Answers to common questions about irrigating soybeans

Frequently asked questions and their answers that can help soybean producers make profitable irrigation decisions.

I have not applied any irrigation water to my soybeans. Will I see an economic return by irrigating them at this point in the season?

Yes, especially if the plants are healthy and irrigation can begin at pod initiation. In general, the most important time to irrigate soybeans is from R3 (beginning pod, one pod 3/16 inch long on one of the upper four nodes on the main stem having unrolled leaves) through R6 (full seed, one pod containing green seed that fills the pod cavity on one of the upper four nodes on the main stem having unrolled leaves). Water applied at R3 to R5 encourages flower and pod retention. This increases yield potential by increasing the number of seeds per acre. Irrigation water applied after R5 (one pod with1/8 inch long seeds on one of the upper four nodes on the main stem with unrolled leaves) is also beneficial as it improves yields by increasing seed size. In fact, if soybeans can be watered only one time during the growing season, it should be at R5.

I began irrigating my soybeans in June to get them through the drought. Can I stop watering them now that we have received some rain?

You may be able to delay irrigation water applications due to recent rain, but plan to apply irrigation water as necessary through seed fill (R6). Failure to continue irrigation may cause more stress on the crop than if no irrigation water had been applied. This is because soybean plants are able to adjust to soil moisture conditions by changing pod number and seed size. Plants may produce fewer and smaller seeds if irrigation is discontinued. The earlier irrigation applications may have also reduced rooting depth. Monitor moisture levels in the top 2 feet of soil closely and maintain the moisture level above 60 percent of the available water holding capacity throughout R6.

When can I stop irrigating soybeans without sacrificing yield?

Research from the University of Missouri showed that terminating irrigation too soon can cause yield losses of 3/4 of a bushel per acre per day on a sandy soil. The Missouri researchers made the last application around September 20, which was well into the R7 growth stage. However, most universities recommend timing the final irrigation run so that the soil moisture level is near 60 percent of the available water holding capacity at the beginning of the R7 growth stage.

Precise timing of all irrigation runs, including the final run, requires a working knowledge of basic irrigation scheduling concepts such as the amount of water required for soybeans at various growth stages to reach maturity, available soil water capacity, allowable water depletions, soybean growth stages, effective rooting depth and estimating soil moisture status. Michigan State University Extension’s Lyndon Kelley and Steve Miller have compiled a list of irrigation scheduling tools that explain and utilize these concepts.

This article was produced by the SMaRT project (Soybean Management and Research Technology). The SMaRT project was developed to help Michigan producers increase soybean yields and farm profitability. SMaRT is a partnership between MSU Extension and the Michigan Soybean Checkoff program.

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