Crop competition the perfect companion to HWSC

5 November 2023 by
Seed Terminator, Kelly Ingram

Do you know your birthday?

Of course you do.

Do you know your partner’s birthday?

This is more important. It’s ok to wake up on your own birthday and forget, but have you ever forgotten your partner’s birthday? Not pretty!

Do you know what your wheat seeding rate is?

Of course you do.

How about your target wheat density?

This is more important.

It blows me away that I can ask every grower in the country their seed rate and they know it, but only a small percentage can tell me how many wheat plants per square metre they are targeting. We’re pretty good at targeting a canola plant density, largely because the seed is expensive. But when it comes to cereals we’re bloody hopeless.

Crop competition is now a critical part of weed control and its awesome because,

1.   On its own it suppresses weed growth and seed set,

2.   Herbicides work better when the weeds are competing with a competitive crop, especially pre-ems., and

3.    Crop competition is the perfect companion to harvest weed seed control. In the immortal words of Forest Gump, they “go together like peas and carrots”. Competitive crops make the weeds grow tall for sunlight, ensuring that the seeds are above the height of the cutter bar on the harvester, and the crop will hold the weeds up for longer to enhance weed capture. A match made in heaven!

Win, win, win!

There’s a few ways to increase crop competition like row spacing, seeding east west, competitive varieties etc., but let’s focus on plant density for now because given that its seeding time in Australia, it’s a bit late to change the other factors.

What is your crop density?

Seeding is the perfect time to get your eye in, and measure a few plant densities, to make an informed decision about what is ideal.

Grab a tape measure off the back of your ute and lock it at 1m. Now put this in a length of crop row and count the plants. It’s best to do this at about the 1 to 3 leaf stage before the plants start to tiller to make them easier to count. Do this in a few spots and decide on an average. If you don’t have a tape measure handy just stand your legs about 1m apart and count the plants between your feet – this will put you in the ballpark.

Now to convert to plants/m2 you need to multiply that number by the conversion factor in the table below for your row spacing. 

Row spacing (inches)

Conversion factor











If you have paired row sowing, count all of the plants in both of the paired rows and use the tyne spacing on the bar to work out your conversion factor.

What is the target plant density?

It’s true that some of the world record wheat crops have been grown at low seed rates as low as 25 kg/ha with 100 plants/m2 or fewer.  Wheat can compensate by tillering, but you must remember that these record crops are grown in clean paddocks, often in places like New Zealand where they grow over about 10 months, so don’t give me that old chestnut, “my best crop ever was a seed crop grown at 25kg/ha”.

Sure, wheat can compensate by tillering, but when there’s weeds about, there’s no substitute for plants/m2. There’s loads of trial data that shows if you double your wheat plant density you will halve the weed seed set.

The old rule of thumb is 50 plants per tonne of yield potential, but I find that for most growers with 10 to 12” row spacing, we rarely get above 180 plants/m2. Above about 180 we can keep pushing up the seed rate, but it’s hard to get a much higher density.

My rough rules of thumb are;

Plants per m2

Wheat yield

Seed rate to achieve this


Up to 2 t/ha


130 to 150

2 – 3 t/ha

52 to 60kg/ha


Greater than 3t/ha


** This is based on average seed size (36mg) and 90% establishment. 


If you want to compete with weeds, 110 plants/m2 isn’t enough. If you’re in a low yielding environment there’s no harm in targeting 150 plants/m2 or more.

My suggestion is to use the targets above, and add 30 plants/m2 for weedy paddocks. 30 seeds/m2 equates to about 12kg seed/ha.


This is all based on average seed size of 36mg (or 36g per 1000 seeds). Large seed is around 42mg and small seed is about 33mg.

If you have small seed, you can cut your seed rate by about 10%, and if you have large seed you will need to increase your seed rate by about 10%. It’s pretty common for there to be a 20% difference in seed size from one wheat seed source to another, so if you’re planting all of your wheat at the same seed rate, which everyone does, you’ll have a 20% difference in establishment.

Let’s do a calc

We’re in a medium rainfall environment targeting 150 plants/m2 so our seed rate for our yield potential is 60kg/ha. The paddock is weedy so we’re going to add 30 plants/m2, now we’re now up to 72kg/ha. And we have large seed (we know this because we paid our kids $5 each to sit at the kitchen table and count 1000 seeds then we weighed them [assuming very young children!]) so we need to put it up 10% taking us to 79 kg/ha.

Easy peasy!

Well it’s not as easy as just saying my seed rate is 70kg/ha from one end of the farm to the other. But it’s a whole lot better.

Happy counting!

The Terminator Agronomist
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Please note this advice is general in nature and not based on your specific circumstances.

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