Dramatic levels of herbicide resistance in annual ryegrass and wild radish in Western Australia have sounded an early warning for producers in eastern states.

In a random survey of the Western Australian wheat belt, researchers Michelle Owen and Dr Michael Walsh found the vast majority of ryegrass (94%) and wild radish populations (85%) were resistant to one or more herbicides. In the northern region of the wheat belt there was only one susceptible ryegrass population and no susceptible wild radish populations found. In this same region almost 90% of the wild radish and annual ryegrass populations were resistant to two or more herbicides.

Dr Walsh offered the statistics at GRDC Grower Updates in the Australian southern grain belt in February, where he commented on the effectiveness of stubble burning as a technique for controlling the weeds.

At Corowa in southern NSW where the GRDC update was also sponsored by the Riverine Plains Farming group, members of the group who had visited Western Australia and seen the problem first hand described it as "a worrying wake-up call".

The aim of Western Australian growers is now to limit the number of weed seeds entering the seed bank and to that end Dr Walsh described a range of techniques designed to harvest the weed seeds for later destruction. But rather than harvest them, a number of Western Australian growers rely on burning stubble to kill the weed seeds in the paddock.

Dr Walsh said that the effectiveness of this strategy depended on the heat generated by the burn and the length of time that heat was maintained. His research suggested that a temperature of 400°C for a minimum of 10 seconds was needed to kill ryegrass seeds and that for wild radish the temperature needed to reach 500°C.

"Under Western Australian conditions those parameters are unlikely to be met in a standing stubble burn," he said. Citing the results of burning a 2.3 tonne standing stubble crop, he said that the required temperature was reached only at 20cm height, well away from seed on the ground.

" Under these conditions only about 80 percent of the rye grass seed was destroyed. Good, but not good enough," Dr Walsh said.

In contrast, where stubble was concentrated in a windrow, the required temperature was reached at ground level with 99% of the ryegrass seed killed by the treatment.

"We estimate that by windrowing the wheat stubble we concentrated 15 tonnes/ha of combustible material for the burn," Dr Walsh said. "It's enough to kill ryegrass seeds everywhere but under the ground."

Similar results were obtained when either standing wheat or lupin stubble was burned in an attempt to control wild radish. In neither case were temperatures hot enough to destroy wild radish seeds on the soil surface even though the burn was conducted at the start of the burning season and at the hottest part of the day.

"However, when we burned windrowed lupin stubble we recorded temperatures of 600°C for two minutes 1cm below the soil surface," Dr Walsh said. "That was hot enough to affect seed buried in the soil. If fire is going to be used as a weed control tool the amount of combustible material available is critical. In Western Australia at least we've found that some form of concentration of the stubble is needed.

"Narrow windrows have a number of advantages beside concentrating combustible material. They're easier to burn without affecting the rest of the paddock, leaving a cover against wind erosion and, where the paddock is lightly grazed over summer, sheep don't appear to unduly disrupt the windrow."