Size Matters

22 February 2024 by
Seed Terminator, Kelly Ingram

Try this.

Using your hand like a tennis racket, whack a ping pong ball as hard as you can.

Now do the same thing with a golf ball.

Which one hurt more? The golf ball right.


That’s because E = 1/2 mv 2

Energy equals half x mass x velocity squared.

The mass of the golf ball is greater than the ping pong ball, and if your hand speed was about the same both times you put a lot more energy into the golf ball because it’s heavier. Similarly, a small ryegrass seed will have less energy put into it than a larger seed when hit with rotor in a seed mill travelling at over 300kph.

This is why weed seed size matters.

Remember the master curve from last month – this told us annual ryegrass needs the magic figure of 10kj/kg of energy to kill the seed. This is the Energy side of the equation. Here’s the link if you need a reminder.

There is just no getting around the fact that we need to put this amount of energy into the chaff to kill the seeds. If the seeds are lighter, less energy goes into them, and the seed kill will drop.

This is an unfortunate truth for all manufacturers of seed mills.

Seed mills are generally tested with ryegrass seed purchased from a ryegrass seed grower because it is of good quality, and easy to germinate for testing purposes.

But commercial seed is big. Around 2.7 to 3mg per seed.

We’re now starting to understand that ryegrass seeds on plants in our crops may be smaller and lighter than this, making the seeds in crops harder to kill than commercial seed that has been grown as a commercial ryegrass “crop”.

We’re still getting a handle on this, because until now, nobody has really bothered to measure ryegrass seed size. There may be times when ryegrass seeds in crop are as big as commercial seed, but there will also be plenty of cases when it’s smaller.

This is a bummer, but it isn’t the end of the world if your seed mill is achieving kills in the high nineties of commercial seed. With small seed, we may see this drop to the low nineties.

Most growers and agronomists will, of course, want the highest kill they can get, and most will be comfortable that over 90% is still a good result.

But where do we draw the line? How much seed kill do we need? 90% seed kill is still a whole lot of weed seeds that are not returning to the seed bank. I was happy with exam results of 60% at Uni, but some of my mates were only happy with 80%. The beauty of a C minus is in the eye of the beholder.

If you have a mill that is starting at a lower seed kill of commercial seed, then it may drop to an unacceptable level. Nick Berry did a great job of explaining this at the 16 minute mark of this video.

I think it’s fair to say that smashing up weed seeds is a whole lot harder than anyone first thought, and not only is annual ryegrass Australia’s most problematic weed, it turns out that it is one of the hardest to kill with a seed mill.

There will always be compromises to the kill rate, so if you have a high seed bank of ryegrass, and dirty crops, you need to choose a mill with the highest kill rate.

There are mill options on the market with known lower levels of seed kill, and they have their place where blockages are an issue, or weeds such as wild radish (that are easier to kill) are the main target. But when conditions are good, and a high kill mill is working well, this should always be the first option.


While we are talking about compromises, let’s talk about moisture.

It won’t surprise you to learn that moist seeds are harder to kill.

In his PhD thesis, Nick Berry found that for every 1% of moisture above 11.3%, seed kill reduced by about 2%. Focus on the red line in the chart below as that is close to what is happening in a mill, and probably only really look at the first couple of data points because I don’t think too many growers will be harvesting at 25% moisture.

Try 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.

Once again, this compromises the seed kill, but there are things we can do such as harvest the weedy paddocks in the driest conditions, and measure harvest losses, this will probably convince you that you shouldn’t be harvesting in moist conditions anyway!


Ryegrass seed is probably smaller than we thought it was, and small seed is hard to kill. And moist seed is also hard to kill. This is why focusing on a mill with the highest kill rate possible is still the A game because we can handle dropping from say 99 to 92% kill if seed size and conditions are against us, but we probably don’t want to drop too much lower than this if we can help it.

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.

Share this post