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Buying alfalfa seed: Seed coatings

Craig Sheaffer, Extension forage agronomist

Seed coating adds weight and color to seed.
The yellow seed is uncoated.
Today, seed coatings are widely used on forage legumes and some grass seeds. The overall goal with seed coatings is to improve germination, seedling vigor, and seedling survival. They also increase seed flowability and help distribute small seeds more evenly.  

Coatings vary in their constituents and coatings affect pure live seed percent within a bag as described by Newell (2018) and Hayward (2017).

A 34% seed coating is widely used for alfalfa seed. Standard components include Rhizobium for nitrogen fixation and fungicide treatments for seed and seedling protection, along with binding and coating polymers, minerals, and colorants. 

Many seed companies add optional treatments based on marketing strategies. These can include specialized components like growth promoters, micronutrients (Mn and Fe), mycorrhizae, insecticides, bio-enhancers, and hydroscopic materials to attract moisture. Descriptions of seed coatings must be included on the seed label. Seed coatings have the potential to increase seedling establishment and reduce risks of establishment failure that can frequently occur with a small seeded forage like alfalfa. However, the 34% seed coating reduces the pure live seed (PLS) percentage within a bag.

Risk mitigation vs. PLS

Is the risk mitigation potential of 34% seed coating enough to overcome the differences in pure live seed (PLS) content? Table 1 shows the effect of 10% and 34% seed coating on seed that has 95% germination. Compared to the no seed coating, the pure live seed in the seed bag would be 42.8 lb for the 10% coating and 31.5 lb for the 34% seed coating. When seeded at the 15 lb/acre there would be 17 seeds per ft2 less for the 34% coating than the 10% coating. To achieve the same 15 lb/acre seeding rate, 6.5 lb/acre more seed would be needed with the 34% coating.

Table 1. Seed and establishment characteristics for seed with different levels of coating.
Characteristic No seed coat 10% seed coat 34% seed coat
Total germination % 95 95 95
Pure seed % 100 90 66
Pure live seed % in bag 95.0 85.5 62.7
Pounds PLS in 50 lb bag 47.5 42.8 31.5
Pounds PLS at 15 lb/acre 14.3 12.8 9.4
Seeds/ft2 at 15 lb/a seeding 71 65 48
Pounds PLS for 15 lb/acre 15.8 17.5 24

In this example, we assumed seed had a 95% germination rate and that there are 220,000 seeds per pound. To calculate seeds per square foot at a 15 pounds per acre seeding rate, use the following formula:

  (Pure live seed% x 15 lb/a x 220,000 seeds/lb) / 43560 ft2/acre

An interpretation

We do not recommend planting alfalfa seeds with no seed coating. This column was only included for comparison. Research has shown beneficial effect of alfalfa seed coatings, which at a minimum contain Rhizobium and fungicides.

In the Alfalfa Management Guide, seeding rates of 12-15 lb/acre of seed are recommended for the Midwest. Research in Wisconsin showed that with natural rates of decline, an alfalfa seeding rate of 15 lb/acre and 200,000 seeds/pound resulted in about 75 seeds/ft2 at seeding but losses due to disease and competition resulted in about 25 plants ft2 the following spring (Rankin, 2008). This population was adequate to provide high alfalfa yields.

If seed coatings do greatly decrease the rate of seedling loss over the first year, then pure live seeding rates can be lowered. Indeed, recent research has shown that there is seldom a stand or yield increase at seeding rates above 10 lb/acre pure live seed (50 seeds/ft2) (Radkin, 2008, Holin, 2018). In our example where we seeded 15 lb of 34% coated seed with 95% germination, it resulted in nearly 50 seeds/ft2 which should be adequate for high yields especially since seedling survival should be increased.

The success of alfalfa establishment has been improved through use of seed coatings of modern varieties. Improved seed coatings containing multiple components to improve seedling survival are often used on seeds of modern alfalfa varieties that include advanced genetics for resistance to multiple seedling diseases and improved seedling vigor. The efficacy of seed coatings in improving alfalfa stands will be dependent of local environment, soil, and seeding technology; therefore, in the absence of independent research trials, growers should determine if adjustments in seeding rates are needed with seed coatings.

Additional readings:

J. Hayward. Alfalfa seed coating versus seed treatments.

F. Holin. 2018. Researchers look at what it takes to establish productive alfalfa stands. Midwest Forage Association clippings.

R Leep et al. Coated Alfalfa Seed, is It Worth It?. Michigan State University.

R. Newell. It’s on the tag and in the bag. 2018. Hay and Forage Grower.

C. Sheaffer. 2023. Alfalfa establishment: management strategies.,on%20yield%20(Table%203).

M. Rankin. Determining the Optimum Alfalfa Seeding Rate. 2008.

D. Undersander, Pure live seed. Univ. Wisconsin

D. Undersander et al., 2014. Alfalfa Management Guide.
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