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Know Your Plant Populations

Jan 06, 2020



Tongue twister parody aside, here's a puzzler for you: When you seed canola, do you set your seeding rate by pounds per acre or target plant population?

It wasn't too long ago that best practice was to use a straight pounds per acre metric, but things have changed and today, even the Canola Council of Canada recommends canola growers set their seeding rates based on a target plant population (TPP) of 5 to 8 plants per square foot.1

In some ways, TPP is simply a refinement of the old pounds per acre recommendation – both are targeted at achieving an ideal plant density, but TPP is a more precise measure because it uses seed weight and field survivability as determining factors.

Now, this is not new in agriculture. Farmers planting large seeded crops, like corn and soybeans, have relied on TPP to set seeding rates and gauge yield outcomes early in the season with a fairly high degree of accuracy. The challenge in applying TPP to canola has always been the small seed size. Bluntly – it's easier to see what you're doing with a large seed going through a gravity-fed planter than it is with a small seed through an air drill.

But research done by scientists with AAFC in Alberta and the Crop Development Centre in Saskatoon showed that TPP is just as important in canola to maximize plant performance and yield. They looked at herbicide-tolerant canola hybrids in a no-till system and found that a TPP of 5 to 8 plants/sq. ft. allowed for some plant mortality over the season without affecting yield potential.2


The TPP for TruFlex canola with Roundup Ready® technology is 6 to 8 plants per square foot. So, how do calculate your seeding rate to achieve that target?

The Canola Council has a great online tool that can do the math for you, but you will need three pieces of data to use it: seed weight, TPP (the Canola Council calls this "target plant density"), and an estimated emergence rate. Sounds simple, but there is a little planning involved.

Thousand seed weight (TSW). This is measured in grams and is found on every bag of certified canola seed. The TSW of current canola hybrids can vary from variety to variety by as much as 3 grams3.

That's important to note because the heavier the seed, the fewer seeds there are in a pound or kilogram. So knowing the seed weight will help you adjust your seeding rate to ensure the optimum number of seeds per square foot get planted.

TPP and field survivability. Basically, you want 6 to 8 plants per square foot to survive, not just emerge, but survive to the end of the season and contribute to yield.

The Canola Council says the average canola seed survival rate is 50% to 60%4, so to end up with your ideal TPP at harvest time, you need to seed at least 10 seeds/sq. ft. in the spring.

This is where the planning comes in. Seed survivability rates are different for every field because every field is, well, different. Perhaps it's low-lying and prone to early frosts and flea beetles. Perhaps it's an uneven field with nutrient levels all over the place. Maybe it's a beautifully uniform field with good moisture retention. The point is, whether you target 6 plants per square foot or 7 or 8 depends on how "survivable" your field is.

If you're not sure what a field's typical survival rate is, the Canola Council's seeding rate calculator includes a plant density assessment tool that can help guide you. To gain baseline metrics, start by doing stubble counts after harvest to see how many plants made it to the end of the season and compare that to your known seeding rate. Keep records to build your field "survivability" database.

Estimated emergence. Emergence is not survival. The plants that successfully emerge won't all make it to harvest but the more that do, the greater your chances of hitting that TPP sweet spot come fall.

If you don't already, start completing emergence counts every year, compare them with the seeding rate you used to build a picture of how each field and variety perform. When you combine this information with field survivability data and TSW, you can fine-tune your seeding rates accordingly the following year.

TPP isn't a new way of calculating seeding rates as plant density has always been important. But it is a more accurate way that will ensure you get the most out of your seed dollar. Talk to your local seed company representative about implementing a TPP approach on your farm.


1,3, 4 CCC best management practices for TPP:
2 Canadian Journal of Plant Science (Response of herbicide-tolerant canola cultivars to four row spacings and three seeding rates in a no-till production system):