Limited Late-Season Irrigation Can Benefit Wheat

Writer: Tim W. McAlavy, (806) 746-6101, email: t-mcalavy@tamu.edu
Contact: Calvin Trostle, (806) 746-6101, email: c-trostle@tamu.edu

LUBBOCK – “How much, if any, should I water my wheat crop?” That’s the question many South Plains
producers are asking now, according to a Texas A&M agronomist based at the Agricultural Research and
Extension Center here.

Recent rains have eased this concern considerably, but questions persist -- especially for late-planted wheat with
marginal yield potential.

“Whether or not your wheat crop will benefit from late-season irrigation depends on three factors,” said Calvin
Trostle, Extension agronomist. “How much nitrogen did you put down? What growth stage is it in? And, what is
the yield potential?

“We recommend feeding wheat 1.2 to 1.5 pounds of nitrogen per acre. So, a crop that received 60 pounds of
nitrogen could produce up to 50 bushels per acre. That crop has enough groceries under its belt to make good
use of a late-season irrigation, whereas unfertilized wheat does not.”

For season-long irrigation on the Texas South Plains, one acre-inch of water typically adds three to four bushels
to a crop’s grain yield. But at what cost?

Using a market price of $2.75 per bushel, a one-inch late-season irrigation that adds 3.5 bushels of yield could
return $9.63 per acre (3.5 bu. X $2.75 bu.), Trostle said. If pumping costs range from $6.50 to $8.50 per acre-
inch, this would produce a slight profit.

An accurate estimate of pumping cost helps figure this potential gain or loss, he added. Growers shouldn’t wait
until they get their energy bill to figure pumping costs. It’s best to call your energy supplier and get the cost
information necessary to make this economic irrigation decision, Trostle said.

“Producers can gauge the third factor – yield potential – by checking the growth stage of their crop and
examining a few flag leaves. A healthy flag leaf is essential to good yield potential,” he said. “The flag leaf
contains 75 percent of the leaf area that contributes to grain filling.”

Trostle said wheat that is still in the boot growth stage can benefit greatly from irrigation.

“The boot stage is an optimum time to water wheat. Yield response to timely, limited irrigation often surpasses
the three to four-bushel boost expected for each acre-inch irrigation,” he said. “Much of the late-planted wheat
sowed last fall is now in the boot stage.”

Recent rains have relieved some moisture stress on wheat. Even so, Trostle suggested that growers with
marginal yield potential should limit their down-side expenses by using a two-fold irrigation strategy.

“Adding about 1.5 inches of water in the mid- to late-boot stage – when heads are just starting to emerge –
should provide a good yield response. Good moisture just before flowering can boost yield potential by
increasing the number of seeds per spikelet,” the agronomist said. “Flowering (bloom) typically occurs four to
five days after heading...on the main stem and on the tillers.

“I would also consider applying another 1.5 inches of water about 14 days later. This moisture will carry the
crop into grain fill and should help enhance seed size, which is the final component of grain yield.”

The decision to irrigate a marginal crop that is already heading may be a harder call to make.

“If the crop has already flowered, there is a very narrow window to gain any benefit from a late-season
irrigation – because grain fill can occur in about 30 days if the crop is stressed,” Trostle said. “Once the kernels
are past watery ripe, there is little benefit in a late-season irrigation.

“If the crop’s yield potential is less than 30 bushels per acre, I would not consider a late-season irrigation –
pumping costs being what they are. Even if a late-season irrigation helps bring your crop to harvest, you will
still have to overcome combining (harvest) expenses.”

“The key to getting any benefit from late-season irrigation is knowing the nutrient status, growth stage and
overall yield potential of your crop. With that information at hand, you can factor in pumping costs and pencil
out a profit or loss,” Trostle concluded. “Even though timing is crucial, much of this year’s crop still has the
potential to add more than 3.5 bushels per acre from limited, but timely, late-season irrigation.”