What’s in Your Alfalfa Seed Bag?

Reading labels isn't just important when you're watching your diet. Knowing how to read an alfalfa seed tag can also reap big dividends, in managing seed costs and ensuring you plant the amount of pure live seed intended. Many alfalfa growers do not realize they may be purchasing heavy-coated seed that reduces the pure live seed they're actually planting. Unless you increase the seeding rate for heavy-coated seed, your stand establishment is at risk.

Many seed companies sell coated alfalfa seed, and the coating material can range from the 9 percent light coat used by DuPont Pioneer, up to 34 percent of total weight or more in heavy-coated alfalfa seed from many other companies. Rhizobium, fungicide and colorant account for approximately 3 percent of the coating weight. Any additional weight listed as coating material is primarily a mineral such as limestone, gypsum or mica, with a polymer that binds these materials to the seed. The most common coating is a 34 percent limestone, referred to here as heavy-coat.

Alfalfa growers should realize these coatings add volume and weight to the bag weight beyond the actual seed. This added weight reduces the number of pure live seeds per pound, increasing the risk of inadequate stand establishment if the seeding rate per acre is not adjusted accordingly. This is especially true with cloddy seedbeds or increased weed pressure during early growth.

DuPont Pioneer uses a proprietary patented light coat of 9 percent, applied in a layering process; a layer of fungicide directly on the seed, followed by a separating layer of mica and polymer, then a layer of rhizobia, with a final layer of mica and polymer. This layering results in separation of fungicide from rhizobia, and provides for optimal rhizobial activity and longevity. We use mica (a mineral with similar flowability properties as the talc that's used as a planter box treatment for improved flowability in seed corn) as opposed to rough limestone or gypsum commonly used in heavy-coated seed.

Adjust Seeding Rates

Growers should adjust seeding rates based on the level of pure live seed (PLS) per unit of alfalfa seed. You can calculate pure live seed from information on the tag.

Pure Live Seed = percent pure seed x percent total germination

Pure live seed is seed you can expect to germinate and contribute to stand establishment. In the case of our light coat seed, multiply the 90 percent pure seed times 90 percent germination, to equal 81 percent pure live seed. A heavy-coat seed would typically show 66 percent pure seed, with a 90 percent total germ. The calculation for this heavy-coated seed is 66 percent pure seed multiplied by 90 percent total germ equaling only 59 percent pure live seed. That's quite a difference, and one that could affect stand density unless seeding rate is adjusted.

Light Coat example:             Heavy Coat example:
     90 percent pure seed         66 percent pure seed
  x 90 percent germ + hard     x  90 percent germ + hard
     81 percent PLS         59 percent PLS

 

What the PLS Score Means

 A good stand of alfalfa will have 20 to 25 plants per square foot following its first winter. To achieve this, plant enough seed to allow for the attrition that occurs as seedlings emerge, contend with diseases and compete with weeds. Throughout most of North America, alfalfa growers have settled into seeding rates that achieve about 80 pure live seeds per square foot. This equates to 13.9 pounds of pure live seed per acre, assuming 250,000 seeds per raw pound. This equals 16 pounds per acre of traditional pink-treated seed at 90 percent total germination, which includes about 3 percent weight gain from fungicide, Rhizobium and colorant. Using this as a starting point, the table below shows the pounds of pure live seed and equivalent seeding rates needed to achieve 80 pure live seeds per square foot:

  9% Light-Coated
Seed
34% Heavy-Coated
Seed
Total germination (from the seed tag) 90% 90%
Pure seed (from the seed tag) 90% 66%
Pure live seed, percent (total germ x pure seed) 81% 59%
Pounds of seed needed for 80 seeds
per square foot
17.2 pounds/acre 23.6 pounds/acre

If cost of bag is:
Then seed cost per acre is:

$225
$77
$190
$90
If cost of bag is:
Then seed cost per acre is:
$350
$120
$300
$142

Without this seeding rate adjustment for pure live seed, you'll be reducing the number of seeds per square foot and increasing the risk of attaining a full stand of alfalfa. Lower seeding rates of pure live seed are associated with reduced yield in the establishment year. Spotty stands hurt production throughout the stand's life. Weed competition can be more impactful in non-uniform stands too, further reducing yield and quality.

In the example above, the heavy-coated seed costs more per acre than light-coated seed when you adjust for pure live seed equivalency.

Find out what's in your bag of alfalfa seed to establish a long-term, viable alfalfa stand.

To learn more, talk to your local Pioneer sales professional.

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