Planting Time Decisions Are Critical for Soybeans

Early season soybean management makes a big difference in yield.

Most of Iowa is now saturated and the very wet soils will take awhile before they dry out. March and the first part of April were generally cooler and wetter than average which is not conducive to early drying. Iowa farmers may be in a similar situation this spring as in 2007.

"Last year, we struggled getting our crops planted on time in central, southern and western Iowa because of the wet conditions," recalls Palle Pedersen, Iowa State University Extension soybean agronomist. "One thing that is important to remember when fields are saturated is that the soil can very easily be compacted. If you 'mud in' your soybeans by planting into soil that is too wet, you get sidewall compaction.

"You are going to pay for it in August from increased incidence of Sudden Death Syndrome disease and from premature senescence - the loss of leaves from plants. These conditions are often aggravated by reduced root growth, and the situation will be worsened if we have a drought, as many weather experts are currently predicting for the 2008 growing season."

Seedbed condition makes a difference

Seedbed conditions are extremely important and are often ignored, notes Pedersen. "True, planting date is also important and you should try to get soybeans planted at an early date if possible. But seedbed conditions should not be ignored. Seedbed conditions should not be overruled by early planting. Planting soybeans early is not advisable until you have good seedbed conditions. 'Mudding-in' your soybeans just to plant early – causing soil compaction and poor seed placement – outweighs any benefit of early planting," he emphasizes.

Stand counts will be important this spring as well. "In a wet seedbed, the probability of damping off from soilborne pathogens such as Pythium is greater than in a dry seedbed," says Pedersen. "There has been a lot of talk about seed quality issues this winter. It seems to me that most of the seed for Iowa should be in good shape. I would guess that less than 15% of our seed planted in the state in 2008 will have germination scores of 85% or less."

"An adjustment of the seeding rate is critical when planting soybean seed with germination of less than 90% so you do not have to replant," he says. "Adjusting your seeding rate if planting into less-than-optimum seedbed conditions with seed less than 90% germination should be taken very seriously. You just cannot afford to make any mistakes here with current high prices for soybeans. There is simply too much to lose out on in our fields."

Pay attention to details from the start

Accurately estimating soybean plant population is important before making replant decisions, he points out. "Plant population should be based on an accurate stand count, along with factors such as yield potential of the existing stand, date of replanting and the real cost of replanting. The existing stand should be determined by evaluating uniformity of stand and overall health of plants. Only some areas of the field may require replanting if the majority of the field seems to have enough viable plants remaining."

Pedersen says it is important to wait several days (three to five) after a crop has been damaged or has emerged before deciding to replant. "Injury can look very serious the day after the event but recovery may be possible," he says. "Previous research from ISU has shown that a final stand as low as 73,000 plants per acre has consistently yielded more than 90% of the optimum plant population that was planted in late May."

The reason is that soybean plants can compensate for missing plants and reduced stands by branching out to make up for a thin stand, explains Pedersen. "Keep in mind the lower the stand count, the more weeds will become a problem due to less shading, especially later in the growing season. If a reduced stand is saved, weed control must be a priority."

For more information on soybean management go to the ISU Soybean Extension and Research Web page at  

New soybean management fact sheets

Six new management fact sheets were finalized in early March 2008, a result of a cooperative agreement between the Department of Agronomy of the College of Agriculture and Life Sciences at Iowa State University and the Iowa Soybean Association. The six sheets cover a variety of selection, soybean planting date, row spacing in soybean, optimum plant population in Iowa, managing soybean cyst nematode and managing soybean for high yield. The six sheets are two-page front- to-back bulletins that can be downloaded from the ISU Soybean Extension and Research Web page at

Limited hardcopies can be requested from the ISU county Extension offices across the state.

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