It has often been said that cotton defoliation is more art than science. This thought is further nurtured when harvest aids perform differently under seemingly similar conditions. As with any pesticide application, many factors come into play when determining harvest aid performance. The first decision that needs to be made is when to defoliate the crop which is what I will focus on below. There are a number of methods that folks use to determine when to apply harvest aids and most have a good deal of merit.
Percent Open Boll: Percent open boll is probably the most common method used to determine when to make a harvest aid application. Generally speaking, it is safe to make a harvest aid application when cotton is 60% open. Keep in mind that in some situations that cotton may be defoliated at 50% open with no yield loss; however, in other instances it may be more prudent to wait until 70% open. Often times, determining how open a cotton crop is on a percentage basis is done from the window of the truck or at the edge of a field. In many cases, we tend to under estimate how open a crop is. In other words, we often judge a crop to be 60% open that may actually be 70 to 80% open. This is the exact opposite of determining leaf defoliation of soybeans (for example) from insect pests. In those situations, we tend to over estimate the level of defoliation present on a given plant or leaf.
Node Above Cracked Boll (NACB): To determine NACB, find the uppermost first position boll on the plant that is cracked. Count that node as 0 and then count each node up the plant until you reach the uppermost harvestable boll. The uppermost harvestable boll will be the one that is solid to the touch, often times has a mottled color, has fully developed cotyledons when the boll is cut in cross section, and has dark seed coats around these cotyledons. Generally speaking, cotton with 4 NACB is safe to defoliate. However, if time and weather are on your side, and the uppermost boll that you feel is harvestable is not mature, wait until that boll fully matures prior to making a harvest aid application.
Node Above White Flower (NAWF) 5 + 850 DD60′s: This method is used to gauge crop maturity; however, it should be used in combination with other methods. NAWF 5 tends to me somewhat of a moving target as a crop may be at this stage for a given period of time which makes determining when to start counting accumulated DD60′s difficult. In addition, there are some who suggest that waiting for 600 – 700 heat units is adequate to fully mature a boll as opposed to 850. Regardless, as with NACB, determine where your uppermost harvestable boll is and cut it in cross section with a knife. If this boll, has fully developed cotyledons and darkened seed coats, it is ready to defoliate.
Sharp Knife Technique: Find the uppermost boll that has a chance of contributing to yield and cut it in cross section with a sharp knife. If the cotyledons and seed coats are fully developed as described above, the crop is ready to defoliate. A word of caution, many folks have received serious injuries from try to cut bolls in cross section.