A PhD on smashing weed seeds

From the Terminator Agro
20 December 2023 by
Seed Terminator, The Terminator Agro

I read a PhD!

All 244 pages of it.

Ok, I didn’t read it all, and I didn’t understand all of it because it was full of equations like this;

It was written by a very clever guy who probably knows more about smashing weed seeds in a mill than anyone else on the planet.

That guy was of course Dr. Nick Berry, the founder of Seed Terminator. 

Nick and his supervisors developed what became the Harrington Seed Destructor mill, and he then took that knowledge to develop the Seed Terminator. Over the next few months, I’ll share a few of the key findings that will demonstrate to you just how difficult it is to destroy weed seeds in a mill attached to a harvester.

I’ll give you the answer in a nutshell. 

You need to whack a ryegrass seed pretty hard at least 4 times to devitalise it. 8 hits is better.

And there’s no free lunch. There’s a given amount of energy that essentially needs to go into the chaff to kill the weed seeds. Unfortunately, if you’re not using horsepower, you don’t kill all of the weeds.

How many hits?

To work out how many times and how hard you need to hit a ryegrass seed to kill it, Nick built a machine that looked like this

…. He then spent weeks feeding thousands of individual seeds into, sometimes once, sometimes up to 16 times per seed, then germinated them to test their RSE – Reduced Seedling Emergence.

The chart below has the answer. If you want to hit at 40 m/s (or 144 kph), you’ll need to hit ryegrass seeds 8 times each to kill them. And if you can go up to 70m/s (252kph), you’ll only need to hit them 4 times.

And if you only hit them twice,  you won’t get to a high level of kill.

For context, a well hit 8 iron has a club speed of about 40m/s (140kph). If you put a ryegrass seed on a miniature tee, and whack it, you’ll need to load it back onto the tee another seven times, and keep whacking it, to kill it.

If you only want to do four whacks, you’ll have to pull out the driver.

My friend Leigh has a driver with a head on it the size of a football, and he’s a big lad. I reckon he’d get away with three whacks.

Although maybe he’d be better off with a small driver with a faster head speed?

But I digress.

The bottom line is, we can only spin a mill so fast, so we need to ensure that the seeds get at least 6 or 8 hits, so we need to design a mill with what the engineers call residency. How long is the weed seed a “resident” in the mill. The longer you can keep it in there, the more times you can hit it, but you need to let it out eventually.

In this study, prototype 1 was modelled to show that ryegrass seeds got 4 hits, and prototype 2 gave them 5 hits. After the PhD, another mill was developed with even higher residency time.

The Mastercurve

After many weeks of belting individual ryegrass seeds with various levels of fervour, Nick developed the


This is the Picasso of seed destruction.


The Mastercurve is an elegant way of saying that there’s a certain amount of energy that needs to fed into the seed bearing chaff to kill the ryegrass seeds in this case.

This is why seed mills simply must use a certain amount of horsepower, or they just don’t work.

If you’ve got a mill that isn’t draining much horsepower, it may not be doing the job you’re looking for.

And, check out the label on the bottom of that chart. (kJ/kg).

Kilojoules per kilogram.


More kilograms of chaff means more horsepower, and there’s absolutely no getting around it.

This is why it’s important to have a mill that doesn’t drain much power when it’s empty. The lower the amount of power you need to put into an empty mill, the more power you can pump into the chaff.


That’s it for this month, and we’ve just scratched the surface. So far we know that we need to hit seeds about 6 or 8 times, really hard, and if we’re not using horsepower, we’re not killing weeds. Next month we’ll learn about the importance of seed size and moisture. They both make a big difference.

The Terminator Agronomist
Proudly brought to you by Seed Terminator 

P.s. Please note this advice is general in nature and not based on your specific circumstances.

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