When to Turn Off the Water

April 29, 2011 09:46 PM
When to Turn Off the Water

Ending Irrigation Too Soon Can Slash Yields

Corn growers can lose as much as 4 bu. per acre per day by ending irrigation too soon. With current commodity prices, that’s like throwing away money.

Calculating when and how much water to apply in the final stages requires measuring or estimating soil moisture, gauging crop maturity and monitoring rain forecasts.

"Don’t use a calendar date or tradition to terminate the season," says Freddie Lamm, Kansas State University irrigation engineer. "Calculate the last irrigation based on crop needs and soil conditions. Most literature on irrigation management is 35 to 45 years old. It was written when irrigated corn yields were 50 bu. to 100 bu. per acre lower than they are today. It’s possible that numerous yield-limiting stresses, such as water, insects and weeds, that were tolerable at lower yield levels are less tolerable today."

While some farmers can conduct a "feel" test to determine soil moisture, today’s sophisticated sensors, when properly used, can provide accurate data for $500 or less per field.

"Farmers should know as much about their soil as they can," encourages C. Dean Yonts, a University of Nebraska–Lincoln biological systems engineer. "Knowing how much water your soil profile holds and how much moisture was there at the start of the season aids in irrigation scheduling."

To track evapotranspiration, atmometers can be set up within a couple miles of each field being tracked. A grassy area close to or within the field (if it’s a short crop) will allow the most accurate assessment of climate conditions. Keep the device out of reach of an irrigation system.
When the corn reaches the R4 (dough) stage, there are about 34 days before it’s mature. Most universities have tables to assist in calculating water needs in the final crop maturity stages. Once that figure is determined, assessing rainfall predictions and evapotranspiration rates will provide the final estimate for water needs.

"If you overirrigate at that point, it’s possible the water you apply won’t be available to your crop next year. It will filter down below the soil profile," Yonts says. "If you severely deplete moisture levels at the end of the season, you risk lowering this year’s yields and next year’s crop could start out with inadequate moisture."

Flip the switch.

Physiological maturity is defined as the time when kernels have stopped accumulating dry matter. Black layer formation at the tip of the kernel is a good indicator of corn maturity. Once the crop stage has been determined, growers need to calculate the difference between how much water is needed to bring the corn to maturity and how much water is available in the field’s root zone, the top 4' of the soil.

Corn roots will penetrate deeper than 4' into the soil, but little water is used beyond the 4' level. If the soil has any root-restricting layers, adjust your root zone assessment accordingly.

The available water in the root zone is considered to be at 100% field capacity when the soil profile holds as much water as reasonably possible after initial drainage. Available water in the top 4' can be lowered to 40% capacity (60% depletion) at crop maturity without reducing grain yields.

Lamm notes that soil types and climatic conditions should be considered in regard to depletion percentages. "Greater values of depletion might be acceptable in milder climates," he says.
Though it’s hard to predict, the amount of expected rainfall should also be considered in the calculation.

"Farmers can form a preliminary decision and then should be willing to adjust plans if the weather is drier and hotter than predicted," Lamm says. "With a 2 bu. to 4 bu. per acre per day potential penalty for ending irrigation too soon, I think flexibility is a reasonable approach."

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