Even as Kansans deal with winter ice and snow, soybean growers know that before long, soybeans will be planted and coming up. And for those who wonder if the timing of irrigation makes a difference, the answer is: yes.

A long-term study at Kansas State University's North Central Kansas Experiment Field near Scandia showed that for maximum yields, irrigation should be timed so plants have water available during the most critical time of reproductive development, which is seedfill, according to Barney Gordon, K-State Research and Extension agronomist- in-charge of the research field.

"The time of peak water use of soybeans is the three-week period from R4 (late pod stage of development), when a three-fourths inch long pod can be seen at one of the four uppermost nodes to R6 or full- seed, which is when the seed fills the pod cavity at one of the four uppermost nodes," Gordon said. "During that time, water use is about 0.3 inch per day."

The study was conducted on Crete silt loam soil in a corn-soybean rotation and was furrow irrigated. The irrigation timing treatments were: no irrigation; blooming stage; podding stage; and levels of 30 percent, 50 percent and 60 percent soil-moisture depletion in the upper 36 inches of the soil. About three inches of water were applied at each irrigation.

In the first six years, the soybeans were irrigated at early bloom (R1); early pod (R3); early bloom-plus-podding; 30 and 60 percent soil-moisture depletion.

The 30 percent and 60 percent soil-moisture depletion treatments with an average of four and two in-season irrigations, respectively, resulted in the greatest yields at 54 and 53 bushels per acre, Gordon said. Average yields for the early bloom, early pod, and early bloom- plus-early-pod treatments were 40, 44, and 43 bushels per acre, respectively.

For the next 11 years, the five irrigation treatments were at late bloom (R2), late bloom plus early pod (R3), late bloom plus late pod (R4), 50 percent soil-moisture depletion, and no irrigation. The 50 percent soil-moisture depletion, which averaged two irrigations and the late bloom plus late pod irrigation treatments had the greatest yields – 54 bushels per acre for both, the researcher said. The average yields for the no irrigation, late bloom, and late bloom-plus- early-pod treatments were 37, 49 and 51 bushels per acre, respectively.

In the last year of the study, the five irrigation treatments were full bloom (R2), full bloom plus full pod (R5), full bloom plus seedfill (R6), 50 percent soil-moisture depletion, and no irrigation. The yields for the full bloom plus full pod and the full bloom plus seedfill irrigation treatments had the greatest yields (68 bushels an acre for both treatments). The average yields for the no irrigation, full bloom, and 50 percent soil-moisture depletion treatments, which had only one irrigation, were 47, 61, and 59 bushels per acre, respectively, Gordon said.

The study also confirmed that irrigation during the early reproductive stages does not maximize soybean yields, but it will result in greater yields than if there was no irrigation, such as in dryland situations, he said.

"Some people will argue that irrigating corn is easier than irrigating soybeans, because corn tasseling, the signal for the start of peak water use, is readily visible. Soybean blooming, on the other hand, lasts for several weeks and continues as pod development occurs," Gordon said. "Also, pods develop from the bottom of the plant so the youngest pods are at the top. For that reason, without careful attention to the stage of plant development, the timing of irrigation may not be at the optimal stage."

K-State Research and Extension is a short name for the Kansas State University Agricultural Experiment Station and Cooperative Extension Service, a program designed to generate and distribute useful knowledge for the well-being of Kansans. Supported by county, state, federal and private funds, the program has county Extension offices, experiment fields, area Extension offices and regional research centers statewide. Its headquarters is on the K-State campus in Manhattan.