Break the glyphosate habit

We are creatures of habit. For some reason I always buy Colgate toothpaste, the other brands don’t even get a look in. Mrs Marsh was just so trustworthy and I daren’t use another brand for fear of my teeth falling out.

Similarly, when it comes to knockdown herbicides, most growers automatically reach for the glyphosate. Such a reliable, brilliant herbicide that rarely lets them down.

But this year may be the first year that glyphosate fails for many Australian grain growers as resistance to this herbicide is going through the roof in some areas.

Random surveys of WA by AHRI researcher Mechelle Owen in 2010 found that 7% of paddocks surveyed had glyphosate resistant ryegrass. This was up from 1% in 2003. Another survey (not random) by Sally Peltzer from DAFWA in the South West of Western Australia in 2013 found that 44% of paddocks surveyed had glyphosate resistant ryegrass. Other surveys by John Broster, CSU, and Peter Boutsalis, Plant Science Consulting, are also turning up plenty of glyphosate resistance.

The short term answer is to use more paraquat based products and don’t automatically reach for the glyphosate. However, this is not a long term solution as paraquat resistance will result. There can be only one true long term solution and that is to farm with a very low weed seed bank.

Take no prisoners.

Map of glyphosate resistant ryegrass 2014

(*) indicates random surveys and (**) indicates targeted / resistance testing surveys.

The Australian Glyphosate Sustainability Working Group led by Dr Chris Preston have documented a huge increase in the discovery of glyphosate resistance in Australia in recent years.

Graph of populations of ryegrass and brome grass with confirmed glyphosate resistance in Australia over 16 years

Figure 2. The number of populations of ryegrass and brome grass with confirmed glyphosate resistance (by testing) in Australia as reported by the Australian Glyphosate Sustainability Working Group. This is the confirmed populations only. The actual number of glyphosate resistant populations is likely to be much higher.

There are two types of resistance surveys, random or targeted.

Table of random vs targeted survey

The random surveys visited randomly selected paddocks and sampled weeds from a range of species which were later tested. The targeted surveys sampled weeds from weedy paddocks that were nominated by growers / agronomists. They were not targeted specifically because glyphosate resistance was suspected, they were targeted because weed numbers were high and resistance to a range of herbicides was suspected.

Table 1. Summary of random surveys testing for glyphosate resistance. Only populations with greater than 20% survival to glyphosate are reported here.

Table of random surveys testing for glyphosate resistance

** Mechelle Owen found 25 populations with survivors to glyphosate in the AHRI survey, however only 3 populations had greater than 20% survival.

Table 2. Summary of targeted surveys testing for glyphosate resistance. Weedy paddocks were targeted (not necessarily where glyphosate resistance was suspected. DAFWA survey is populations with 1% or greater survival. Charles Sturt University data is 10% survival or greater.

Table of targeted surveys testing for glyphosate resistance

207 paddocks (20%) with glyphosate resistant ryegrass. This should serve as a big wake up call.

The results

The random surveys found low levels of glyphosate resistance. On average, only 2% of populations tested as glyphosate resistant (20% survival or greater). 2% may sound very low and that there is nothing to worry about, BUT, this is actually a significant result for a random survey. We also need to keep in mind that several of these random surveys are a few years old so resistance levels will have increased since.

The targeted surveys found much higher levels of resistance. 20% of populations from targeted surveys had glyphosate resistant ryegrass. In some of these surveys we are reporting 1% survival or greater and in others it is 10% survival or greater so the definition of resistance is different to the random surveys. Keep in mind that the targeted surveys were not specifically targeting glyphosate resistance, they targeted weedy paddocks where resistance to a range of herbicides was suspected.


Testing for glyphosate resistance is increasing. Growers who send weeds to resistance testing services are now often requesting for glyphosate to be tested. Testing for resistance is a great idea, especially for rare resistance genes such as glyphosate.

Bar chart showing number of samples being tested for glyposate resistance

Short term answers

Use more paraquat based products as either a single knock or in the double knock technique. Paraquat based products are most cost effective at controlling small weeds. We should use more paraquat based products in these situations to give glyphosate a break.

For example, in Western Australia this year there was a perfect opportunity for a paraquat based knockdown. A long, dry summer with a distinct break at the end of April. Growers were faced with only small weeds. There is no need to use glyphosate in this situation.

The double knock of a full rate of glyphosate followed by a full rate of paraquat based product one to seven days later is the ideal herbicide option to take the pressure off glyphosate. However, it simply isn’t possible to achieve a double knock on every paddock every year. This practice should be used whenever it is practical and cost effective.

Long term answers

The problem with the short term answers above is paraquat resistance. The first herbicide resistance paper that Professor Stephen Powles published in 1986 was on paraquat resistance. Rotating to paraquat is a good idea but is only a short term solution. See past edition of AHRI insight, Rotation plus.

The only real, long term solution to glyphosate resistance in ryegrass (and all resistance problems for that matter) is to farm with a very low weed seed bank. Once growers declare war on weeds, and adopt a take no prisoners approach using a combination of chemical, cultural and mechanical weed control, they have a win. It is difficult, and takes considerable motivation, but it can be done.

Declare war on weeds (guerrilla warfare is best, confuse the enemy), and take no prisoners.

What to do if you suspect glyphosate resistance this year

Step 1. Test. Sample some plants from the field and send them to Peter Boutsalis of Plant Science Consulting in Adelaide for quick testing.

Step 2. If resistance is confirmed jump hard on the problem. Don’t let them set seed.

Step 3. Develop a long-term plan to declare war on weeds. Chemical, Cultural, Mechanical.

Follow the links below for further information:



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