Cotton defoliation can be tricky

Cotton acreage may be down a bit in the Southeast, but cotton yields look like a sure bet to increase in 2009.

High yielding cotton from Virginia to the Florida Panhandle may come down to harvest season weather and how efficiently growers can get their crop out of the field.

Record September rains have put much of the big cotton crop at risk in parts of the Southeast, delaying defoliation and picking throughout much of the southern Cotton Belt.

Picking cotton in October in much of the Southeast this year has been at best done under cool, damp conditions — not exactly ideal for most defoliants to work properly. The risk of getting too much ‘green’ in the lint is high under these conditions.

Defoliants work best on mature cotton under warm, humid and sunny conditions. Cool temperatures prior to applica¬tion and for the three to five days afterward can reduce the activity of most defoliants. Getting timing right can make the difference in profit and loss, even with such a heavy set crop as is the case across much of the Southeast this year.

When to defoliate, especially under adverse weather conditions, is tricky at best, even for veteran cotton growers.

Some proven defoliation timing systems include:

• Five nodes above white flower + 850 DD60s

Calculating DD60s after NAWF5 is a good way to gauge crop maturity, but should be used in combina¬tion with other techniques. Generally, bolls are safe for defoliation after they have accumulated 850 or more DD60s, but some studies have shown that 950 DD60s may be safer.

The big knock on NAWF5 is that growers tend to defoliate too early. For the 2009 crop this could be particularly troublesome because of the dramatic swing from hot, dry weather to a prolonged period of cooler than normal temperatures and heavy rainfall.

• Percent Open Bolls

Measuring percent open bolls has been the standard defoliation technique for many years and is still the “old standby.” It is generally safe to defoliate when 60 percent of the bolls are open.

Up until the third week of September most growers in the upper Southeast had near ideal growing conditions for cotton and a fairly uniform crop. Getting percent open bolls correct is always difficult, but for those growers waiting until late September and later, percentage of bolls opened changed almost daily.

• The node above cracked boll (NACB) method

In contrast to the percent open boll method, NACB focuses on the unopened portion of the crop. NACB is determined by locating the uppermost first-position boll that is cracked open with visible lint and counting the number of main-stem nodes to the uppermost harvestable boll.

By focusing on the unopened portion, NACB takes into account potential fruiting gaps. Most recommendations call for defoliation at four NACB. Low plant population and skip-row cotton, however, are often more safely defoliated at three NACB. Lower plant population usually means a later-maturing crop, with a significant portion of yield coming from outer-position bolls and bolls set on vegetative branches.

North Carolina State University Cotton Specialist Keith Edmisten says timing of defoliation is always critical, but can be especially difficult in cotton fields with skippy stands.

Using nodes above cracked bolls is a good system. However, on fields with skips throughout the field a similar system counting vegetative branches can work better.

Speaking at a recent field day at the Tidewater Agricultural Research and Extension Center in Holland, Va., Edmisten showed growers in attendance how to count vegetative branches to determine optimum maturity for uneven cotton fields. He took a first position boll and counted five branches to find the last harvestable boll.

“If that was just one of a few plants in a field, I wouldn’t be worried about it. But, if you have a skippy stand throughout the field and lots of vegetative cotton, you might find as much as 40 percent of the total bolls coming from vegetative bolls,” Edmisten said.

The NACB method is really like a calendar. Every 2-3 days bolls are set, so you would have 12 days or so from the time the first node above the first cracked boll was set until the fourth node above the first cracked boll was set, he explained.

A new technology, at least new to cotton defoliation in the Southeast, is thermal defoliation. While cotton growers commonly use chemicals to defoliate plants, chemicals can be difficult to apply in wet or windy weather, and typically require 10 to 14 days to take effect.

Good farming practices and environmental regulations also restrict the use of chemicals near urban areas and sensitive crops due to the risk of chemical spray drift.

Advances in thermal defoliation with propane are creating a new option for farmers looking to effectively remove leaves for a more timely harvest, while reducing insect populations and fiber degradation.

Thermal defoliation is of particular interest to organic and sustainable growers (who strive to

reduce or eliminate harvest-aid chemicals. but may also offer benefits to conventional growers.

A new propane defoliator developed by researchers in California improves on slow-moving designs that relied on still air for radiant heat transfer. The new defoliator propels a stream of moving air, heated to 380 degrees F through the cotton canopy. The moving air more efficiently transfers heat to kill the leaves while preserving the cotton. The design includes a return air path that boosts air penetration while reducing fuel consumption.

Once cotton is properly defoliated, the next challenge is getting it efficiently out of the field without losing valuable lint.

The new technology would be particularly valuable in parts of eastern North Carolina and southeast Virginia this year where excessive mid-season rain produced a tall crop of cotton.

Subsequent late season rain has left some growers needing to defoliate twice to get the job done.

A typical cotton field in these areas has a lot of bolls open and fluffed out. The best alternative is to apply a light rate of defoliant to remove the top leaves for better canopy penetration, then come back with a full rate.

North Carolina State’s Edmisten says there are no new defoliants on the market that will help growers overcome these adverse conditions. When to spray is a much more critical question than what to spray in most cases, he stresses.

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TAGS: Cotton
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