June 18, 2002

LINCOLN, Neb. — Timing is everything in soybean irrigation. Farmers might get anxious to water, but patience is rewarded with better yields, a University of Nebraska agronomist said.

Many farmers mistakenly irrigate soybeans too early – during flowering and vegetative stages, said Jim Specht, soybean geneticist in NU's Institute of Agriculture and Natural Resources. Early irrigation results in tall, lanky plants that produce smaller seeds and are likely to topple over in storms. It also promotes shallow root systems that concentrate near the surface and are susceptible to drought stress.

"If you can't avoid irrigating during flowering and vegetative stages because of sandy soils, you can't stop," he said. "You must continue irrigating thereafter or yield responses could be less than if you had not irrigated at flowering at all."

It's best to irrigate during pod elongation, when soybeans are most responsive to irrigation. In 15 years of research, Specht discovered that heavy irrigation during pod elongation always led to large yield responses.

"We've found that this gives us the biggest bang for the buck," he said.

Specht said soybeans always should be irrigated during pod elongation, regardless of weather forecasts. Waiting for rain deprives soybeans of water during this critical growth stage.

"You can't afford to miss the pod elongation period – ever," he said. "If you do, you'll miss the prime opportunity to put bucks in your pocket. Get out there and irrigate, even if it's drizzling. Mother Nature doesn't always deliver on request."

At the pod elongation stage, pods on the upper four leaf nodes are one-half inch long and very few flowers remain on the plant. This usually occurs in late July and early August for soybeans planted in mid-May, but varies by planting date, Specht said.

At this stage, irrigate heavily for two weeks to replenish moisture, Specht said.

"If you replenish moisture back to 75 percent or more of normal field capacity during the pod elongation period and then irrigate soybeans like corn thereafter, you're in good shape," he said.

After pod elongation, Specht recommends scheduling irrigation as needed during the seed enlargement stage because water loss can hasten maturity.

Soybeans can benefit from irrigation through early September, Specht said. If a center pivot is split between corn and soybeans, keep it on the corn during June and most of July, then intensely irrigate the soybeans during the last week of July and first week of August, he said. Apply equal water to both crops thereafter, he said.

Again, be patient.

"A lot of people shut off the water on soybeans too soon, when they're finished with their corn," Specht said. "Soybeans can actually use water for one more week. This is critical for some farmers."

In a split pivot field, the corn to soybean yield ratio should be roughly 3-to-1, he said. If corn yields 180 bushels per acre, then soybeans should yield at least 60 bushels per acre, Specht said.

"If you're getting a different ratio, you're not irrigating at least one of the two crops right," he said.

In the end, it's better to wait to irrigate, Specht said. "Our research has shown that it doesn't pay to irrigate early."

Specht's research indicates soybean yields seldom respond to irrigation during the flowering and vegetative stages. Moist soil and high humidity during flowering also make plants vulnerable to white mold infection.

Rather than irrigating during these growth stages, Specht recommends maximizing off-season precipitation infiltration, and minimizing evaporative loss of soil moisture before and during the growing season.

Storing as much preseason moisture as possible usually helps postpone irrigation until well into the flowering stage. Soybean roots can reach five feet into the soil. Central and eastern Nebraska's silty, clay loam soils can hold two inches of soil moisture per foot. That means soybean roots can tap up to 10 inches of moisture in the soil during the season if the soil moisture profile was full at planting.

Specht's research is conducted in cooperation with IANR's Agricultural Research Division.

06/18/02-SH

James E. Specht - Ph.D.
Agronomy and Horticulture
Professor
(402) 472-1536

Vicki Miller
Research Communications Coordinator
(402) 472-3813

Department: Agronomy & Horticulture