Corn and soybean production and profitability were not improved by deep tillage in U of M studies conducted between 1981 and 2002 in southeastern and south central Minnesota, Randall says.

In many instances, Randall says deep rippers have replaced chisel plows as the primary tillage tool for corn and soybeans. The rippers operate at greater depths and are pulled at faster speeds than previously. The perception is that deep tillage (subsoiling to between 10” and 18”) will promote better internal drainage, greater root growth and higher yields.

However, Randall says deeper and faster tillage almost always buries more residue, leaving less residue on the surface to minimize soil erosion.

Randall cites two deep-tillage research studies conducted in southern Minnesota. The first was at Rochester on well-drained Port Byron loess soils. Deep tillage (15”) was performed using an in-row subsoiler each fall (except for one wet fall when the subsoiling was done in the spring).

The deep-till system was compared to chisel plow, strip-till (eight” deep in the fall), and no-till systems from 1997-2000. Four-year continuous corn yields averaged 163 bu/acre for the deep-till system compared to 166 for the chisel, 162 for strip-till and 155 bu/acre for no-till. Surface residue coverage after planting averaged 54, 26, 64 and 87% for these tillage systems, respectively.

When corn followed soybeans, yields were not statistically different among tillage systems, Randall says. Yields averaged 186 bu/acre for the deep-till (subsoiling) system compared to 182, 183, and 182 bu/acre for the one-pass (field cultivate), strip-till and no-till systems, respectively. Surface residue after planting averaged 41, 23, 57 and 67%, respectively.

"These data show no yield enhancement for this aggressive, high-horsepower-requiring, deep-tillage practice above other tillage systems on the well-drained silt loam soils of southeastern Minnesota," Randall says.

The second study was on the poorly drained Nicollet-Webster clay loam soils at Waseca. Deep tillage (14”) was performed using an in-row subsoiler on a 30” spacing each fall for corn after soybeans. The deep-till system was compared to one-pass (field cultivate), strip-till and no-till systems. Three-year (2000-2002) corn yields averaged 156 bu/acre for deep, in-row subsoiling compared to 151, 157 and 150 bu/acre for the one-pass, strip-till and no-till systems, respectively.

Surface residue coverage after planting was least with deep subsoiling (43%) compared to 47% for the one-pass, 56% for the strip-till, and 70% for the no-till systems. Soybean yields in the following year were not affected by the tillage systems for corn. Yields ranged from 55.8 to 56.4 bu/acre for the four tillage systems.

Results were similar to those on the well-drained soils. "The data in this study do not show deep subsoiling to enhance corn or soybean yields compared to less aggressive tillage systems on these tiled soils with poor internal drainage," Randall says.

Soil compaction studies at Waseca during the 1980s also did not show beneficial corn or soybean yield responses to deep subsoiling (up to 18”) regardless of compaction level. The only significant response was negative, with corn yields being reduced in a very dry year by subsoiling the previous fall.

If you do choose to use deep tillage implements, operating at depths deeper than 10” is not recommended, Randall says.