What exactly is herbicide resistance anyway?

Which of the following statements about herbicide resistance are true?

A plant that is not killed after a herbicide application, then sets seed, must be resistant

Resistance is inherited from one generation to the next

Poor application techniques cause resistance to get worse

Weeds become resistant after surviving a herbicide application

Resistance can be present in a weed population even before any herbicide applications have been made

What exactly is herbicide resistance anyway?

A grower applies herbicide at a reduced rate, and gets a poor kill. Does this mean the survivors must be resistant?

Yes

No

What exactly is herbicide resistance anyway?

After spraying Roundup® in a fallow, a grower notices some weeds have survived. Worried about the possibility of resistance, the grower decides to take action. Which of the following are likely to help reduce the risk of resistance? Select all that apply.

Do nothing - to avoid making the problem worse

Till the paddock to kill survivors

Spray with a different type of herbicide

Spray more Roundup® to get the ones that were missed

Preventing and managing resistance

Which of the following tactics and strategies can help to prevent or slow herbicide resistance? Select all that apply.

'Double Knock' (two different herbicides used within a few days of each other)

Using several different types of herbicide

Attacking multiple parts of the weed life cycle

Using tillage for weed control

Preventing and managing resistance

Which of the following are useful parts of a strategy for preventing or slowing down herbicide resistance? Select all that apply.

Stop weeds from setting seed

Keeping a refuge of unsprayed plants near the field

Keeping the herbicide rate as low as possible

Monitoring fields after spraying to check for survivors

Preventing and managing resistance

A friend suggests to you that you should use low herbicide rates in order to avoid getting herbicide resistant weeds. Will this work?

Yes

No

Preventing and managing resistance

If it took 15 years of glyphosate use on a weed population to cause it to become completely resistant, how long would you have to avoid using glyphosate before the weeds could be killed with it again?

Much less than 15 years

About 15 years

Much longer than 15 years, possibly never

Preventing and managing resistance

You decide to do a follow-up application a few weeks after spraying glyphosate to kill some survivors you noticed. Which of these options will help reduce the risk of glyphosate resistance occurring?

Spraying with glyphosate at twice the original rate

Spraying with a different type of herbicide

Doing a full disturbance tillage operation

None of the above

Herbicide groups and how they work

Fusilade® and Verdict® are both Group A herbicides. This means that they:

Are made by the same company

Have the same safety rating for operators

Kill weeds in the same way

Are both supplied as liquids

Herbicide groups and how they work

A consultant warns you that you are at risk of pendimethalin (Group D) resistance due to frequent use. He advises you to switch to trifluralin (also Group D). Will this help you avoid pendamethalin resistance?

Yes

No

Herbicide groups and how they work

A company releases a new herbicide, DefinitKill. On the label it is marked as a Group M herbicide. Group M contains herbicides that have been used for many years. Could there already be DefinitKill-resistant weed populations out there?

Yes

No

Where does resistance come from?

In which of the following ways might resistant weeds get into a paddock? Select all that are true.

Wind blown from neighbours farm

Seed trapped in harvest machinery

In or on livestock

Hay purchased from interstate

Hay from another paddock on your own farm

Have always been there in low numbers

Where does resistance come from?

Using a quick-test, a plant from a grower's paddock is identified as being resistant to glyphosate. Where did this plant get its ability to resist the herbicide?

From the herbicide application it survived

From one or both of its parents

Where does resistance come from?

Which of the following are important factors in determining how quickly resistance to glyphosate might evolve in weed populations?

Frequency of use of glyphosate

Frequency of use of other herbicides or tillage

Annual rainfall

Irrigation and fertiliser amounts applied

Where does resistance come from?

Which of the following paddock characteristics are likely to increase the chances of resistance evolving?

Light textured soil type

Heavy textured soil type

Low weed population (sparse weeds)

High weed population (dense weeds)

Detecting resistance

Which of the following is most likely to indicate a potential resistance problem?

Plants surviving in wheel tracks some weeks after spraying

A mixed patch of healthy and dead plants some weeks after spraying

Plants surviving under the thick stubble after spraying

Species that have never been controlled well with a particular herbicide surviving some weeks after spraying

Detecting resistance

Which of the following are possible indicators that a herbicide failure was not due to resistance? Select all that apply.

Multiple survivors in one or more long strips

Weeds were very stressed at the time of spraying

Surviving weeds that have suffered some damage but are regrowing

Knockdown herbicide was applied in very dusty conditions

Surviving plants of one species (that is usually well controlled) surrounded by dead plants of different species

Selection pressure

Which of the following scenarios is likely to have the lowest selection pressure for glyphosate resistance in summer grasses?

A rotation that contains winter cereals and Roundup Ready Flex® cotton, with a residual herbicide used before planting cotton and inter-row tillage used in the cotton crop to control Roundup Ready® herbicide survivors

Winter grains only, with glyphosate used in every summer fallow to control grass weeds and Glean® or Ally® used to control different weeds in the grain crop

A fence line with a highly mixed weed population sprayed twice a year with glyphosate and no other weed control used

Selection pressure

Consider the rotation in the diagram. Roundup Ready Flex® cotton is grown for four years, followed by wheat. The diagram shows where in-crop and fallow glyphosate applications (red arrows) are made and where non-glyphosate herbicides are used (purple arrows).

Which of the following weeds is likely to be under the highest selection pressure for glyphosate resistance?

A winter grass

A summer grass

A broadleaf that appears throughout the year

Bonus Question 1

From the drop-down box, choose the mode of action group that each product belongs to:

Bonus question 2

Which of the following are possible ways that a plant can resist the action of a herbicide? Select all that are true.

Target site resistance (a change in the enzyme affected by the herbicide)

Translocation or exclusion resistance (keeping the herbicide molecules away from the parts of the plant that need to be affected for the herbicide to work)

Metabolic resistance (breaking the herbicide down into non-lethal molecules before it can affect the plant)

Novel mutation (contact with the herbicide causes a genetic change in the plant making it not susceptible to the herbicide)