Knockdown Sprays. Dry-Sowing. New Pre-Emergents. What you need to know.

Once again, it’s autumn, and we’re sitting with tractors ready, looking to the skies for something to fall down. So, once again, we are posed…
April 13, 2021Seed Back to All

Once again, it’s autumn, and we’re sitting with tractors ready, looking to the skies for something to fall down. So, once again, we are posed with the age-old question: Do we delay seeding to get a knockdown spray in, or do we go in sowing without one?

The quick answer: In most cases, research supports that we go straight in and not wait – but ensure other weed management strategies are organised.

Before we get into it though, we need to clarify what is meant by dry sowing. Dry sowing is when there is no moisture in the seeding bed profile, which doesn’t allow anything to germinate. If there is some moisture to get something to germinate, then it isn’t truly dry sown, and thus the following information won’t apply to your situation. Also, if there are any green weeds present, then this article is also irrelevant.


Chris Preston, and his team from The University of Adelaide, has shown that early sown crops can compete better with weeds. This is because they are growing in warmer conditions and putting pressure on ryegrass and other weeds. Their research shows ryegrass will set a similar amount of seed regardless of whether sowing was early or delayed to get a knockdown. 

The key points are this: Sowing your crop in the right sowing window for the variety will maximise yield and boost crop competition. Don’t worry about knockdowns. We can fight weeds with a robust pre-emergent herbicide package. Preston’s trial results demonstrate how sowing early maximises yields regardless of ryegrass. See more below.


Research has shown that, due to years of waiting for the ryegrass to emerge and then getting a good knockdown, it has caused a shift in how ryegrass germinates. In essence, in the past, we have killed off all the ryegrass that emerges on the opening rain, but now the ryegrass emerges staggered and often weeks after the opening rains. Waiting for the ryegrass to emerge and then killing it with the knockdown will only lead to disappointment, with much of the ryegrass still emerging later. Every day we wait for the knockdown is another day for the soils to cool and cause the crop to be less competitive and vigours.

Table 1: Ryegrass density in crop, headcounts in spring and wheat yield for two times of sowing at Hart, SA 2014, where Sakura was applied pre-sowing (118g/ha). (Australian Herbicide Resistance Initiative 2019)

So, if you take away one thing from this article, make sure it’s this – don’t let weeds determine when you sow. Sow to benefit your variety. And if that means going in without a knockdown, then go for it. Your yields will be better for it. Just ensure you have a pre-emergent herbicide plan to tidy up any weeds. Also, take any opportunity to kill the ryegrass that does germinate before the crop by applying paraquat to very small ryegrass before crop emergence. This is a must if the opportunity arises as it will take some pressure off the pre-emergents. 2020 showed some excellent results from using this technique. 


If you start letting weeds call the shots in your system, you’ll lose money. Working around weeds means changing rotations, delaying sowing, and even leaving paddocks out—all to the detriment of our profits. 

Remember that the factor that allows us to get away without a knockdown spray is using a robust pre-emergent herbicide package. Mix at least two herbicides, and make sure they’re herbicides that suit the situation. 

In true dry sowing situations, some of the older chemistry, such as Trifluralin and Avadex, are excellent mix partners with something like Sakura. All these products will happily sit in the dry soil and activate once the break/rainfall comes. These products are also safer for the crop. However, damage can still occur if significant rainfall of 50mm+ occurs in rapid succession, particularly on lighter soils. 


A warning on new pre-em chemistry:

Be extremely cautious with some of the new chemistries such as Luximax and Overwatch. We wouldn’t recommend using Luximax in dry sowing conditions, or Overwatch in lighter soils in dry sowing. In extreme cases, some pre-emergents like Boxer Gold may be a better choice once there is some rain around the corner, as we will lose activity of the product if it sits in the dry soil for weeks. So look to apply the Boxer Gold when the rain starts. Also, a heavy rain onto dry soil can push chemical quickly through the soil profile and into the farrow, which can increase crop damage. Ones to be careful with are Luximax, Edge in canola, and Overwatch/Sakura in very sandy soils. 


You could use the money saved from skipping a knockdown on pre-emergent herbicides that may even do a better job. Just remember to keep rotating chemistry to stop resistance. 

Also, try to maximise a competitive crop’s growth with the following; utilise narrow row spacings, use higher seeding rates, try east-to-west sowing, competitive cultivars, and improve healthy soils.


If you’re still itching for a knockdown spray, remember if you miss the knockdown at the start of the season, you can always try to get one at the end of the season. You can stop the weed set cycle with crop topping, or hay cutting if appropriate, and using harvest weed seed control techniques when harvesting. 

We wish everyone the best of luck seeding this year!