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Published January 22, 2009, 12:00 AM

Comparing soybean planting population and profits

WORTHINGTON — Following the trend of most agricultural inputs, soybean seed costs have tracked upward as well. In the quest to reduce input costs, growers may be asking themselves if greater profits could be achieved with reduced seeding rates.

By: Lizabeth Stahl, U of M Extension, Worthington Daily Globe

WORTHINGTON — Following the trend of most agricultural inputs, soybean seed costs have tracked upward as well. In the quest to reduce input costs, growers may be asking themselves if greater profits could be achieved with reduced seeding rates.

The answer depends in part on your starting point. The effect of soybean seeding rate on yield was evaluated in University of Minnesota research trials conducted across southern Minnesota in 2007 and 2008.

These trials, supported by the Minnesota Soybean Research and Promotion Council, demonstrated that very low seeding rates under good conditions could result in maximized yields.

Starting with a low plant population, however, can be risky. Stand loss can be expected throughout the growing season each year. Enough “cushion” should be built in for when conditions like the weather do not cooperate. A guideline commonly used for estimated stand loss throughout the season is 20 percent.

Losses can vary considerably, however, as demonstrated in the University of Minnesota soybean population trials in 2008. Differences between planting and final populations ranged from a loss of 8 percent to 47 percent depending on the target population and location. Losses were greatest where seedbed conditions were very wet at planting.

Previous soybean population research and studies looking at the effect of hail injury on soybean show that soybean stands of 100,000 plants per acre or more at harvest are sufficient to obtain optimal yields. To reach this harvest population, Seth Naeve, U of M Extension Soybean Specialist, recommends that soybeans be planted in southern Minnesota at a rate of 140,000 live seeds/acre under optimal conditions. If conditions are less than ideal, a higher seeding rate should be considered to help compensate for an increased risk of seedling loss.

What effect can tweaking seeding rates have on a producer’s bottom line? Consider an example where a bag of soybeans containing 150,000 seeds/bag at 90 percent germination sells for $45. This translates to 135,000 live seeds/bag (150,000 x 0.90).

If a grower currently plants at 160,000 live seeds/acre, this equals a seed cost of $53.33/acre. Switching to a seeding rate of 140,000 live seeds/acre equals a seed cost of $46.67/acre, or a savings of $6.60 per acre. If the grower plants 500 acres of soybeans, this equals a difference in input costs of $3,330. At a seed cost of $50/bag, the difference equals $7.41/acre or $3,705 over 500 acres.

Keep in mind if you are currently seeding soybeans at a rate greater than 140,000 live seeds/acre and are considering switching to a lower rate, it is recommended to adjust downward gradually over time versus making dramatic changes.

It is also recommended to take stand counts after emergence and prior to harvest to see what is happening in your field.

Also keep in mind seeding rate recommendations are based on planting under optimal conditions. If seedbed conditions are less than ideal, cool and wet weather is in the forecast after planting, disease issues are common in a field, or your planter distributes seed poorly vertically or horizontally, you may need to adjust the seeding rate above 140,000 live seeds/acre in order to achieve a harvest stand of 100,000 plants per acre.

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