Planting date strongly influences soybean yield. In 2013 and 2014, we conducted a planting date trial at the Western Agricultural Research Station near South Charleston, Ohio. In both years, soybean yield decreased by 0.6 bu/ac per day when planting after mid-May.
The greatest benefit of planting May 1 to mid-May is canopy closure which increases light interception, improves weed control by shading out weeds, and helps retain soil moisture.
However, planting too early (before field conditions are adequate) comes with a risk. Factors such as damping-off and pressure from bean leaf beetle are concerns to keep in mind, as well as the possibility of a late spring frost. (Our early May planting date in northeastern Ohio in 2013 was damaged by bean leaf beetle and two frosts that occurred mid-May.)
Before heading to the field, consider the conditions you will be planting into. Soybean germination begins when soil temperatures reach 50°F and moisture is present at the planting depth of 1-1.5 inches. Do not plant early if the soil is excessively cold or wet.
Slower germination and compaction can negate the benefits of the earlier planting date. Timely planting is critical for maximizing yield in soybeans, but using good judgement on field conditions plays a role that is equally important to determining yield potential.
When soybeans are planted in May, a final (harvest) population of 100,000 to 120,000 plants per acre is generally adequate for maximum yield. Final soybean population depends on germination, emergence, disease and insect pressure, competition from other plants, etc. In most situations, 140,000 seeds per acre should result in at least 100,000 plants per acre at harvest.
In Ohio, most soybeans are planted in row widths ≤ 15 inches. Soybeans grown in narrow rows (≤ 15 inches) tend to out-yield soybean produced in wide row width (30 inches) due to increased sunlight interception in narrow rows. Row width should be narrow enough for the soybean canopy to completely cover the interrow space by the time the soybeans begin to flower.
In our 2016 row width study, soybeans grown in 7.5 and 15-inch rows yielded similarly while soybeans grown in 30-inch rows yielded on average 15-20% lower. Our trial located at the Western Agricultural Research Station in Clark County was planted the end of May (pictured below).
In June, the soybeans planted in 30-inch rows looked better than the soybeans planted in 15 and 7.5-inch row widths. However, the soybeans planted in 30-inch rows did not achieve canopy closure until after July 15.
The 30-inch plot pictured below yielded 59 bu/acre while the 15 and 7.5-inch plots pictured below yielded 81 and 85 bu/acre, respectively.