Over the last 20 years there has been a growing trend for more and more alfalfa and clover seed to be coated. I am often asked “Is it worth buying coated seed?” I almost always answer yes to this question, especially in the case of alfalfa and clover seed. This question is usually followed by another question “Since the coating means that there’s less seed in the bag, should I plant at a higher seeding rate?” My normal answer is no, you should plant at the same rate as recommended by the forage establishment publication in your state. In Kentucky we normally recommend planting alfalfa at 20 lbs./acre for both coated and uncoated seed.

Simply put, the reason to plant coated seed is to improve the establishment process. Most seed coating is done to improve the number of seeds that germinate, produce healthy roots and young shoots, and then go on to produce established plants. In other words, seed coating improves establishment success by improving establishment percentage. 

There is an interesting history to coated seed. The first recorded seed coating was practiced hundreds of years ago by villagers in Southeast Asia coating each rice seed in a ball of mud. The mud balls were then spaced evenly in a flooded rice paddy and the rice plants rooted as the mud ball held the rice seed in position. This eliminated the problem of seed drift when the dry rice seed was sown on the surface of flooded paddies by simply using a seed “coating” to improve establishment success. By 1958 there were 35 patents that proposed different types of coatings and processes to apply them. One patent from 1866 used a paste of wheat flour on cotton seed to slick down the lint so the seed would flow in the planter. In 1905, Aristotle Rey received a patent to use glue to stick fertilizer to cereal seed. More recently, in the late 1950s, New Zealand Agricultural workers developed a coating for clover and grass seed. These coated seeds could be broadcast by airplane in the rugged hills of New Zealand. The use of coated seed proved to be highly successful in establishing stands of clover and grass due to improved inoculation and ballistics. The coating was put on the clover seed to hold the inoculum, help give a uniform seeding pattern and to provide a micro-environment for quick and effective nodulation. The grass seed was coated to help with a uniform seeding pattern and to provide added weight to ensure good seed to soil contact as the seed fell from the airplane. The New Zealand process began the modern science of seed coating, but all of the seed coatings listed above had the same thing in common. The goal of coating seed is to improve establishment success.

Seed coating has come a long way from the New Zealand practice of the 1950s. The majority of improved varieties of alfalfa and clover seed are now coated. The standard coating involves pre-inoculating the seed with rhizobia inoculum to improve the rate and success of nodulation and N-fixation. This is followed by a thin coating to protect the live rhizobia bacteria in the inoculum, then a layer of fungicide is added to reduce the chances of “damping off” and similar seedling diseases. The fungicide is then protected by a second layer of coating. Additional components are added to modern coated seed (see Table 3), but with legume seed the most important are the inoculum and a fungicide.

A classic set of field experiments were conducted 15 years ago by Dr. Joe Burns, professor emeritus from the University of Tennessee, when he was working for Cel-Pril. Fourteen field trials were conducted in TN, KY and AL comparing coated alfalfa seed (Rhizo-Kote XL) plus Apron fungicide to pre-inoculated, uncoated seed plus Apron at the same seeding rate. Note that both the coated and uncoated seed were inoculated and had Apron applied. The average seeding rate was 18.5 pounds per acre for the Rhizo-Kote XL and 18.4 pounds per acre for uncoated seed. The Rhizo-Kote XL seed had about 33% fewer seeds per pound as compared to the uncoated seed. The seed for the two treatments were from three improved alfalfa varieties and were taken from the same seed lot in order to eliminate genetic differences.

The two treatments in each strip test were planted at the same seeding rate per acre with the same drill and the drill was calibrated for each type of seed. The coated seed flowed at a higher rate than the uncoated seed at the same drill setting. Five of the 14 strip tests were conducted as no-till plantings. All trials were planted in strips adjacent to each other. Fertilizer and lime were applied according to soil test recommendations. Seeding dates were near or within the recommended seeding dates for each state. Results of these field experiments are shown in Table 1 and Table 2 for establishment of coated (Rhizo-Kote XL) vs. uncoated pre-inoculated alfalfa seed six weeks and six months after seeding.

