Production Technology - PT97-25, April 1997
Oklahoma Cooperative Extension Service
Ron Sholar, Extension Crops Specialist and Lewis Edwards, Professor of Agronomy

Research conducted in the past two decades has consistently shown yield increases for row spacings of 30 inches and narrower. The previous practice of using 40 inch rows has been largely abandoned in all soybean growing areas. Southern soybean research frequently shows yield increases for row spacings of 20 inches and narrower. Yield increases from narrow rows are more consistent with early or late planted soybeans and with early maturing varieties.

The opportunity to eliminate row cultivation and the potential for yield increases of 1 to 3 bushels per acre make narrower rows an attractive alternative to row soybeans. Production factors that influence whether soybeans will be drilled or planted include the availability of broad-spectrum weed control, cost-benefits of row cultivation, and benefits of the greater seeding precision of a row planter.

The cost of broad spectrum herbicides and the possibility of poor or erratic soybean stands have prevented more growers from seeding in 7-14 inch rows. But the availability of improved drills, highly effective broadcast herbicides, and improved yield prospects for narrow row soybeans have resulted in increased drilled soybean acreage. Despite these improvements, there are still distinct differences in these tools and growers must make an informed decision between a planter and a drill.

Planters
There have been a number of recent improvements in soybean planters. Staggered planting units or double toolbars allow rows to be planted as narrow as 10 inches. A few models now have side by side planting units with row spacings of less than 20 inches.
 
 Planters can be used for planting in reduced tillage situations and seeding in heavy residues. Seeding rate monitors, row guidance sensors, and hopper box augers speed planting operations and improve precision. 
 
 A major advantage of planted soybeans is the opportunity for row cultivation. Many producers like to use one or two cultivations to provide cost-effective control of a broad spectrum of grass and weeds before soybeans form a canopy. Rows planted at least 15 inches apart can be cultivated once with 5-inch sweeps. Several recent improvements on high-residue cultivators provide simpler adjustments and less trouble with operations. Guidance systems permit cultivation speeds of 6 to 8 miles per hour. With 6- to 15-inch row spacings, the soybean stalks move across the header smoothly, resulting in reduced harvesting loss.

Air Seeders
Air seeders work well on mellow soils where good stands of soybeans are typically obtained. For tilled soybean production, an air seeder is probably the quickest way to seed a large acreage. A variety of seed metering and furrow opening systems are available. Press wheels are needed to place the seed in an area of moisture. Otherwise, during hot weather, the soil may dry before soybean plants emerge.

Drills
The easiest method of planting narrow row soybeans is to use a grain drill to plant 7-10" rows or by blocking alternate flutes for 14-20" row widths. To obtain an ideal plant population for maximum yields, the seed must be uniformly spaced within the row and planted at a uniform depth of 3/4 to 1 and 1/2" deep. Fluted feed metering systems on grain drills do not space soybean seed uniformly in the row and may split or damage large seed. A double-run feed mechanism will distribute the seed more uniformly than a fluted feed mechanism but neither will match the accuracy of a unit planter using a seed plate or an air planter. 
 
Recently introduced improvements allow grain drills to be used for reduced tillage and permit use on a greater range of soils. Heavier drill frames, weight racks, and down-pressure springs on the seed openers help to place seeds into good soil contact and into moisture. 

Seeding Depth
Most drills are not capable of maintaining the seeding depth uniformity that is obtained with planters. To obtain good seed depth placement with a drill, the springs should be set properly to force the double disk openers into the soil. Some drills now have adjustable-depth gauge wheels beside the openers to regulate seeding depth. Without the depth gauge wheels, seed is usually placed too shallow in firm or cloddy soils and too deep in loose, fluffy soil.
 
 The openers on modern drills follow ground contours and place seeds at a consistent depth. Firming wheels with beveled perimeters provide cover over seed in almost any soil which has sufficient moisture for soybean emergence. Clay soils require timely planting to place soybeans into moist soil and obtain enough friable soil to firm around the seed. Press wheels that can follow surface contours independent of the seed openers cover seed more uniformly. Some of the newer drills apply force to the press wheels independent of the seed openers. As a result of these improvements, better soybean stands and more consistent in-row plant spacings are obtained.
 
 If soybeans are drilled at a shallow depth and depth control is not precise, the seed is more likely to be damaged by soil applied herbicides.

Seed Metering
Drills provide less accurate seed metering than row planters. Several seed metering mechanisms are available including fluted, double-run and wobble-slot and all require repeated adjustments to obtain the correct seeding rate. Typically, drills provide less uniformity in seed spacing and seeding rates than a recently purchased planter with all the modern attachments.
 
 Drills usually have simpler seed metering systems, furrow openers and, furrow closing mechanisms. This reduces their initial cost and maintenance expense.  However, the costs will be greater when features are added to improve seeding precision and soybean stands. An advantage of drills is that they weigh less than equipment producing the same number of planter rows. Consequently, drills require less hydraulic lift capacity.

Some adjustments will cause a grain drill to be closer in performance to a unit planter.

1. Remove tractor tracks ahead of the drill.  A leveling-firming tool between the tractor and the drill will firm the seedbed and enable a more uniform planting depth when changing from one soil type to another or when planting in cloddy areas. Uniform depth placement can be improved if double disk openers are equipped with depth bands. When depth bands are used, maximum down-pressure should be applied to ensure adequate depth placement in hard spots and clods. Equip the drill with press wheels capable of exerting sufficient down-pressure to ensure good seed to soil contact for rapid emergence.
2. Adjust the metering mechanism to drop 2-3 viable seed/foot in 7" rows or 4-6 viable seed/foot in 14" rows. Generally, less seed damage occurs with 14" rows due the large flute opening. Using a wider gate opening and slower rotation of the flute will usually give better distribution of seed in the row. Always calibrate the drill on the basis of seeds per row foot. Seeds per pound can vary tremendously between varieties and even within varieties depending on growing conditions under which the seed was produced.
3. Whenever possible, avoid large seed. Seed damage increases as seed size increases. Use seed having at least 2400 seed/pound and increase the seeding rate to compensate for the seed damaged by the metering mechanism.
4. Increase seeding rate by 10% for a poor seedbed.
5. Increase seeding rate by 10% for early maturing varieties.
6. Increase seeding rate by 10% when planting late or after wheat.  

Planter or Drill - The Bottom Line
Drills and planters have both become very costly. Choosing a planter or drill that will seed multiple crops effectively will conserve capital expense. Planting remains the most precise method where soil and other factors require it. The efficacy and cost of post-emergence herbicide weed control must be considered along with the benefits of cultivation. The decision on whether to buy a planter or a drill must consider all crops that will be grown along with the soils, other equipment, and management.

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Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1913, in cooperation with the US Department of Agriculture, Sam E. Curl, Director of Oklahoma Cooperative Extension Service, Oklahoma State University, Stillwater, Oklahoma.  This publication is printed and issued by Oklahoma State University as authorized by the Dean of the Division of Agricultural Sciences and Natural Resources.