts remain stable, delaying defoliation in hopes of maturing more of the top crop is near hopeless beyond October 1. Additionally, we know that these upper, younger bolls are likely going to be more difficult to open than the older, lower bolls. This becomes even more challenging when trying to defoliate and open bolls in cool weather. Most of our boll opening products are hormonal products, and are thereby less active in cooler temperatures than in warm weather. Therefore, we should probably take action at least 2-3 days ahead of October 1, while we still have some heat needed to enhance harvest aid activity and to promote boll opening.

When defoliating, growers should consider using a high rate of ethephon in their tank mixtures. The highest rate allowed is 2.67 pints/A of 6 lb a.i./gal material. Growers may also want to consider using products like Finish or Terminate (ethephon + cyclanilide) which is more active in cooler temperatures and more effective at opening bolls than ethephon alone in cooler weather. Although it is not used as much as it once was, CottonQuik or FirstPick (ethephon + urea sulfate) is also very active in cooler temperatures and more effective at opening bolls in less than ideal conditions. In cases where growers have already defoliated, and most leaves have been removed but some upper bolls are hardened but still will not open, the use of paraquat can sometimes promote boll opening at that point. This should NOT be done in the first defoliation, but rather only when some bolls refuse to open following our best efforts to defoliate and open bolls with ethephon-containing materials and nearly all leaves have been removed. This strategy may or may not work, and will not further enhance boll maturity, but occasionally it can be effective at opening near-mature bolls that otherwise would not open on their own due to cooler weather. If this strategy is used, keep in mind that the application can not be made within three days of harvest. With that said, desiccation is to be expected, therefore waiting longer than 4-5 days after application to harvest will likely increase the chances of desiccation, bark, or poor leaf grades, etc.