Researchers study effects of drip tape irrigation

Staff Writer

HARBESON, Del. — Researchers at the University of Delaware shared their latest findings on irrigation Aug. 20 as farmers, industry professionals and the general public gathered at the Warrington Irrigation Research Farm.
The corn and soybean fields there wrap around a small cemetary where graves of the Warrington family date back as far as 1840.
The 130-acre farm has 66 acres under center pivot irrigation. On the section surrounding the cemetery, subsurface drip irrigation is being studied. Of the 18 acres there, with center pivots, only 12 acres could be irrigated, explained James Adkins, associate scientist at the university.
The project was begun because the university needed farmer management guidelines for subsurface drip tape irrigation, first used in this area in 2002 and 2003.
Researchers at first thought it would be used as in vegetable production. One major difference is that vegetable farmers use less expensive drip tape that is discarded after one season. “We hope to get 20 years from this tape,” Adkins said.
The field was divided into 42 individual zones for research. Drip tape was installed down to 14 inches at every other row of a corn field using a machine from Rainflow in Pennsylvania. The machine isn’t as elaborate as much more expensive models — it doesn’t cover the tape back up — but it does save labor.
The header lines lay flat on the ground. Adkins said workers are able to plant the end rows without cutting most of the lines with a no-till planter. Only two of 300 lines have been damaged.
Flush values are located at the opposite end of the field to make possible flushing any sediment from the lines. This water quality measure is done twice a year.
The filter is the most important part of the system, but the water quality on the Warrington Farm is excellent, with nothing being found in the filters.
Root intrusion is another problem, but acid can be used to clear them from the lines.
Adkins said they had learned it takes just as much volume to irrigate with drip irrigation as with center pivot, although it isn’t applied all at once. “Drip irrigation is not a solution to not having enough water,” he added.
Ten inches of water was applied last year, when there was 26 inches of total rainfall. Tensiometers are used to determine when water is needed.
Yields were the opposite of what was expected, Adkins continued. The highest yields were where less water was applied. “We injected 90 pounds of fertilizer through the drip tape the day before a 7-inch rain,” he explained. The fertilizer leached through the sandy soil. “Nitrogen was already borderline too deep. We could definitely tell there was nitrogen deficiency later in the year.”
As a result, the researchers determined nitrogen should be sidedressed only, so that it would remain available to the plants. “We wanted to fertigate, but it just wasn’t feasible.”
One problem with drip irrigation is the potential for leaks. “If there’s a hole in the tape, you must fix it,” Adkins said. Four holes have been found in three years of research, one caused by a groundhog. “You need an intensive rodent management program,” he said.
Damaged areas of the tape are easy to splice, but difficult to find. “Do you have enough time to micromanage and walk your fields regularly?” he asked.
In answer to questions from the onlookers, Adkins said the cost per acre ranges from $1,500 to $2,000, depending heavily on the dimensions of the field. A long, narrow field is best. Labor is the biggest part of the cost.
The researchers are also studying variable rate pivot irrigation in another section of the Warrington Farm. Adkins is working on this with Extension agent Corey Whaley.
“We’re using a range of strategies to trigger irrigation,” Adkins explained. “The neighbors think we’re nuts. We irrigate every day, but only part of the field.” Some of the test areas get no water, except to provide an equal playing field when planting, for example, with double crop soybeans in the middle of a very dry month.
Adkins said the system being used slows or speeds up the pump to maintain constant pressure.
“There are a host of considerations,” he continued. “How variable is your field. How many acres need to be treated differently?” Two acres out of a field is not worth $27,000 worth of technology, he suggested.
“A $2,500 panel can improve irrigation using sector control. You need $900 return on a 3-acre plot. (In some corn price scenarios) that’s a 100-bushel difference. Can you improve that much?”
He summed it up, even before visitors got to the test area: “Variable rate is a neat tool. You can do it, but can you afford it - now?”
Anyone who does use variable rate should have more than one tensiometer per field. “My recommendation is you need three in the field. I prefer four. If you’ve got only one and it goes down, then what do you do? What if it’s not installed properly?”
As the visitors climbed down from their hay wagon transportation, Whaley explained the variable rate plot studies, now in their third year, were funded by the Maryland Grain Producers Association and Delaware Soybean Board.
Data is sent back to the base station by transmitters, he said. Rather than transmitters with batteries, the researchers use solar-powered transmitters. Information from 240 transmitters are monitored by a single person. Sensors measure moisture levels at 4, 10 and 16 inches. With soybeans, the most water use is in the top 6 to 12 inches.
Plots within the field measure 60 feet by 60 feet. A new “prescription” for irrigation is developed every morning. It could be done by computers, Whaley acknowledged, but he doesn’t trust them. What if there was a lightning strike or power outage? You’d need a backup system, he said. For now, labor is cheaper, he said.
With unusual rainfall patterns, performing research has been challenging. In 2012 there was well below average rain until mid-July. There was no significant difference in double crop soybean yields in irrigation and non-irrigated.
In 2013, there was 16 inches of rain from mid-June to mid-July. “It was a rough year for corn, but soybeans turned out good,” Whaley said.
He jested that a study done under a huge dome might provide the most accurate information.
Row spacing is also part of the studies. “In all the studies, row spacing did not respond differently,” Whaley said.
Dr. Amy Shober is conducting studies at the Warrington Farm also, with a goal of learning how nitrogen rates and timing affect yields and nitrogen efficiency in corn. Studies last year were “abysmal” because it was too wet.
This year, even when adding 1.2 pounds N, she is seeing chlorosis in some fields. She uses a lysimeter to sample water from the field to see how much N is below the root zone.
During the question and answer period, Shober advised the farmers to watch for manganese and potassium deficiency. Soil tests are essential, and tissues samples will confirm deficiency. A foliar application will help with manganese deficiency, but farmers should use chelated manganese, rather than manganese sulfate if Roundup Ready beans are used.
“There is some indication that Roundup Ready beans are not as efficient as conventional (non-Roundup Ready) beans at taking up soil manganese, Taylor said.”