Last Call

OMAHA (DTN) -- Late-season irrigation is one of the more important water management decisions farmers will face the entire growing season. Watering crops in August can help finish crop development during a time that tends to be dry in many regions.

However, many irrigators may apply more water than is necessary, because crops are using less water per day late in the season. That extra irrigation may be wasting 1 to 3 inches of water and 2 to 5 gallons of diesel fuel per acre, according to research from the University of Nebraska-Lincoln (UNL).

USE CURVES, PROBES

Jeremy Olson, an agronomist for Pederson Seed and Service in Hiawatha, Kansas, said he uses corn water-use curves to determine how much water should be applied. A corn water-use curve illustrates the long-term average water-use pattern for corn as well as evapotranspiration (ETc) levels through the growing season based on the average daily ETc numbers over a 10-year period. He usually recommends lessening irrigation depth per pass after the R5 growth stage.

From R5, beginning dent in corn, farmers need to apply 4.5 inches of water to finish the crop, he said. On half-starch-line corn, it takes 2 inches of soil water to finish.

"On full irrigation, usually we water until 50% starch line unless the field is completely saturated," Olson told DTN.

With soybeans, it is a little trickier to water late in the season compared to corn due to standability concerns and sudden death syndrome (SDS), Olson said. As with corn, he follows a growth water curve. At R5 beginning seed stage (40 days to maturity), he will apply 5.5 inches of soil water to finish, or at R6 full seed (30 days to maturity), he will apply 3.5 inches of water to finish.

Brittany Bolte, an agronomist at Rock County Agronomy Service in Bassett, Nebraska, told DTN she watches several different factors when considering late-season irrigation for growers. These would include location of active roots, crop growth stage and weather forecasts for temperature, humidity and winds, etc.

"I'm spoiled in that a good chunk of the growers I work with have soil-moisture probes," Bolte said.

If the roots are deep, they are able to pull water as the season goes on, she said. Because of this, there is a bigger "pond" to pull from, and thus, farmers can reduce the irrigation cycle, because those roots are pulling from a bigger source of water.

On the other hand, if the roots are shallow or even slow to pull down water, it will affect the recommendations of timing and amount of water in late season, Bolte said.

Growth stage also dictates how much irrigation water is needed later in the growing season.

Bolte gave an example with corn. When corn is in late season, farmers know they only need so much water to finish out the irrigation season. Using the probes and knowing the active root depth, if there's 2 inches of plant-available water in the soil and if the growth stage standard says only 0.75 inch is needed to finish out the crop, then the farmer would be done irrigating, she said.

Another example would be if the standard says 2.5 inches is needed to finish the crop, but the profile is showing only 2 inches of soil water present. That would require running the pivot to make up the 0.5 inch of water needed, she said.

WATCH THE WEATHER

Bolte said knowing the effects weather can have on the crop will also affect how much irrigation will be needed in late season. A week of 90-degree-Fahrenheit temperatures with 20-mile-per-hour winds in a cornfield starting to dent could suck the soil moisture up faster than a week with cooler temperatures and less windy days.

Quentin Connealy, a corn and soybean farmer from Tekamah, Nebraska, said he keeps a close eye on the weather to make late-season irrigation applications. While he hasn't made the jump to utilizing moisture probes, he does watch field conditions.

"We try to stay ahead so there are no signs of stress showing in the crops," Connealy said. "Making sure the ground isn't cracking open is an obvious sign."

Connealy has about half of his acres irrigated and said he usually puts an inch of water on at a time. With his irrigated corn acres, he is currently finishing irrigating a couple of late-planted fields in the dent stage, which should help gain some test weight.

In addition, he has put 2 inches of water on all of his irrigated beans over the last couple of weeks.

LATE-SEASON IRRIGATION DIFFERENT

Connealy said irrigating late in the growing season is different from irrigating earlier in the season. Less water is used in late season, while earlier in the season, you are pouring more water to the plants to stay ahead of the heat, and the plants are using more water, he said.

"We had 5 inches of rain the first few weeks of July, so plants were made," he said. "We are just trying to fill our ears from there, which has been the priority."

Olson, the Kansas agronomist who is also a farmer, said the last irrigation of the year is just as -- or maybe even more -- important than early season irrigation, especially on late-fill cornfields.

In most cases, he recommends two to three trips of 1 inch irrigation-water passes early in the growing season. Then, later on in the growing season, he runs a 0.50 inch pass to hold standability, he said.

"An accurate determination of growth stage is needed on each hybrid in the field to make sure we don't short those late-fill corns," he said.

Bolte said she has learned over the years that early and late-season irrigation is different. Normally, she has her growers match water amount to active roots, gradually increasing to continue to push roots deep.

"There is a little something to 'stress the plants to root down,'" she said.

The majority of soils in her north-central Nebraska location are sands, and a half-inch of rain or irrigation water can move 8 to 10 inches in the sandy soil. Fertigation (fertilizer applied through the center pivot) starts at V5 in corn, and most roots are likely only 6 inches deep.

Bolte said that, before they started using soil probes, farmers in her region always used 0.75 inch of water with the fertilizer. If the water moves 8 to 10 inches, the fertilizer and the water sometimes move too deep.

Now, most farmers only apply 0.30 to 0.40 inch of water with their first few rounds of fertilizer to keep the water and fertilizer in a zone where the plants can utilize it more efficiently.

"It's amazing the difference even just 0.20 to 0.30 of an inch of water makes for nitrogen movement in the profile, and I feel it's helping in yields," Bolte said. "Also, a faster water lap takes less time, thus saving energy costs. It's a win-win."

UNL's CropWatch Newsletter from August 2017 had an article on how to determine late-season irrigation needs. To read the article, visit:

https://cropwatch.unl.edu/…

Russ Quinn can be reached at russ.quinn@dtn.com

Follow him on Twitter @RussQuinnDTN

(BAS/AG)

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