In two years of planting seed corn at different depths of 1,2, 3 and 4 inches, the Indiana Prairie Farmer and Precision Planting trials never saw a disadvantage for planting 4 inches deep, but in one year out of two saw a statically significant disadvantage for planting too shallow. Yields were lower for the shallow planting.
The studies were conducted at the Throckmorton Research Center near Romney in cooperation with the Tippecanoe County Extension Service. Jeff Phillips, Tippecanoe County Extension, designed the replicated experiments.
The only problem was that in both years, planting was mid-May or after. Weather conditions and logistics prevented an earlier planting date when soils would have been cooler at deeper depths.
The year the shallow depth was lower yielding was a year when it was dry for several days after planting, and very hot, before it rained. The theory at the time was that the seeds had enough water to get started, but not enough to keep going until rain came. Some may not have started until rain came. The plants in those plots planted at one-inch deep ran about two growth stages behind all other plots in the test all season long.
Work by DuPont Pioneer in demonstration and test plots at Tipton and other locations confirm the same results. David Gregory, an agronomist in northwest Indiana, says that in 2011, 27% of the plants planted at one-half inch were considered runts, or weeds. Only 3% at the deepest depth, 3 inches, were runts. The yield difference was 20 bushels per acre in favor of planting deeper than one-half inch.
In 2012 the results were even more convincing, with a 70 bushel per acre yield difference.
Gregory says Pioneer's work confirms that 2 inches is probably the ideal planting depth. If you're in the situation next spring and have to make a choice, he says going too shallow instead of deeper on seed placement can cost you bushels and dollars.