Research Highlights

Research Highlights
Kentucky Soybean Board funds seeding rate research

By Jason Sarver

Soybean seeding rates are a popular topic among Kentucky producers and researchers, with the issue becoming more relevant as seed costs continue to rise. With these costs in mind, those 40-50,000 extra seeds planted per acre ‘just-in-case’ begin to have a bigger impact on your bottom line. While the first question is “How many seeds should I plant?” the next one should be “How will these reduced seeding rates affect my weed control program?”  To help sufficiently answer this inquiry, the University of Kentucky conducted field studies in 2007 and 2008 at both Lexington and Princeton to determine the effect of several glyphosate application timings on crop canopy development, weed pressure, seed quality, and seed yield at three seeding rates: 75,000, 125,000, and 175,000 seeds per acre.

In the study, weed control protocols were planned to reflect those commonly used by Kentucky producers. Each plot was treated with a burndown herbicide application two weeks prior to planting, then followed with glyphosate at the labeled rate at either 3 weeks after planting (WAP), 5 WAP, 3+7 WAP, or 3+5+7 WAP, along with check plots which received no weed control and others that were kept completely weed-free.  The weeds present represented those commonly dealt with in each geographical area, with Crabgrass, Yellow Nutsedge, Johnsongrass, Marestail, and Smooth Pigweed being the most problematic in Princeton, while Yellow Nutsedge, Yellow Foxtail, Smooth Pigweed, and Common Ragweed showing up most often in Lexington.

A final stand of at least 101,000 plants per acre resulted in maximum yield across all environments in the study.  This falls in line with the current recommendation from the University of Kentucky that a final stand of 100,000 plants per acre is sufficient at maintaining maximum yield potential in full-season soybean planted on or prior to the first week of June. When full-season planting becomes later (as seen in 2009) or we get into double cropping situations, the smaller plant size associated with the more rapidly maturing soybean crop generally commands a higher plant population to achieve full canopy by R1, a requirement to maintain yield potential.

Sequential applications of glyphosate at 3 and 7 weeks after planting resulted in the highest yields in the study, but single applications at either 5 weeks or 7 weeks after planting proved to do just as well across all four environments.  A single application at 3 weeks was equal to the sequential application in only one environment, where weed pressure was heavy very early in the season. 

The main indication of this study was that plant population had no effect on the optimum glyphosate application timing in any environment as it related to yield. In these environments, no change in weed control protocol was beneficial when reducing seeding rates. 

Each situation is unique, however, so scouting for weed emergence is vital for weed removal timing, especially in a single treatment scenario.  Knowing your fields, their weed histories, and emergence patterns are vital for success in your weed control program. For more information, email

Published: Jan 1, 1970