The yield potential of any soybean crop production system can be greatly enhanced by planting as early as possible, a University of Nebraska agronomist reports.

James Specht says Nebraska research demonstrates that for each day that soybean planting is delayed after May 1, the yield penalty per day ranges from 0.25 to 0.63 bushels per acre.

Specht cautions that the yield reward arising from early planting should not be used as a reason to plant seed into seedbeds that are too wet to plant. Other than trying to plant early, he says you should exercise good judgment relative to other seed-planting practices.

Specht says there are three primary reasons why early planting of soybeans can enhance soybean yields.

"First, you want to have your soybean crop collect as much of the seasonally available solar radiation as possible, simply because plants require the energy of sunlight to convert carbon dioxide into carbohydrates, protein and lipids," Specht says.

Allowing the soybean crop canopy to cover the ground sooner in the growing season allows the plant to collect nearly all of the incoming sunlight from that day forward. Specht also says no-tillers should keep in mind that day length increases from the equal day-equal night cycle of the spring equinox to the longest day-shortest night cycle of the summer solstice.

"A soybean crop, when planted in late April or early May, is likely to close its canopy within a week or so after the summer solstice," he says. "Later-planted soybean crops will be deprived of the opportunity to collect as many hours of sunlight compared to earlier planted crops, and thus will invariably have less yield potential."

Second, Specht says you want to have your soybean crop transpire a greater fraction of the seasonally available water, simply because there is a linear relationship between the amount of total water transpired by the crop and final crop yield. The seasonally available water includes off-season rainfall that was stored as soil water prior to planting, plus all of the in-season rainfall.

"In order for plants to acquire carbon dioxide to produce plant and seed organic dry matter, the pores in the leaves must open, allowing water inside the leaf to escape," he says. "In effect, plants must exchange water for carbon dioxide."

As a general rule, the soybean-exchange ratio translates into about 1 acre-inch of water, or 27,154 gallons, being required for every 3 bushels of seed produced per acre.

Specht says crop water use includes water lost via evaporation directly from the soil, as well as water lost as transpiration from the leaves. Crop water-use efficiency can be improved by reducing evaporative water loss as this means more water will be available for transpirational water loss.

Early planting helps in this regard because:

• The cooler soil and air temperatures prevailing in late April or early May are much less conducive to soil water evaporation than the temperatures in late May and early June,
• The canopy closes earlier in the season, which reduces the interception of solar radiation by the soil surface. That lessens the heating of soil surface that drives soil water evaporation
• The higher humidity that often prevails in a closed soybean canopy minimizes the degree of evaporative soil water loss.

"In addition to allowing plants to collect more seasonal solar energy for use in photosynthesis, early planting also increases the yield potential by allowing the crop to use more of the seasonally available water for transpiration because less soil water is lost to evaporation," Specht says.

Finally, Specht says you want to have your soybean crop produce as many plant stem nodes as possible, simply because plant nodes are where the plant produces its flowers, then pods and ultimately seeds within those pods

The rates of soybean germination and emergence are temperature sensitive, so these processes are slower in cooler soil temperatures that prevail during early plantings, the agronomist says. However, once soybean plants reach the V1 stage, temperature sensitivity is much less, given that a new node is produced on the main plant stem about once every 3.7 days, until node accrual ceases at the R5 stage, when seed enlargement begins in the uppermost stem nodes.

The node accrual rate between V1 and R5 is not impacted much by the calendar date of planting, Specht adds.

"What is impacted by planting date is the calendar date when V1 occurs," he says. "This is quite important, given that the V1 date establishes the earliest date that linear node accrual can start. Moving the planting date earlier typically results in an earlier V1 date, even though an earlier planting lengthens the number days from planting to V1 due to the sensitivity of soybean germination and emergence to soil temperatures."

Specht says later-planted soybeans simply do not have the opportunity to catch up to the soybean node development of earlier planted soybeans. Thus, earlier soybean planting can increase crop yield potential by allowing plants to generate more stem nodes. It also induces the beginning flower (R1) stage to occur nearer the date of the summer solstice.