Cost of soybean seed has increased over the years due to the introduction of genetic traits that impart herbicide resistance and the common practice of using seed-applied pesticides.  This has resulted in increased attention to reduced seeding rates.

Before the above increased seed costs, excessive soybean seeding rates were often recommended and used in some environments to ensure an adequate stand if and/or when poor conditions for emergence occurred, and to increase the potential for rapid canopy closure.  This management option is no longer economical.

In a recently published article in Crop Management, Kentucky scientists report the results from research that was designed to evaluate the influence of soybean seeding rates on the effectiveness of glyphosate applied at various times after planting.  A detailed presentation of the experiment and its results are contained in a Master’s Thesis published by the senior author.

The study included experiments that were conducted during 2007 and 2008 at both Lexington and Princeton KY.  MG 3.9 varieties were used each year in plantings that were made in early to mid-May at both locations.  Seeding rates were 75, 125, and 175 thousand seeds per acre each year, and plant stands were counted at about R1 to confirm actual populations in each treatment.  Glyphosate was applied at 3, 5, 7, and 3 followed by 7 weeks after planting (WAP).  The study had both a weed-free control and a weedy check with no weed control.

Weed control resulting from glyphosate application at any of the application times was not affected by seeding rate.  However, weed control was affected by glyphosate application timing in three of the four environments.  Generally, weed control and yields resulting from the glyphosate application at 3 WAP were less than those from all the other treatments.  The authors surmised that this likely resulted from the emergence of weeds following the application of glyphosate in the 3-WAP treatment.

A seeding rate of 125,000 seeds/acre with an average final stand of 89,000 plants/acre was sufficient to achieve maximum yields in all environments.  Glyphosate application timings resulting in the greatest yields were consistent across seeding rates, which means that a reduction in soybean seeding rates within the boundaries of this experiment will not require a change in the strategy for glyphosate application to soybeans.

Take Home Message

• A seeding rate of 125,000 seeds/acre was sufficient in 4 Kentucky environments to produce maximum yields.  This confirms general recommendations for Mississippi soybean production.


• Glyphosate application timings that resulted in the highest yields in this study were consistent across the 75 to 175 thousand seeds/acre range that was used.  This means that the weed control strategy for soybeans grown in lower populations need not be changed when glyphosate is used for post-emergence weed control.


• As stated above, this study was conducted in 2007 and 2008 when there was less attention being given to glyphosate-resistant (GR) weed populations.  Therefore, the above results should only be considered where post-emergence glyphosate is used as the primary tool for weed control in soybeans.  Since this is not as prevalent in today’s soybean production systems because of the increased presence of GR weeds and the evolvement of weed control programs in soybeans to include residual herbicides, these results should only be considered pertinent for environments that are similar to those in this study.

Composed by Larry G. Heatherly, June 2013, larryheatherly@bellsouth.net