Growers face huge range of cotton defoliation challenges

When do you know you are in for a troubling San Joaquin Valley cotton defoliation season?

  • It's the middle of September and people are wearing jackets to a cotton field day.

  • There are cotton plants with a bottom crop, no middle crop and green bolls on top that may not open until next spring.

  • Plants are lush and green in the middle of September because they have reduced fruit load from either lygus or heat damage, and it is no warmer than 80 degrees during the day you start considering defoliation.

  • A University of California farm advisor says he plans to defoliate his test plots on the same day a Pest Control Adviser (PCA) asks how to defoliate a field that is getting its last irrigation at the same time.

  • When that same farm advisor says forget all the nodes above cracked boll (NACB) science he has been espousing for years and tells producers and PCAs to count backward 21 days from when they want to pick a field and then defoliate on that date.

Steve Wright, University of California farm advisor for Tulare and Kings counties, offered up a wide range of scenarios to defoliate this year's SJV cotton crop because that is what fits the “huge range” of field conditions awaiting growers as they face defoliation season.

“This season fields are all over the board,” said Wright at the recent West Side Research and Extension Center, Five Points, Calif., cotton field day.

Wright's counterpart in Fresno County, Dan Munk, said variability is most dramatic between fields of Acala and Pima. It is not uncommon to find Acala fields with bottom crops, no middle crop and little top crop. However, Pima plants are often loaded from top to bottom, visual testimony to Pima's higher heat tolerance. When the heat was shedding the Acala crop in July, Arizona-bred Pima was setting fruit.

Typically, SJV producers defoliate in September to pick in October, said Wright. However, that doesn't hold true when there is a field that is not going to open up until Christmas, if then.

“Temperatures are already on the edge,” said Wright at the Sept. 15 field day where some people arrived wearing jackets. Temperatures must be above 80 degrees during the daytime and above 52 degrees at night.

Defoliants do not work well at temperatures below these numbers.

Wright said in many Valley fields, the four Nodes Above Cracked Boll (NACB) measurement, used to determine when to defoliate, should be replaced by calendar date defoliation and/or percentage of open bolls.

Set a harvest schedule and stay on it, Wright recommended. “Take a hit and get going,” he said. Otherwise, the open bolls could be soaked by late fall's rains and fog. For Pima, this could be economically disastrous, as much as 50 cents per pound if the cotton is significantly rain damaged.

“Fields are not going to be white this year,” Wright warned.

With wide ranging field conditions, Wright and fellow UC cotton team members are offering different strategies. Most involved pre-conditioning and defoliation.

“It may take two, three or four applications to get the job done this year,” said Wright.

Wright said this may be the year a pre-treatment of 4 to 6 ounces of Ginstar at six nodes NACB, followed by a later treatment at about three to four nodes NACB of Ginstar at 8 ounces, or a combination of Ginstar with a boll opener (Prep, Cotton Quick, Finish or others) or Def/Folex, plus a boll opener should be considered.

UC cotton specialist Bob Hutmacher and the Valley's farm advisors warn that to avoid freezing leaves on plants, Ginstar rates should be adjusted to air temperatures at application or temperatures anticipated following application. In many cases in both Acala and Pima, a final application of sodium chlorate and paraquat, Shark or ET will also be useful in desiccating remaining leaves and improving opening of the last remaining bolls. Shark or ET also will help dry remaining broadleaf weeds.

Another strategy in vigorous, late-maturing non-Roundup Ready variety fields involves using glyphosate as a pre-treatment. A 1 pound per acre glyphosate treatment at eight, nine or 10 nodes above cracked boll followed seven to 10 days later by standard defoliation treatment of Def/Folex or Ginstar, with or without boll opener, could open some of the later-developing bolls.

The cotton advisors warn that glyphosate should not be applied before about 8 NACB for Acala or 10 NACB for Pima. This could result in significant yield loss.

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