E. P. Christmas
Agronomy Extension Specialist, Purdue University
When grown under high populations, individual plants produce fewer pods, fewer branches, grow taller, and pod higher off the soil surface than when grown at low populations. Yield potential is maintained with high populations since there are more plants per acre. Soybean populations that are too high also undergo a natural thinning process due to the intense competition between plants, which reduces the stand to a more acceptable level. In other words, plants are eliminated after emergence. In summer, soybean populations can vary perhaps as much as 50 percent from recommended levels without affecting yields, as long as missing plant spaces are not too large and weeds are controlled.
There are varietal differences in soybean response to over- or under-population. Taller varieties that are lodging prone are likely to have reduced yields if populations are too high. Shorter varieties are more likely to have reduced yields if populations are too low. In general, fewer problems occur when stands are established at or near recommended levels.
Soybean plant population recommendations for Indiana are shown in Table 1 (columns 3 and 4) for various row widths. At the wider row spacings (20- to 36-inch), stands that vary +/- one plant per foot will differ little in yield. However, as row width is narrowed (20-inch and below), establishing the stand to within +/- ½ plant per foot becomes more important.
The population guidelines in Table 1 are based on responses of different public soybean varieties. Most varieties grown in Indiana will produce maximum yields at these populations but there are exceptions.
For determinate varieties, you may need to increase plant populations to realize full yield potential. An extreme example is the determinate variety, Hobbit 87, which is recommended for solid seeding (6-, 7-, or 8-inch rows) at populations of 200,000 to 250,000 plants per acre, or about three plants per foot of row. You can see from the table that this is 50 percent greater than the standard recommendations. When planting semi-dwarf types, follow your seedman's population recommendations. These suggested populations have a large built-in safety factor for unusual conditions. There is no need to adjust the seeding rate for soil type or planting date. Recent studies show that soybeans can produce a normal seed yield with populations down to about 60,000 plants per acre when planted by June 7 (80,000 required for later planting) so long as the plant stands are reasonably uniform and weeds are not a serious problem.
Col.6
Col.4 Col.5 Lb. seed
Col. 3 Recommended Required needed if
Col. 1 Col.2 Suggested total plant seeding rate at 2500 seed/lb.
Row Feet of number plants population 90% germination (81% final
width row/a. /ft. of row (Col.2 x Col.3) and 90% emergence emergence*)
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in. ft. plants/ft. plants/a. seeds/ft. lb./a.
36 14,520 7.0 101,640 8.6 50
30 17,424 6.0 104,540 7.4 52
20 26,136 5.0 130,680 6.2 65
18 29,040 4.5 130,680 5.6 65
16 32,670 4.0 130,680 5.0 65
15 34,848 3.7 130,680 4.6 65
14 37,336 3.5 130,680 4.3 65
12 43,560 3.0 130,680 3.7 65
10 52,272 2.8 143,750 3.4 71
8 65,340 2.5 163,350 3.1 81
7 74,674 2.2 168,020 2.8 83
6 87,120 2.0 174,240 2.5 86
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*Final emergence = percent warm germination x percent expected emergence
The basic formula for calculating the seeding rate necessary to establish a particular desired population is as follows:
Suggested plants/ft. of row
Seeding rate = (Col.3 from Table 1)
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Pct. Pct. expected
germination x emergence
Example: A farmer is planning to plant Resnik
soybeans in 30-inch rows. He has purchased seed
labeled "90 percent germination." Previous experience indicates
that, with his soil type and equipment,
he can expect to lose about 10 percent of the seedlings from
crusting problems, leaving a 90 percent
live seed emergence. What should be his seeding
rate to establish the desired stand?
6 plants/ft for 30-in. rows = 6 = 7.4 seeds/ft.
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.9 (germ.) x.9 (emerg.) .81
Thus, our farmer would set his planter to drop
about 74 seeds per 10 feet of row.
Information about seed drop, soybean plates, and sprocket settings is given in the owner's manual for planting equipment. Some manuals use pounds per acre when referring to seeding rates. Therefore, to select desired settings corresponding to seeds per foot, it is necessary to convert to an acre basis. This is done using Table 1 (column 2) and Table 2 in conjunction with the following formula:
Ft. of row/acre x Seeding rate
Lb. seed per acre = (from Table 1) (from above)
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Seeds/lb. (from Table 2)
Referring to our previous example, the farmer's seeding rate on a pounds per acre basis using Resnik soybeans would be:
17,424 ft of row/acre x 7.4 seeds/ft.of row = 128,938
------------------------------------------- ------- = 45.6 lb. seed
2,830 seeds/lb. for Resnik 2,830
Regardless of the planter used, seed drop should be field-verified regularly. The number of seeds per pound also should be calculated for each lot of every variety planted because the number of seeds per pound will vary from lot-to-lot depending upon growing conditions.
The most difficult figure to accurately determine when using the above seeding rate formula is percent expected emergence. Percent warm germination is the standard measure for seed quality; every bag of commercially-processed seed in Indiana must be labeled with this information. The warm germination test is run under ideal conditions (i.e., 7 days at 70° F and high humidity) and thus is essentially a test for live seed.
Maturity group Variety Ave. seeds per lb.* --------------------------------------------------------- Group II Archer 2600 Burlison 2330 Century 84 2520 Chapman 2170 Group III Bass 2800 Edison 3070 Harper 87 2280 Hobbit 87 2650 Linford 2380 Pella 86 2320 Resnik 2830 Williams 82 2570 Winchester 2330 Group IV Flyer 3170 Corsica 2530 Spencer 2600 Delsoy 4210 2320 -------------------------------------------------------- *Number of seeds per pound will vary from year-to-year, depending on growing conditions.
Purdue studies show that there will likely be less difference in emergence between early-and late-planted soybeans with high quality seed than with low quality seed. For instance, at 90 percent germination, we might expect a 3-8 percent emergence difference between a May 10 and a June 10 planting date; but for seed testing 80-85 percent, this difference increases to 8-13 percent.
Secondary tillage affects the condition of the seedbed and influences the accuracy of planting depth, crusting tendency of the soil, and moisture retention. Therefore, review your specific cultural practices, and consider if and how they might influence positively or negatively-seedling emergence when calculating your seeding rate.
The weather during germination also can affect the seed treatment-emergence relationship--i.e., as planting is delayed and conditions for germination improve, there is less emergence response to seed treatment. Remember, too, that fungicide treatment rarely increases yields, so long as adequate minimum populations are established.
Calculating desired plant populations can be approached several ways. Table 1 goes through the exercise of determining seeding rates, expressing them on the basis of seeds per foot of row and pounds of seed per acre.
The accuracy of your seeding rate decision depends on your ability to predict percent emergence. Initially, your prediction as to emergence probably will be based on personal observation. But for future decisions, do a few field checks to confirm or improve your accuracy.
Brand names are used simply for clarity with the understanding that no discrimination is intended and no endorsement by the Indiana Cooperative Extension Service is implied.