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Corn+Soybean Digest

Late-Season Water Tops Off Yield

Drive by a Baugh Plantation soybean field during the first week of September and you'll likely see a center-pivot irrigation system watering the nearly mature crop.

Is that too late to be irrigating soybeans?

"We found out many years ago that we need to water our soybeans much longer than we once thought," answers Coleman Connell, who operates the Sherard, MS, plantation with his father, Oscar.

Connell says they usually gain at least 5 bu/acre from that late-season irrigation.

During last year's hot, dry growing season, a 160-acre field of Group V beans was irrigated 14 times, with 1" of water applied each time.

"The last circle was put out on Sept. 7," reports Mike Williamson, Connell's farm manager. "We averaged 62 bu/acre."

In 1997, a more typical year, a similar field was irrigated six times. A 1-lb shot of Benlate was applied about Aug. 1, and the last watering came in early September. Connell says the late irrigation-Benlate combination "really filled out the beans - caused them to be heavy and healthy."

The field averaged 64 bu/acre. "Our dryland average wasn't all that bad - 43 bu/acre," Connell reports. "But that didn't compare to the late irrigation-Benlate soybeans."

Larry Heatherly, USDA-ARS research agronomist at Stoneville, MS, says most Midsouth soybeans are grown on shrink-swell clay soils.

"Based on much research and other information, if the soybeans are surface-irrigated, flood or furrow, we prefer to see the last irrigation applied between mid-pod-fill and full seed," says Heatherly.

"With these clay soils that shrink when dry, the grower applies 2 1/2-3" of water with each irrigation event," he explains. "That would be enough water to carry the crop to full maturity."

Since most pivots don't put out as much water as surface systems, Heatherly suggests that the last sprinkler application be done at full seed to ensure there's enough water in the soil to completely fill out the seeds.

"The real key is educating growers to understand what full seed is," he states. "The seed must be touching in the pod. And we want the soil to be reasonably moist at full seed to carry the plants to full maturity."

It's better to water a bit too late than to stop too early, he says.

"If there is any doubt in the grower's mind - the seed are almost touching in the pod and there are dry conditions - then he should water. Every bit of material that will go into the seed at the end of the growing season will amount to several bushels per acre."

Heatherly remembers inadvertently missing one late-pod-fill irrigation in some April-planted Group V beans.

"The result was quite profound - the loss of 10 bu/acre," he reports.

The research agronomist says cotton is much more sensitive to overwatering than soybeans.

"In reality, soybeans are quite simple to irrigate. Most problems with soybean irrigation are mismanagement or undermanagement," he says.

"The information is out there, but many soybean producers have other agendas or don't believe in the irrigation of soybeans strongly enough. I have found that soybean farmers who really manage their irrigation make good yields year after year after year," Heatherly concludes.

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