Soybean with signs of cold injury

Soybean with signs of cold injury after planting.

Editor’s note: The following was written by Mark Licht, assistant professor and Extension cropping systems specialist with Iowa State University, for the university’s Integrated Crop Management News website April 14.

Farmers in Iowa have been experimenting with very early soybean planting dates, from late March to mid-April.

Based on reviews of the yield data, there is not a consistent yield benefit to planting early as opposed to planting the first week of May. But one benefit of very early planting is that it extends the planting window, allowing farmers to take advantage of ideal soil conditions in April.

The data suggests — more so for corn than soybean — that early planting also reduces the risk of yield penalties for late planting.

But there are significant risks to planting soybean very early. Below is a list and how to mitigate those risks.

Imbibitional chilling

This occurs when soybean seed imbibes very cold water, under 45 degrees, in the first 12 to 24 hours after planting. Imbibitional chilling is often associated with a cold front that brings not only colder temperatures but also cold rainfall within 24 hours of planting.

Cold injury can still affect the seed and seedling beyond 24 hours after planting, however symptoms are typically less severe. Visible symptoms of imbibitional chilling and cold injury include uneven emergence and dead tissue on the exterior of the cotyledons.

Reduce the potential for imbibitional chilling and cold injury by utilizing these management practices:

  • Look for seed varieties that have better early season vigor or cold tolerance.
  • Plant high-quality seed having intact seed coats that are free of wrinkles or cracks.
  • Avoid planting seed having low initial moisture levels.
  • Plant in the early afternoon to allow soils to warm up — ideally 50 degrees and rising.
  • Avoid planting when a cold rain is forecasted within the next 24 hours after planting.

Frost/freeze damage

One of the biggest risks of very early planting is that emerged plants could be damaged by freezing temperatures. Luckily, planting soybeans in cooler soil temperatures will delay germination and emergence.

Additionally, soybean cotyledons are more resistant to freezing temperatures because they are thicker than corn tissue. Typically, temperatures must dip below 30 degrees for damage to occur to emerged cotyledons. When unifoliate leaves — along with the first trifoliate leaves — develop, the plants become susceptible to damage from temperatures at or below 32 degrees.

Removing residue from the row has mixed results when it comes to very early planting. It will cause the soil to warm-up sooner and promote earlier germination and emergence, but could also expose the newly emerged plants to greater risk of frost injury.

Reduced populations

Attaining a soybean harvest population of 100,000 plants per acre leads to maximum yield potential. However, one of the issues associated with very early soybean planting is a loss of plant population. This loss could be an additional 10-20% reduction in the emergence rate compared to what is normally expected.

This reduction can be caused by imbibitional/cold injury, seedling diseases and insect feeding.

Increasing the seeding rate by 10-20% can compensate for the loss of plants per acre, but early planted fields may still have less uniformity across the field.

Seedling diseases

The risk of disease pressure will be greater with very early planted soybean due to being confronted with cool and wet soils. For this reason, it is recommended to use fungicide seed treatments when planting early. Select fungicides that are effective against fusarium, pythium, phytophthora and rhizoctonia.

Variety selection can also be used as a tool to reduce risk associated with sudden death syndrome and phytophthora when planting very early.

Bean leaf beetles

The potential for damage from bean leaf beetles is increased by very early planting as the beetles tend to be more problematic on the earliest emerging soybean fields in a given area. However, the use of insecticidal seed treatments diminishes this risk.

The overwintering adult bean leaf beetles would be an indicator of first- and second-generation risk later in the summer.

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