Understanding the way plants evolve across generations and acknowledging a plant's incredible will to survive is the best way to begin any weed control strategy.
Very simply put, Australian Herbicide Resistance Initiative (AHRI) director Hugh Beckie says weeds are much smarter than we think.
"The weeds in our paddocks are the winners of the selection-adaptation-evolution marathon," Dr Beckie says.
"As we have witnessed over the past 40 years, herbicide resistance is weed evolution in motion.
"But the creeping evolution in response to recurrent non-herbicidal weed control is also happening in the background, often without us noticing until it is too late."
Survival of the fittest
Dr Beckie explains the first step in the evolutionary process is 'selection' - where only the most robust offspring survive all the stresses imposed upon them.
These stresses are numerous throughout their life cycle (from seed germination to seed maturation) and can include:
- herbicide action;
- soil disturbance; and
- crop and other plant competition - which could mean limited access to nutrients, water and sunlight.
Over time, often years, weedy adaptation results in evolutionary changes in populations with permanently altered phenotypes and genotypes.
If a plant can survive all those stresses, it then passes on this genetic 'robustness' to its offspring - and the evolutionary process has begun.
Warming up
An important stage in this marathon is adaptation, and Dr Beckie says this could be in the form of any morphological, physiological development - or behavioural character or trait that allows these more-robust plants to survive and continue to reproduce.
An example of this type of adaptation is lower wild radish pod heights that can evade harvest weed seed control measures.
"Over time, often years, weedy adaptation results in evolutionary changes in populations with permanently altered phenotypes and genotypes," Dr Beckie says.
Over the past 20 years, he says, researchers have learnt how plants can perceive the world around them without eyes, ears, or brains, and respond accordingly for self-preservation.
Tackling weeds head-on
Australian Herbicide Resistance Initiative director Dr Hugh Beckie says weed control is like a marathon - with a range of stages - and requires multiple tactics. Photo: Evan Collis
"The field of plant perception or 'gnosophysiology' is fascinating, with rapidly expanding insights and knowledge," Dr Beckie says.
He says plants are capable of reacting to a wide variety of stimuli or threats, including:
- chemicals;
- gravity;
- light;
- moisture;
- temperature;
- oxygen and carbon dioxide concentration;
- pathogens and pests;
- physical disruption; and
- sound and touch.
As an example, Dr Beckie says plants have different types of photoreceptors that sense and respond to light quality and quantity.
For example, if a weed is being overshadowed by a bully neighbour, it will sense the drop in light intensity or change in light spectra and divert resources to increase its stem height.
Plants also systematically use hormonal (chemical) and electrical signalling pathways to regulate processes affecting their physiology, morphology, growth, movement (for example, leaf, stem, root) and development (phenology).
With these tools or abilities, plants are able to adapt to evade, to varying degrees, weed-control practices such as a prostrate growth habits in response to recurrent mowing or cutting to keep 'under the radar' or speeding up their lifecycle to shed more seeds before annual crop harvest.
What all this means for growers, Dr Beckie says, is the need to stay one step ahead of these intelligent life forms, with a constantly changing set of control measures that interrupts this adaptive or evolutionary process.
More information: Peter Newman petern@planfarm.com.au; Dr Mike Ashworth, mike.ashworth@uwa.edu.au