Doing the Math on TKW of Canola: Should Farmers Pay by Seeds Per Bag?

Big seed results in only slightly better germ rates — not enough to balance the pounds/acre seed number spread.
Big seed results in only slightly better germ rates — not enough to balance the pounds/acre seed number spread.

A couple weeks ago there was a good discussion on Twitter discussing the varying thousand kernel weights (TKW) of canola coming from suppliers this season. I hear this discussion pop up every spring, it seems like, and no wonder. Average TKWs on seed fluctuate from year to year, yes, but there’s been a trend to larger canola seed in recent years. Bigger seed offers a slight germination advantage, but that bump is slight.

Because farmers are charged by the pound for canola seed, the heavier or bigger the seed, the higher the TKW, and the fewer seeds per pound. It means, for the same pound of seed you’re paying the same for fewer potential plants. This can be an issue for a few reasons: one, it’s potentially costing you more to plant that bigger seed on a per acre basis; and, two, it could also inadvertently hinder your plant stand counts, which, in turn could have an effect on yield, maturity, disease management and more.

Let’s look at two different seed sizes, as examples, so we can illustrate how big of a difference there is between TKW:

4.0 g TKW = 113,500 seeds per pound

6.5 g TKW = 69,846 seeds per pound

If we assume a seeding rate of 5 lb/ac (typical for many) here are the differences:

113,500 x 5 lb = 567,500 seeds planted on one acre

69,846 x 5 lb = 349,230 seeds planted on one acre

567,500 seeds per acre divided by 43,560 (square feet in an acre) = 13 seeds/ft2

349,230 seeds per acre divided by 43,560 = 8 seeds/ft2

That’s five seeds difference in every square foot of field — and brings a great target plant stand down to a marginal one, all because of TKW and nothing more.

Now we haven’t taken into account seedling mortality which can vary from 30% to 60% (or more), but you can see the old standby of 5 lb/acre isn’t necessarily the way to go in every situation. These numbers show first and foremost that there needs to be the TKW taken into account when determining a seeding rate as it can have a big impact on your final plant stand. The difference of five seeds per square foot could be anywhere from two to five plants, more or less, in any area of the field which means you could have a less than ideal plant stand through no fault of your own (except for not re-calculating your seeding rate).

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If we look at the big seed (6.5 TKW)we can see that there is only eight seeds/ft2, assuming a 50% mortality that means you may end up with only four plants/ft2 which is not in the ideal range of seven to 14 plants that the Canola Council has established. This is right in the range where the Canola Council starts to see a decrease in yield due to a lack of plants.

My question for everyone is whether or not farmers and industry should be looking for a better way of doing things. Should companies be selling based on seeds per bag like in corn or soybeans? Should growers be sent their TKW numbers earlier so they can better determine their exact amount of bags needed? It’s not necessarily economic to up seeding costs by a third for the same number of plants. Is there a better strategy?

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