Before implementing variable-rate irrigation, test the pivot to ensure there is an even watering rate across the entire length of the pivot.
Bushels respond to variable-rate irrigation
Driving through the countryside, you’ll notice more pivots springing up as farmers try to keep a tight rein on Mother Nature.
"Farmers want to maximize their water efficiency to reach their productions goals," says Cole Fredrick, Valley Irrigation product manager.
To learn more about efficient water application and use, the Farm Journal Test Plots program is studying variable-rate irrigation (VRI) technology. In 2011, the Valley VRI system was outfitted on Mason County, Ill., farmer Dan Meeker’s center pivot. The system uses existing electrical lines on a 1,296' half-circle pivot to send signals and information along the span.
The multi-year study consists of an 80-acre field with soil types that range from sandhills to bottom loams. The sandhills in particular are a focus for Isaac Ferrie, who oversees this effort. Those areas have suffered a significant yield decrease in recent years.
Since soil is the lifeblood of a farm, knowing the different soil types and soil characteristics in a field can be the difference in producing a low-yielding crop versus a record breaker.
Zone in on need. In its first year, the Farm Journal Test Plots crew learned each zone or soil type was not getting the appropriate amount of water using a standard pivot. The sandhills require more water than the bottom loams, but without VRI there is no efficient way to cater to the specific needs of soil types.
To help gauge how much water each zones requires, Irrometer soil moisture sensors were placed at three depths—6", 18" and 24"—in a representative zone for each of the primary soil types. An in-field modem wirelessly transmits the data to the crew so they can monitor the soil.
Real-time soil moisture data is crucial to gauge field conditions and write VRI prescriptions. Based on a test conducted with a water infiltration test kit from Cornell University, the plots crew found the infiltration rate in the bottom loams was twice as much as the sandhills.
"The results were opposite of what we assumed for the given soil types," Ferrie notes. "We believe the water was running off the sandhills down to the bottom loams causing overwatering in the bottom loams and underwatering in the sandhills."
The sandhills have a fine texture and low cation exchange capacity (CEC), resulting in a diminished hydraulic conductivity of the soil, Ferrie says. The low hydraulic conductivity of the sandhills diminishes the soil’s ability to pull water into the soil profile. In this particular case, soil characteristics trump gravity.
The VRI and non-VRI sandhills received the same total amount of water and showed a 30 bu. average by applying more often with less water. The VRI loams received a 16 bu. gain with 5" less water than the non-VRI loams.
"We realized through our soil moisture data and infiltration tests that more water across the board wasn’t the issue; it was the amount of water each zone was receiving per watering," Ferrie says.
A matching game. To reach yield potential, the infiltration rates of each zone need to match the actual watering rate. Based on data and in-field evaluations, the VRI prescription was altered to cut back on application rates but apply water more often.
Using standard irrigation, each zone was watered 17 times, receiving a total of 14.8". When using VRI, each zone received water 25 times throughout the growing season. The sandhills received a total of 14.8" and the loams received 9.2".
The VRI and non-VRI sandhills received the same amount of water, but the water was applied more efficiently. The VRI loams received less water compared to the non-VRI loams, yet yields increased in both areas using less water more frequently.
"Having the ability to adjust the application rate across the field allows good use of water," Fredrick says. "When water goes down through the soil, it is being picked up by the crops instead of being washed away."
The loams showed a 16 bu. average gain. By matching the infiltration rates, the sandhills showed an average 30 bu. increase. Even with the boost in yield, the sandhills are still the lowest yielding spots in the field but considerably improved with VRI technology.
"It’s all about making a high-dollar input produce the most possible," Fredrick says. "By using water efficiently, you are able to increase your overall yield and bottom line."
Valley has made updates to their VRI system to make it more user friendly. Using a tablet or smart phone, Valley telemetry provides real-time field visuals and system monitoring.
"You have the ability to start, stop or change VRI prescriptions at the touch of a button," Fredrick says. "The program sends the new prescription to the pivot, and it immediately transitions."
With two years of data under our belts, plans are to continue Farm Journal Test Plots efforts on VRI technology next growing season.
The thermal images by AirScout and the NDVI maps provided by GeoVantage show that variable-rate irrigation helps minimize the areas affected by drought-stress in the sandhills.
- September 2014