SOYBEANS
Plant Populations and Seeding Rates

Plant Populations

      Although plant population can vary considerably without affecting final yield, there can be significant effects upon plant growth and development. For instance, thin stands may cause shorter plants with large stalks, more branches, lower podset, and more pods per plant. Thick stands may be the exact opposite, contributing to taller plants with smaller stalks, fewer branches, higher podset, fewer pods, and more barren stalks. Considering soybean seed costs and their ability to compensate for wide plant spacings, utmost attention should be given to seeding rates.

Seeding Rates

      Adjusting planters to plant a specific number of seeds per foot of row is the key to obtaining a proper stand. Check planters before you go to the field to determine the actual number of seed per foot of row. However, actual counts in the field are necessary to make final planter adjustments.

      Table 1 lists the number of seed per foot of row required to achieve a certain final plant population. Because seed quality varies, three levels of germination are used as examples in this table. It is important to know that most purchased seed are tagged (labeled) at 80 percent germination. Eighty percent is normally the lowest germination acceptable by certification standards and establishes the lower limit for seed beans unless extenuating circumstances cause this to be lowered. You may purchase seed labeled 80 percent that actually have a higher germination rate, resulting in a thicker stand than expected. Of course, the opposite may also be true. To be certain of actual germination, you can conduct your own germination test before planting or request the actual germination from your dealer.

      Although a seed lot may actually germinate 80 percent, this does not insure that 8 out of 10 seed will produce vigorous seedlings in the field. Because field conditions are generally less favorable than the laboratory, the seeding rates in Tables 1 and 3 assume 90 percent emergence of 80- and 90-percent germination seed. If you are planting into ideal soil conditions and using high-vigor seed, seeding rates can be reduced by 10 percent. To figure the rate of seed per foot of row, use the following formula:

desired plants/foot of row    Percent germination x Percent expected emergence

=

seed/foot of row

Example:

Desired stand equals 6 plants/foot of row. Seed germination equals 85 percent. Expected emergence equals 90 percent.

Therefore:

6          85% x 90%

=

6    .765

=

7.8 seed/row foot required to achieve desired population

Pounds of Seed Required

      Table 2 gives the approximate number of seed per pound for several popular soybean varieties. Although incomplete, this list shows the variability among varieties. This information should serve only as a guide, since seed size varies from year to year. If this information is not provided on the bag, contact your dealer for this information; use the size given for seed to purchase. If you wish to determine the number of seed per pound, count the number of seed in one ounce (of seed) and multiply by 16. Check several bags to obtain a better average.

      To determine the amount of seed needed, use Table 1 to obtain the suggested plant population per acre for your desired row width and Table 2 to get an approximate seed size. Then you can use Table 3 to find approximately how many bags of seed will be needed to plant a certain number of acres. Use this only as an estimate of needs since there can be considerable variation.

To calculate the approximate pounds of seed per acre for situations not covered in Table 3, use the following formula:

Pounds of seed/acre

=

Row feet/acre x seed/foot of row        Number of seed/pound

Example: Row feet/acre for 30-inch rows = 17,424 (from Table 1). Seed per foot of row = 6. Seed per pound = 3,200.

Therefore:

17,424 x 6   3200

=

32.67 pounds per acre

Note: The formula on the following page should prove helpful in identifying the number of row feet per acre for various row spacings.

Cost of Planting Seed

Germination Requirements

Early Planting

      Early planting is becoming a major part of Mississippi soybean production. The majority of acreage planted early are Group IV maturity varieties; however, grower interest in planting maturity Group V's early is increasing. Although not documented for every variety, early maturing varieties may express a higher degree of vigor than late maturing varieties, enabling them to emerge under cooler conditions.

No-Tillage Planting

Final Stands

      1. Soil conditions--Cloddy, crusted, or dry soil will usually reduce emergence. Soil temperature should be within a range of 68 to 86 °F at the desired planting depth for the fastest and greatest percent emergence.

      2. Seed treatments--Fungicides may increase emergence of lower quality seed, especially under less than ideal soil conditions. Seed with 80 percent or higher germination may be affected slightly, if at all. A fungicide containing an active ingredient effective against pythium will prove beneficial for early plantings. Remember, seed treatments with fungicides may increase stands but rarely increase yields as long as final stands are uniform and within the desired range. Seed treatments are a form of insurance; sometimes they pay, and sometimes they do not. The key is to use the proper seed treatment to address the predominant concerns. Cropping history, soil temperatures, and the weather forecast following planting will all aid in this decision.

      3. Depth of planting--Plant soybeans 1 to 2 inches deep, depending on the soil type and the moisture situation. Deeper planting slows emergence and may reduce final stands. Place seeds in firm, moist soil and cover them adequately; soybeans must absorb 50-55 percent of their weight in water to germinate, so good seed-to-soil contact is critical.

