"Defoliation is as much an art as it is a science." says farm advisor Ron Vargus, at the Madera County Cooperative Extension Service office. "What we do with defoliants is try to control the physiological processes such as growth, boll opening and leaf drop, and timing and application are critical to making that work."
As a grower you face two critical decisions before defoliating a cotton crop, he says. You must select which harvest aid material to use, and when to make the treatment.
Put simply, defoliation is the process during the latter part of the cotton plant’s growth cycle leading to the ultimate loss of growth and functions in the green synthetic tissues. Ideally you want the leaves to fall off the plant within 14 days of defoliation with minimum desiccation. However, plant growth, available nitrogen, and environmental conditions are all working against you when it comes to defoliation.
"You can’t expect defoliants to work well when you’ve got high nitrogen levels and a crop that is still growing and flowering," Vargus says. "In general, cotton plants that have reached the cutout stage, and have a heavy, uniform boll load are easier to defoliate."
Impacting how the harvest aid product you select is going to work are boll load, available moisture and nitrogen levels. Vargus also recommends tankmixing harvest aid products and adding adjuvants for increased activity.
Growers should remember, he says, that most harvest aid products work best when the temperature is warm, and humidity and light intensity are relatively high. "There are minimum temperature requirements for the optimal performance of most harvest aid materials. Sodium chlorate works well when the temperatures are lower. However, for most other products, daytime temperatures should be above 80 degrees and nighttime temperatures should be above 60 degrees, if possible."
While defoliants may work best under hot, humid and sunny conditions, in California and Arizona that may be easier said than done.
"We have high seasonal temperatures, but we usually have lower humidity levels than they do in the Mid-South and Southeast parts of the Cotton Belt. That really contributes to thickened leaf cuticles in our production system, which can reduce the activity of some of the defoliants," Vargus says.
Cloudy weather, which often is accompanied by cooler temperatures and lower humidity levels, also may decrease product efficacy.
Other crop conditions that affect defoliation include plant density, plant height, plant growth regulators, plant diseases, and of course insect feeding and weed control.
An ideal defoliation plan, Vargus says, requires moderately high temperatures, relatively low nitrogen levels, moderate soil water levels, adequate insect and disease control, and uniform crop development which has matured at least to the cutout stage.
The activity of harvest aid products may vary according to variety.
"We’ve really seen some differences by variety, especially with the verticillium tolerant Acala varieties and Pima varieties. The California uplands, in general, are a little bit easier to defoliate," Vargus says. "In 1992, long before we were able to use some of these other cotton varieties, we found that 21 days after just one application of sodium chlorate on the California upland variety Delta Pine 90, we had about 80 percent defoliation. In comparison, we achieved only 20-25 percent defoliation with the same treatment on a verticillium tolerant variety. Those varieties with verticillium tolerance seem to be a little bit more difficult to defoliate."
The earlier you terminate a cotton crop, the better your chances are for favorable weather and the more potential hours available for harvest. However, those shouldn’t be the only factors in determining when to defoliate.
An effective defoliation treatment is much easier in fields that have reached cutout, and have a heavy, uniform boll load than it is in fields with continued vegetative growth.
When your cotton crop reaches cutout may depend on your cotton variety. For the upland and Acala varieties, Vargus defines cutout at four nodes above white flower. Pima varieties, he says, reach cutout at about two nodes higher at about six nodes above white flower.
"What you have to do look at first position bolls," he says. "Count down to the first position cracked boll and when you reach four nodes above white flower, it’s safe to begin defoliating. Cracked bolls are a somewhat subjective evaluation, though. You have got to spend time going through the fields because you aren’t going to find those on every plant."
To determine if a boll is mature, Vargus recommends trying to slice it with a sharp knife. "A mature boll of upland cotton will be difficult to cut, and the boll will be difficult to dent or depress with your fingers. In addition, an immature boll has a whitish seed coat," he says.