Specialists Speaking - August 2008
Heading Toward Defoliation
Preparing for defoliation is a season-long endeavor. Choices of variety, plant nutrition and irrigation will impact defoliation and harvest. Insect and disease management and the use of growth regulators will keep the crop on track.
The goal is to have the cotton crop shut down at the end of the season by virtue of a heavy boll load. For the best defoliation, we would like to see the nutrient levels depleted and just enough moisture to aid in defoliation.
Producers and consultants often have favorite formulas for defoliation. However, it’s good for on-farm experimentation to see what works best under your own conditions. What has really worked well one year may not work as well under different climatic conditions.
We have a wide range of products that are available in many price ranges. Under warm conditions, all products work very well. But when it cools down, defoliation is slower, and a second application is often necessary for best results. A mixture of boll opener, defoliant and regrowth inhibitor is recommended for best results in cool weather. It is good to read the product labels to see which adjuvants are recommended for the products.
Timing of application can be determined by several methods. In Missouri, producers and consultants often use the Hal Lewis method of defoliation timing. This system relies on micronaire values to determine when the field needs to be defoliated. This has worked well, and many of those who use this method rarely have bales in the penalty range.
Late season management and preparing for defoliation is a critical time to preserve or enhance season-long management of cotton leading to yield. Like most crops, the majority of management decisions are finished after the first two months, and then it is critical to keep pests at bay, as well as provide needed water.
However, as cotton begins to open in late August, irrigation should be curtailed to reduce new growth and prevent boll rot of newly opening bolls. In fields where foliar diseases are prevalent and the cotton crop defoliates early, consider using a foliar fungicide with an insecticide application while leaving a control strip through the field to determine responses to the pesticides.
Fungicides have had no impact on how well the crop defoliates as has been noted from tests over the past seven years. Keeping healthy leaves on plants in fields that normally defoliate early may contribute as much as 150 pounds per acre to the cotton crop.
In Florida, the last effective bloom date is usually at the end of the first week of September, and those blooms may actually contribute very little to yield as compared to the blooms set in the first few weeks of July.
A lot of Georgia cotton will be made in the month of August, but it will take some management to do it. Rain showers have been scattered, so it is important to take advantage where there has been some rain and save on irrigation costs.
Make sure to supplement properly with irrigation where available and where needed. Stinkbugs and corn earworms both need to be scouted in the field. It is important to protect young bolls from stinkbugs for both yield and lint quality. Even Bt cotton needs to be protected from worm damage.
Foliar fertilization with nitrogen and potassium will likely be more popular this year because of producers cutting back on preplant and sidedress fertilizer due to costs. Foliar feeding N and K is a great way to adjust in-season and make up where you might have come up short, and especially to help on dryland fields that have received rain and show good yield potential.
Managing growth regulators is still important, depending on planting date, variety and fertilizer management to this point. Tankmixing combinations of insecticides, foliar fertilizers and growth regulators is also a way to save on application costs.
June-planted cotton after winter wheat looks good under irrigation or where rains have been received. It is important to manage June-planted cotton carefully (for insects, weeds, fertility etc.) because there is not as much time for errors and recovery. Harvest season is not that far away so tips for preparing for harvest will be in the next issue.
insecticides, foliar fertilizers and growth regulators is also a good way to save on application costs. June-planted cotton after winter wheat looks good under irrigation or where rains have been received. It is important to manage June-planted cotton carefully (for insects, weeds, fertility etc.) because there is not as much time for errors and recovery. Harvest season is not that far away, so tips for preparing for harvest will be in the next issue.
The amount of rainfall and heat we have in August usually determines when cotton defoliation will begin in northern Alabama. Typically, we would like to begin defoliation in early to mid-September as the temperatures begin to decline.
Hot dry weather, however, can greatly speed up this process. When defoliation begins in August, as it did last season, this usually results in a below normal cotton crop. Making cotton defoliation decisions involves many factors, including air temperatures, crop size, re-growth potential, amount of bolls to open and the general condition of the crop.
