When evaluating soybean seeding rates, the University of Illinois' Emerson Nafziger says there is rarely a yield response above the 100,000 plants per acre threshold.
But that's plants, not seed, and he doesn't recommend planting at this base level. He estimates most Illinois farmers are in the 140,000 to 160,000 range for soybeans. He's heard reports of folks moving down into the 125,000 range.
"Seed is expensive," he notes. "It's a way to cut costs, but we have to be smart about it."
As seed prices tick up and early-season seed treatments become the norm, the lower end of what's a comfortable seeding rate could be trending downward. In his experiments, Nafziger has seen maximum yield from as little as 80,000 plants per acre.
If yield can be maximized with such low rates, why isn't the average range much lower? According to Nafziger, many still worry that emergence might suffer, causing stands to drop below the minimum. Planting high rates doesn't carry much downside risk. Thus, higher rates can serve as insurance against low emergence.
"It takes a lot to hurt yields with high populations," he adds.
Still, some fall into the trap of thinking a higher planting rate will mean fewer instances of replant. Nafziger says this is a fallacy. "Poor stands usually follow heavy rainfall after emergence, and I haven't seen many cases where using a high seeding rate prevented having to replant," he notes.
Nafziger says a reasonable emergence rate is around 80% for high-germination seed. To calculate seeding rate, take the expected stand and divide it by 0.8. So, if 100,000 plants are the goal, divide 100,000 by 0.8. In this instance, the prescribed seeding rate would be 125,000. If germination is listed in the low 90s or less, further divide by that percentage to increase the drop rate to account for seed that won't germinate.
30 vs. 15-inch rows
Last year, soybean yields were a bright spot for many Illinois farmers. Still, it seems corn gets all the attention. As a result, many don't mind popping for a top-notch wide corn planter. These planters are already heavy, so many prefer not to add more weight by splitting rows. That means more acres of 30-inch row soybeans.
However, Nafziger says that widening rows from 15 to 30 inches often means slightly lower yields. In trials conducted by U of I (with funding from the Illinois Soybean Association) over four sites in 2010 and 2011, there was on average a 1.5-bushel advantage for 15-inch rows over 30s.
"Response to widening rows ranged from 0.7 bu more to 3.3 bu less yield, with about half the sites showing a significant response in favor of the 15s," Nafziger explains. "So, not everyone will see this every year."
Though there may be good reasons to move from 15s to 30s, Nafziger says folks should expect yield will change as a result.
"One should pencil a bushel-and-a-half lower yield if they're switching from 15- to 30-inch rows," Nafziger notes. "A bushel and a half loss is not that noticeable, but it's not free."