      4. Herbicides--Many herbicides can affect stands and seedling vigor if rates are excessive or if there is excessive rainfall and otherwise poor growing conditions.

      5. Cultivation-- Cultivation in row middles should not affect stands. Rotary hoes are effective in breaking a soil crust and removing small weeds; however, stand losses may occur if you operate hoes in the morning hours when small seedlings are brittle.

Replanting

      Remember, soybean plants have a tremendous capacity to compensate for thin stands. If the stand is within 50 percent of the population given in Table 1, replanting would probably not be advisable if weed control is good and the remaining plants are healthy. Usually, stands of three to four plants per foot in 30- to 40-inch rows are better than replanting. Populations even less than this are adequate if irrigation is available.

Steps to Good Soybean Stands

1. Start with good seed:
   - at least 80 percent germination
   - pure and free from contamination
2. Plant to achieve 75,000 to 125,000 plants per acre (125,000 - 175,000 drilled).
3. Adjust planter according to seed per foot of row, not pounds per acre. Remember: we tend to overplant most crops we grow.
4. Plant into a good seedbed:
   - moist
   - warm (68-86 °F)
   - firm
5. Consider using the proper seed treatment.
6. Plant 1 to 2 inches deep, taking moisture into consideration.

Number of square feet in an acre = 43,560 (Constant)

Number of square feet in an acre ÷ row spacing in feet = number of row feet per acre

Example:

      Row spacing = 30 inches

             30 ÷ 12 = 2.5 row spacing in feet

             43,560 (Constant) ÷ 2.5 = 17,424 row feet per acre in a 30-inch row

Table 1. Suggested plant populations and seeding rates for soybeans planted in Mississippi.
Row width inches	Feet of row per acre	Plants per foot of row	Plant Population per acre	Seed per foot of row* Germination
				90%	80%	70%
40	13,068	8.00	104,544	10.00	11.10	12.70
36	14,520	7.00	101,640	8.60	9.70	11.10
30	17,424	6.00	104,544	7.40	8.30	9.50
20	26,136	4.00	104,544	4.90	5.60	8.30
15	34,848	3.75	130,680	4.60	5.20	6.00
12	43,560	3.00	130,680	3.70	4.20	4.80
10	52,272	2.75	143,748	3.40	3.80	4.40
7	74,674	2.25	168,016	2.80	3.10	3.50
6	87,120	2.00	174,240	2.50	2.80	3.20

*Assuming 90-percent field emergence of the live seed.


Table 2. Approximate number of seed per pound of soybean seed for selected Group IV, V and VI Maturity Groups.*

Variety	Brand	Seeds/pound
Group IV
4880	AgriPro	2800
AG4702	Asgrow	2600
4715	Asgrow	3000
4860	Delta King	2900
3478	Delta Pine	3200
4994RR	Hartz	2800
4755	Hornbeck	2700
Manokin	Public	3200

Variety	Brand	Seeds/pound
Group VI
6297	Asgrow	2900
6785	Asgrow	3800
66	Buckshot	2900
723	Buckshot	3200
3640	Delta Pine	3200
6255	Hartz	3100
HBK69	Hornbeck	2500
HY663	Hyperformer	2600
62-62	Northrup King	2900
59-V6RR	Northrup King	3400
65-50	Northrup King	2700
9631	9631	3100
9692	Pioneer	3000
Dillon	Public	2700
Young	Public	3300
RVS678	Riverside	3200
RVS699	Riverside	2700
6792	Terral	2900

*Seed size may differ from year to year.
Consult state variety trials or company representatives for additional varieties.

Table 3. Pounds of soybean seed required for various desired stands at two germination levels.*

Desired final plant population per acre
	100,000	110,000	120,000	130,000	140,000	150,000	160,000	170,000	175,000
Number of seed	Pounds of seed per acre
per pound	90 percent tag germination
2,800	44	48	53	57	62	66	70	75	77
3,000	41	45	49	54	58	62	66	70	72
3,200	39	42	46	50	54	58	62	66	68
3,400	36	40	44	47	51	54	58	62	63
3,600	34	38	41	45	48	51	55	58	60
	80 percent tag germination
2,800	50	55	60	65	70	75	80	85	87
3,000	46	51	55	60	64	69	74	78	80
3,200	43	48	52	56	60	65	69	73	75
3,400	41	45	49	53	57	62	66	70	72
3,600	39	42	47	51	55	59	62	66	68

*Assumes a final stand of 90 percent of the seed. For example, the final stand equals 90 percent germination or 81 percent of the seeds planted. Under ideal conditions and with high vigor seeds, reduce rates by 10 percent.

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Publication 1194

Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. Ronald A. Brown, Director