Tag Archives: Yield

It’s Time to Increase Soybean Seeding Rates

Wheat harvest has begun.  Remember that soybean yield potential declines with delayed planting date.  During the first two weeks of June, this decline is barely noticeable.  After mid-June, expect yield to decline about ½ bushel per acre per day.  So to minimize soybean yield loss due to late planting, immediately (or as soon as you can) plant.  I like to see the combine and the planter or drill in the field at the same time.

Soybean planted after mid-June will not always develop enough leaf area to capture 90-95% of the available sunlight by early pod development (R3).  Only if four layers of leaves are formed by that time will yield not be affected.  Of course the growing season makes a difference.  Last year in many parts of Virginia, even the latest-planted soybean were able to produce enough leaf area.  But, on average, June- or July-planted soybean are unable to do so.  Therefore it’s important to take actions to minimize this decline.

First, plant as soon as possible.  With the recent rains, soil moisture should not be a problem.  Even if the soil is dry on top, I suggest to go ahead and plant.  You may have to plant a little deeper to hit moisture (don’t plant much deeper than 1.5 inches), but planting depth is not as big of an issue with double-cropped as with full-season soybean.

Second, narrow your row spacing.  Narrow rows will produce more leaf area and canopy over faster.

Finally, increase your seeding rates gradually as time goes on.  I’m suggesting the following plant populations for Virginia in 2013:

  • June 2-8 – 120,000 plants/acre
  • June 9-15 – 140,000 plants/acre
  • June 16-21 – 160,000 plants/acre
  • June 23-29 – 180,000 plants/acre
  • June 30-July 5 – 200,000 plants/acre
  • After July 5 – 220,000 plants/acre

See the seeding rate chart below to determine the seeds per foot of row needed to obtain a certain number of plants per acre.  Note that these seeding rates assume 80% emergence.  For different emergence assumptions, divide your desired plant population by the percent emergence that you expect.


Desired Plant Population

Row Width

Early-June Planting Date

Late-June/Early July Planting Date








(Seed per foot of row assuming 80% emergence)




































As planting is delayed, increase seeding rate.

Use lower seeding rates on more productive soils.

Calculation: Desired Plant Pop. ÷ (43,560 sq. ft./acre ÷ row width in ft.) ÷ % emergence

Example: = 180,000 ÷ (43,560 ÷ (15 ÷ 12)) ÷ 0.80 = 6.5 seed per foot of row

In addition to planting date, I’ve found that more productive soils will tolerate lower seeding rates, and less productive soils need higher seeding rates; a more productive soil will produce more leaf area than a less productive soil over the same time period.

Full-Season Soybean Seeding Rates

My soybean seeding rate recommendation for full-season production systems is to plant enough seed to insure 80,000 uniformly spaced plants/acre.  If you cannot uniformly space the plants within a row, then my recommendation rises to 100,000 plants/acre.  Based on numerous seeding rate experiments conducted in Virginia, I feel very confident in this recommendation.

Of the 27 full-season seeding rate experiments conducted from 2004 to 2009, I can group soybean yield response to plant population into the following three categories: 1) no response to plant population; 2) optimum plant population of 75-100,000 plants/acre; and 3) optimum plant population of 100-140,000 plants/acre.  Below are individual tests that represent these categories.

Full-Season Seeding Rate Study - Virginia

First, note that these examples only show the response of yield to plant population and do not take into account seed costs.  When seed costs are included, the optimum plant population is lower than is shown on the graphs.  Also note that these graphs show yield response to plant population, not seeding rate.  To convert to seeding rate, adjust these numbers to reflect your expected percent emergence.  For example if you assume 75% emergence, you would need to adjust your seeding rate to 133,000 seed/acre to obtain 100,000 plants/acre.

We conducted these experiments with maturity group 4 and 5 varieties.  While one may think that more seed might be required for early-maturing varieties, this was not the case (i.e., group 4 and 5 varieties responded similarly).

There may be some correlation with yield potential as listed below:

  • 30-40 Bu/A Yield Potential (14 tests)
    • 6 required 100-130,000 plants/acre
    • 2 required 70-100,000 plants/acre
    • 6 had no yield response
  • 40-60 Bu/A Yield Potential (7 tests)
    • 2 required 70-100,000 plants/acre
    • 5 had no yield response
  • > 60 Bu/A Yield Potential (7 tests)
    • 2 required ~130,000 plants/acre
    • 4 required 70-100,000 plants/acre
    • 1 had no yield response

So, what do these data mean?  It means that every environment (year & location) is a little different and there is no way that we can predict with 100% accuracy the exact seeding rate that will be required for your field in the coming growing season.  However, we do know that if we can obtain full canopy closure (90-95% light interception) by full flower (R2 stage) to early pod (R3 stage), we can maximize soybean yield potential.  In a dry year or under a droughty soil (low yield potentials), greater seeding rates will help insure this.  Still in most cases (30 to 60 bushel yield potentials), 70-100,000 plants/acre are adequate.

What about fields with greater than 60 bushel yield potential?  In this case, we need to look beyond adequate leaf area and need to start thinking about how many pods the soybean plant can support.  For instance, at 40 bushels/acre and 100,000 plants/acre, we only need to produce 72 seed/plant (using 3000 seed/lb) or about 30 pods/plant (using 2.5 seed/pod).  But, at 60 bushels/acre, we need to produce 108 seed or 45 pods per plant; at 80 bushels/acre, we need 144 seed or 60 pods per plant.  Considering that 12 reproductive nodes per plant are possible, 4 to 5 pods per node on a rather tall plant would be required.  Although branching will also contribute to yield, that seems a lot to ask of one soybean plant.  So, if you are trying to win the yield contest or are irrigating soybean, I suggest planting enough seed to obtain 120-140,000 plants per acre in a full-season system; otherwise 80-100,000 plants are adequate.

