Western Australian growers with mixed farming systems are urged to be particularly vigilant in preventing the development of triazine (Group C) resistance in the weed silver grass (Vulpia spp.).
A study conducted as part of a Grains Research and Development Corporation (GRDC) Regional Cropping Solutions Network (RCSN) project recommends growers explore the use of herbicides with alternative modes of action, consider carefully the timing of their spray-top operations in crops and pastures, and test suspicious populations for resistance.
ConsultAg agronomist Trent Butcher, who led the study, said the extensive use of simazine to manipulate clover-based pastures and the popularity of triazine tolerant canola crops meant silver grass was under high selection pressure from Group C herbicides.
“Silver grass is less competitive than other annual grasses such as wild oats or annual ryegrass but at high densities it can severely reduce crop yields and compete with pasture species, and its residue can be allelopathic—reducing crop and pasture establishment and growth,” he said.
“Simazine resistant silver grass threatens current cropping rotations because it will survive and set seed in most break crops and pastures under commonly used agronomy practices.
“This is because it is late germinating, quick maturing—often enabling it to avoid control from spray topping—and relatively unpalatable to stock.
“Silver grass also has the potential to act as a host to pests and root diseases and to allow the carryover of disease through the pasture phase.
“Early identification of the scope of this looming threat is critical to establish if new management practices need to be developed for silver grass control.”
Mr Butcher said WA’s first population of silver grass resistant to simazine, atrazine, and metribuzin—all Group C herbicides—was identified in 2014.
“However, diuron, while also a Group C herbicide, is in a different sub group (urea) and was found to have efficacy against the triazine resistant population,” he said.
Mr Butcher said the GRDC RCSN study aimed to identify the extent of triazine resistance in silver grass in the medium-to-high rainfall, mixed cropping zone, and identify any obvious trends in the weed’s resistance status and triazine use patterns.
“We found many crop and pasture rotations are very reliant on Group C chemistry for weed control, which results in a great deal of selection pressure on silver grass and other weeds,” he said.
“The results are a warning that growers should look at all weeds which act suspiciously to herbicides, and not just the well-known problem weeds.”
Mr Butcher said the GRDC RCSN study, initiated by the Kwinana West and Albany port zone RCSN groups, involved surveying and collecting samples from 42 different paddocks where growers were concerned about populations of silver grass.
In many cases these paddocks had been extensively manipulated in the pasture phase with simazine, and atrazine and simazine in the canola and lupin phases.
“Of the 42 samples tested only two samples had resistance to both simazine and metribuzin,” Mr Butcher said.
“Both samples belonged to the same grower who found WA’s first resistant sample in 2014.
“It is likely the resistant population arose from a mutation in one paddock and was moved to others via farm machinery or livestock.”
Mr Butcher said the grower would need to use alternate chemistries in the manipulation phase and pay close attention to spray-top timing to prevent survivors from setting seed.
“It is also likely multiple spray tops will be needed and on-farm biosecurity measures will need to be taken to avoid the movement of resistant plants between paddocks or farms,” he said.
More information about silver grass is available at https://grdc.com.au and http://ahri.uwa.edu.au/simazine-resistant-silver-grass/.