See the (late) June photos of four strips of soybeans that were planted about two weeks apart from late April or early May to mid-June (Figure 1). Clearly, 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. Why waste free sunlight by letting it hit the ground? Indeed, the goal each year for a soybean producer is simple: “Get that soybean canopy green to the eye by the 4th of July.”

Also 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 (see Figure 2). 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. 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.

2. 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 (known as stomata) must open, allowing water inside the leaf to escape. In effect, plants must exchange water for carbon dioxide. As a general rule, the soybean exchange ratio translates into about one acre-inch of water (27,154 gallons) being required for every three bushels of seed produced per acre.

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 (see Figure 1), which reduces the interception of solar radiation by the soil surface, thereby lessening the heating of soil surface that drives soil water evaporation , and
• the higher humidity that often prevails in a closed (versus open) 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.

3. 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. However, once soybean plants reach the V1 stage (shown in Figure 3), temperature sensitivity is much less, given that a new node is produced on the main plant stem about once every 3.7 days (i.e., about two nodes per week), until node accrual ceases at the R5 stage (shown in Figure 3), 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, (see parallel node accrual trend lines for each planting date in Figure 4).

What is impacted by planting date is the calendar date when V1 occurs. 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 (Figure 4), 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.

Later planted soybeans simply do not have the opportunity to catch up to the soybean node development of earlier planted soybeans. This is evident when you compare the V-stage node numbers in the pictures of the four planted strips in Figure 1 with the linear V1 to R5 node number accrual lines observed for those same four strips in the central graph of Figure 4. 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.

Summary

So, what kind of yield advantage does a producer gain by planting soybeans early? Nebraska research reported in the Agronomy Journal demonstrated that for each day that soybean planting was delayed after May 1 (Figure 5), the yield penalty per day was as much as 5/8 (0.63) bu/ac in a “great” soybean year (like 2004), and still a substantive ¼ (0.25) bu/ac in a “not so great” soybean year (like 2005). Multiplying these yield penalties by the current soybean price provides a clear indication of the importance of planting date in terms of optimizing the net profit potential in a soybean production system.

The yield penalties accruing from delaying soybean planting beyond early May in Nebraska have also been documented in other states:

• Wisconsin 
• Indiana       (see Table 2) 
• Iowa 

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, exercise good judgment relative to the other seed planting practices.

James Specht, Professor of Agronomy and Horticulture