Integrated Crop Management

Soybean planting decisions

When the weather warms up, soybean producers become anxious to get on the tractor and start planting. But the key to success is proper soil conditions so that compaction does not occur. If compaction occurs, the effects can linger throughout the growing season.

In Iowa during 2000, approximately 28 percent of soybean was planted into no-till fields. However, fall 2001 was warm and dry so many producers did more tillage than normal; therefore, the percentage of no-till soybean acres this year may be reduced. With the herbicides currently available, tillage is less necessary for weed control, and studies have shown that tillage has little or no effect on soybean yield or profit. The amount of tillage necessary before planting soybean will depend on the soils in each field.

Planting date

The average date of soybean planting in Iowa is May 17. Table 1 indicates that the preferred soybean planting dates are between late April and mid-May. However, spring weather may push the preferred dates earlier or later, depending on soil temperature and rainfall frequency. Ultra-early soybean planting (before mid-April) may result in the seed lying in the soil for more than 3 weeks before germination and emergence. This lengthy period allows insects and pathogens an opportunity to damage the seed and possibly reduce the percentage of stand established. If ultra-early planting occurs, and the field has a history of seedling diseases, the seed should be treated with a fungicide to protect it from pests that are present in cool, wet soils.

Table 1. Effect of planting date on soybean yield in Iowa.


Planting Date



(% of maximum yield)
Late April 100* 96* 98*
Early May 96* 100* 100*
Mid-May 99* 96* 98*
Early June 81 93 89
Mid-June 61 59 82
Early July 33 45 47

*Yields are statistically similar.

Seeding rate

Table 2 indicates that harvest stands of 110,000 and above produce similar soybean yields. So how many seeds must be planted to harvest a desired plant stand? Studies have shown that field stand losses of 15 to 30 percent are common. These field losses are due to lack of seed germination, insect and disease damage, barren plants, lodging, and many other factors. Each field will likely have a different stand loss. Therefore, if a producer wants to harvest 130,000 plants and has a 23 percent stand loss before harvest, the seeding rate should be 159,900 seeds per acre. If seed size is 3,200 seeds per pound, the field would require approximately 50 pounds (or one bag) of seed per acre.

In recent years, producers have been concerned about insect damage to early soybean growth. This potential problem would be eliminated if more producers planted soybean

early so more seedlings were available at the same time. Therefore, the damage due to insect feeding would be distributed over larger areas and the yield and economic loss from individual fields would be reduced.

Table 2. Effect of row spacing and harvest stand on soybean yield in Iowa.

Narrow Rows (7.5 and 10 inches) Wide Rows (30 inches)

Grain Yield


Grain Yield

94,000 47.9 73,000 46.3
133,000 50.2* 110,000 49.3*
157,000 50.9* 135,000 49.8*
186,000 51.6* 165,000 50.6*
221,000 51.4* 189,000 49.8*

*Yields followed by an asterisk (*) are statistically similar.

This article originally appeared on pages 45-46 of the IC-488 (5) -- April 15, 2002 issue.

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