Australian Herbicide Resistance Initiative (AHRI)

The rate debate

Plants in flowerpots

Many a late night has been spent over a glass of red at weeds conferences around the world debating whether high or low herbicide rates lead to faster resistance evolution. All weed scientists have an opinion on this issue, some of which are held very tightly. To some extent, the debate is still raging because the answer is not straight forward. The answer is both. As you can imagine, both sides of the debate are claiming victory!

What we do know is that low herbicide rates have been documented to lead to rapid resistance evolution to Hoegrass®, Roundup® and Sakura® in annual ryegrass. The problem is that we need to do the research for each weed and each herbicide, one at a time, to determine if this is a recurring theme, and if there are exceptions to the rule.

In a world first, AHRI postdoctoral fellow Roberto Busi evolved resistance to the new herbicide Sakura® before it was even commercially released. Roberto found that ryegrass evolved resistance (8 fold) to Sakura® after just three generations of recurrent selection at low rates. A similar study by Dr Paul Neve at AHRI in 2005 found that ryegrass developed 55 fold resistance to Hoegrass® after three generations of recurrent selection at low rates. This tells us that weeds find it easier to evolve resistance to some herbicides more so than others, but the end result is still a resistant weed.

Given that we can’t put the rate debate to bed for all weeds and all herbicides yet, we should give the weeds the benefit of the doubt, and assume that low herbicide rates are bad news until proven otherwise. Guilty until proven innocent!

Dr Paul Neve was the first scientist to confirm resistance evolution in ryegrass to low doses of Hoegrass®. Paul started with 107 ryegrass seedlings to which he applied 10% of the label rate of Hoegrass®. He then took the survivors, allowed them to cross pollinate with each other and grew them out to seed. He then grew seedlings from this seed and sprayed them with 20% of the label rate of Hoegrass® and once again allowed the survivors to cross pollinate and set seed. For the third generation he sprayed the full label rate of Hoegrass®. At the end of this process, Paul had evolved ryegrass to have 55 fold resistance (that is 55 times the rate at which 50% of the population is controlled – LD50). The reason for this resistance evolution was later confirmed to be due to metabolic resistance caused by multiple gene mutations. Since this original experiment, Paul’s work has been repeated several times and has also been conducted in a field experiment. These consistent results confirm resistance evolution in ryegrass to low doses of Hoegrass®.

Enter Dr Roberto Busi, winner of the Perth marathon and passionate Italian who has lived and breathed the effect of low herbicide dose on resistance evolution over the past seven years. Roberto used an approach similar to that of Paul Neve as described above, only this time he used the herbicides Sakura® and glyphosate. The result was that resistance evolved, but at a much lower level (only 1.7 to 1.9 fold resistance compared to 55 fold for Hoegrass®). However, when he started with ryegrass that had already developed resistance to Group A (ACCase), Group B (ALS) and Group D (trifluralin), and exposed it to three generations of low rates of Sakura®, the populations developed 8 fold resistance to Sakura®. The somewhat surprising result was that this ryegrass was also resistant to Boxer Gold®. This resistance was later confirmed to be due to a single gene mutation.

Table 1. Summary of low dose resistance evolution research in annual ryegrass. Hoegrass® active ingredient Diclofop methyl. Sakura® active ingredient pyroxasulfone. Boxer Gold® active ingredients prosulfocarb and metolachlor.

Table about resistance shifts

What about broadleaf weeds?

There has been limited work on broadleaf weeds in this area. However, the research that has been done shows similar results. Mike Ashworth is currently doing his PhD at AHRI on wild radish and has put low doses of glyphosate and 2,4-D to the test. The preliminary data of this research suggests that where he exposed wild radish to low doses of glyphosate he measured a small shift towards glyphosate resistance. Where he exposed wild radish to low doses of 2,4-D he saw a bigger shift and observed that wild radish were resistant to field rates of 2,4-D after just three generations of recurrent selection at low dose.

Scientists may continue to argue whether low doses or high doses cause faster resistance evolution. What is clear is that there is now well documented evidence that low herbicide doses do select for resistance after just a few generations and there are differences between weeds and herbicides as to how fast this resistance evolves.

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