Planting Rate
Maximum soybean yield from May-planted soybeans is generally provided by planting rates which result in 6 to 8 plants per foot of row in 30-inch rows, 4 to 6 plants in 20-inch rows, and 3 to 4 plants in 10-inch rows. Higher population densities may result in ex-cessive stem elongation and lodging if the variety planted does not resist lodging. Weather that favors
excessive vegetative development on the plant can also result in lodging. Breeders who select soybeans with stronger stems see problems with lodging dimin-ish.
An insufficient plant population will limit yield, as plants fail to form the complete canopy of leaves needed to fully use the available sunlight. It is par-ticularly important to soybean yield that a complete canopy be in place by the time pods begin to form. Thin stands also allow more weed competition to de-velop in the crop, and low plant densities encourage plants to branch and pod closer to the soil line, possi-bly adding to harvest losses.
Studies have demonstrated that the productive ca-pacity of soybean is surprisingly good at rather low plant densities. At extremely low densities, a consid-erable amount of the production may not be effi-ciently harvested with a combine due to low podding and excessive branching low on the main stem of the plant. Precipitation and planting date actually deter-mine what the ãidealà plant density may be in a given year. In a dry year, when vegetative development of plants is restricted, higher densities of soybean are desirable so that a full canopy can develop. In con-trast, a year with abundant rainfall following timely planting can result in excessive vegetative growth, possibly leading to lodging. At planting we cannot predict weather during vegetative growth, so a com-promise in seeding rate offers the most yield poten-tial.
Seeding-rate trials conducted in 30-inch row spac-ings suggest that a wide range of seeding rates will produce good yields. Seeding rates that result in 110,000 to 150,000 plants per acre tend to produce best yields (Figure 3.03). Soybeans in narrow row planting (drilled or otherwise) are often planted at a seeding rate resulting in densities greater than 150,000 plants per acre. If lodging resistant, plant density can likely be increased to the range of 180,000 to 200,000 plants per acre without risk of lodging. Benefits to yield are often questionable, however, when plant densities ex-ceed 150,000 plants per acre in a timely planted stand with uniform plant distribution.
The more rapid full canopy, which develops in fields planted to narrow rows at higher plant densi-ties, has often been reported to aid in weed manage-ment through the shading imposed on weedy species. Shade pressure on weeds will help reduce weed growth but should not be relied upon solely as a means to suppress weedy competitors in narrow-row soybeans.
For seed of average size, planting 40 to 60 pounds per acre can achieve a stand of 110,000 to 150,000 plants per acre. Planting at rates toward the higher end of this range helps ensure a full stand; planting toward the low end might fail to produce adequate stands in an unfavorable environment, which limits emergence. It is generally wise to plant at a rate that achieves a stand toward the upper limit in plant den-sity which the soybean variety will tolerate without lodging. Research on planting rates and yield poten-tial indicates that virtually all varieties respond simi-larly to changes in seeding rate until the plant density reaches a level that results in lodging.
As previously mentioned, soybeans that are not timely planted in spring, and especially those planted after harvest of winter wheat, will have reduced veg-etative development (fewer leaves per plant and a shorter stem) and will tend to be more resistant to lodging. Soybeans planted late or double-cropped need to be established at higher densities per acre and in narrow rows to allow the crop to fully intercept sunlight by the time pod development begins. Rec-ommendations on planting rate therefore change as seeding is delayed from May to June or early July.
As row spacing narrows, fewer plants per foot of row are needed to achieve a given population of plants per acre (Table 3.03). The actual amount of seed needed per acre will be determined by the popu-lation density desired, seed size, and seed quality, as well as by field conditions and equipment consider-ations which relate to emergence of a viable seed. The extent to which seeds are dropped in excess of the de-sired plant density per acre depends on how probable it is for a viable seed to emerge. Seed drop rate per acre can be determined by the calculation.
Text and formula by:
Gary E. Pepper, Associate Professor, Soybean Extension, Department of Crop Sciences.
Taken from:
Chapter 3 of the 1997-1998 Illinois Agronomy Handbook. University of Illinois at Champaign-Urbana - College of Agricultural, Consumer and Enviromental Sciences - Department of Crop Sciences - Cooperative Extension Service - Circular 1344