Ag engineer advises crop-specific irrigation

Proper irrigation is a science, but implementing it on a farm is less precise. Moisture for a crop serves two purposes; it cools the plant and transports nutrients needed for development. When nature doesn't provide the water through rain, technology can by irrigation.

Jim Thomas, agricultural engineer with the Mississippi State University Extension Service, said there are no fixed initiation or termination dates for irrigation in the Mississippi. “You need to know the needs of the crop, the soil moisture status and the crop stage of growth and requirements, then trust your instincts,” Thomas said. “If you wish your fields would get an inch of rain, then it's probably time to turn on the irrigation.”

In Texas, cotton farmers start irrigating at square set and continue until they can no longer meet crop demand from the soil and supplemental irrigation. In Mississippi, cotton farmers usually start irrigation the first of July or around first bloom. They usually terminate irrigation in mid-August or within two weeks of the first open boll.

Some producers take infrared temperature readings or feel the temperature of the leaves to help determine crop stress, but Thomas warned producers to watch for cloud cover.

“A cloud cover instantly and arbitrarily reduces crop temperatures for 30 to 45 minutes,” Thomas said. “Know what's going on in the field when you take these measurements or they may not be accurate.”

Thomas said Mississippi producers usually do a better job with furrow irrigation than with pivot irrigation.

“We typically don't put out as much water with pivot as we do with furrow,” Thomas said. “We turn the pivots off after three or four days because the ground looks wet, but pivots are usually designed to meet daily crop demand for water and should be run 24/7.”

Thomas recommended producers with pivot irrigation make at least three circles, then check soil moisture in the field. On average, three circles is equal to one furrow irrigation.

“Pivot irrigation making a three- to four-day circle on a four- to five-day return cycle gets 80 to 85 percent of the water to the plant, and meets only 60 to 65 percent of the plant's needs,” Thomas said. “Cotton irrigated with a pivot three days out of seven can run a deficit for three days or more on average.”

Mississippi's major row crops each require a quarter inch, or 6,500 gallons, of water per acre each day during peak demand. Set the pivot to spray a minimum of 5 gallons per minute per acre to just meet this demand. This spray rate does not supply extra moisture to carry the plant through days when the pivot is turned off.

“One inch of pivot water does not equal 1 inch of rainfall because of the cloud cover and cooler temperatures that accompany rainfall,” Thomas said.

Thomas said there is a misconception that corn requires more moisture than cotton, soybeans or other row crops require.

“Corn demand peaks earlier than cotton, but is not higher,” he said. “It is vital from reproduction to maturity that plants get the moisture they need to set a good crop.”

The corn plant starts setting yield potential as early as the six- or eight-leaf stage.

“The reproductive period of corn is the most sensitive to stress. Four days of severe water stress at tassel and silk can reduce final yield by as much as 70 percent,” Thomas said.

Over-irrigating is rarely a problem in Mississippi. Larry Oldham, Mississippi Extension soil specialist, said too much moisture affects the plant's ability to take in the nutrients and oxygen it needs to survive. Runoff has worse effects.

“Drainage is the movement of excess moisture away from plants. Runoff occurs when water moves rapidly across the soil surface,” Oldham said. “Effective irrigation creates the slow movement of water in or across the soil, the sediment stays in place better and nutrients are more likely to stay in the soil.”

Bonnie Coblentz writes for Mississippi State University Ag Communications.

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