Ryegrass is the most damaging weed in Australian grain systems because of its competitiveness, prolific seed production and widespread herbicide resistance.
The effect of ryegrass costs growers an estimated $93 million per year, including grain yield losses and control expenses.
Researchers at La Trobe University examined how herbicide-resistant and herbicide-susceptible populations of ryegrass responded to various soil moisture levels under controlled glasshouse conditions.
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 shoots compared with saturated conditions.
While severe water limitation 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.
The 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 means that ryegrass could remain a serious threat even in drier seasons, particularly where herbicide-resistant populations already occur.
Senior lecturer in weed science and agronomy and lead author Ali Bajwa 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.”
Dr Bajwa said understanding how different populations of ryegrass responded to drought conditions could help design targeted strategies.
“Resistant populations 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.
Dr Bajwa emphasised the need for further research to future-proof weed management programs against seasonal variabilities and changing climate.
