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!

With herbicide resistance weeds are now driving the farming system in many areas, reducing crop yields and limiting enterprise options for growers AGRONOMO and Plant Science Consulting P/L are offering professional herbicide susceptibility testing using the Quick-test backed up with a professional management advice service.

No grower wants to be painted into a corner and forced to make management decisions that don’t suit your short term and medium term goals. The question is how can you be sure the herbicides you are using this season are going to work and are you spending thousands of dollars on herbicides that are giving poor levels of control?

Faba beans sprayed one month earlier with clethodim. Ryegrass happily growing. What to do now? Image:AGRONOMO

Alternatively, are you using expensive herbicides to tackle resistance when older and cheaper herbicides could still be working because you ruled them out thinking you have resistance to them already? This may not be the case.

The only sure way to be certain is to TEST!

For example many growers are now experiencing reduced levels of control from clethodim (e.g. Select®) due to herbicide resistance. Will butroxydim (Factor®) give better control of these problem grasses? Maybe yes or maybe no. The only way to be sure is to either spray the paddock with butroxydim or do a herbicide resistance test.

The service provided by AGRONOMO offers the Quick-test backed up by relevant personalised management advice.

What is the Quick-test?

The Quick-test can be conducted NOW by collecting live plants which are then expressed posted to the laboratory, trimmed, re-potted then sprayed with the herbicides of your choice following discussion of the relevant options.  Results are available in 3 to 4 weeks enabling effective management decisions to be made this season to prevent viable seed being produced by these resistant weeds. This is the ONLY way to manage herbicide resistance.

The Quick-test is ideal for pre-seeding or early post emergent herbicide survivors. Why didn’t those weeds die? What will you do about it?

Please note that the Quick-test is only suitable for post emergent herbicides such as glyphosate and paraquat or in-crop selective herbicides. To test for susceptibility to pre-emergent herbicides, particularly trifluralin, you must use the seed test later in the year.

For more information on this new service go here or phone AGRONOMO on 0428 423 577.

It had to happen. In the latest of a long line of “wake-up” calls, there is confirmation of resistance to four herbicide modes of action in a ‘Top 100 World’s Worst crop weeds’ population. The scary thing is that three of these modes of action are our major knockdown herbicides glyphosate, paraquat and glufosinate. The fourth is Group A.

The research confirming 4 modes of action in the one population of crowsfoot grass (Eleusine indica) from Malaysia has just been published in a paper by Jalaludin, Yu and Powles (Team AHRI) in the journal Weed Research.

Crowsfoot grass is a C4 tropical-sub tropical grass that behaves as either an annual or perennial and can produce up to 50,000 seeds per plant. It is a rampant weed of pastures, annual crops, horticulture/plantations and non-crop areas, such as roadsides. This species is also in the top 200 worst environmental weeds in south east Queensland. In 2012 David Thornby et al rated crowsfoot grass the 16^th most likely weed to develop glyphosate resistance in Australia.

 The map below however shows that complacency should not be entertained as this species is widely distributed across Australia, and is particularly vigorous above 30 degrees lattitude.

Distribution of Eleusine indica in Australia. Source: Australian Virtual Herbarium

Resistance in crowsfoot grass

Crowsfoot grass has a trail of herbicide resistant populations across 8 countries to 7 different modes of action. It was the second species in the world to develop resistance to glyphosate. Table 1 below shows the year the mode of action was first confirmed by country.

Year first confirmed	Mode of action	Herbicide common name	Country
1973	D	trifluralin	USA
1989	B	imazapyr	Costa Rica
1990	A	fluazifop	Malaysia
1990	L	paraquat	Malaysia
1997	M + A	glyphosate + fluazifop	Malaysia
2003	C	metribuzin	USA
2009	N	glufosinate	Malaysia
2009	L + N	paraquat + glufosinate	Malaysia
2014	A, L, M, N	fluazifop, paraquat, glyphosate, glufosinate	Malaysia

 Table 1. First case of herbicide resistance confirmed in crowsfoot grass to different modes of action.

To date no populations of crowsfoot grass in Australia have been confirmed resistant to any herbicide mode of action.

Why is it so??

Malaysia is obviously a hot-spot of resistance with this species so what is it about this country that makes it this way?

• Wet tropical climate that stimulates multiple weed emergences through the year
• Large populations of rapidly growing weeds.
• Major reliance on herbicides to control weeds – up to 12 applications per year
• No seed bank management of weeds
• There is a large area of ‘plantation’ agriculture, with 5 million hectares planted to oil palms (2011) alone. This creates large areas where weeds can grow with little competition from the crop.

Oil palm (Elaeis guineensis) bunch that can weigh up to 15 kg.

What will be the next action to manage this weed in Malaysia? There are two modes of action remaining - amitrole from Group O and 2,2-DPA and flupropanate from Group J. None of these have the effectiveness or flexibility of the herbicides already lost. Non hebicide options will also be less effective and difficult to implement.

Remember that in late 2013 annual ryegrass from a vineyard in Western Australia was confirmed as being resistant to both glyphosate and paraquat? This occurred due to the sequential use of both herbicides over a decade or more, with no action preventing surviving plants from setting viable seed.

How do we avoid going down this 'one-way elevator'?

As outlined in the GRDC Integrated weed management manual controlling the survivors of any herbicide application is the key stone of resistance management.

Simply going from one herbicide mode of action to the next produces weeds with resistance to multiple modes of action. Imagine losing the effectiveness of glyphosate and paraquat in your major weed species!

To manage these seed banks and drive down weed numbers we need to use a wide a range of weed control techniques as possible. This has been the message since 1990. For a bit of nostalgia here are some of the slogans that have been used in the past....

                      

 And now.........

                                                      

This is the same message in different words and continues to be relevant today.

For further information on the multiple resistance research go here.

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).