Table 1. Establishment of coated (Rhizo-Kote XL) vs. uncoated pre-inoculated alfalfa seed six weeks after seeding.

14 locations in Alabama, Kentucky and Tennessee-1993-2000. * Average seeding rate, uncoated 18.4 lb. /A, Rhizo-Kote XL 18.5 lb. /A. 200,000 seed per pound. ** 280 measurements per treatment. Burns, et al., 2002

Table 2. Establishment of coated (Rhizo-Kote XL) vs. uncoated pre-inoculated alfalfa seed five to six months after seeding.

13 locations in Alabama, Kentucky and Tennessee-1993-2000. *Average seeding rate, uncoated 18.4 lb. /A, Rhizo-Kote XL 18.5 lb. /A. 200,000 seed per pound. **130 measurements per treatment. Burns, et al., 2002

The number of plants that emerged as a percentage of the seed planted was higher for the coated vs. the uncoated seed. Most importantly the number of the plants per sq. ft. was equal for the coated seed in comparison to the uncoated, therefore the coated seed emerged at a significantly higher rate. At all locations the coated seed had a significant higher survival rate (established plants vs. seed planted) vs. uncoated seed. No-till planting also showed the same trend with coated seed at 32.6 plants per sq. ft. and uncoated seed at 33.8 plants per sq. ft. on no-till. Five to six months after seeding, there still was no significant differences in the number of plants per sq. ft. for the coated vs. the uncoated seed.

These results and similar studies conducted in recent years show that coated legume seed has a higher percentage of plants that survive to become an established stand. In other words, planted at the same rate, there is no difference in the final stand using coated vs. uncoated seed. Why go to the trouble to coat the seed if there is no difference in the final stand? Two main reasons: 1) coated seed of improved varieties is usually the same price per pound as uncoated seed, and 2) more importantly, coated seed provides a better microenvironment around the seed for germination and growth. This is especially important when planting under less than favorable conditions like cool, wet soils or soils with inadequate moisture conditions.

In grass seed, the primary advantage of coating is to ensure easier and more uniform seeding. This is especially true when planting warm season grasses like bermudagrass, crabgrass and teff that have very small, light seed and therefore low seeding rates. Seeding these grasses with a broadcast spreader becomes almost impossible without coated seed. Even with a drill, the coated seed makes it easier to set the desired seeding rate because the coating in essence “bulks up” the size of the seed. Unlike legumes, most recommendations call for seeding at higher seedling (lbs./acre) with coated vs. uncoated seed.

The seed industry continues to improve seed coating technology. A recent article in Hay and Forage Grower by Cory Catt lists some of the traditional and new products now available for coated seed (Table 3).

Table 3. Common Industry Seed Treatments

Each seed company will generally use a base combination of a fungicide and bacterial inoculant. Seed companies are putting out some unique seed treatment formulas based on new research. Each treatment can change the drill flowability, so it is advisable to calibrate the drill for each seed lot. This is a common seed treatment; there are additional biologicals, micronutrients, and treatments that can be beneficial. Information adapted from Seed Treatments: A Worthwhile Investment for Alfalfa.

In conclusion, seed coating is a proven technology that shows equal establishment success to uncoated seed until ideal seedbed conditions and improved establishment success when seeding under less than ideal conditions.

References

Burns, Joe; Bennett, Brad; Roney, Kelly; Walsh, John; Hensley, Jeff. Proc. 57th Southern Pasture and Forage Crop Improvement Conference, Athens, GA April 23-25, 2002 Coatings for Legume and Grass Seed. http://agrilife.org/spfcic/annual-proceedings/57th/coatings-for-legume-and-grass-seed/. Accessed February 5, 2016.

Catt, Corey. 2014. Seed Treatments-A Worthwhile Investment for Alfalfa.http://www.progressiveforage.com/forage-types/alfalfa/seed-treatments-a-worthwhile-investment-for-alfalfa. Progressive Forage Grower. Accessed February 5, 2016.

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