Agronomics of Over Irrigating
A couple weeks ago our Agronomic Services Lead Nick Lammers wrote an insightful article “Should we be applying water now? June 1st” on the CropMetrics Blog. As I got to thinking, I thought it was interesting that not only was I seeing the same thing in the Midwest as Nick, but also that I was receiving questions from our Certified Dealers and growers on other components relating to water management. With hotter temperatures coming in the next weeks, growers may be asking “Should I start irrigating yet?” As plant leaves start to roll due to hot temperatures, it may be difficult not to turn that system on, but doing so may have impacts that need to be addressed before saying “Yes”.
The first component of answering the question of “Should I start irrigating yet?” is to understand if our soil moisture is limiting. Plants in extreme temperatures will roll regardless of if they have adequate soil moisture, as a response to reduce transpiration by reducing surface area of the leaves. Doing so in a soil that does not need irrigation has multiple detrimental effects. Not only will the crop roots be hindered for future growth, but needed inputs such as nitrogen and chemicals may be moved through our profile. Yield reduction and stress from reduction in soil moisture on corn is very limited in the early stages of the crop. Up until V9, corn has less than 1% of yield reduction per day of water stress and is 0% up until V6-7 corn. In many areas of the Midwest, not only are we seeing fields with adequate moisture, even in the top 12”, but also water migrating from lower areas of our profile where moisture is abundant due to our above average spring of moisture events. These lower areas in our fields are acting like a drip tape system, replacing the drying moisture at the top 2-6” from below, through adhesion of water molecules to the soil particles and cohesion of water molecules together in the soil.
This image shows the effects of early irrigation on root development. These fields are less than a mile away from each other and were at the same crop stage planted less than 3 days apart. Early irrigation resulted in root mass on June 15th, 2017 to be 32% smaller than the non irrigated field.
Running an irrigation system when water is not a limiting factor can deter root growth, and make it difficult later in the season when water is key to yield success by creating a smaller management zone of roots to access water that is stored and placed in the profile through irrigation or precipitation. Shorter roots are hard to manage in the heat of irrigation season, but also can leave plants prone to wind events and lodging. By holding off early when we do not need to irrigate, and allowing our roots to grow we are able to have a deeper profile and keep up with the crops high demands of over .30” per day during pollination time periods while storing the free irrigation that comes from precipitation. Ensuring proper water availability later in the season is also key, as during tasseling yield reduction in corn from water stress can be 7% per day of stress! At CropMetrics, our Certified Dealers are reading soil moisture graphs every day to ensure growers have adequate water and are allowing for root growth and development for necessary timeframes. Not only are we encouraging root growth, but also impacting key yield factors for later in the season.
In the image below from last year, we are comparing probes in fields only a half mile apart from one another. The grower on the right irrigated 2 times in May, and 1 additional time during rain events in late June, compared to the grower on the left who did not irrigate until July. By not irrigating early, the grower on the left had active roots down to 24″, compared to only 16″ with the grower on the right. An extra 8″ of root mass in this silt loam soil provided an additional 0.73″ of available water during the heat of the season resulting in 3 more days against stress. When the pivot has trouble keeping up with 0.20″ of daily crop water use later in the season, the shallower rooting depth resulted in lower yields and decreased profits from simply irrigating when not necessary.
The 0.73″ may not sound like much, but remember that we can see up to 7% yield reduction per day of water stress during tasseling. The 3 days of additional water availability created from proper early season water management could be responsible for saving 21% of our yield! At 220 bu/ac corn, that loss could equal 46.2 bushels or $161.20 lost profit ($3.50/bu corn).
In the example below we have a field with multiple soil types and the pictures were taken just last week. The corn in all soil types was at V6, but the sandier soil (left corn plant) had deeper roots down to 12”, while the heavier textured soils (corn plants on right) had restricted rooting depths due to the saturation from early precipitation.
The probe graph on the right is from the sandier soil with the 12″ roots from the left picture. This soil is the majority soil type in the field, and we are seeing soil moisture week to week increasing from the wetter profile replacing what is needed. Even though on the topsoil, we can kick and see some dust, the majority root mass is actually increasing in water content!
Watch for Part 2 of this blog focused on fertility management in relation to the “Agronomics of Over Irrigating”.