Timing your defoliation treatment can be a tricky balance between crop maturity and your need to get the crop harvested in a timely and efficient manner. “If you do nothing else, make sure at least 60 percent of your harvested bolls are open when applying a defoliant to your cotton crop,” suggests Charles Snipes, Extension cotton specialist at Delta Research and Extension Center in Stoneville, Miss.
Timing an early defoliation application is critical, he says, because the fewer open bolls in a field, the less lint you'll find making its way into that trailer or module builder parked on your turnrow.
Studies at the Stoneville research facility have shown a 14 to 16 percent reduction in yield in two out of three years and an eight to 12 percent decrease in micronaire in all three years when defoliating cotton with only 20 percent open bolls. In comparison, applying a defoliation treatment to a cotton field with 40 percent open bolls reduced yields six to 14 percent in two out of three years, and dropped micronaire eight to 12 percent in each of the three years.
According to Mississippi cotton defoliation guidelines, it's generally safe to defoliate when 50 to 60 percent of the bolls are open and the youngest boll you expect to harvest is mature. “A boll must be 36 to 40 days from, from the day of white bloom, before it is safe to defoliate the plants. Bolls less than 36 days old when a harvest aid is applied generally will be reduced in weight, micronaire and fiber strength,” the guide says.
Another way to determine whether your cotton crop is ready to defoliate is to count the nodes above the uppermost first position cracked boll. Snipes recommends using a sliding scale of four to eight nodes above cracked boll, based on environmental conditions, and your individual production management plan. Keep in mind that as that node above cracked boll number decreases, micronaire increases.
Calculating the node above cracked boll figure for a field is a matter of sampling plants to find the uppermost first position bolls that are cracked or open on each plant. Count the cotton boll as cracked if you can see any white fiber. Then, count up a minimum of four nodes above that point and everything from there down is considered safe to defoliate. If you can't count at least four nodes, remember that it takes an average of an additional three days per node for a boll to reach that maturity stage.
Inspecting the last harvestable boll that will contribute economically to your yield can also help assess crop maturity.
“The first position boll at the fourth of fifth node down from the terminal should be mature. If it is hard to cut cross-section, if fibers string out, or if the seeds have well defined leaf folds and no jelly present it's mature. If any jelly is present in the seed it is an immature boll and it's too early to defoliate,” Snipes says. “Any bolls above what we consider the last harvestable boll will likely either fall off when you defoliate or will not contribute enough economically to wait for it to mature.”
Although all of these tools will help with defoliation timing, growers must also consider their individual harvest schedule.
“You've got to give some thought to things like upcoming weather patterns, how many pickers you have available for harvest, and how much acreage you need to cover,” Snipes says. “Right now, on average, 10 days out of 14 are suitable for fieldwork. Later in the fall, that number declines to three or four days for every 14 day period.”
Which defoliant product you select, Jost says, is based on a variety of factors.
“Each year it is somewhat of a guessing game as to which product to use, and it's not uncommon for one product to work well one year and be a near miss the next year,” he says. “Specific rates of any of the available products should be adjusted according to temperature, humidity, day-length, plant leaf condition and maturity, expected weather, and desired effects such as defoliation, regrowth control, boll opening and weed control.”