Increased Soybean Rates make the case for weed suppression


The benefit of higher soybean seeding rates in terms of suppressing weeds is well documented.  Especially during the 1970’s and 80’s, it was commonly recommended to plant 200K soybean seed per acre in narrow rows to reduce weed control. As new, more effective herbicides were introduced those seeding rates were reduced.


The current seeding rate recommendations were developed following the introduction of Roundup Ready soybean seed. Due to the significant increase in soybean seed prices and the simplicity in managing weeds, seeding rates were based solely on economic returns.


Herbicides are no longer the main tool in the weed control basket. However, due to the spread of herbicide resistance it made weed management difficult. Alternative weed management tactics are now a must, one relatively easy tactic to adopt is enhanced crop competition.  Row spacing, planting density, cultivar characteristics, and row orientation all influence how well a crop can fight against weeds.


Iowa State conducted a study that evaluated the influence of seeding rate in 15” rows on biomass of weeds in glyphosate resistant soybean.  They learned that weed management which consisted of a single glyphosate application applied at different timings. This program was selected since they knew it was susceptible to weed escapes, primarily weeds that emerged following the glyphosate application.


As soybean seeding rate increased the amount of weed biomass decreased. Since water hemp seed production is closely related to biomass, higher soybean seeding rates also reduce weed seed production.

The relationship between seeding rate and weed growth probably would have been much less evident if we had used sequential glyphosate applications, the standard program during the popular days of Roundup Ready soybeans.  The evolution of glyphosate-resistant weeds doomed that simple system, creating the need for alternative tactics.


They went on to discuss the phrase ‘many little hammers’ which has been used to describe weed management systems that use multiple tactics, each providing relatively small contributions. Due to the spread of herbicide resistant weeds, we are now using herbicides much less effective than glyphosate.


These herbicides would benefit from other tactics that contribute to weed control.  Increased seeding rate is a tactic easily incorporated into systems of growers planting soybean in narrow-rows, and offers an opportunity to include another ‘little hammer’ in the production system.

Seeding rate recommendations are based on a single year, weed management needs a long-term view.


Another thing to consider is that the soybean seeding rates recommended by most agronomists are based on single year economics. Weed management needs to be viewed as a long-term issue since weed seed produced by weed escapes impact future management.


While the enhanced suppression provided by higher seeding rates may not enhance soybean yield that year, the long-term benefits due to improved weed control, lower weed management costs, and reduced risks for resistance should be considered.


So, what is the optimum soybean seeding rates for growers using narrow-row soybean? If a grower can consistently maintain full-season control at the lower recommended seeding rates, they could probably continue with this approach. However, they should scout fields closely to detect any weed escapes.


Iowa State stated that growers that struggle in maintaining full-season weed control probably would benefit from increasing their seeding rates (if they are at the current seeding rates).  As with every aspect in crop production, there is no ‘one-size fits all’ approach and individual needs will vary from farmer to farmer and even between individual fields on the same farm.