April 23, 2015
Little old lady
Little old lady who?
I didn’t know you could yodel!
It is double knock season in Australia. Glyphosate resistance has been confirmed in many fields across the country in at least 10 weed species. Public enemy number one is the world champion of herbicide resistance, annual ryegrass. Glyphosate resistant ryegrass has been confirmed in hundreds of fields in Australia and is suspected in thousands.
The double knock technique of glyphosate followed by paraquat is a very useful tool…when done right. The right sequence – glyphosate followed by paraquat; the right timing – anywhere from one to fourteen days between knocks; the right rates – full rates of both herbicides.
Does it work for glyphosate resistant ryegrass? Yes. But you have to do it right, plenty of paraquat at the right time.
Unfortunately, paraquat may be in short supply in some areas of Australia this year. Where this is the case we must prioritise where we use it and be prepared by having product on farm.
Glyphosate resistant ryegrass coming to a paddock near you if you don’t do something about it.
The double knock is a great place to start.
Former AHRI researcher, Paul Neve (now based at Rothamsted Research) and Art Diggle from the Department of Agriculture and Food of WA built a computer model in 2003 to determine the probability of glyphosate resistance evolving for a range of knockdown strategies. Their model appears to be quite accurate. The model predicted that after 12 to 15 years of continuous glyphosate the probability of resistance began to increase. Twelve years on and glyphosate resistant ryegrass is starting to escalate in Australia, just as the model said it would if we over-relied on glyphosate. The model ran a Wheat: Lupin: Wheat: Canola rotation sown with no-till seeding equipment.
Paul and Art also modelled some alternative strategies. Firstly, rotating between glyphosate and paraquat. This certainly reduced the probability of glyphosate resistance evolving.
Secondly, they modelled using the double knock technique – glyphosate followed by paraquat.
The computer model certainly suggests that the double knock technique is a winner. One could argue that the model was pretty accurate when it came to predicting the evolution of glyphosate resistance under a continuous glyphosate regime. This gives us some confidence that the model can accurately predict how good the double knock is to prevent the evolution of glyphosate resistance.
What is the ideal timing between knocks? Back in the day, the recommendation was to apply the second knock 5 to 10 days after the first. However, growers found that in some situations they were ready to seed before 5 days was up, hence the question, what is the shortest interval between knocks without compromising weed control?
Following is a quick review of a number of research trials.
The trial below was conducted by Peter Newman in 2003 while working at DAFWA with GRDC funding. Ryegrass as the site was 3 to 4 leaf (623/m2) and capeweed (116/m2) was 5 to 15cm diameter. Gly/SS refers to glyphosate followed by Spray.Seed® and so on at various intervals between knocks from 1 hour to 4 days. This trial shows that all knockdown intervals worked for ryegrass. Excellent growing conditions persisted and the Spray.Seed® rate was more than adequate. Keep in mind that this ryegrass is not glyphosate resistant. The trial also shows that applying Spray.Seed® first in the sequence worked well, however, as you will see later, this is not the case for capeweed.
Gly = glyphosate 540 @ 900 mL/ha; SS = Spray.Seed® (paraquat / diquat) 1.4 L/ha.
The trial below was also conducted by Peter Newman while working at DAFWA with GRDC funds in 2004. This trial looked at a range of single and double knockdowns for large wild radish (up to 60cm diameter). This trial shows that following glyphosate with Spray.Seed® within one day can antagonise glyphosate. It is likely that the Spray.Seed® shuts the plant down before glyphosate has had a chance to translocate. One day between knocks is OK, but two days is better. The trial also shows that Spray.Seed® first in the sequence is a bad idea, as is half rates of herbicide, and there are a number of ways to kill large wild radish with Spray.Seed® mixes.
Gly = glyphosate540 @ 1.2 L/ha; SS = Spray.Seed® 2 L/ha; Ester = 2,4-D Ester 80.
