Establishing uniform stands with plants that emerge at close to the same time will increase the probability of maximizing corn yields -- assuming fertility and pest issues are addressed and the weather cooperates. Deeper planting often has been mentioned as a way to increase uniformity of emergence.
A study conducted in 2011 at Manhattan examined the impact of planting depth on speed of emergence, stand uniformity, and yield. Two hybrids each were planted at six depths, from 1 inch to 3.5 inches, on April 11. The study was repeated with a planting on May 3. The two plantings provided very different soil temperatures. The average 2-inch soil temperature for the two weeks after planting on April 11 was 61°F and was 66°F after the May 3 planting. The two hybrids responded similarly so the results below are averages of both hybrids.
Timing of emergence
Figure 1 shows that the time it took for 90% of the plants to emerge increased by about two days for every additional inch of depth in the April 11 planting. The corn planted on May 3 all emerged within 7 to 9 days of planting, regardless of planting depth. Uniformity of emergence was greatest for the 1.5- and 2.0-inch planting depths with early planting and cooler soils. With later planting and warmer soils, emergence was most uniform for the 3.0-inch planting depth, but planting depth didn’t really cause much difference in timing of emergence compared to early planting.
Figure 2 shows that plant stands declined by about 1,500 plants per acre with each additional inch of planting depth in the April 11 planting. Planting depth did not influence final stands in the May 3 planting. Stand uniformity was greatest with the 1.5-inch planting depth in the April 11 planting and the 2.5-inch depth in the May 3 planting (Table 1). In this table, the lower the number of the standard deviation, the greater the uniformity of stand.
A number of irregular or deformed plants were noted in the April 11 planting, with a greater frequency occurring with the 3.0- and 3.5-inch depths (15% on average) compared to planting at 2.5 inches or less (3% on average). These deformations likely arose because plants at the deeper planting depths had to push leaves that were not protected by the coleoptile to the surface.
Although these plants were not tracked during the rest of the season, they likely contributed to non-uniform plant development within the stand.
Figure 3 shows that yield was similar for planting depths from 1.0 to 3.0 inches, but dropped off at 3.5 inches in the April 11 planting, likely related to the lower stands associated with that planting depth. Yields were highly variable in the May 3 planting due to conditions unrelated to planting depth (heat during silking, rooting depth), but did not change appreciably with planting depth. The 1.5 to 2.0 - inch planting depths resulted in some of the most consistent yields with both planting dates.
These results support current recommendations that corn be planted at 1.5 to 2.5 inches depending on soil conditions. Corn can be planted a bit deeper than 2.5 inches and shallower than 1.5 inches in certain conditions with a minimal impact on yield.