April 17, 2013

It’s the third week of April and the welcomed moisture we received has halted any corn planting progress from the first week of April. Often soybean planting is delayed until after corn planting is complete so it’s important to realize how planting date affects soybean yield.

Since 2005 UNL has conducted research on early and late planted soybeans. UNL research conducted by UNL soybean physiologist Jim Specht and funded by the Nebraska Soybean Board found that producers could lose 1/4 to 5/8 of a bushel per day for every day planted after May 1. The amount of yield lost depended on whether the spring was cold (1/4 bu) or warm (5/8 bu). Their research also found that a soybean plant acquires a new node every 3.75 days once the plant reaches V1.

Because producers wanted to know if planting date made a difference in their fields, UNL Extension conducted on-farm research with producers to test the yield differences of early versus late planted soybeans. “Early” were planted April 18 to May 3 and “late” were planted May 14 to May 24.

From 2008 to 2010 the early planted soybeans always out-yielded the later planted soybeans by 1-10 bu/ac, regardless of whether the spring was cold and wet or warm and dry.

When yields from all locations for the three years were averaged, early planted soybeans showed a significant yield increase of nearly 3 bu/ac (Table 1). For research consistency, both the early- and late-planted fields were treated with a fungicide-insecticide seed treatment. We do recommend a fungicide and insecticide seed treatment with early planted soybean, especially in a cool, wet spring like we're experiencing this year.

So why does early planting produce a higher yield in soybeans? The soybean crop needs to collect as much of the seasonally available solar radiation as possible because plants require the energy of sunlight to convert carbon dioxide into carbohydrates, protein, and lipids (oils).

With earlier planting, a soybean crop canopy will cover the ground sooner in the growing season, collecting nearly all of the incoming sunlight from that day forward. The longest day of the growing season is June 21. The soybean crop needs to harvest as much sunlight as possible to create pods, seeds, and ultimately yield, and to do that, its leaves have to start collecting sunlight as soon as possible. The goal for a Nebraska soybean producer each year should be to “Have the soybean canopy green to the eye by the 4th of July.” Later planted soybean crops will be deprived of the opportunity to collect as many hours of sunlight as earlier planted crops, and thus will invariably have less yield potential.

Planting Risks

Today's soybean varieties have much greater germination cold tolerance than older varieties. This is why producers can now push the limits of early planting by sowing the new varieties into 40°F rather than 50°F seedbeds. Germination failure in early planting is not so much due to cold temperature, but rather soggy wet conditions coupled with cold temperatures immediately after planting. Soggy soil conditions favor fungal pathogens and, if accompanied by cooler temperatures that slow soybean seed germination, can give pathogens a favorable environment and more time to infect the seedlings before they emerge. The yield reward from early planting should not be used as a reason to plant seed into seedbeds that are too wet to plant.

With the strange April weather we’ve had, you may also wonder about frost risk. The key to assessing risk is to consider expected date of seedling emergence rather than seeding date, since damage only occurs when emerged soybean tissue is exposed to freezing air temperature of 32°F. Soybean typically emerges in 7-10 days (although we’ve seen that vary depending on year). Planting 7-10 days earlier than the percent frost risk factor for which you are comfortable is a good rule of thumb.

Planting Recommendations

Based on five years of research, UNL’s recommendation is for producers to plant their soybeans earlier than they traditionally have. A good guideline is for the southern two-thirds of the state to plant in the last week of April and the northern third of the state to plant the first week of May. Use a fungicide plus insecticide seed treatment to reduce insect and disease risk. When planting rainfed soybeans early, extend the growing season to take advantage of August rains. Change to a quarter or half longer relative maturity (RM) than you would use for a later May planting.

With corn planting to resume full-speed when soil conditions are right, soybean planting may be delayed. This can cost you  yield. Some producers have found that the extra yield pays for itself in hiring someone to custom plant soybeans while they plant corn. Consider penciling this out for your operation. The increased yield benefit of planting early with a custom planter or designating one of your planters to beans may pay for you as well!

Conducting Your Own On-Farm Research

If you’re interested in testing soybean planting dates or conducting any other on-farm research study, please contact your local UNL Extension educator for help in designing an on-farm research comparison. For more information regarding on-farm research, please go to the Farm Research section of CropWatch.

More Information

For more information on this topic, see these resources:

• Soybean Planting Date When and Why (EC145)
• Three Reasons Why Planting Date Matters
• Data Show Nebraskans Planting Soybeans Earlier Each Spring
• Producer On-Farm Research Report: UNL Corn and Soybean Planting Date, Planting Rate, and Depths

Jenny Rees, UNL Extension Educator
Jim Specht, UNL Professor of Agronomy and Horticulture