This trial was also conducted by Peter Newman while working at DAFWA with GRDC funds in 2004. The grass weeds in this trial were primarily large tillering barley grass. Once again, glyphosate followed by Spray.Seed® is the best sequence with one or two days between knocks. One hour between knocks was too close and Spray.Seed® benefited from cultivation after spraying.
Gly = glyphosate540 @ 1.5 L/ha; SS = Spray.Seed® 2.4 L/ha
Does it work on glyphosate resistant weeds?
All of the trials above were conducted on glyphosate susceptible weeds. So begs the question, will the double knock work on glyphosate resistant weeds? This is the whole idea after all.
The pot trial below was conducted by Paul Neve while working at AHRI with GRDC funds. He tested three populations of ryegrass. VLR 1 is susceptible. NLR 50 is a glyphosate resistant population from NSW. WALR 50 is a glyphosate resistant population from WA. This trial has the potential to make life hard! It shows that perhaps the best interval between knocks for glyphosate resistant ryegrass is either one day or 14 days, and not the period in between. However, a higher Spray.Seed® or paraquat rate may fix this problem. This pot trial was conducted on grass weeds that were 4 weeks old. The same pot trial was conducted on 3 week old ryegrass and all knockdown intervals worked well.
The next trial was truly ground breaking stuff. This was the first double knock field trial to be conducted on glyphosate resistant ryegrass. The trial was conducted by Andrew Storrie, AGRONOMO consulting (formerly NSW DPI) and Tony Cook, NSW DPI. This trial concluded that the double knock certainly does work on glyphosate resistant ryegrass. The trial also showed that Spray.Seed® followed by Spray.Seed® gave 100% control of glyphosate resistant ryegrass. This is not a double knock, but it is a useful salvage option for glyphosate resistant weeds.
Andrew Storrie commented that the key is to ensure that the paraquat or Spray.Seed® rate is high enough. Don’t skimp on the second knock.
Glyphosate first is the best sequence for the double knock. In some trials, grass can be controlled well when paraquat or Spray.Seed® is applied first but it doesn’t work for broadleaf weeds. The data below is from the 2003 trial by Peter Newman already mentioned above. All of the capeweed died when glyphosate was applied first in the sequence but it did not when Spray.Seed® was applied first. Further evidence of this was seen in the wild radish trial above.
It is imperative that lethal rates of herbicide are used for both knocks. The whole idea of the double knock is to ensure that no resistant weeds survive. Cutting the rate of either of the knocks jeopardises this. In addition to this, many double knock trials have been conducted on glyphosate susceptible weeds. The starting point for paraquat as the second knock on annual ryegrass should be 1.5 L/ha and 2 L/ha for Spray.Seed® for 3 leaf plants increasing with plant size and time between knocks. For glyphosate resistant weeds, it is imperative that the paraquat or Spray.Seed® rate is adequate to completely control the weeds based on when they germinate.
In the ideal world, the double knock would be applied to every field every year. However, this is not always possible, particularly given the likelihood of paraquat shortages in some years. Below is a priority order list of fields that are best suited to the double knock:
- Fields where glyphosate resistance has been confirmed or is suspected,
- Fields that will be sown to a glyphosate tolerant crop this year e.g. Round Ready®
- Fields that were glyphosate tolerant crops or glyphosate chemical fallow last year
- Fields with high ryegrass burden
- Fields with weed species that benefit from the double knock for good control e.g. Fleabane (Conyza sp.)
- The list could go on forever, but you get the picture. Plan to double knock the fields listed above and make sure that you have product on hand.
In a nutshell
Computer modelling and farmer experience shows that the double knock works.
At least one day between knocks, two or more is ideal.
Plenty of paraquat or Spray.Seed® with excellent application.
The double knock works on glyphosate resistant weeds, don’t skimp on the rates.
Follow the links below for further information:
- Scientific paper
- Double knockdown timing trial (2003 Peter Newman)
- Single and double knockdowns for large wild radish (2004 Peter Newman)
- Glyphosate resistant ryegrass trial (Andrew Storrie & Tony Cook)
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