No. 4 Article 11/April 15, 2005

Crop Conditions and Soybean Seeding Rates


The cool, dry spring weather has been very good for the winter wheat crop, and warmer temperatures are beginning to get the crop growing well. Color is good in most fields, but those that were planted late last fall may still be showing somewhat uneven growth as tillers continue to be added, at least in the northern part of the state. Statewide, the crop is rated at only 62% good or excellent, which is a mediocre number. As long as conditions don't turn warm and wet, this number might improve some as we move toward heading. Heading normally begins in the southernmost area in Illinois in late April, and with average temperatures it takes about 4 weeks to move to the Wisconsin line. While we hear reports of various rusts in areas south and west of here, continued stretches of dry weather should be good for the Illinois crop.


Corn in the planting-date study that we planted at Urbana on March 30 emerged by Monday, April 11. Growing degree-day (GDD) accumulation from March 30 through April 10 totaled 130. We normally expect emergence in about 120 or so GDDs, but it can often take more than that, especially when planting is early and soils are cool. The additional 21 GDDs on Monday brought the crop up fully. From April 1 to 10 of 2004, we had only 55 GDDs at Urbana, while this year we accumulated 110. That will be favorable for the emerging crop, unless of course low night temperatures drop to near freezing later, which will injure top growth. As soils warm, their ability to protect small plants through radiational warming will increase, but we'd much rather have temperatures stay above freezing for the 14% (as of April 10) of the corn crop already planted in Illinois.

Soybean Seeding Rate

The question of soybean seeding rates has taken on more importance through the combination of higher seed costs, large seed size of the 2004 crop, and the use of insecticidal seed treatments that can add as much as one-third to the cost of seed. Because most soybean seed is sold by weight, large seed can easily cost a third more than small seed, on a per-seed basis. I've heard of seed sizes as large as 2,000 per lb this year, which compares to sizes more commonly around 3,000 per lb for some varieties, especially when produced under less favorable conditions than we had last year. Insecticide/fungicide-treated seed may cost around $30 or even more for a 50 lb unit this year, and at a seed size of 2,200 per lb, one unit is only 110,000 seeds and won't even plant an acre. Seed costs per acre for soybean are approaching or even exceeding those for corn in some cases.

The most pressing question for most people is how many soybean seeds we really need per acre in order to establish enough plants to maximize yield. A great deal of research has been done to try to answer this question, and while results tend to be more inconsistent than we might expect, I can summarize such work with the following statement: The chance of getting yield increases as plant population moves above 100,000 plants per acre is small.

Please note that, while chances of getting yield increase from raising plant populations above 100,000 are small, there can be conditions under which that many plants are not enough. This is more commonly the case with late planting, when plants don't get as large and may fail to produce a full canopy. Even then, narrowing rows would more likely increase canopy cover and yield than would having more plants per acre. Dry conditions or too-wide row spacing with earlier-planted soybean can also mean that more plants per acre are necessary to maximize yield. Such conditions are uncommon in Illinois.

If we accept that we need at least 100,000 plants per acre when planting in May under good conditions, how many seeds does it take to get to this population? When soybean seed was less expensive than it is now, planting 180,000 to 200,000 seeds per acre was commonly seen as a type of "insurance"--not necessary in most cases, but not very expensive. Seed companies spend a great deal of time and effort handling and processing soybean seed to try to improve its ability to emerge, and as a result we should in some cases raise our expectation for percent establishment. This, along with more expensive seed, should point some toward lowering seeding rates.

As a rule of thumb, I think we should expect establishment percentage to be about 10 percentage points less than the warm test for the soybean seed we're planting if planting is done under good conditions and if we're using row units to place seed. In other words, expect 83% establishment if the seed tests 93% germination. Realistically, most of us will want to establish more than the 100,000 population previously discussed. For our example, if we want to establish 120,000 plants and we expect 83% of seeds to establish a plant, then we should plant 120,000/0.83 = 145,000 seeds.

If we are using a drill to plant soybean seed, it may be appropriate to lower the expected establishment percentage some, perhaps to 15 percentage points less than the warm germ of the seed. Most studies have failed to show a consistent need for more plants per acre in narrower rows, though from a visual standpoint, having only 120,000 or so plants emerging in drilled rows often looks like a deficient stand. We need to try to erase impressions of what a sufficient stand "looks like," though, and go on stand counts after emergence to assess adequacy.

The point is often emphasized that certain plant populations are adequate only if plants are "healthy and evenly distributed." We know that's seldom the case, but it holds true for those parts of fields that don't drown out or get destroyed by some other means. The chances of having stands too low to keep are not as closely related to planting date as we might think. Instead, low stands are almost always the result of heavy rainfall just after planting, with soils saturated for a few days or formation of a soil crust that prevents emergence. These conditions can develop regardless of planting date and can be even worse in warm soils following late planting, because of more rapid death of seeds without oxygen in warm soils and more rapid formation of soil crusting under hot, dry conditions.

Adjustments in planting rate thus might be appropriate depending on soil conditions and expected weather at the time of planting, but planting more seeds to escape a disaster seldom pays. If emergence is only 20% because of saturated soils or soil crusting, the field will need to be replanted whether we first planted 150,000 or 225,000. The high cost of soybean seed has raised the stakes, but we need to approach soybean seeding rate with the goal of establishing a reasonable number of plants rather than as an "insurance" decision. Such insurance is no longer inexpensive.--Emerson Nafziger

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