Irrigation Soybeans in 2017
When it comes to irrigation, we often allocate our water management tools and technology to our corn acres vs other crops like soybeans. The thought behind this is that the repayment for investing in high quality irrigation management on soybeans isn’t as lucrative as it is with corn. Certainly, there is an exceptionally high return on the investments made to insure we have precise water management on corn, but soybean acres can also pay very high dividends on those same dollars. For example, each year we hear of growers that say their pivot corners yielded the same or more than the irrigated acres on a pivot. If precision based irrigation isn’t being used that can certainly happen. Without having the ability to measure soil water in the majority acre and then distributing irrigation water based on the needs of the crop in each area of the field, we will over water. Soybeans are sensitive to water management especially in the later part of the crop year. So, investing dollars that prevent over or under watering will have an excellent return on the investment in both soybeans and corn. (See yield decreases from drought stress – Iowa State University)
Overall, soybeans will require around 20” of water during the growing season. But, the watering strategy requires two different action plans. In the vegetative part of the growth cycle (prior to flowering), we generally should only water to prevent severe drought stress. In fact, watering soybeans during their vegetative growth period can reduce yields because of excessive top growth (lodging) and the increased potential of white mold development especially under sprinkler irrigation. Even watering during the early R stages can be costly. If we receive more rain or water more than needed during flowering we also increase the chances of decreased yields. The abundant water at flowering triggers more seed development and any stress later in the season will reduce the seed size and ultimately the yield below what it would have been without the water. Sensors placed in the majority soil type and monitored to assure we aren’t stressing the crop during vegetative growth, insure we are only watering enough to prevent yield reducing stress.
Once the crop has reached R3 (at least one pod on top 4 nodes is 3/16” long), we need to manage the water so our AWC (Available Water Content) does not drop below 50% in the top 3 feet of the profile (2 ft in sandy soils). Soybean roots will go deeper than that, but by far most of the water being taken from the profile occurs in the top 3 feet. Measuring water uptake below that is not needed and if the sensors are measuring water every 4-6” through the 3-foot profile, you will have what you need to make good water management decisions during reproduction.
Studies done by the University of Nebraska have shown that there was no statistical difference in yield with soybeans that received full watering (12.5” of water based on 50% AWC depletion throughout the growing season) and those that received water at pod elongation (7.5” total), and those watered at full flower (10.5” total). If there is adequate soil moisture at planting, irrigation can probably be withheld until the R3 stage as long soil moisture is monitored on a regular basis during the growing season. Researchers also determined that if we are growing soybeans on fields that have irrigation wells with limited capacity, and we can only apply water twice, then R3-R4 and R5-R6 would be the most economical times for that to happen. Soybeans generally will need around 10” of water during reproduction and most (4.5”) of that will be used during the R5-R6 stage. Daily water use (ET) will top out at about .32”/day at R5 but still will be using .25” per day at R7.
Leaves on the lower nodes will be turning yellow at R6 so it’s not unusual to see pivots running on soybean fields that are starting to turn. In field moisture sensors, along with careful staging of crop growth will tell you how much water will be needed specifically for each field. However, a general guide line is in the chart below.
Adding water at the right time and in the right amount can make a significant difference in soybean yields at the end of the year. But, with just those two components we haven’t accounted for the spatial variations that appear in every field where soybeans are grown. Texture and topography variances will affect how much water is available for plant growth. There are certain parts of a field that maybe lower in elevation or higher in clay content and thusly require less water. And certain parts might be sandier or have higher elevations that may need more water. Variable Rate Irrigation address the spatial variation that exists in most soybean fields.
Once we adjust the speed of the pivot to modify the water application to account for the variability, it’s easy to see the effects. Many of the lighter soils may not show an immediate change in yield with spatial water management because many growers are watering based on what they see in those parts of the fields anyway. However, by cutting down on the overwatering in the medium and heavy soils, we see the VRI advantage right away because over watering has limited the root development in years prior. Now we are applying water based on the needs of the crop and water holding capacity of the soil. Because of this we develop a deeper more robust root system that taps into reservoir of stored nutrients that has leached past the root zone from years of overwatering in the heavier soil textures.