Spring wheat planting season is approaching South Dakota growers this month, at least according to early plant insurance coverage, March 16 (northern SD) or 26 (southern SD). Seeding rate is a routine management decision each year that is a critical component toward achieving maximum profitability on your farming operation (revenue less seed cost). What seeding rate will you plant this spring?

With funding from the South Dakota Wheat Commission, new spring wheat seeding rate research has been conducted by SDSU Agronomists from 2010-2013 in eastern South Dakota (Table 1). This research showed a seeding rate of 1.2 million pure live seeds (PLS) per acre maximized yields in 3 out of 4 years (Figure 1). The 2.3 bushel/acre increase in yield in 2012 would have covered the cost of increasing the seeding rate from 1.2 to 1.8 million PLS/acre in that particular year. However, averaged over the last 4 years, profit per acre was slightly reduced by increasing seeding rates above the 1.2 million PLS/acre.

Assuming an average germination of 95% (viable seed) and purity of 99%, the 1.2 million PLS/acre (28 PLS/ft2) equates to a 1.3 million seeds/acre (Table 2). However, during the study seed size varied from 12,812 to 26,374 seeds per pound, which can change the planting rate from 1.69 bushels to 0.82 bushels per acre achieve the same number of seeds per acre. SDSU Seed Testing Lab and other labs offers germination (viable seed), purity, and seed count services to help growers achieve their targeted seeding rates. Informal surveys suggest most growers are currently closer to the 1.5 to 1.8 million PLS/acre seeding rate. The updated seeding rate recommendations based on this recent study will leave the previous recommended seeding rates unchanged (Table 2).

Calculation: bushels per acre seeding rate =

Seeding Rate Recommendation (PLS/ac)
%Viability/100 x %Purity/100 x Seed size x 60lbs/bushel

Example: = 1,200,000/(0.95 x 0.99 x 16,000 x 60)
= 1.3 bushels/acre seeding rate

New SDSU research on spring wheat seeding rates Similar to soybeans, wheat has a good ability to flex components of yield to achieve equal yield with less plants (Table 3). In this recent study, components of yield such as heads per live seed planted, heads per square foot, seeds per head, and seed size were measured to help explain the adjustments spring wheat plants can make under a range of plant populations. As seeding rate increased, the number of heads per live seed planted decreased, and therefore the number of heads per square foot were very similar at 60-61 heads. At lower seeding rates, plants increased the number of seeds per head and produced larger seeds. 

In summary, with good seedbed conditions and normal planting dates, 1.2 million PLS/acre will achieve equal yields as higher seeding rates because spring wheat plants can adjust yield components (heads per plant, seeds per head, seed size) with the resources available. Averaged over the last 4 years, profit per acre was slightly reduced by increasing seeding rates above the 1.2 million PLS/acre.