First Cut: The seeding rate conundrum

By Mike Rankin

Not long ago I was sitting in a farm office talking to a dairy producer about his forage cropping program. Amongst the flurry of questions I asked was, “What is your alfalfa seeding rate?” He shot back, “Eighteen pounds per acre.”

Next question, “That variety you’re using has a 34 percent seed coating by weight, so is that pounds of actual seed or product as it comes out of the bag?” He looked at me quizzically and said, “That’s a good question. I think it’s product; it must be product. I’ll have to check.”

Here’s the thing: I’ve had this same conversation more than once over the past few years.

What used to be a standard agronomic term with pretty universal understanding — seeding rate — has now taken on some additional baggage. Seeding rate now may mean pounds of product (seed plus everything that comes with it, including the coating), pounds of actual seed and whatever other small amount of nonseed components that might come in the bag (if there is no coating), or it may mean pounds of pure live seed (PLS), as it usually does in most university seeding rate research.

The concept of PLS rips away everything but viable and hard seed. If it’s not alfalfa or it doesn’t germinate, it doesn’t count. When you hear a university specialist say that they didn’t see significant advantage to seeding rates over a given number of pounds per acre, PLS is usually what they are talking about.

For a long time the difference between pounds out of the seed bag and pounds of PLS wasn’t too significant. That’s changed with the introduction and wide use of seed coatings. Now, many companies use seed coatings that comprise one-third the weight of the bag. Add to that any seed that doesn’t germinate, and it’s not uncommon to have a bag of seed with 60 percent PLS.

Alfalfa seed coatings are usually clay and/or polymer based and may contain Rhizobium bacteria, fungicide(s) and perhaps a growth regulator. They are often marketed as providing improved germination and growth. Research results are somewhat mixed on this point. Though it seems counterintuitive, coated seed tends to flow out of the drill faster than uncoated seed.

How much or how little seed coating is on the seed varies with marketer and is probably not a good primary variety selection criteria. What is important is knowing if you’re seeding too little or too much. At the farm level, perhaps the focus should be turned to the number of seedlings or plants established at a given point in time. After all, we don’t harvest seeding rates.

Every one pound of pure seed planted equates to dropping about five seeds per square foot. Lots of factors contribute to final stand density. These include: drill performance, seed placement, seedbed conditions, weather following seeding and issues related to actual seeding rate. Your optimum seeding rate may differ from your neighbor, who has a different drill, seeding method or soil. If stand counts are lower than desired, then try to determine the cause. It may be seeding rate; it may be something else.

Realize that, if you have always seeded 16 pounds per acre of non-coated seed product and now seed 16 pounds of a high-volume coated seed, your actual rate is likely somewhere below 10 pounds of PLS per acre. Maybe that’s good enough for a full, productive stand; maybe not.

This editorial appeared in the November issue of Hay & Forage Grower on page 4.

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