Posts in Herbicide resistance
Fine tuning narrow windrow burning

It is that time of year again when a farmer’s thoughts turn to burning. Like most things there is more than one way to skin a cat. However I am going to talk about some of the dos and don’ts for narrow windrow burning.

Although at this point you cannot do anything about chaff windrows that have already been produced what you will see is that windrows from a well set up header will have withstood considerable amounts of rain over summer and still be ready to burn. The windrow you can see below has had over 100 mm of rain between harvest and burning.

Windrows that aren't over-threshed remain open and aerated like this one.

Chaff still on top of windrow despite 100 mm rain.Things to note are that the windrow appears prickly and this shows that during harvest the straw was not over threshed. This has allowed the chaff portion to remain high in the windrow which also aids drying after rain. The crop was harvested at a height of approximately 10 cm.

Another critical factor in burning windrows is the meteorological conditions when you start burning.

Starting the burn when the FDI is about 7 which in this case was about 18:00.The use of a fire danger index (FDI) has made stubble burning more of a science than a mystical art. The FDI is determined by temperature, humidity, wind speed and dryness of the fuel. These need to be measured and put into the fire index app to calculate FDI. This means that you need a weather meter, either hand-held or cab mounted, and your mobile phone with the app installed. A very useful app for both Android and Apple is Fire Tools by Mountain Pine Studios.

Doug Smith measuring wind speed, temperature and humidity to feed into the fire app. Also use the Bureau of Meteorology’s MetEye® site for monitoring local predicted changes to wind speed and direction so you can decide whether to press on or call it a night.

This is the sort of result you aim for.As a rule of thumb for the fire index:

Greater than 15 is too high

8 to 10 is ideal

Less than five is often too cool or damp

Starting when the fire index is too high means that the file will rarely stay in the windrow and the fire will burn significant portions of the paddock. See image below.

Too low and the windrows won’t burn as hot as needed to kill weed seeds in the row.

This paddock on another property was cut a little high and the burn started when the Fire index as too high. The fire didn't remain within the rows.Important pointers from Doug & Kerry Smith, Pingrup, WA

  • Always over-estimate the problem and under-estimate your ability when it comes to fire control
  • Your aim is to burn the rows not the farm
  • Buy a good fire lighter as it makes a big difference to the amount you can light per night
  • Conditions change by the hour so keep measuring conditions as well as knowing what is coming for the next couple of days before you light up.
  • Check for smouldering rows early the next day so they can be dealt with before the day heats up.
Paraquat resistance is gaining momentum in Australia

Another three weed species in Australia have just confirmed resistant to paraquat – Cudweed (Gamochaeta pensylvanica), blackberry nightshade (Solanum nigrum) and crowsfoot grass (Eleusine indica) taking the total number of species to 9 in Australia (Table 1).

Paraquat resistant crowsfoot grass regrowing following spraying. Image: P. Boutsalis & C. Preston

Overseas there are now 24 species resistant to paraquat, from the Middle East through to New Zealand, comprising of 6 grass and 18 broadleaf species.

Table 1 Species that have developed paraquat resistance in Australia
(Australian Glyphosate Sustainability Working Group)


Common name

Year first confirmed



Resistance to other herbicides / MOAs

Arctotheca calendula





Diquat (L)

Hordeum glaucum

Northern barley grass




Diquat (L)

Hordeum leporinum

Barley grass




Diquat (L)

Vulpia bromoides

Silver grass




Diquat (L)



Small square weed




Diquat (L)

Lolium rigidum

Annual ryegrass


South Australia

Pasture seed

A / M - 2 populations

Gamochaeta pensylvanica




Tomatoes, peanuts, sugar cane, avocados


Solanum nigrum

Blackberry nightshade



Tomatoes, peanuts, sugar cane, avocados


Eleusine indica

Crowsfoot grass



Tomatoes, peanuts, sugar cane, avocados



As can be seen from Table 1, paraquat resistance hasn’t developed in broadacre cropping yet and the listed rotations, or lack of them, were highly reliant on paraquat for weed control.  However the widespread adoption of paraquat either as a second knock or an alternative to glyphosate over the past 10 years we probably don’t have long to wait. There are rumours of a glyphosate-paraquat resistant population of annual ryegrass (L. rigidum) about to confirmed from southern Western Australia.

In 2013 we confirmed a ryegrass population from a Great Southern vineyard that is strongly resistant to both glyphosate and paraquat which was the result of a vineyard manager rotating between these two important herbicides. Rotating herbicide modes of action buys you time, but doesn’t prevent resistance.

The answer?

The only way to stop herbicide resistance in its tracks is ensure no survivors of a herbicide application are allowed to set fertile seed.

This means drive weed numbers down and use diverse crop rotations, which in turn gives you plenty of options to use a range of non herbicide weed tactics. Combined with competitive crops, good timing of operations and effective spray practices you are on the way to putting herbicide resistance well into the future.

