Planting Dates & Populations

Pawel Wiatrak, Jim Dunphy, and Jason Norsworthy

In South Carolina, Group V and VI varieties offer top yield performance when full-season planting occurs from May 1 until June 10. Group VII and VIII varieties are recommended for later planting dates to allow adequate stem elongation prior to flowering. However, research has shown about a one-half bushel loss in yield potential for each day of planting delay after mid-June.

For soybeans planted after June 10, seeding in row widths >30 inches is discouraged because the crop is not likely to lap the row middle and produce yields equivalent to drill-seeded soybean under a shortened growing season (see Figure 1). Regardless of row width, planting after July 1 will not allow adequate growth prior to flowering and yields will be greatly suppressed.

Fig. 1. Schematic diagram of canopy development of "late" planted drilled and wide row soybeans. Because canopy development ceases near pod initiation, wide-row soybean fails to harvest sunlight from the row-middles, thus limiting yield.

Drilled soybeans planted after June 10 in South Carolina have been shown by research to consistently provide a 10- to 15-percent greater yield than soybeans planted in wide rows.

Farmers should use plants per row foot as a seeding rate guide rather than pounds per acre. Significant differences in seed size occur among varieties. Seeding rates based on pounds per acre can cause an inadequate or excessive population. In addition, it may lead to an added expense, especially when using Roundup Ready varieties in which a technology fee is accessed to each bag of seed purchased.

For top yields, seeds should have a minimum germination of 80%. Table 1 shows the number of seeds per row and seeding rates for different row spacings. Under optimum soil moisture conditions, the current seeding rate recommendations for various row spacings are:

• 8 seeds per row foot for 38-inch rows
• 7 seeds per row foot for 30-inch rows
• 3 to 4 seeds per row foot for 15- to 20-inch rows
• 2 to 2.5 seeds per row foot for drilled rows less than 10 inches wide

For maturity group V through VII, recent research at the Edisto REC has found that drill-seeded soybean yields were similar when seeded at 150,000 seed/acre (2 seed per row foot) compared with 250,000 seed/acre (3.3 seed per row foot) which has been commonly used by many growers. The only time seeding rates should be increased is when planting with conservation tillage into dense residue that limits seed-soil contact, and seeding rates should only be increased by 15% in this instance.

Soybean seed should be planted at about ¾ to 1 inch deep in soil moisture adequate for both germination and emergence. Follow this rule of thumb for determining if moisture is ideal for planting: a handful of a typical sandy loam soil with optimum moisture for planting should "ball up" and then fall apart as you form and then release a tight fist.

Table 1. Number of seeds per row and seeding rates for different row spacings.