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When should soybean planting season begin?

HUTCHINSON -- This spring we have seen a significant percent of corn acres already planted prior to May 1. Thus many growers are faced with the decision of when to begin their soybean planting. Soil conditions are of primary importance when considering "early planting" while planting by the calendar date and the effect on final yield should be taken into account when planting later in May.

Soil conditions and soil temperature

Soil conditions at and after planting usually make a difference in how successfully the crop establishes. Soybean has "delicate" seed, and as such it benefits when planted about 1½ inches deep, modestly firmed into the seed furrow, covered by relatively loose soil, and into soils with temperatures at 60 to 70 degrees Fahrenheit.

Soil temperatures at the 2-inch depth as of April 27 averaged 48 degrees at Lamberton and 51 degrees at Waseca at the university outreach centers there. These soil temperatures cycle up and down as a reflection of the air temperature, and as a result, local soil temperatures are still fluctuating.

Remember that soybean seedlings need to pull their cotyledons through and out of the soil and that this feat is helped a great deal by good surface soil conditions. Of course, several inches of rain immediately after planting can quickly turn surface conditions from good to poor, especially in some soils.

The lack of oxygen in saturated soils and the formation of a soil crust of even modest strength can almost eliminate soybean emergence. Therefore it is important to know the five-day weather forecast prior to planting.

Early planted soybeans (last week of April and the first week of May) in cool and wet conditions may lead to poor germination and seedling diseases such as pythium. These problems are magnified by extended cold and rainy periods after planting.

Soybean plant populations do not create yield; yet, maximum yields require sufficient populations. Soybean stands must be large enough to maximize light interception throughout the growing season and provide an abundance of fruiting sites (leaf axils) so that pod set can be maximized.

More plants allow more potential places for seed to set and mature. For this reason, the minimum plant stand at harvest to maximize yield is the critical number to strive for. Initial seeding rates help to determine springtime stands. These spring stands then help to determine the number of plants that will ultimately bear seed and produce yield.

Dr. Seth Naeve, University of Minnesota Extension soybean specialist, recommends that under ideal conditions, soybeans in southern Minnesota should be planted at about 140,000 live seeds per acre.

It appears that soybeans grown in central and northwestern Minnesota require harvest stands of 125,000 to 150,000 plants per acre to maximize yields. This is likely due to shorter-statured soybeans with fewer total nodes that are often produced in these regions. Increased seeding rates are required in central and northwestern Minnesota.

Therefore a system based on soybean maturities has been developed to point producers toward reasonable soybean seeding rates:

- Maturity Group II soybeans -- 140,000 live seeds per acre;

- Maturity Group I soybeans -- 150,000 live seeds per acre;

- Maturity Group 0 soybeans -- 160,000 live seeds per acre;

- Maturity Group 00 soybeans -- 170,000 live seeds per acre.

The use of soybean seed treatments may be more easily justified if soybeans are seeded at low populations early in the growing season when it is important to ensure successful stand establishment.

Seed rots, damping off and seedling blights of soybeans are generally worse under wet conditions. Poorly drained and compacted soils are especially prone to these diseases.

Economic returns most often occur when soybeans are planted under cool, wet conditions. Very early planting, wet or poorly drained soils and minimal tillage are cases where a seed treatment is most likely to pay for itself.

David Nicolai is a crops educator with the University of Minnesota Extension Service Regional Center, Hutchinson.