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 soybeans — that early planting also reduces the risk of yield penalties for late planting.

Major risks exist if planting soybeans very early. Here’s a list and how to mitigate those risks:

Imbibitional chilling injury. This occurs when soybean seed imbibes very cold water, under 45 degrees F, 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. You can reduce the potential for imbibitional chilling and cold injury by using these management practices:

Frost and 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. Also, 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 it could also expose the newly emerged plants to greater risk of frost injury.

Reduced plant 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% to 20% reduction in the emergence rate compared to what is normally expected.

This reduction can be caused by imbibitional or cold injury, seedling diseases, and insect feeding. Increasing the seeding rate by 10% to 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 soybeans due to cool and wet soils. 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.

Licht is the ISU Extension cropping systems agronomist. Contact him at [email protected].