The rates and combinations of defoliants and boll opening products should be adjusted each season based on these factors. To assist cotton farmers, we conduct early season strip defoliation trials across the area so farmers can see which combinations are working or not working this season. More defoliation trials are conducted as the weather or crop conditions change. Let’s hope I am not putting these trials out until September.
Much of our crop is or will be approaching NAWF=5 by the first week in August. As we roll into August, the number of questions concerning the potential of the crop and when to safely stop spending money on it are common.
Tracking NAWF values from first flower to cutout can offer great insight on the condition and potential of the crop. This time of the season we are interested in using this tool to help us in crop termination decisions.
We have a great amount of diversity in this crop in terms of planting dates. It is not uncommon to see a month or more difference in planting dates on a farm this season. However, there comes a time in the season when we can’t count on a white flower to contribute significantly to yield and profit. This point in time is referred to as the latest possible cutout date. This date is based on the likelihood of accumulating 850 heat units through the remainder of the season.
In Arkansas, the latest possible cutout dates using a 50 percent probability of collecting 850 heat units on a 30-plus year data set are: Aug. 9, Keiser; Aug. 14, Marianna; and Aug. 17, Rohwer. If we use the last five years’ weather data, these dates may be extended slightly (three to five days).
Establishing the dates of cutout is important in identifying the last group of bolls that will contribute significantly to yield and profit. It is this group of bolls and their development on which we base our end-of-season decisions.
General termination guidelines include plant bugs, bollworm, and tobacco budworm, cutout plus 350 heat units (HU); stink bugs, cutout plus 450 HU; irrigation, cutout plus 350 to 500 HU; and defoliation, cutout plus 850 HU.
The Arizona cotton crop is progressing well into peak bloom in most areas of the state and in western Arizona is fast approaching cut-out. Decisions regarding crop termination and defoliation will need to be made soon.
Deciding when to apply the final irrigation should be made based upon crop growth and development in an effort to optimize economic return. Once cutout is identified, one should begin to decide which bolls on the plant are the ones that will be taken to maturity and harvest, keeping in mind that approximately 600 heat units (HU 86/55F) are required for a fresh bloom to develop into a mature harvestable boll.
In most Arizona locations, this equates to approximately 21 days in mid- September. Proper boll and fiber development require adequate soil moisture during the entire 600 HU from fresh bloom through mature boll. Depending on soil water-holding capacity and environmental conditions, this might require two additional irrigations to fully mature the intended bolls for harvest.
Once this is complete, then we should begin looking at defoliation timing. Applying a harvest aid at the proper time is critical in achieving adequate defoliation and ultimately high quality lint. This is typically achieved when the crop reaches approximately 60 percent open boll. This stage of development may also be estimated by counting the number of nodes between the top most cracked boll (NACB) and the last fruiting branch with a harvestable boll.
Correlation of NACB to percent open boll data has indicated that when the crop reaches 60 percent open boll there are approximately four NACB. At this point, a decision on harvest preparation material and rate must be made. There are many different types of harvest prep materials available, and one must determine which material works best for your operation.
Cotton fields that are “blooming out the top” with nodes above white flowers (NAWF) of three or less are a common sight in many areas of Louisiana. Whatever your definition of cutout, it is safe to say that many fields have reached that point.
Having cutout cotton in Louisiana on July 21 is not normally what we expect. When it happens this early, you can usually say yields are reduced, and we’ll be picking “bumblebee cotton” with only a few bolls on very short plants.
Certainly that is the case in areas that have received very little rain, particularly on clay soils. However, the yield potential of many of these fields is still decent, with 10-12 bolls per plant.
A lack of moisture has been a factor, but the decline in NAWF and onset of cutout is also partly a function of high fruit retention and a good overall boll load.
There still will be some management challenges with cotton that has cutout so early. Most of these challenges will revolve around residual nitrogen that we can assume is still in the soil.
Even in fields with a good
boll load, there is probably some residual nitrogen left – the
result of drought conditions.