Farmers who grow corn or soybeans generally know, and have checked, the seeding rate they're planting. However, when asked about the rate at which they seed alfalfa, they may be a little less certain. In fact, many of those that think they know their alfalfa seeding rate may be off by several pounds.
Studies have shown large differences between seeding rates of different varieties of alfalfa when seeded through the same seeder. Coated seed flowed faster than uncoated seed which resulted in higher seeding rates than was expected (considering the reduction in seed numbers per pound because of the increased seed size associated with seed coating).
With uncoated seed, the seeding rates ranged from 14 to 21 pounds per acre without changing the settings on the seeder. While this range in seeding rates does surround the normally recommended alfalfa seeding rate (15 to 18 pounds per acre), the variation can be very expensive. If a farmer thought he was seeding 15 but was actually seeding 20 pounds per acre the additional cost per acre for seed would be substantial.
The difference between variety seeding rate was attributed to small differences in seed size and shape. It is a general rule-of-thumb that a pound of alfalfa contains 220,000 seeds but studies have shown a range from 196,000 to 224,000 seeds per pound. In addition, some seeds were slightly rounder and flowed through the metering device faster than other varieties. Recommended alfalfa seeding rates will normally result in 75 to 90 seeds per square foot. However, the range in seeding rates that was observed resulted in ranges from 70 to 105 seeds per square foot.
Unfortunately, varieties showed no consistent trends in variation. Therefore, seeder calibration is an important consideration when alfalfa varieties are changed. This can be done rapidly by driving the seeder over a tarp spread on the ground, and counting the seeds that are dropped in a couple square foot areas on the tarp. Seeders which drop between 75 and 90 seeds per square foot are planting at recommended rates. Seeders should also be calibrated annually before starting to seed since slightly worn seed metering devices on the seeder can cause large changes in alfalfa seeding rate.
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