While tank mixing solid rates of different modes of action can be effective, many growers will be too late for this tactic as they already have resistance to at least one of these modes of action. For tank-mixing to work as a resistance management strategy both herbicides MUST BE fully effective on the weeds in question.

Mixing glyphosate and paraquat is not a viable option because of antagonism (biological) in the plant. When mixed together these products are not complementary and the paraquat works too rapidly for the glyphosate to be effectively translocated. This is the reason the double knock is utilised. Both products can be utilised on the same weed population without antagonism of the mix.

But before you go, ask yourself the question, “How do I know which of my herbicides still work?”

Blackberry nightshade Image: AGRONOMO

Why herbicide resistance tests don’t always reflect paddock experience

Some growers and agronomists feel the results from herbicide resistance testing services don’t accurately reflect what is happening in the paddock. Some samples are coming back susceptible to the tested herbicides while the farmers are finding poor control in the paddock.

Glyphosate resistant annual ryegrass in winter fallowThis phenomenon was investigated in northern NSW in 2008 where there were differences between in-paddock control of wild oats compared to the testing service results. What came back as susceptible in the “lab” tests were still not being well controlled in the field. So what is going on?

Differences occur due to:

  1. Sampling
    How were the samples taken? From one spot in the paddock, several dense patches or averaged over the whole paddock. Different sampling methods will give different test results. This needs to be known by whoever is interpreting the test. 90% resistance in one patch can be quite different to 5% resistance is a bulked sample from across the paddock. Results are only as good as the original sampling.
  2. Differences in herbicide coverage between field and lab
    Testing services spray the weeds in spray cabinets using 110° 01 twin jet nozzles. These nozzles produce a very fine to fine droplet spectrum and under the controlled conditions of a spray cabinet give excellent coverage. If a grower tried this at least half of the herbicide would not reach the target.
    Spray application by the grower has to contend with widely varying temperatures, humidity, wind, droplet interception by stubble and crop, much higher travel speeds, varying boom height and potential moisture and temperature stress of the target weeds.
    By necessity growers use coarser spray quality and often lower application volumes giving less than 15% coverage compare to the lab’s over 40% coverage. Add to this herbicide rate, adjuvant, water quality there is no wonder there are differences.
    Testing pre-emergent herbicides such as atrazine add another layer of complexity. Due to atrazine’s high water solubility the composition of the potting mix and level of watering by the testing service can make huge differences to the results. Too much watering can leach the herbicide from the pots. Add to this atrazine’s sensitivity to light, insufficient light means the herbicide doesn’t work, giving a false positive to resistance.

Blank pots were susceptible samplesBottom line

If an Australian testing service says you have resistance, you have resistance. No doubt about it. What that means to you as a grower or adviser is:

  • Make sure you take a representative sample.
    Seed from a harvest sample is more likely to be an average. Take some photos of the infestation and send them in with the samples so the service can give you some more accurate advice.
  • So you have resistance? But do you know which herbicides still work?
    Next time think more broadly and test for susceptibility.
  • Make sure you use the best spray application techniques available to you and don’t cut corners. Poor application will only make ALL of your weed problems worse!
Forget séances and reading animal entrails – your last chance to REALLY know which herbicides still work in the 2016 season

With so many farmers dry seeding crops to maximise potential yield, the loss of key post emergent herbicides through resistance makes it is even more important that weed control is planned before seeding.

Unfortunately many growers are unknowingly spending tens of thousands of dollars on ineffective herbicides. Gut-feelings and wishful thinking are no substitute for hard data and sound analysis. 

This is no way to develop a weed management plan!

To put this in perspective the 2010 pre-harvest survey in Western Australia by AHRI showed that over 80% of wild radish plants tested were resistant to Group B SU’s, 50% to Group B ‘imis’, 50% to Group F, and 70% to 2,4-D. Also Group B and Group A ‘fops’ were virtually useless on most annual ryegrass populations and clethodim was failing at an alarming rate. Five years later, the situation is much worse.

AGRONOMO and Peter Boutsalis at Plant Science Consulting offer you the chance to beat the resistance challenge in 2016 by conducting weed seed testing for which herbicides still work! Combined AGRONOMO and Plant Science Consulting have 50 years of weed and herbicide resistance expertise.

Seed testing requires the collection of weed seed samples before harvest and mailed to the lab. Once received dormancy of the seed is broken and seedlings grown before spraying with the herbicides determined as relevant to your farming system. Level of susceptibility for each herbicide tested is then measured. Results are normally available in early March.

Seed testing is an effective for both pre-emergent (trifluralin, Sakura®, Boxer® Gold) and post emergent herbicides.

A range of packages are available. Packages start with DIY seed collection and submitting samples for testing with report showing the effective herbicides.

Premium packages includes on-farm consultation, seed collection, advice on herbicide selection for testing, susceptibility testing, and a detailed report and management plan for the next three to five years.

For more information on Herbicide Susceptibility Testing go to /herbicide-suscept-testing/

Or phone on (Western Standard Time)

Andrew Storrie 0428 423 577