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, demonstrated that very low seeding rates under good conditions could result in maximized yields.
Starting with a low plant population, however, can be risky as stand loss can be expected throughout the growing season. A guideline commonly used for estimated stand loss throughout the season is 20%. Losses, however, can vary considerably. Differences between planting and final populations ranged from losses of 8-47%, depending on target population and location in 2008 University trials. Losses were greatest where seedbed conditions were very wet at planting.
To achieve desired stands at harvest, the following guidelines have been developed by Seth Naeve, University of Minnesota Extension soybean specialist. Recommendations are in live seeds/acre:
For Group II soybeans, a seeding rate of 140,000 is recommended.
For Group I — 150,000
For Group 0 — 160,000
For Group 00 — 170,000
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 grower's bottom line? Consider an example where a bag of soybeans containing 150,000 seeds/bag at 90% germination sells for $45. This translates to 135,000 live seeds/bag (150,000 × 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/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's best to adjust downward gradually over time vs. making a dramatic change. It's also recommended to take stand counts after emergence and before 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/acre. Soybeans planted in high-pH areas prone to iron deficiency chlorosis will also likely benefit from higher seeding rates, particularly in 22-30-in. rows.