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While seed size is influenced partially by genetics, the environment during seed fill plays a larger role. Small and large seeds of the same soybean product have the same genetic material and therefore, the same yield potential. Under most conditions, seed size does not affect germination or emergence. However, in extreme situations differences may be observed. Large seed, due to a larger energy reserve, may be able to survive a longer period of time prior to emergence under adverse weather conditions. However, the size of cotyledons increases with seed size and therefore large seeds require more energy to emerge from the soil. Larger seeds, which require more moisture for germination, may also suffer from reduced emergence relative to small seed in extremely dry soil moisture conditions.
Figure 1. Soybean seeds began to imbibe water; however, dry conditions arrested germination. Soybeans require 50% of their weight in water before germination can occur.3
Both historically and currently, a healthy, consistent, and uniform stand of 100,000 plants per acre is generally considered the threshold for replant situations.1 Often 100,000 to 125,000 plants per acre at harvest has been shown to allow optimum profitability and maximum yield potential.1 While the threshold for replant situations may be around 100,000 plants per acre, the seeding rate recommendations to help attain maximum profitability and/or yield potential vary by geography and local conditions. Follow local university recommendations for desired plant populations.
Since germination in some seed lots may be below the normal 90%, seed tags should be checked for each seed lot planted to determine the germination percentage and seed size. Drills and planters should be adjusted for each seed lot to help provide an adequate plant population.
A general formula to determine seeding rate is:2
The percent live seed emergence estimate is not provided on the seed tag and is an arbitrary number determined prior to planting. The estimate should be determined on a field by field basis and is dependent on variables such as the type of planter or drill, field conditions, and planting depth. Typically 90% is a good rule of thumb, but should not be used in all situations.
For example, let’s say a grower wants to establish a stand of 165,000 plants per acre in 7.5- inch drilled rows. The seed tag indicates that the seed has an 89% germination rate and is 95% pure live seed. Assuming a 10% loss in germination due to a clay soil that crusts, what seeding rate will achieve the desired stand?
When managing low-germination soybean seed, it is important to achieve a good stand the first time and avoid situations where it becomes necessary to replant. It is also important to reduce the stresses when planting low germination soybean seed, particularly in cold and/or wet soils. Seed lots with higher germination percentages should be planted first leaving low germination seed lots for later in the planting season when conditions are more ideal.
Figure 2. Cotyledons must have enough energy reserve to get the first true leaves to the surface for photosynthesis to occur. Poor soil conditions like crusting or compaction can hinder emergence.
Soybean seedling diseases can result in reduced plant stands, poor plant vigor, and lower yield potential. Including seed treatment products, such as Acceleron® Seed Applied Solutions, can be effective at protecting seeds and young seedlings from many seed-borne and soil-borne pathogens. Fungicide treatments will not improve germination of seed that has reduced quality, but may protect the seed from soil pathogens if the seed is planted into cold and/or wet soils.
Seed size is largely influenced by environmental conditions. However, seeds of the same soybean product have the same genetic yield potential. It is important to read the seed tag for each bag or unit and adjust seeding rates as necessary for low germination seed lots. Plant soybeans when soil conditions are suitable and consider a seed treatment to help protect soybeans from seed and soil- borne fungi.
1 Pedersen, P. 2007. Optimum Plant Population in Iowa. Iowa State University Department of Agronomy. http://extension.agron.iastate.edu/. 2 Robinson, A.P. and Conley, S.P. 2007. Plant populations and seeding rates for soybeans. AY-217-W. Purdue University Extension. http://www.extension.purdue.edu/. 3 Nielsen. R.L. 2010. Visual indicators of germination in corn. Purdue University. http://www.agry.purdue.edu/. Christmas, E.P. 2001. Soybean seed quality and planting date. Purdue University Department of Agronomy. http://www.agry.purdue.edu/. Pederson, P. 2006. Soybean seed quality in 2006. Integrated Crop Management News. Iowa State University. http://lib.dr.iastate.edu/. Pederson, P. and Robertson, A. 2008. Will 2008 be the “perfect storm” for soybean seedling disease? Integrated Crop Management News. Iowa State University. http://lib.dr.iastate.edu/. Web sources verified 03/03/16. 141211160312