Are your weeds fit or suffering

For some weeds, the ability to survive herbicide treatment comes with a ‘fitness penalty’ that makes them less able to thrive when faced with strong competition.

Glasshouse experiments show that some herbicide resistant weeds suffer a ‘fitness penalty’ that reduces their ability to compete with other plants when no herbicide is applied. Unfortunately not all herbicide resistance causes a fitness penalty.

AHRI research fellow, Dr Martin Vila-Aiub studies the ability of herbicide resistant weeds to survive when faced with other environmental pressures in the absence of herbicide application.

“What we have found is that some weeds that are selected for their ability to survive herbicide treatment are less able to grow well and reproduce—this is called a ‘fitness penalty’,” says Dr Vila-Aiub. “However, some herbicide resistant weeds do not suffer any ‘fitness penalty’ and these ‘super weeds’ can adapt well to every control tactic applied to them.”

For the weeds that do suffer a fitness penalty, Dr Vila-Aiub says there are opportunities for growers to exploit this new weakness and to greatly reduce seed production of these herbicide resistant weeds without using herbicides.

There are many variables involved in this field of study that make it difficult for researchers to issue general recommendations. Importantly, it is not possible to observe resistant weeds in the field and make assumptions about what might be causing their apparent lack of fitness or otherwise.

Dr Martin Vila-Aiub studies the ‘fitness penalty’ associated with herbicide resistance in some weeds to find ways to exploit this weakness using environmental factors.

What does it mean if weeds survive a herbicide treatment but look sick?

Short answer: There may be an opportunity to exploit this weakness.

Longer answer: Sometimes herbicide resistant weeds can survive a treatment but are very damaged by the herbicide. These plants are less likely to compete well against a vigorous crop or pasture. In a competitive environment, these plants are less likely to set seed and so the population size can be significantly reduced.

If I think my weeds might be herbicide resistant, what should I do?

Short answer: Get samples tested to determine if they are resistant and to identify any herbicides that are still effective.

Longer answer: Providing a competitive environment is usually a beneficial tactic in a weed management plan. Weed populations that suffer a fitness penalty associated with their herbicide resistance trait can be significantly reduced in size. Resistant weeds with no fitness penalty are likely to still grow well but may be inhibited by the rapid early growth of a suitable crop cultivar.

What other fitness penalties can be inferred with herbicide resistance?

Short answer: Increased susceptibility to insect and disease attack.

Longer answer: The herbicide resistant biotypes of some weed species become more attractive to pests and diseases compared to plants that are susceptible to herbicides. Some species use lowered photosynthesis activity as a mechanism to survive herbicide treatment. This has a side effect of reduced seed production, which reduces the resistant population’s ability to thrive and spread.

Do herbicide resistant weeds have other weaknesses that growers can exploit?

Short answer: Probably, but more research is needed.

Longer answer: Some herbicide resistant weeds may have a new ‘weak link’ in their ecology. For example, glyphosate resistant ryegrass and wild sorghum are susceptible to glyphosate applied at low temperatures. In another experiment, researchers showed that some herbicide resistant plants produce seed that does not germinate without light. Burying these seeds just a few millimetres was enough to prevent germination. These responses have been shown experimentally but can not be relied on as management tactics until the processes are better understood.

How to ask a WeedSmart question

Ask your questions about taking advantage of reduced fitness to help manage herbicide resistant weeds, using Twitter @WeedSmartAU or leave a comment below.