In critical growth stage: Many soybean fields need irrigation

The focus for a lot of Arkansas producers is irrigating soybeans, now that most have gotten over the hump on watering rice. The lucky ones who hit the planting window right for early soybeans are pretty much winding up irrigation and have received some pretty good rains to help out. However, most of the soybeans are just getting into the critical stage of bloom and pod development and have a ways to go before the combine rolls.

At mid-August some areas of Arkansas received some welcomed showers — most of it was pretty light. Thank the Lord the temperatures haven't been in the 100s like we are subject to having. Temperatures in the upper 90s projected for the next several days, the critical growth stage of soybeans and the reduced chances for rainfall make irrigation critical for achieving good yields.

At this time of the season it is likely that the soybeans will use 0.2 to 0.3 inch of water per day. This works out to be about 1.5 to 2 inches per week. Producers who have adequate water and labor for flood, furrow or border irrigation will irrigate a lot of their soybean fields every seven to 10 days if they don't get any rainfall.

Soybean fields under center pivot sprinklers probably will need to be watered about every three days or twice a week if possible.

There is always a temptation to delay irrigation and hope that for once that 30 percent chance of rain will actually fall on your farm. If your fields have good drainage, a better plan is to start irrigation and shut it off if you are fortunate enough to get a good rain. Waiting for a rain that doesn't come during this critical growth stage can result in the crop being stressed and losing yield potential.

If soybeans are first drought-stressed and then irrigation (especially with levees) keeps some areas wet for too long, the plants are more subject to suffer.

Starting on time — or even a little early — with flood, furrow and border irrigation, usually results in the field watering better and quicker and less likelihood of damage to the beans. Keeping up with the crop's water use under center pivots is also critical, and most times starting a little early can help.

It is critical that soybeans have adequate moisture to assure that the beans fill the pods. This can become difficult to do if rice harvest is in full swing and everyone is busy. The full yield potential of the soybeans will not be realized if the last needed irrigation is not applied. Research and demonstration studies have shown that the final irrigational usually adds 5 to 10 bushels per acre.

The recommendation is that the final irrigation should assure that there is good soil moisture when the crop is at R6 — pods at one of the four uppermost nodes have seeds that fill the cavity — seed is touching in the pod. At this point the crop is about three weeks from maturity. With flood, furrow and border irrigation there should be adequate water for the remaining pods to fill out during this period.

Since center pivots don't wet as deep, it may be necessary to sprinkle irrigate at and past R6 to assure complete pod fill.

Irrigation cost information shows that the operating cost associated with the last irrigation is $3 to $6 per acre. If one more irrigation is needed, and it often is, a yield increase of 5 bushels per acre would more than cover the cost.

This is one of several articles on drainage and irrigation water management. If you have questions or suggestions on topics please contact me: Phil Tacker, 501-671-2267 (office), 501-671-2303 (fax), 501-944-0708 (cell), or [email protected] (e-mail).

Phil Tacker is an Arkansas Extension ag engineer.

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