Soybean plant population

The question of how many soybean plants per acre are enough to produce maximum yield has been debated for decades. In recent years, we have heard conversations about increased soybean populationif no-till planting is used, increase soybean population; if narrow rows are used, increase soybean population; if late-season planting occurs, increase population and numerous other reasons. However, based on research conducted at Iowa State University in the past two years, the question of soybean plant population may be less confusing than first thought.

In 1994, ISU conducted research trials at four research stations in northeast, southeast, southwest, and central Iowa. In 1995, research was conducted at the same four locations, with an additional site in northwest Iowa. Plots were no-till planted directly into standing cornstalks, with 30-inch wide-row spacing and a 10-inch narrow-row spacing (7.5-inch row spacing at the central Iowa location). Plant population would not differ for maximizing yield in either tilled or no-till planting situations.

The 1994 and 1995 results are presented in Table 1, with low harvest populations equal to or less than 100,000 plants per acre (PPA); medium populations ranging from 101,000 to 150,000 PPA; high populations ranging from 151,000 to 200,000 PPA; and very high populations being greater than 200,000 PPA. Because of adverse weather and planting conditions in 1995, very high harvest populations were not attained in half of the plots, even though more than 250,000 seeds per acre were planted for the very high treatments. Three sites in 1994 produced low-population, wide-row yields that equaled any other population. The northeast Iowa site produced its highest yield at the high population.

Table 1. Yield response of soybean to harvest populations.

Row spacing
Location of trial 7-to-10-inch rows 30-inch rows
1994 1995 1994 1995
NW (Sutherland) _* M _* LMH
NE (Nashuea) LMHV MHV H MH
SE (Crawfordsville) LMHV LMHV LMHV LMH

+Harvest population range: L<=; 100,000 plants per acre (PPA); M=101,000 to 150,000 PPA; H=151,000 to 200,000 PPA; and V>=200,000 PPA. When more than one population is listed, they are not statistically different.

*no trial in 1994

Row spacing comparisons in 1994 showed no significant difference at two of the four sites. One site produced an advantage with wide rows and the other site produced an advantage with narrow rows.

Results were obtained from five sites in 1995. Weather conditions delayed planting in 1995. In 1995, wide rows produced high yields with low populations at two of the five locations; medium populations produced high yields at the other three locations. Narrow rows produced high yields with low populations at two locations; medium populations produced high yields at two other locations, and the high population produced top yields in central Iowa.

Narrow rows outyielded wide rows at three of the 1995 locations. Wide rows outyielded narrow rows at one location. There was no yield difference between row spacings at the fifth location.

In summary, 10 of 18 location-by-year trials produced their high yield with a low harvest population of 100,000 PPA or less (see Table 1). Another location-by-year combination produced its high yield with a medium population of less than 150,000 PPA at harvest, and only two location-by-year combinations (1995 central Iowa narrow-row and 1994 northeast Iowa wide-row) produced their high yields at a high population of 151,000 to 200,000 PPA.

This is evidence that low to medium populations of less than 150,000 PPA at harvest are sufficient to produce high yields throughout Iowa in most situations. When comparing across row spacings and years in southern Iowa locations, low population is sufficient in seven of the eight trials listed in the table.

The picture is more confusing for northern Iowa. Two of the six row spacing-by-year sites produced high yields with low harvest populations, and three produced high yields with medium harvest populations. In one case, a high population was required to produce high yield.

How many seeds must be planted in order to have a harvest population at a certain level? Experience has shown stand losses of 10 to 15 percent between emergence and harvest. An additional loss occurs between seed planting and emergence. The loss from seeds planted to plants harvested may range from 15 to 25 percent, depending on the environment, management, and seed quality at planting.

Seed cost is another consideration when determining optimum harvest populations for high yield. Seed cost for 50,000 seeds per acre will range from $3.00 to $10.00, depending on seed size and cost of seed.

Table 2 shows a range of seed sizes and cost per 50 lb. bag, with the cost of 50,000 seeds calculated for each combination. Seed cost is not high compared to herbicide and other costs in soybean production, but every few dollars count. If the producer can attain high yields by harvesting between 100,000 and 150,000 PPA, then it should not be necessary to plant 200,000 or more seeds per acre at planting.

Table 2. Cost of 50,000 soybean seed.

Price of 50-pound bag of seed
Seed size (seeds/lb.) $12 $14 $16 $18 $20
Seed cost ($) per 50,000 seeds
2,000 6.00 7.00 8.00 9.00 10.00
2,500 4.80 5.60 6.40 7.20 8.00
3,000 4.01 4.68 5.34 6.01 6.68
3,500 3.43 4.00 4.58 5.15 5.72
4,000 3.00 3.50 4.00 4.50 5.00

Seed size in 1996 is frequently smaller than in 1995. More seeds are found in each bag of soybean seed, meaning more acres can be planted with each bag and overall soybean seed cost can be reduced.

Another factor relative to seed size is that planters and drills must be calibrated, before planting, for the seed size that each producer has for this season. Do not assume that the machine is properly calibrated based on the previous years planting practices. Study calibration tables from the planter manual and check your seeding rate prior to planting time.

This article originally appeared on pages 38-39 of the IC-476 (6) -- April 22, 1996 issue.

Updated 04/21/1996 - 1:00pm