In the case of alfalfa, the coatings usually consist of a rhizobium-peat mixture, a lime coating, a fungicide or a combination followed by a “glue” to hold it all together.
Most legumes have a symbiotic (mutually beneficial) relationship with bacteria in the genus Rhizobium. The bacteria infect the roots of legume plants from which they obtain food, and the bacteria obtain nitrogen from soil air and “fix” it in a form usable by the plants.
The nitrogen is accumulated in small appendages called “nodules” which form on legume roots. Rhizobia have always been required by legumes for inoculation (putting bacteria on the seed) and proper growth, and the other ingredients in the mixture help the seedling in its very vulnerable state.
According to some seed companies, the lime coating affects the soil pH surrounding the seed as it germinates and counteracts the acidity of other fertilizers added at the same time.
This allows the rhizobia to survive and infect the root of the legume. It has also been suggested that the coating acts as a wick for water in times where moisture conditions are less than ideal.
According to other seed companies, coated seed is less likely to be eaten by birds and rodents because of its larger size and the lime or phosphorus coating.
The coating can also protect rhizobia when coated seed is mixed with a granular fertilizer and broadcast applied in a one-step process.
Fungicides can also be added to the pelletization process. Fungicides can give the seedling an added boost by protecting against root rot during the seedling stage.
These diseases can kill young alfalfa seedlings when they are very vulnerable, especially when the environmental conditions favor the diseases.
Care should be used when planting coated seed. Coated seed will weigh almost one-third more than uncoated seed because of the added coating and will flow through the planting equipment at a different rate than uncoated seed.
It has been suggested that coated seed flows 30 percent faster than uncoated seed (opposite of what many people would expect). Other studies showed no change in rate of seeding. In any case, planters should be calibrated for the specific brand and type of seed used.
The rhizobium in coated and pre-inoculated seed can die if it is stored too long or in warm temperatures for extended periods of time.
The seed itself does not lose viability, but the rhizobium in the seed coating may no longer be viable. As with all rhizobia inoculants, the coated seed must be kept cool and dry.
Checking the expiration date on all inoculants and coated seed when planting will eliminate problems later. If in doubt of the age or viability of the inoculants, add fresh inoculant. A few ounces of prevention might be all that stand between a seeding success and a failure.
By placing rhizobia next to the seed then surrounding that package with a systemic fungicide, seedling survival can be greatly increased if environmental conditions for pythium and phytopthera root rot are present.
The fungicide helps the alfalfa seedling when it is most vulnerable to pythium and phytopthera root rot, while placing the rhizobia close to the germinating seed almost guarantees infection of the legume’s root and nodulation.
With a higher survival rate for the seedlings, less seed is needed to establish a good stand. That is the rationale used to recommend planting coated seed – up to one-third less seed is used.
In theory, less dollars spent per bag compared to uncoated seed is a quick way to save a little money when establishing a stand of alfalfa. The seed companies are selling less seed but the same numbers of plants become established.
According to seed companies, the benefits of planting coated seed are better stands when compared to uncoated seed. An increase in revenue per cutting could be achieved without incurring any additional production costs.
However, in several independent university studies, the effects of seed coating on alfalfa establishment and yield were inconsistent over the years at different locations, so it is important for producers to ask questions when making decisions about purchasing seed. FG
References omitted due to space but are available upon request to editor@progressiveforage.com.
—Excerpts from Michigan State University website
