Annual ryegrass (Lolium rigidum) has evolved into the most problematic and economically damaging weed in Australia because of its competitiveness, prolific seed production and widespread herbicide resistance. It is estimated to cost grain growers $93 million each year including grain yield losses and control expenses.
To better understand how the weed adapts to drought stress, researchers at the La Trobe Institute for Sustainable Agriculture and Food (LISAF) and the School of Agriculture, Biomedicine and Environment (SABE), examined how herbicide-resistant and herbicide-susceptible populations of ryegrass responded to various soil moisture levels under controlled glasshouse conditions.
They found that moderate water limitation did not simply suppress ryegrass growth, but in some cases stimulated the weed’s development, with the resistant population producing up to 132 per cent more leaves and 153 per cent more tillers compared with saturated conditions.
And while severe drought reduced plant height, root and shoot biomass, and seed production of the herbicide-resistant ryegrass population by up to 50 per cent, the plants were still able to survive and reproduce.
In addition, the annual ryegrass showed greater flexibility in flowering time and maintained higher leaf water content under stress, suggesting it may be better equipped to persist in water-limited environments.
This suggests annual ryegrass may remain a serious threat even in drier seasons, particularly where herbicide-resistant populations already occur.
The research, Morphological and physiological adaptations underpin drought resilience in contrasting populations of annual ryegrass (Lolium rigidum), has been published in the journal Scientific Reports.
Lead author Dr Ali Bajwa, Senior Lecturer in Weed Science and Agronomy, said the findings highlighted the need to rethink weed management in the face of climate variability.
“Annual ryegrass is already one of the most difficult weeds for Australian grain growers to control, and our study shows it has a strong capacity to adjust to moisture stress,” Dr Bajwa said.
“The fact that the resistant populations can grow more vigorously under moderate drought and still produce seed under severe stress means growers need management strategies that are tailored to local weed populations and seasonal conditions. Resistant population did not show any major fitness cost and in fact, performed better under stress, which could be a double whammy for grain growers who are having to deal with herbicide resistant plants that are also drought hardy”.
Dr Bajwa said the findings reinforced that there was no one-size-fits-all solution to managing annual ryegrass.
“Understanding how different ryegrass populations respond to drought can help growers and advisers design more targeted strategies to reduce seedbank build-up and limit the spread of difficult-to-control populations.”
“We need further research on greater number of populations under field conditions in order to future proof our weed management programs in grain production in the wake of seasonal variabilities and changing climate.” said Dr Bajwa.
The research was conducted by Roshell Warnakulasuriya, Babar Shahzad, Muhammad Adnan, Aakansha Chadha, Muhammad Farooq and Dr Ali Bajwa.
