Grain growers: don't cut herbicide rates

EDITOR’S NOTE: Steve Powles, director of Western Australian Herbicide Resistance Initiative (WAHRI), is recognized as an international expert on herbicide resistance. Earlier this year while visiting the United States, he granted an interview with Delta Farm Press ( In the interview, Powles explained why he believes the United States will soon become the world’s leader in weed resistance. Recently returned to his homeland, Powles continues his work.

Australian grain growers are acutely aware that they need to keep their costs well under control in the cropping systems we know and love. Despite herbicides being one of the major costs incurred in Australian cropping, not all in our industry will be aware that herbicide costs in Australia are the lowest that prevail in any developed nation.

Additionally, the rates of herbicide used in Australia are the lowest in the world. This is primarily due to economic realities in our relatively low output, low profitability broad-acre farming systems. In comparison with other countries we apply much less herbicide per hectare.

As one of many examples, the recommended rate for the well-known herbicide diclofop (Hoelon, Illoxan, Hoe-Grass) for ryegrass control in the United States is double that recommended for ryegrass control in Australia.

It is important for all in the Australian grain industry to realize that we use herbicides at low rates, by world standards.

It is very unfortunate that in Australian cropping there are cases where users choose to cut herbicide rates below the recommended rate. Presumably, individuals cut rates because they believe that they are saving money by using less herbicide. Some almost automatically cut rates below recommended rates.

This is false economy for at least two reasons.

First, by cutting rates there is a far greater probability of killing fewer weeds. The survivors reduce crop yield and inject much fresh seed into the seedbank to become future weed problems.

Second, cutting herbicide rates can speed up resistance in weeds.

To demonstrate this, a very simple but elegant experiment was conducted at WAHRI. A known herbicide, diclofop, was sprayed on susceptible ryegrass at a range of rates. As expected, cut rates allowed some ryegrass plants to survive. The survivors were kept and allowed to produce seed. These progeny plants were then sprayed and compared with their parents. Researchers found more of the progeny survived diclofop.

Using the same plants, they repeated the experiment. The next generation was resistant! The bottom line: just four generations of ryegrass plants treated at cut rates of diclofop yielded a herbicide-resistant population.

Even worse, the population was found to be not only diclofop-resistant but it showed cross resistance to some other herbicide groups.

The implications of these experiments are clear. Cut herbicide rates can rapidly lead to resistance. We should not cut rates of herbicides below the label-recommended rate. This is especially true in Australia because we already use herbicides at low rates, by international standards.

Cutting rates below the recommended label rate can rapidly lead to resistance, especially in a cross-pollinated weed like ryegrass.

This experiment is a great example of research that is immediately applicable. The response is clear — use herbicides at label-recommended rates and do not cut herbicide rates.

(For more information, the paper can be downloaded from the WAHRI Web site (http//

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