Deferring the First Soybean Irrigation until Stage R3

Deferring the First Soybean Irrigation until Stage R3

July 5, 2012

SoyWater Updated to Account for 2012 Soil Water Conditions

Also see in this week's CropWatch:  How to Register For and Use UNL SoyWater

The UNL SoyWater website is an easy-to-use tool to predict when any given soybean field will reach the R3 growth stage (begin pod). UNL research has shown that R3 is a water-critical reproductive stage. Irrigating soybean at R3 provides the greatest yield bang per irrigation buck of any timed irrigation event. (This research was funded by the Nebraska Soybean Board.)

Soybean R3 stage

A field is considered to be at Stage R3 (beginning of pod elongation) when at least 50% of the soybean plants in the field have at least one pod 3/16 inch long at any of the four uppermost main stem nodes. Producers using an irrigation strategy that defers the first irrigation until just before or at R3 can use the SoyWater website to view a projected R3 calendar date for each field.

With early planted soybean on a silty clay loam or silt loam soil recharged to field capacity at planting, the yield response to pre-R3 irrigation is essentially zero. After applying the first irrigation at R3, continue irrigating as needed, based on crop water use, to capture the yield potential for the field. SoyWater can help you track and project soybean development and water use in any field (see related article) to determine whether irrigating before R3 is necessary.

The 2012 cropping season is unusual in that many fields did not have fully recharged soil moisture levels at planting, due to below-normal spring rainfall. To deal with this scenario, we have updated the UNL SoyWater website to include an extra input box so producers can indicate amounts less than 100% field capacity (FC) soil water. The available FC choices (for an initial soil water field capacity) are 100%, 90%, 80%, 70%, 60%, and 50%. Keep in mind that a 50% FC choice tells SoyWater that your soil has only "half" of the total plant-available water (PAW) it could potentially hold. That remaining "half" is more difficult for the crop to extract, and at 50% FC, irrigation would likely need to begin soon after planting. Be prudent in selecting your initial soil water choice.

Any current SoyWater user who wishes to change the field startup soil water FC from 100% to less than that, can simply go to the SoyWater list of fields, click on the given field, and then click on the red text Modify Selected Field, and make your choice (menu) for percent field capacity (% FC). Note: You can always change your % FC later. (After changing the % FC, the SoyWater table will be updated accordingly.)

Jessica Torrion
Research Associate, Department of Agronomy and Horticulture
James Specht
Professor, Department of Agronomy and Horticulture