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How early is too early? When should irrigation end in cotton field?

In a brutal drought and buying expensive diesel, many producers and consultants have been asking Bill Robertson about terminating irrigation.

“I can understand why they want to stop the pumps,” says the Arkansas Extension cotton specialist. “And there's been some information floating around about how it's okay to terminate early. However, that's only advisable in a very narrow sense.”

Robertson has visited a Lee County, Ark., cotton field throughout the season. It was planted on April 10 and reached cut-out on July 15. By Aug. 1, the field had 350 heat units.

“At the point it reaches 350 heat units and is clean for bollworms and budworms, insecticide sprayings can be terminated.

“I was in that field Aug. 8, and it was being irrigated for the last time. By today, it had 550 heat units. Our guideline for irrigation termination in central Arkansas is 500 to 550 heat units.”

Guidelines for defoliation call for 850 heat units.

“So, if you follow that field on out and the 30-year weather average holds, that field will hit 850 heat units by the end of August.”

It's unlikely to surprise anyone that Arkansas cotton is accumulating heat units faster than the 30-year average. That means defoliation can likely be moved up.

“If everything holds, the picker should be in the field Sept. 15 on the early irrigated cotton. The (Lee County) farmer's goal is to have his picker in there at that time.”

In other fields, Robertson is watching a field first planted on May 1. A poor stand meant a second planting was needed between May 15 and May 18. That cotton has just now reached cut-out — 10 days behind the Lee County field.

“We're still using the same heat unit calculations behind last effective boll population. ‘Last effective boll population’ is a mouthful but, basically, what that refers to are the white flowers on the plant when nodes above white flower (NAWF) get to five. The squares and flowers that came after that will boll-up some. But those are typically smaller and the quality of the lint isn't as high.”

When fields reach cut-out, that's the last group of bolls that will contribute significantly to yield and profit.


There is a difference in irrigation termination when moving from south to north Arkansas. Robertson believes this isn't a function of geography as much as different soil types, effective rooting zones, nematodes and other things.

In Mississippi County, where much of the northeast Arkansas research has been done, there are many years when no benefit is seen from irrigating after 350 heat units beyond cut-out. Those fields are often sandier soils, and “I feel we have good water infiltration with a good, effective rooting zone.”

Moving south into St. Francis and Lee counties, “we run into some tighter, heavier soils — places where hardpan can be a problem. My base heat unit count beyond cut-out for irrigation termination is 450 to 500.”

Move farther south and the plants' ability to take up soil moisture becomes even more paramount. “In southeast Arkansas, I'm most comfortable with heat units being between 500 and 550 heat units.”

Insecticide termination at 350 heat units is “pretty cut and dried. That advice is based on the physiological aspects of boll maturity — lignin formation and things that make it harder for an insect to penetrate the boll wall and damage lint. That's heat unit driven.”

However, get into irrigation termination and the advice jumps around, depending on your situation. In some places, pH drops off very quickly below 6 inches.

“I've seen some numbers from 6 to 12 inches in the profile that drop to the lower 5s. When you have 5.2 pH in a 6-inch to 12-inch soil sample, there won't be a lot of root activity. Without that, the plants won't be harvesting much water at such depths.”

In such cases, “if you've got a half-mile pivot with little rain, you'll get farther behind every day. You turn it on and just keep feeding that monster diesel. I've visited producers who claim they've ordered more tankers of diesel this year than they ever have before. The drought won't let them stop.”

In southeast Arkansas, even with furrow irrigation, producers must consider whether there is deep moisture in reserve. Many times with a pivot, “if you get behind early, those reserves will have been used.”

Consider this scenario: a producer doesn't believe he's got good deep moisture in reserve, he's at 490 heat units and forecasted temperatures are high. Does he water again?

“I'd have to go ahead and do it. If I was on the irrigation bubble in southeast Arkansas, my benchmark would be 500 to 550 heat units. Without deep moisture, I'd have to water once more.

“Usually the cotton producers down there are on a seven-day schedule. But many have tightened that up because furrow irrigation wouldn't hold that long. I know some are now on a five-day and six-day schedules.”

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