Australian growers spend an average of $40,000 a year on crop and pasture chemicals, with the extensive herbicide use leading to more than 40 known glyphosate resistant annual ryegrass populations in Australia.

To address growing herbicide resistance issues, the Grains Research and Development Corporation (GRDC) is supporting research by Bill Roy, Agricultural Consulting and Research Services, into using integrated weed management as a tool to prevent herbicide resistance from impeding sustainable farming.

"By demonstrating viable integrated weed management strategies, growers will be able to adopt programs that will allow intensive cropping to continue in paddocks with resistant ryegrass populations," Mr Roy said.

"At the same time, such programs will enable the industry to become less dependent on herbicidal inputs."

According to Mr Roy, for success and good returns when growing wheat, the ryegrass population must be reduced to a level where the crop sown at high seeding rates is, in itself, an excellent competitor.

Following the completion of his eight year GRDC-supported project, Mr Roy recommended three key management principles to observe after herbicide resistance develops: preventing seed return to the seed bank; reducing the seed bank before sowing and maximising the competitive capacity of the crop.

"In adopting integrated management programs for the control of herbicide resistant weeds, and therefore maximising the opportunity to crop, it is imperative to reduce the weed population to the lowest possible level at the earliest possible time.

"One to three year periods of seed bank reduction, combined with seed set control, were established and ryegrass seed set was prevented during a pasture phase by grazing with sheep and timely paraquat applications each spring.

"The value of preventing the return of resistant ryegrass seed to the seed bank using a pasture phase is reflected in the six year gross margin returns .

"The reward is an extended period of time in crop and more years paddocks can be continuously cropped before another time-out period is necessary, with the added benefit that grass selective herbicides are no longer needed, unless required for other grass weeds such as wild oats," Mr Roy said.

The GRDC research showed the time taken to run down a resistant ryegrass seed bank influenced the initial in-crop ryegrass population and the number of years in crop before needing another ‘time out' period.

"The quick fix, being out of crop for one year, was not effective, as the seed bank was not run down enough and a seed set control operation had to be imposed.

"By contrast, with two to three years of seed set control the seed bank was exhausted to the extent that a sustained period of cropping resulted in low ryegrass numbers kept in check by competitive wheat crops," Mr Roy said.

Preventing seed returning to the seed bank continues to be the key to successful integrated weed management programs directed at herbicide resistant ryegrass and other annual weeds.

"Benefits from using a pasture phase are similar to those achieved by brown manuring to prevent seed set.

"Subsequent ryegrass population level relates to the length of time invested in such techniques.

"Making silage to prevent seed set, followed by three years of pasture where no ryegrass was allowed to set, lowered total ryegrass numbers, with continued absence after resuming cropping," Mr Roy said.

Using stimulation cultivation under dry conditions provided no advantage in terms of ryegrass stimulation and a sizeable negative impact on grain yield relative to the direct drilled, unstimulated areas.

Brown manured trials handled dry conditions better than untreated blocks and had a positive effect on grain yield and gross margin the following year.

"Herbicide resistant weeds are not just a genetic selection problem. Like all weed management, it is a numbers game.

"Integrated weed management works if a highly disciplined approach is taken and the principles embodied in this project can also be applied to other annual weed seeds with extended seed banks," Mr Roy said.