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Timing is key to watering soybeans

07/27/2011 @ 10:04am

Building on years of field research, University of Nebraska agronomists are rewriting the rules on how to irrigate soybeans. Their approach is to delay irrigation on beans until early pod formation. The crop, instead, relies on stored soil moisture and early-season rainfalls to get by until that point.

“We've learned a lot about how to irrigate soybeans that is quite remarkable compared to how most farmers now irrigate,” points out University of Nebraska agronomist Ken Cassman. His observations build on years of research by Cassman's colleague, soybean geneticist Jim Specht, into soybeans' drought resistance and the best methods of irrigation.

Too much too soon

Typically, farmers plant soybeans in early May and begin irrigating in June. In years with average or above-average early-season rainfall, that can result in too much water being applied to plants. Too much moisture can result in taller and leafier soybean plants that are more susceptible to disease and lodging at harvest.

Avoiding too much early irrigation, on the other hand, encourages plants to develop stronger, healthier root systems that grow deeper. “You defer irrigation because there's typically enough stored moisture and enough rainfall even in dry years to create a plant that can achieve high yields,” Specht explains.

Soybeans need about 19 inches of water from planting in early May through harvest to yield about 85 bushels. If soybeans don't get any of that from early-season rains, producers will have to catch up with watering once deferred irrigation begins in early July. But if early-season rains are normal, the deferred approach could reduce the amount of irrigation water applied throughout the season.

Specht and Cassman's research has been incorporated into an online irrigation scheduling program called SoyWater. Developed by Specht and Jessica Torrion at the University of Nebraska, it is available online at www.soywater.unl.edu. This online spreadsheet offers producers access to the two most popular irrigation strategies in practice.

Soil water balance method

With the first approach, called the Soil Water Balance Method, the user chooses an irrigation trigger, which represents a percentage depletion of the maximum water (field capacity) in the crop root zone. This depletion is caused by crop withdrawal of soil water.

Water balance method

The Water Balance Method, based on Specht's research, is advisable for fields with soils that have a high water-holding capacity (such as silty clay loam soils) and in years when the field water content is at full capacity at planting or emergence. In this strategy, the producer allows the soybean crop to extract the water it needs for vegetative growth and early reproductive development from the stored soil water present at emergence and any postemergence rainfall stored in the root zone prior to the R3 stage.

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