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Drip irrigation under cotton

The Forrest City, Ark., cotton producer put in about 185 acres of drip tape on two fields last spring, after three years of thinking about it and two years of serious discussion with a drip manufacturer.

The first lesson he learned was not to wait until the last minute to install a system. Whittenton was planting tape when he should have been planting cotton, putting the producer behind the eight-ball from the beginning.

"But it wasn’t by choice," he stressed. "It’s just the way it happened. The cotton wasn’t planted until May 27. That’s not a good program to be on for cotton."

Whittenton placed the drip tape 9 to 10 inches deep under each row on 38-inch spacing. The drip includes 7 to 8 miles of buried PVC pipe and 492 miles of drip tape, a little longer than the distance from Forrest City to Atlanta. The emitters are 18 inches apart.

Whittenton considered putting the drip tape down in alternate middles, which would have cut installation costs by about 30 percent. "But if you go every other row, you’re married to cotton. Right now, cotton’s our crop. But 10 years from now, who knows what it might be. I wanted to leave the option open, so we could go to whatever we needed to."

Soil type was another consideration for placing the tape under every row, according to Robin Franks with Netafim, the manufacturer of the drip system. "The infiltration rate on these soils is extremely low. To move the water laterally would sometimes necessitate an over-application of water. We wanted to avoid that."

The company, which is based in Fresno, Calif., has installed drip systems in New Mexico, west Texas, Arizona, Arkansas, Missouri and southern Alabama. "But this is the first large-scale testing that we’ve done in this area," Franks said of Whittenton’s operation.

Whittenton owns the smaller of the two drip-irrigated fields, about 30 acres, and he and two partners, Jim Lindsey and Jim Hughes, own the larger one, about 155 acres. In the past, the larger field had been watered with a towable pivot.

Under drip, Whittenton expects "to use water more efficiently" and fertilize through the system. "All nutrients, except for what was put out dry preplant (phosphorus, potassium and boron), went through the tape last year," Whittenton said. That included all his nitrogen, plus phosphorus, potassium, sulfur and zinc.

Fertilizing through the system is one way for Whittenton to address the biggest obstacle for drip irrigation — cost. "There are options for insecticides and many other things besides water," said Bill Robertson, Arkansas Extension cotton specialist, who’s been following Whittenton’s experiment. "But it’s going to take a very high degree of management to accomplish it, and that’s not going to be for everybody."

Whittenton noted that drip systems work best in an arid environment where growers have more control over how much moisture the crop receives. Drip in the Mid-South is a unique experiment.

"Rainfall can throw out your timing," he explained. "But if we size our fertilizer pumps right, we can put our chemicals out and put only 0.05 inch of water."

In fact, "in wet growing seasons, the drip can be used purely as a means to apply chemicals," Franks noted.

Eventually, Whittenton hopes to vary both water and fertilizer rate with the system. He’s divided the 155-acre field into 18 zones based on elevation. Each zone has a valve to open or close water flow. "But I need a more sophisticated controller at the pump house."

The maximum amount of water the tape can apply in the root zone is about one-third inch per day, according to Whittenton. "The most I put out was 0.24 inch and the least was 0.07 inch. I was feeding the cotton what it needed."

To help him finetune his water applications through the drip, Whittenton plans to install near the field a weather station, including soil probes that can provide up-to-the-minute information on evapotranspiration.

"Accurate day-to-day, hour-by-hour information can make the drip that much better," Franks said. "You have the ability to irrigate the entire field every day. But you want to finetune the system to fractions of an inch of water applied by day."

However, Whittenton wants to modify Netafim’s approach by watering less frequently in larger amounts to compensate for varying elevations in his fields. "I’ll increase the amount of water I put out with each application and make 15 to 20 applications during the year."

One reason for that approach is that yield maps of the drip fields indicated lower yields in lower areas of the field. "This is just theory, but when you start filling tape with water, the low ends of the field fill first. Then when you stop watering that area, it drains to the low end. So when you’re watering every day, it’s keeping the lower part of the fields wetter. This year, unless I’m convinced differently, I may irrigate every third day."

Whittenton, who farms 1,250 acres of cotton, continues to experiment with cotton varieties to find those best-suited for drip. "We have six varieties planted on this field," Whittenton said of the larger drip field. "Most of it is Stoneville ST 4892BR. We have strips of other varieties out there, Deltapine DP 451B/RR, Sure-Grow 501BR, Sure-Grow 521R (for the refuge) and Paymaster PM 1218BR. But as far as finding the right variety for this system, it may take us three years before we know."

What can he tell so far? "Early on, the earlier-maturing cottons looked better. But in the end, I think you need a fuller-season cotton on it to push for the extra yield. With some of the newer varieties, if you stub your toe, you’re out. You need a more-forgiving cotton."

Yield under drip "did all right for when we planted it," the producer said. "But we still have room for improvement. This year is going to be the best test. Unless something happens, the cotton on the drip fields is going to be the first cotton we plant."

Water conservation is another issue for Whittenton, who is on the local soil conservation board. "Within the next few months, we will be declared a critical groundwater area," the producer said. "There are problems with water supplies just 10 miles to the west. I need another two years to know what the real savings on drip are going to be. But I think I’m saving about 30 percent."

"You really have to look at the future," said Robertson. "As water becomes more and more critical, the bottom line for the producer is going to change. There was an article in the paper recently that said that water in the coming century is going to be what oil was to this last century."

"TMDLs (total maximum daily loads) are another issue," Whittenton said. "EPA has mandated that the state of Arkansas clean up its rivers and streams in certain watersheds, including the L’Anguille watershed (where Whittenton farms). Drip irrigation will help keep nutrients on the farm unless we have a rain. Our water won’t leave the farm and add to the silt in the river."

"Drip irrigation also help you manage your nitrates within the soil profile," Franks added. "If you have the data to precisely control the water in the root zone, you’re not pushing your nitrates into the water table. Drip irrigation is root-zone management."

Netafim’s drip irrigation system will last 15 to 20 years if the grower follows the maintenance schedule, according to Franks.

e-mail: Elton Robinson

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