SA brome grass found to be glyphosate resistant

A population of Great brome grass (Bromus diandrus) from South Australia has been confirmed resistant to the world's most important herbicide, glyphosate.

This is the first time that this highly competitive annual grass weed of crops and pastures has evolved resistance to glyphosate and is the third Australian weed species confirmed as resistant to glyphosate in the past 12 months. This highlights the need for growers to be on-the-lookout for any weeds that should be controlled by glyphosate but which survive.

The resistant brome grass was found surviving in a paddock where an old fence had been removed and cropped over and a pre-sowing application of glyphosate had been applied. The fence line had previously been sprayed with glyphosate for many years with no other weed control tactics used.

"This is a huge concern to Australian grain growers because this highly competitive weed has been becoming a major problem in reduced-tillage farming," said Associate Professor Chris Preston, chair of the Australian Glyphosate Sustainability Working Group and leader of the University of Adelaide team which confirmed the result.

"Currently the number of glyphosate-resistant ryegrass populations evolving along fence lines is exploding. That and this new discovery are a real 'wake-up call’ and show that any weed might develop glyphosate resistance and growers need to be vigilant."

Brome grass is a major weed of crop and pasture on lighter textured soils across the southern and western Australian cereal belts. In wheat, there are few effective in-crop herbicide options for this species, and it can reduce yields by 30 to 40%. It also emerges after crop establishment enabling it to compete strongly with the crop and produce large amounts of seed.

Brome grass is also a host to a range of cereal crop diseases including take-all and barley net-blotch, while the mature seeds can injure stock. Populations of brome grass are already resistant to grass selective Group A and Group B herbicides in Victoria and Group B and Group C herbicides in Western Australia.

Dr Preston knows that managing brome grass and other weeds along fence lines requires a diverse approach. Herbicides with different modes-of-action to glyphosate need to be included. On erosion-prone sites ground cover needs to be maintained so control along fences and firebreaks should take place late winter to early spring. The older tactic of sowing crops to the fence line then baling and spraying an area along the edge of the crop for a firebreak is a valuable way of stopping weeds moving into the crop while protecting the soil.

The management of brome grass in winter cereals is complex and requires many different tactics to get weed numbers down and lessen the risk of developing herbicide resistance. Which tactics are used will be determined by location, farming system and the farmer's aspirations.

"Farmers need to use a rotation that enables a sufficient range of tactics to be used so that the weed seedbank can be driven down and kept low. Tactics such as spray-topping, brown or green manuring, hay, crop competition and windrow burning are useful to take selection pressure for resistance off the herbicides," concluded Dr Preston.

If you suspect glyphosate resistant brome grass on your property, contact your relevant state expert. Details of who to contact in each state are available from the Australian Glyphosate Sustainability Working Group.

The Australian Glyphosate Sustainability Working Group is supported by the Grains Research and Development Corporation (GRDC) and key R&D-based crop protection companies with an interest in the sustainability of glyphosate. Its web site has a range of information about glyphosate resistance including a register of glyphosate resistant weed populations and guides and links for management of glyphosate resistance in different crops and management situations.