Third in a series of video from the 2021 University of Arkansas Cotton and Rice Field Day
Out of all the decisions a farmer has to make to finish out his cotton crop, deciding when to terminate irrigation may be one of the most difficult, according to University of Arkansas System Division of Agriculture Extension specialists.
Not only can watering cotton past the optimum cutoff add expense for additional diesel fuel, natural gas or electricity, it can also delay growers putting pickers in the field, says Dr. Bill Robertson, Extension agronomist for cotton with the University of Arkansas.
“There are a ton of factors that go into irrigation termination,” said Robertson, a speaker along with Ray Benson, Extension staff chairman for Mississippi County in northeast Arkansas, for the University’s Corn and Cotton Field Day. The field day was held virtually this year because of Covid-19 pandemic concerns.
“I’ve been in fields where you could take a soil probe, go three feet deep and have good soil moisture, a lot of cotton roots. We can terminate irrigation on those, some of the research says, as early as 350 heat units after cutout. But I’ve been in fields that had a lot of nematodes, compaction, hot weather like today and some of those have gone to almost 600 heat units.”
Many times, he said, the long-term data for Arkansas shows irrigating cotton in September doesn’t pay “because we’re running out of heat. When we turn the calendar from September into October our heat units drop off really fast.
“Even if we water, and we try to make the bolls, we don’t have enough heat units to mature them out. It doesn’t make us much cotton, but it costs us time getting the picker in the field. Wouldn’t you agree that most fields I look at somewhere around the 500 heat-unit mark to 550 heat units is about when that cutoff point occurs?”
“I think that would be a fair estimate,” said Benson.
“But I also find that nodes above white flower five or heat units beyond cutout is a difficult tool to use by itself to terminate irrigation,” said Robertson. “But those are things that we can do or we hope we can do more of some day on our specific irrigation termination guidelines.”
Robertson and Benson also discussed using heat units after nodes above white flower for terminating insecticide applications for defoliating insect pests and for terminating the cotton crop itself.
The benchmark for terminating insecticide applications for defoliators is about 500 heat units after cutout to help protect leaves on the cotton plant.
“We still need some leaf area to finish filling out some of the bolls,” said Benson.
“When a cotton leaf gets 50 to 60 days old, it’s ready for retirement,” said Robertson. “It’s still there; it still looks OK; but it’s not doing much work. The younger leaves are the ones doing the work. So we have those young leaves on the outside of the plant that are doing all the heavy lifting to make energy so we want that sugar factory running as long as it can to fill out those bolls.”
University of Arkansas Extension Service guidelines recommend defoliating cotton at 850 heat units after cutout. But the decision-making may not always be so cut and dried, according to Robertson and Benson.
“To me, it’s always the best place to start,” said Benson. “Then you look at percent open. If you want more confidence then slice some bolls and get some ideas. But, generally, 850 heat units is going to get you in the ballpark to where you can start trying to time defoliation.”