Why We Have Lost So Much Soybean Yield Potential

I had really hoped to have our soybeans planted by now. Our potential yields have been lost and keep slipping every day.

All joking aside, the soggy saga continues and it has gotten past the critical
point for area farmers. We have not dodged the “rain bullet" and still can't get
in the field.

The decisions of what to do first will be quite varied. Do we just concentrate on getting corn planted and wait on soybeans? Do we try to do both at the same time to
cover more acres? Do we consider switching away from corn altogether(especially if we are still really wet into nextweek)?

The last couple of weeks we have thought about corn planting and burndown programs.
So, what about soybeans? What can we do to help the yield potential in a crazy year such as this?

Agronomists agreethe obvious potential of any soybean crop production system can be greatly enhanced by planting as early as possible. That never happened this year in the eastern corn belt.

There are three key reasons why we want to plant soybeans early. First, you want your soybean crop to 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 (oils).We don’t want to waste sunlight but we haven't had much this year.

The ultimate goal is for our soybean canopy to be green to the eye by the 4th of July, like we had last year. Also keep in mind that day length increases from
the equal day/equal night cycle of the spring equinoxto 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. Later planted soybean crops will be deprived of the opportunityto collect as many hours of sunlight compared to earlier planted crops, and thus will invariably have less yield potential.

Secondly, 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 producedper 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 lateMay and early June.

• The canopy closes earlier in the season, which reduces the interception of solar radiation by the soil surface, thereby lessening the heating of
soil surface that drives soil water evaporation.

• The higher humidity that often prevails in a closed (versus open) soybean canopy minimizesthe degree of evaporativesoil water loss.

Finally, you want tohave 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, (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. 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. 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.

So, what kind of yield advantage does a producer gain by planting soybeans early? In Nebraska, research reported in the Agronomy Journal demonstrated that for each day
that soybean planting was delayed after May 1, 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 1/4 (0.25) bu/ac in a “not so great” soybean year (like 2003).

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 and 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.

I thought this would explain why we will have less soybean yield potential this year and be helpful to farmers and our non farming consumers.

Ed Winkle