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)

Species	Common name	Year first confirmed	State	Crop	Resistance to other herbicides / MOAs
Arctotheca calendula	Capeweed	1984	Victoria	lucerne	Diquat (L)
Hordeum glaucum	Northern barley grass	1983	Victoria	lucerne	Diquat (L)
Hordeum leporinum	Barley grass	1988	Victoria	lucerne	Diquat (L)
Vulpia bromoides	Silver grass	1990	Victoria	lucerne	Diquat (L)
Mitracarpus hirtus	Small square weed	2007	Queensland	mangoes	Diquat (L)
Lolium rigidum	Annual ryegrass	2010	South Australia	Pasture seed	A / M - 2 populations
Gamochaeta pensylvanica	Cudweed	2015	Queensland	Tomatoes, peanuts, sugar cane, avocados
Solanum nigrum	Blackberry nightshade	2015	Queensland	Tomatoes, peanuts, sugar cane, avocados
Eleusine indica	Crowsfoot grass	2015	Queensland	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

Update on 2016-09-20 03:56 by Andrew Storrie

An update on the paraquat resistance story - it has now been confirmed by the University of Adelaide and the Australian Glyphosate Sustainability Working Group that a population of flaxleaf fleabane (Conyza bonariensis) has been found to be resistant to 4 L/ha of paraquat.

The population developed in wine grapes in the Murrumbidgee Irrigation Area in New South Wales.

Paraquat resistant fleabane (L) and susceptible fleabane (R). Image: P. BoutsalisThis poses significant issues for weed management in vines and tree crops as more paraquat is being used to manage glyphosate resistant weeds.

Over-reliance on glufosinate will ultimately lead to glufosinate resistance.

A diverse range of weed management tactics needs to be considered which will most likely include the use of pre-emergent herbicides.

For more information on paraquat and glyphosate resistance go to the Australian Glyphosate Sustainability Working Group web site - http://www.glyphosateresistance.org.au/index.html

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!

The first edition of "Integrated weed management in Australian Cropping systems" was published in 2006. This manual was a world first and it broke new ground in the fight against herbicide resistance. This manual was a huge collaboration under the umbrella of the CRC for Australian Weed Management.

The GRDC saw the value in the project and helped fund the development and printing of the manual.

Eight years later the revised manual is available online at the GRDC.

While it follows the same structure as the first edition it has some new sections and different emphasis.

There are now 23 weed profiles with the addition of fumitory (Fumaria spp.), feathertop Rhodes grass (Chloris virgata) and windmill grass (Chloris truncata).

Brome grass is one of the two “weeds-to-watch” for southern farmers.  The other is wild radish in its many combinations of herbicide resistance.

Brome grass swamping a fire break - a great source of weedsI talk about brome grass and not rigid or great brome, as I do not know anyone who can tell them apart in the paddock. In a lab under a binocular microscope, it is possible. For management purposes they should be considered the same species.

Brome grass is on the increase with the increase in minimum tillage because of limited availability of effective in-crop herbicides, delayed germinations and an ever expanding spread of herbicide resistant populations, particularly to Groups A and B. There are now 3 populations of brome identified resistant to glyphosate – two in South Australia and one from the Victorian Mallee.

The other problem is its ability to shatter. I passed a barley paddock near Kojonup, Western Australia, in late November which had a massive infestation of brome grass visible. When I passed the same crop two weeks later, most of the brome seed had dropped to the ground (see below). For harvest weed seed management to work here, the barley should have been harvested as soon as it was ripe.

Now you see it, now you don't - heavily infested barley crop where brome has shed after two weeks.Follow the link below to read a comprehensive interview with Chris Preston, University of Adelaide, regarding management of brome grass.