Planting depth recommendations for corn in Ohio are 1.5 to 2 inches deep to ensure adequate moisture uptake and seed-soil contact. Shallower planting depths increase the potential for injury from certain preemergent herbicides although widespread use of safer herbicides (e.g. glyphosate) has probably decreased this effect. Deeper planting may be recommended as the season progresses and soils become warmer and drier. Planting shallower than 1.5 inches is generally not recommended at any planting date or in any soil type.
When corn is planted 1.5 to 2 inches deep, the nodal roots develop about 0.5 to 0.75 inches below the soil surface. However at planting depths less than 1 inch, the nodal roots develop at or just below the soil surface. Excessively shallow planting can cause slow, uneven emergence due to soil moisture variation, and rootless corn (“floppy corn syndrome”) when hot, dry weather inhibits nodal root development. This latter situation has led some field agronomists to speculate that shallow plantings increase stress and result in less developed roots, smaller stalk diameters, smaller ears and reduced yields. Nevertheless, many corn growers plant at depths less than 1.5 inches. The rationale for this shallow planting is that seed will emerge more rapidly due to warmer soil temperatures closer to the surface. This is an important consideration as corn growers across the Corn Belt are planting earlier so they can complete planting before yield potential begins to decrease after the first week of May. Particularly in soils that crust, speed of emergence is critical in order to establish plant stands before heavy rainfalls “seal” the soil surface. While previous research has generally documented faster emergence rates with shallower planting depths, the comparisons have often included deeper planting depths than the recommended ranges and results are highly influenced by temperature and rainfall in the given season.
Recent studies comparing planting depths that are within the depth ranges commonly used by growers are limited, and none have attempted to compare hybrid differences to varying planting depths. Hybrids with higher levels of drought tolerance may provide improved yield stability in shallow planted situations while also providing improved performance at normal planting depths, though this has not been documented.
In 2011 and 2012 we a conducted a study supported in part by a Pioneer Crop Management Award to determine the response of hybrids with different drought tolerance ratings to varying planting depths. Plots were established at the ten Ohio State University Ohio Corn Performance Test locations (Hebron, Washington Court House, S. Charleston, Greensville, Van Wert, Hoytville, Upper Sandusky, Bucyrus, Wooster, and Beloit). Three Pioneer brand hybrids with relative maturities of 109 CRM, 108 CRM and 107 CRM were planted at three planting depths (0.5, 1.5, and 2.5 inches). The drought tolerance ratings for the three hybrids were 7, 8, and 6, respectively; based on scale where 1 is poor and 9 is excellent.
Did planting depth affect corn yields?
In 2011 at eight of the 10 sites, yields of the 2.5-inch planting depth treatment exceeded those of the 0.5-inch planting depth treatment. At five of the 10 sites, yields of the 1.5 and 2.5-inch treatment were similar. The 1.5-inch treatment out yielded the 2.5-inch treatment at one site. Grain yields, averaged across locations and hybrids, were 13% and 15% greater for the 1.5- and 3-inch planting depths, respectively, than the 0.5-inch planting depth. The lower yield of the shallow planting treatment was associated with a reduced final stand (27,200 plants per acre for the 0.5-inch depth verses 34,180 and 34,000 plants per acre for the 1.5-inch and 2.5-inch planting depths, respectively) and a greater percentage of late emerging plants or “runts” (28% for the 0.5-in. depth verses 5% and 4% for the 1.5-inch and 2.5-inch planting depths, respectively).
In 2012, a much hotter and drier growing season than 2011, at nine of the 10 sites, yields of the 1.5-inch and 3-inch planting depth treatment were greater than those of the 0.5-inch planting depth. At six of the 10 sites, yields of the 1.5- and 2.5-inch treatment were similar. Grain yields, averaged across locations and hybrids, were 40% greater for the 1.5- and 2.5-inch planting depths than the 0.5 inch planting depth. The lower yield of the shallow planting treatment was associated with a lower final stand (19,460 plants per acre for the 0.5-inch depth verses 32,000 and 31,000 plants per acre for the 1.5-inch and 2.5-inch planting depths, respectively) and a greater percentage of runts (31% for the 0.5-inch depth verses 6% and 3% for the 1.5- inch and 2.5-inch planting depths, respectively).
Did yield response to planting depth differ among hybrids?
Although differences in yield were present among hybrids, the three hybrids exhibited similar yield responses to varying planting depth. In 2011, yields of the 109 day hybrid which had the highest drought tolerance score, averaged across locations and planting depth treatments, were greater than 107 day hybrid (which had the lowest drought score) at four of the 10 sites in 2011. In 2012, yields of the 109 day hybrid, averaged across locations and planting depth treatments, were greater than 107 day hybrid at seven of the 10 sites.
Results of the evaluation support current recommendations to avoid planting shallower than 1.5 inches. There was no evidence that yield response to planting depth was affected by hybrid.