Plant Health Initiative

Optimum Soybean Plant Population

Start to reduce your seeding rate on a few acres this year. You are the only one who will know how low you can go on your farm to achieve an even stand of 100,000 plants per acre at harvest. Photo credit: Natural Resources Conservation Service

Seed has become one of the most expensive input costs for soybean growers, so the question of soybean seeding rates has taken on more importance in recent years.

The seeding rate refers to the number of soybean seeds planted per acre to obtain an early season stand. These spring stands then help to determine the final plant population — the number of soybean plants per acre that will ultimately bear seed and produce yield.

High plant populations can have some advantages: canopy closure is quicker, light interception is greater, and weed competition is lower. However, yield and economic return does not always increase as plant population increases.

 

In much of the Midwest, aim for a population of 100,000 plants per acre at harvest

Based on many years of research and field observation, most soybean agronomists agree that a final stand of 100,000 soybean plants per acre will produce the maximum economic return, regardless of row spacing, when soybean is planted in a timely manner in good soil conditions. In most of the Midwest, the chance of getting yield increases as plant population moves above 100,000 plants per acre is small, and outweighed by the increase in seed cost.

A recent multi-year research study completed in Iowa confirmed that an even-stand population of 100,000 plants per acre at harvest is enough to maximize yield and economic return (Figure 1).  A plant population higher than 100,000 plants per acre tends to decrease the yield per plant, while the overall yield per acre remains the same. They concluded from these studies that increasing the seeding rate to try to achieve higher yield is not a sound economic decision. However, a population less than 100,000 plants per acre can result in significantly lower yields.

 

Figure 1. Yield response to four harvest plant populations in 15 inch row spacing (top) and 30-inch row spacing (below). Data collected from eight environments from 2004 to 2006. LSD (5%) for seeding rate is 1.4 bu/acre. Yield values are not significantly different from each other when the difference between values is less than the LSD. From: Soybean seeding rates: The balance between cost and yield — Palle Pedersen, Iowa State University

 

Farmers in the extreme upper Midwest may have to a adjust upwards. Minnesota agronomists have noted that for the northernmost soybean growers in the region, slightly higher harvest stands of 125,000 to 150,000 plants per acre appear to be needed maximize yields. This is likely due to shorter-statured soybeans with fewer total nodes that are often produced in these regions.

 

Generally, a seeding rate of 125,000 to 140,000 seeds per acre (MG II and higher) planted with a planter is enough to provide a final stand of approximately 100,000 plants per acre at harvest using good quality seed and planting in a good seedbed.

How to determine seeding rates for optimal final stands

So how many soybean seeds do you really need in order to establish 100,000 healthy plants in your field?

Seeding equipment plays a big role in determining emergence and final stands. Seeding equipment that distributes the seed poorly  within the row or in the soil will lead to a smaller and less vigorous stand. If you use a drill, you will likely need to use a higher seeding rate because drilling does not provide the same seed-to-soil contact as planters do. Other factors that determine the difference between the number of seeds planted and the final stand count are:

• seed quality, seed treatment, and germination rates
• seedbed conditions
• planting depth and speed
• plant-to-plant competition during the season
• weather
• pathogens and insects

Some general guidelines based on soybean maturity groups have been suggested in Minnesota to point producers in northern regions toward reasonable soybean seeding rates (Soybean Seeding Rates in Minnesota ):

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

Fortunately, establishing precise plant populations is not critical because the soybean plant readily adapts to a thinner stand.

Traditionally, higher seeding rates have been recommended when using narrow row spacing (less than 30 inches). However, this recommendation is changing. As long as a planter is used and weeds are managed early, the same seeding rate can be used regardless of row spacing.

 

Take stand counts in the fall

Although the general recommendation in the region is to seed between 125,000 and 140,000 seeds per acre, regardless of row spacing, planting date, or seed treatment, it is important to recognize that stand counts will vary from year to year, field to field, and farmer to farmer. The only way to determine what seeding rate is required to achieve 100,000 plants per acre at harvest on your farm is to take stand counts in the fall. This will help you zero in on the optimal spring seeding rate for your fields.

This will require taking stand counts in all fields each year, but the pay-off will be an eventual fine-tuning of seeding rates that reduces input costs while avoiding replant situations in unfavorable spring weather.

 

Sources

Plant Populations and Seeding Rates for Soybean (pdf format) - Purdue and University of Wisconsin

Soybean Seeding Rates in Minnesota - University of Minnesota

Minnesota Soybean Field Book (pdf format) - University of Minnesota

Crop Conditions and Soybean Seeding Rates - The Bulletin, University of Illinois

Optimum Plant Population in Iowa (pdf format) - Iowa State University

Soybean seeding rates: The balance between cost and yield - Iowa State University

How many seeds does it really take to get 100,000 plants per acre at harvest? - Iowa State University

Soybean Production - Ohio State University