The 2025 NC Cotton Field Day was another huge success thanks to the support from our Industry Partners, as well as NC Cotton Producers Association and Cotton Incorporated who support our research through grower check-off dollars. Without that support, we would have essentially no way to conduct applied research or extension activities to address critical challenges facing growers. This event was well attended once again, affording us the opportunity to discuss and showcase our research to growers.

How late in the season can we develop recent blooms into harvestable bolls?

One subject that was discussed was Last Effective Bloom Dates in NC cotton, which has also been discussed in several meetings and other field days recently. The last effective bloom date refers to how late in the season do we have a reasonable chance of making a bloom into a harvestable boll. In most cases, this would only apply to situations when a grower is enticed to “chase” the top crop as late as possible. Such situations include any situation where the bottom crop is less than ideal for whatever reason. This could be due to losses from insects earlier in the season, boll rot in August or hardlocking in August into September. It is important to note that, along with these situations, current conditions should be favorable for the development of a strong top crop. Late season rains in late August or September, especially on later planted cotton that is still actively blooming and hasn’t reached cutout, are examples of situations that are likely to result in a strong top crop. A crop that has already reached a true cutout and has a decent boll load may experience a resurgence in blooming if late season rains occur, however in many cases, blooms at that time are likely regrowth blooms that are likely to abort. Likewise, if we have a strong bottom crop and the boll load is still developing but a true cutout has occurred, it is difficult to justify waiting on just a few bolls on top to develop if a resurgence in blooming occurs. Most of the crop around the state experienced noticeably dry weather, albeit cooler, during late August and early September. For most of the crop this year, growers are not likely concerned with last effective bloom dates, and are therefore not enticed to chase the top crop, as most of this crop has reached a hard cutout and ceased blooming altogether. However, there is quite a bit of late May planted cotton that may still be actively blooming, especially if it irrigated. Likewise some regions experienced rains in late August, unlike the rest of the state, and/or noticed some severe boll rot in the bottom crop. The top crop may still have strong potential in these areas, which may entice growers to chase it. Therefore, there are likely to be a few cases where last effective bloom dates may be of interest to our growers this year. Likewise, some of the principles we discovered through this research apply to all cotton, which will be discussed later

Generally speaking, we use to say that our last effective bloom date was around August 25th in NC, which could be extended to Labor Day in some years. Our recent research on this subject was rather eye-opening, and clearly suggested that late season blooms can develop into harvestable bolls at a much later date than we previously ever thought, however this is dependent only very timely management.

Historically, when growers “chase” a top crop, they would push it all the way until the end of the season (first frost) in hopes of giving every available heat unit to developing bolls, so that the chances of maturing and opening those bolls would be greater. The graph below illustrates our findings when this practice was conducted. The data below is from 2024, which was an abormally cool Fall in NC, however data from a warmer year wasn’t much different. In this situation, where we pushed the crop all the way to the bitter end then defoliated just before our first meaningful frost (usually around November 1st), results were mixed. In this case, we had to count both opened bolls as well as bolls that easily cracked when little pressure was applied. As seen on the graph below, in a couple of places in NC, we had a reasonable chance (greater than 50%) of developing harvestable bolls if it bloomed by September 5th or 6th (to include both of these locations). In a couple of other places in the state, we never reached 50%.

So what happens if we defoliate earlier than that? Logic would tell you that you would likely open even fewer bolls that bloomed late in the season. Our research consistently found the opposite, which was eye-opening and very interesting. The graph below illustrates our findings when we defoliated on October 15th. October 15th is generally the point at which we observe a noticeable decline meaningful heat unit accumulation. In other words, we dont gain much in terms of maturity beyond mid October, and it pays less and less to wait on bolls to mature at that point. Likewise, we often notice very little progress in boll maturity after mid October, and any gains in maturity are very slow at that point. However, we are usally still a couple of weeks away from our first meaningful frost, and those last remaining heat units may be of value in a different way. Results were very consistent across 2 years, one of which experienced a warm Fall and the other experienced an abnormally cool Fall (only results from the cool Fall are shown below). In this situation, we did NOT push the crop to the bitter end, therefore we robbed these upper bolls of a few last remaining heat units. Therefore, in this situation, although we are stealing a few heat units away from developing bolls, we are also giving those heat units, and the extra time, to our boll openers. As seen in the graph below, by defoliating in mid October, we allow some heat and more time for our boll openers to work before a meaningful frost. In this case, we could very consistently develop opened harvestable bolls if they bloomed as late as September 15th across the entire state, which is much later than we ever previously thought.

So how does this apply to my crop this year?

This would be applicable in a couple of ways. First, those few areas mentioned earlier, where cotton is late planted and irrigated or in an area that happened to receive rains recently, there may be potential for a strong top crop. Likewise, a few areas experienced excessively wet conditions in early August and lost some of the bottom crop to boll rot while the top crop continued to grow. This would certainly be applicable to those areas.

