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Getting Soybean Planting Right

Emerson Nafziger
Emerson Nafziger
Professor of Crop Production and Extension
Department of Crop Sciences
(217) 333-4424; ednaf@illinois.edu

In a research project that we started at five Illinois locations in 2001, we are trying to provide answers to questions concerning soybean planting date and rate. Specifically, we want to know if planting soybean very early—say, even before we finish corn planting—might boost yields in some cases, or whether it’s a dangerous practice that can reduce yields more often than it increases them. We also want to know if seeding rates should change as soybean planting is delayed. Furthermore, we added seed treatment as a variable to see if it lets us lower the seeding rate. Funding support for this work came from DuPont/Pioneer Hi-Bred International.

Response of soybean yield
Figure 1. Response of soybean yield to seeding rate and
planting date at Urbana, 2001-2002.

In our Agronomy Day presentation, we will talk about the results of the 2001 and 2002 trials at Urbana. We are also running this study in 2003, and at four other locations, but results in central Illinois—Urbana and Monmouth—have been relatively consistent, and so these results should be somewhat predictive for central Illinois. There were four planting dates: early April (April 5 both years), late April (April 25 and 24 in 2001 and 2002, respectively), mid May (May 12 and 10), and late May (May 28 and 30). At each date, we planted four seeding rates—75,000, 125,000, 175,000, and 225,000 viable seeds per acre—of an adapted (Pioneer 93B67, Maturity Group 3.6) variety, with and without ApronMaxx seed treatment at each seeding rate. We also planted 175,000 treated seeds of an earlier variety (Pioneer 93B26) and of a later variety (Pioneer 93B84) at each planting date. Due to available planting and harvesting equipment, we are conducting the study in 30-inch rows.

Figure 1 shows the response of yield to planting date and seeding rate, averaged over 2001 and 2002. Two things are clear: It is more detrimental to yield to plant before mid April than it is to plant in late May, and the response to seeding rate does not change a great deal as planting date changes.

Seed treatment produced almost no response in the two years of this study at this location (Figure 2). It did not affect stand in most cases, regardless of planting date, and averaged across planting dates, it had no effect on yield. While this is not evidence that seed treatment is never needed, it shows clearly that there will sometimes be no response to seed treatment, even when planting very early.

Response of seeding rate and fungicide seed treatment
Figure 2. Response of seeding rate and fungicide seed treatment,
averaged over planting dates, Urbana, 2001-200

Our results (Figure 3) show that using a latermaturing variety might be a useful strategy if planting very early. The late variety also produced the most stable yield across planting dates, but the mid-season variety produced the highest yield averaged across planting dates. We do not see a reason to change from midseason varieties when planting is delayed, though our results suggest that simply choosing a high-yielding variety may be more important than its maturity.

Yields of three different maturities of soybean varieties
Figure 3. Yields of three different maturities of soybean varieties
planted at different times at Urbana, 2001-2002. Early = Pioneer 93B26
(3.2); Mid = Pioneer 93B63 (3.6); Late = Pioneer 93B84 (3.8).

It is possible to apply economics to find optimum soybean seeding rates based on the data shown here (Figure 4). With soybean seed at $22.00 per 50-lb bag (150,000 seeds at 3,000 seeds per lb.), the optimum planting rate calculates to about 131,000 viable seeds per acre (shown by the square on the line), or about 145,000 seeds per acre at 90 percent germination.

Soybean response to seeding rate
Figure 4. Soybean response to seeding rate at Urbana, averaged
over two years, four planting dates, and with and without seed
treatment. The square marks the calculated optimum seeding rate.

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