Soybean Crop is Still at Risk to Drought & Pests

Our soybean crop is moving right along.  Much of our May-planted crop has hit or it approaching the R6 stage.  Our double-cropped soybeans are in the R4-R5 stage depending on their maturity, planting date, and whether or not they experienced early-season drought.  There’s a tendency for us to believe that the crop is made after the seed have met in the pod.  However, as shown below, only 50% of our yield has been made at the R6 stage. 

Dry weight will continue to accumulate in the seed for the next 3 weeks until the crop reaches the R7 stage (physiological maturity), which is defined as one pod on the plant that has reached its final mature color.  Much of this seed weight is due to translocation from other plant parts, especially the leaves and petioles (remobilization).  Sometimes soybean is called a self-destructing plant.  This is largely true.  As photosynthate is moved from the leaves to the seed, the leaves will turn yellow and drop from the plant.  If the leaves are yellowing and dropping due to this natural process, then we shouldn’t be concerned.  However, if the leaves are dropping due to another reason such as dry weather or disease, we should take notice.  It is important to keep the leaves green as long as possible to maximize yield.

It takes about 2 weeks to move from the R5 to R6 stage and another 3 weeks for to move from R6 to R7.  This is a long time and much can happen during this period.  First, we must protect the developing seed from insects such as corn earworm and stink bugs.  After R6, these two pests are not as much of a concern, but defoliators such as soybean looper can still remove green leaf area, which I’ve already stressed, is vital for maximum yield.  We also need to be concerned about foliar disease, which can also rob us of green leaf area

Lower Seeding Rates & Maintain Yields with Narrow Rows & Uniform Stands

Before the cost of soybean seed jumped with the introduction of the Roundup-Ready trait, soybean was planted at much greater rates than was needed for maximum yields. Back then, seed was cheap. It only cost a few dollars to insure adequate stands – even if only 50% of those seed emerged. Today, planting extra seed is a luxury that we can no longer afford, especially if the seed includes one or more of the many seed treatments now available. Furthermore, with better planters and drills that easily handle residue and place the seed at the proper depth, we tend to get better stands. But, just how low can we go? How many seed per acre does it take to maximize yield?
Over the past 8-10 years, I’ve collected a lot of seeding rate data on different soils throughout Virginia. The main conclusion from that research was that seeding rate recommendations were too high for full-season (May-planted) soybean and barely adequate for double-cropped (late June- to July-planted) soybean. Over the past three years, I’ve also collected quite a bit of data following a barley crop (early June planted) and I feel comfortable with relatively low seeding rates for this cropping system. My current recommendations are listed below. Note that the table lists the desired final plant population in plants per acre and the seeding rate calculations (in plants per row foot) are based on 80% emergence. Your final stand may vary depending on emergence. In general, for May plantings, 80,000 plants per acre are adequate for all row widths.

Suggested Soybean Seeding Rates for Virginia

Don’t Widen Rows to Save Seed. I’ve heard talk of widening rows to save seed. It’s true that if you move back to 30 or 36” row spacing, you’ll not use as much seed. That’s because you can only crowd so many plants into a row and get a benefit. Basically, too many plants within the row are competing with each other. But, you’re losing the benefit of narrow rows, which is, once again to capture 90% or more of the sunlight by flowering and early pod development. Sure, it’s possible that in a good year on a good soil, you’ll be able to meet these requirements. But over time, it’s not likely. On the other hand, row spacing of 15 to 20 inches should provide adequate canopy if planted in May through mid-June. Keep in mind that the later you plant, the greater the benefit from narrow rows. Also, narrow rows benefit early-maturing varieties more than late maturing varieties. And, as I’ve alluded to earlier, there is less of an advantage to narrow rows on more productive soils. Therefore, some fields will likely respond more to narrow rows than others.

Here’s a good way to check and see if your row spacing is narrow enough for a particular field. This summer, when the soybean crop is in full bloom, walk your fields. As you’re walking, look into the canopy. Can you see any ground? If so, then you’re rows are not narrow enough. If you’re already planting in narrow rows, then consider raising your seeding rate. This will insure that you’re meeting the leaf area requirements.

Uniform Stands Matter. We generally think that soybeans will compensate for poor stands, wide rows, and/or gaps within rows. Soybean will compensate much more than other crops. However, I think that uniformity of stand within the row matters.  The more uniform the spacing between plants within a row, the greater the yield potential. This was shown with double-cropped soybean in research conducted in Virginia that compared a 15-inch planter with a standard drill and a drill that uniformly distributes the seed in the row (see figure below). I’ll refer to this drill as a “precision drill.” Details of the drills used are shown below.

Meteriing wheels for various seed types in the Great Plains 1510P or 1520P Precision Seeding System

Type, make and model, seed singulation method, and row spacing of seeding equipment used in this study.

Stand uniformity with the precision drill was equal to the vacuum-meter planter and better than the standard drill. Yield results are also shown below.  Soybean yield with the standard drill was equal to the vacuum-meter planter in three of four years and less than the vacuum-meter planter in one year. Soybean yields were greater when planted with the precision drill than when planted with the vacuum-meter planter in 2 of 3 years, and averaged 10% over three years of study.