However, the same principles could be applied to all other areas but for different reasons. As mentioned earlier, noticeably dry weather has prevailed during late August and early September in areas where most cotton is grown. A sharp cutout has long been reached, and the bottom crop is relatively strong. The reason we think these research findings are also applicable to these situations, is because we often observe cotton that is sufficiently mature for defoliation in early to mid September, but the grower may not be prepared for harvest yet. In many cases in the past, and likely the case this year for early planted cotton, cotton may be ready for defoliation but the growers’ attention is focused on some late corn, peanuts, etc or they simply havent prepared their picker yet, therefore they wait until late September or early October to defoliate. These are the cases where cotton couldv’e been defoliated and harvested much earlier, which allows for yield to be preserved if and when Tropical weather comes. In years past, cotton such as this would often be lost to Tropical weather, but it couldve been avoided, at least on the earlier maturing acres.

Therefore we recommend defoliating as soon as the crop is ready and mature enough to be defoliated. It is advised to cut open some of the upper bolls to observe maturity of those bolls when deciding when to defoliate. We also advise defoliating only the number of acres that you know you can harvest 2 weeks later. Using appropriate rates of boll opener can aid in this. We realize that weather, breakdowns, etc. interfere with harvest operations, and therefore not all acres can be harvested exactly 2 weeks after defoliation. However, it does us no good to intentionally defoliate alot of acres that we know cannot be harvested for at least 3 to 4 weeks, especially early in the Fall. Fully open and exposed cotton is very vunerable to Tropical weather, whereas closed bolls are protected bolls. If some bolls remained closed when Tropical weather arrives, they have a good chance of being harvestable later, but that is not the case if those bolls are opened and waiting on the picker. Defoliating as soon as the crop is ready, then spraying a few acres every sutiable day and staying on a defoliation schedule such as this allows for timely harvest and prevents weathering losses in the early crop, but also helps to leave some acres protected in the meantime. This sequence can often follow the same order as planting date, but summer weather can also dictate the order of defoliation as well.

With all of that said, we also recommend defoliating all remaining acres by October 15th or as close to that date as possible. This would most likely be on the latest planted or otherwise latest maturing cotton you have. In some cases such as June replanted cotton when rains/irrigation allowed for continued late season growth, cotton may not be sufficiently mature by that time. Likewise, if your chasing the top crop, it may be full of immature bolls and only 20-30% open at that time. Still, we recommend defoliating your last acres in mid October, because our research has consistely shown that the chances of opening and harvesting those later set bolls, are much greater if we do this.

But you just said dont defoliate more than you can harvest in 2 weeks time…

Yes we did. But if you defoliate the early crop as soon as its ready, and follow a daily/weekly sequence for defoliation as your fields become ready, harvest can commence earlier (preserving yield) while some fields with unopened bolls can remain somewhat protected from weathering. Most importantly, if you start on time, you shouldnt have many acres left to be defoliated by mid October. Ideally, this would be the case, but even if it isnt and you have quite a few acres left to defoliate and harvest, the odds of experiencing Tropical weather after mid October are less and diminishing (although it could still happen…we are never immune from storms during the Fall). Along those same lines, our research has also been clear in that pushing a crop to the very end and defoliating just before a frost doesnt consistently result in adequate boll opening, whereas earlier defoliation in mid October does.

Sidenote: Has this recent cool weather hurt our yield potential?

This has been a common question lately. The logic behind this question is sound. We often think that we need heat to make substantial progress in filling out upper bolls, which will therefore be reflected in yield. That is true to a degree, but only when we have moisture along with ample heat. Likewise, too much heat can be punitive, especially when conditions are dry, and they certainly have been for upwards of 3 weeks in some areas. Temperatures that reach the mid 90’s can stress developing cotton whereas the high 80’s are favorable for development. But it has been noticeably cooler than normal since late August and intermittently thus far in September.

Yes we have been cooler lately, but not excessively cool. We’ve had several nights down into the upper 50’s which is less than ideal (60’s), and many daily highs are hovering around 80 degrees. That is cooler than normal, but we are still experiencing positive heat unit accumulation so it’s certainly not the worst thing right now. We would argue that this cooler spell has likely helped us to some degree. If it had been noticeably warmer during this same timeframe, more of the top crop would’ve likely shed (due to dry weather) to a greater degree than it has. Had we gotten any rain during this time, it wouldve lasted longer with cooler weather, so we cant really say that cooler temperatures have hurt us. If this trend continues, we may see some lower micronaire, and given that alot of this earlier planted crop is very mature, the recent dry weather could’ve resulted in some high micronaire, also due to dry weather late in the season. Time will tell on that, but we arent overly worried at the moment.