Soybean can be planted later than corn and not suffer the same level of yield loss. Nevertheless the ideal time to plant soybeans is the first 7 to 10 days of May. After that, yields losses increase exponentially with later planting.

Planting date research conducted by Emersion Nafziger at the University of Illinois enabled him to construct a response curve showing the increase in yield loss as planting is delayed past May 10.

• Yield loss is less than 10% by mid May
• Yield loss is about 16% by the end of May
• Yield loss continues to accelerate when planting is done in June

If you worry that your soybeans will be planted later, say after May 20 or after June 1, there are a couple strategies you can implement to keep your yield prospects high.

Variety: Switching out corn hybrids as planting is delayed is important with corn because corn needs to blacklayer before the first frost. That doesn’t really apply to soybeans as the seeds will continue to fatten until the first frost that kills the plant. The rule of thumb with soybeans is to plant the same variety regardless of planting date for full-season varieties. If you want to plant a shorter maturity group, any gain in yield will probably be lost by planting an unadapted or lower yield variety. However if it gets beyond June 15, planting full-season soybeans is like planting double-crop soybeans, so perhaps shorten the maturity group by 0.5 to 1.0. However your full-season variety might do just as well because it was selected for adaptation and high-yield potential.

Because soybeans are photoperiod sensitive, they will flower at the same time despite planting date and relative maturity, and that really kicks off around summer solstice (June 21). Earlier planted soybeans have more nodes and branches with node and pod potential, and thus greater yield potential.  And later planted soybeans are shorter with fewer nodes.

Row spacing and population: You have more opportunity to retain your yield potential if you narrow the rows and increase the plant population. Later planted soybeans are shorter and have less nodes and branches and fewer potential pods. Since yield is really a function of pods and seeds per acre the more plants you have in the field the greater pod production and yield potential. If planting after the first of June, increasing seeding rate by 10 to 15% can be an advantage. Increased plant density or narrowing the rows will close the canopy over sooner, make the plants taller and increase number of pods and seeds per acre. Earlier row closure also helps with weed control.

Harvest: Remember that delayed planting doesn’t necessarily delay harvest. A three-week delay in planting equates to at most one-week delay in physiological maturity.

Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com.