Before you buy a bag of alfalfa seed, find out just how much of the seed is likely to germinate. In other words, learn how much pure live seed (PLS) a bag holds, says Dan Undersander, University of Wisconsin Extension forage specialist.

“Most people don’t take the time to look at pure live seed,” he says. Seed-bag labels only carry the percent of inert material – weed seeds and other material – as well as the percent germination on their labels. But it’s easy math to figure the PLS.

Say your seed bag only has 1% inert material. Its purity is the remaining amount, translating to 99%. Multiply that 99% purity times the percent germination – let’s say that’s 90%. The amount of PLS is 89%.
That’s a pretty good percentage, Undersander says.

It’s especially important to calculate PLS when buying coated alfalfa seed, he says. The coating on seed could account for about 30% inert material, which means its purity is around 70%. If the germination level is at 80%, that gives a PLS of 56% (0.70 x 0.80).

“If you’ve got 80% germ, only half of your bag is germinatable seed,” he says. If it were at 90% germination, PLS would be at a more acceptable 63%. Because growers tend to seed heavier – averaging about 17 lbs/acre rather than the 10-12 lbs that universities recommend – an alfalfa stand should establish well with 60% PLS. If PLS falls below 60%, the seeding rate should be adjusted accordingly.

“Our premium alfalfas are going to be good, but some of our farmers are looking at cheaper seed,” he says. Coated seed with low germination selling for $150/bag may not be saving a grower money or provide the stand establishment that a $300 bag with a higher PLS count can, he adds.