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Tri-State Soybean Forum When to stop soybean irrigations

It happens every year: producers terminate irrigation and then cross their fingers, hoping they haven't jumped the gun. To bring more information to those who fret over that annual decision, Phil Tacker, Arkansas Extension engineer, spoke at the Tri-State Soybean Forum in Delhi, La., on Jan 6.

“The last watering is just one of many decisions we're faced with,” said Tacker. “But it's gotten even more important now that water supplies are shorter and pumping costs have increased.

“Towards the end of the season, a producer is hoping he can quit, but he needs to make sure beans are at their maximum size before shutting the water off.”

In 2005, Tacker was involved in two irrigation termination tests in Arkansas. In the southeast part of the state, a plot was set up at Rohwer, Ark. The second test was in northeast Arkansas at Keiser, Ark.


At Rohwer, Larry Earnest and Scott Hayes ran the test on a field previously in corn. The beans were planted in 19-inch rows around May 10 and came up on May 18.

“When they irrigated, water was sent down every other middle. The Arkansas irrigation scheduler program was used to trigger a watering at 2-inch deficit.”

The test involved two varieties — a Group 3 and a Group 4. In previous years, “we've also included a Group 5. But in all the previous years, the 5s didn't show us much so we zeroed in on the 3s and 4s.”

The Rohwer test had two treatments: quitting irrigation at R6 and quitting at R7. In previous years, “we've tried quitting irrigation at R5. Not surprisingly, that was too soon. Stopping irrigation at R5 won't work well unless timely rains hit.”

The growth stages are determined by finding a pod at one of the four uppermost nodes of the plant that satisfy the following criteria:

  • R5 — seed in pod is 1/8 inch long.
  • R6 — green seed fill the pod cavity.
  • R7 — pod has reached mature pod color.

Large plots, replicated four times, were used. “So we feel good about them being representative of many field conditions producers face.”

The Rohwer plots were watered twice in June. “That's early. One of the hopes with planting early beans is to try and get away from early waterings. But last year that wasn't the case.”

The R6 irrigation for the Group 3s was on Aug. 8. The last irrigation, at R7, was on Aug. 15.

“The Rohwer fields didn't need irrigation during July which was an exception for the state — there were a few spots that got rain and these test plots were one of them.

“There's another point about growing early beans. They've got a short window to do their work in and they can't afford to stub their toe. If they need watering early to maintain growth, you've got to turn it on. The early watering is just as important as that at the end of the season. You're protecting the yield potential.”

Because it was dry and hot, the Group 3s responded well to the last watering at R7. There was a 5 bushel advantage for those beans.

Does the $7 cost of a watering — “and $7 is a pretty good estimate: tubing is already out, so the cost will be diesel and a little labor” — pay with a 5 bushel increase? Even at $5 beans, “that's a $25 return. So you're close to a $20 net. Given those conditions, I do think that last irrigation was worth it.”

The R6 watering on the Rohwer Group 4s was on Aug. 15. The R7 irrigation was on Aug. 22. About five days following the final watering, “there was a pretty decent rain that might have affected our data. Plus, temperatures dropped into the high 80s. Conditions were more moderate for the Group 4s to finish in.

“As you'd expect, the Group 4s didn't respond as much to the final watering. At most, there was about a 1 bushel increase.”

Comparing Group 3s and Group 4s, the 4s “clearly” have high yield potential when planted and irrigated properly, said Tacker. “The 4s hit the high 60-bushel range. The 3s weren't bad, just not capable of doing as much as the 4s in this study.”

In Keiser, the irrigation study was overseen by Chris Tingle, Arkansas Extension soybean specialist, with help from Shawn Lancaster and Alan Beach. The test plots were placed under a lateral sprinkler. The previous crop was also soybeans and the plots were planted on 19-inch rows the first week of June.

“This study was also set up to water on a 2-inch deficit. Three varieties were planted — a Group 3, 4 and 5.

“We looked at irrigating a slightly wider range from R5 through R7 and included an R6-plus-10-days test to get a better idea of what might be happening between R6 and R7.”

There were several early irrigations. In fact, the crop had to be watered just to get it to emerge. “That's one of the benefits of using the sprinkler — it wasn't hard to water the beans up.”

In late August, almost 4 inches of rain fell over a four-day period. The rain seemed to affect results of the study.

“The yields for the R6 to R7 irrigations for all three groups weren't statistically different. But there was a trend toward a yield response to the later irrigations.”

The cost for watering under the sprinkler was about $5 per acre.

“As in the Rohwer study, the Group 4s again expressed the most yield potential.”

So what to take home? “You're out in the field trying to make a call on whether you're through irrigating or not. The final decision often hinges on whether the producer is prone to water more or not.

“We talk about having ‘good moisture’ at a certain growth stage. Well, that's subjective. If a guy doesn't want to water again, he can say, ‘Man, I've got plenty of moisture.’ A producer who tends to water late will see his ground as lacking moisture: ‘Yeah, it's dry enough to water again.’”

At Rohwer, the Group 3s seemed to respond to the R7 irrigation more than the Groups 4s. “The 3s were maturing under hotter conditions, though.”

At Keiser, both the Group 3s and 4s tended to show a response to irrigations after R5 but yields weren't statistically better. The 5s didn't seem to respond to any irrigation past R5 due to the ran received in late August.”

Regardless, in both studies the Group 4s expressed the most yield potential.

Early-planted soybeans may still need to be irrigated early. If there's no rain within two or three weeks after emergence, “you're going to usually have to put some water on it.

“Without question, there must be good moisture at R6. So what's good moisture? Be honest with yourself. Is it dry enough that a rain would really help? Or, if it rained, would the moisture not really show any benefit?”

Beans attempting to mature in dry weather with 95-plus degrees are most likely to respond to irrigation past R6. “You want the size of the beans to be maxed out before they begin shriveling. When that doesn't happen, the pods produce BBs. You'll normally get enough yield from a watering at the R6 stage to pay for the diesel or electricity it requires.”

Once irrigation is in place — “whether pivot, row, or even levee, to a degree” — the cost of a last irrigation isn't too prohibitive.

e-mail: [email protected]

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