Study adds to resistance arsenal

30 Sep, 2004 07:00 PM
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FARMERS and researchers are battling the challenge of herbicide resistant weed populations because of the increase in continuous cropping over the past decade.

Leading the charge is consultant Bill Roy who has been managing an eight-year study, finishing at the end of this year, into the viability of non-herbicide weed control options.

Mr Roy's work has provided tremendous insight into managing weed seedset, reducing the size of the weed seedbank before seeding and maximising crops' competitive capacity.

"Although the old adage that prevention is better than cure is still true, in most cases the challenge is to provide an economic way out for farmers once their ryegrass population has become highly resistant to herbicides," Mr Roy said.

The introduction of a non-crop phase provides the opportunity to drive down weed densities to manageable levels and enable sustainable cropping in following years.

Non-crop phases can include grazing, fodder hay crops and green or brown manuring.

Mr Roy said the challenge with managing herbicide-resistant ryegrass is to monitor the ryegrass population and determine the appropriate stage to take the paddock out of grain production to enable a year or more of complete seed set prevention.

"In the trial work we have carried out, we always found that the best base from which to start an integrated weed management program was to implement a management practice such as brown-manuring to prevent any ryegrass seed set for the following crop," he said.

An example of the programs implemented began with a chickpea crop being brown manured with a double knock program of glyphosate applied in mid-August followed a month later by paraquat.

The following year a late sown barley crop did not compete well with the ryegrass population that germinated with the crop.

Peas were the following legume crop as they offered the chance to crop top.

The pea crop was reasonably competitive but the ryegrass count in the following wheat crop showed the topping operation was not a great success.

Subsequent chaff carting operations and windrow burning did not make up for this.

"Given an in-crop ryegrass count value of 193 plants/m2 it was a straightforward decision to plan a brown-manuring operation following the wheat crop," he said.

"The in-crop population in that year provided justification for brown-manuring and the area is now reasonably clean.

"What should be noted from this example is that an incorrect decision in year two had lasting implications."

Mr Roy said the challenge with managing herbicide resistant ryegrass populations is to monitor the ryegrass population and determine the appropriate stage to take the paddock out of grain production to enable a year of complete seed set prevention and provide for long term sustainability.

The leading site and success story to date of the GRDC- funded project is based on a simple plan.

"The break to the ryegrass cycle was made in year one with the brown manuring of a chickpea crop," Mr Roy said.

"In the following five years, excellent wheat crops were grown with the yield reflecting the season.

"In years of high yields, the volume of stubble produced allowed hot burns on stable soil, which maintained ryegrass numbers at a level that was readily contained by dense wheat crops sown at 100kg/ha."

The longevity of Mr Roy's experiment meant all season types were included in the trial years.

For example, the 2002 drought bought forward some interesting results showing that brown manuring can help wheat crops better survive the dry conditions.

These advantages are likely due to biomass retention (manuring) and moisture conservation under dry conditions.

"During 2002 wheat grown in the brown manured block survived the dry conditions better than the untreated crops.

"Yields in brown manured blocks were higher, especially in more fertile sites."

The study has also highlighted there is no advantage in using cultivation to stimulate the seedbank to control ryegrass.

In fact, it is possible that by not using stimulation cultivation, ryegrass germination was reduced.

Cultivating to stimulate ryegrass germination is based on ryegrass seeds not burying themselves.

If seed from the previous year is left on the surface then sowing will bring seed into close contact with the soil where it germinates with the crop.

"Our data from earlier years of the project show that cultivation to stimulate the seedbank has mixed results, particularly at non-wetting sites," Mr Roy said.

"The obvious explanation is that unless the soil surrounding the ryegrass seeds is wet, nothing will happen.

"Under the dry conditions of 2002, history repeated itself, there was no advantage in terms of ryegrass stimulation.

"The data suggest that ryegrass germination was reduced in the absence of stimulation cultivation.

"The extra cultivation also reduced wheat yields, possibly due to the operation causing additional drying out of soils already suffering from severe moisture deficiency."

Mr Roy's extensive study also investigated taking paddocks out of cropping production for varying lengths of time what affect this had the effect the ryegrass population.

The study found that one year out of production, or one year of paraquat-manipulated pasture, is not long enough.

In contrast, where two to three years of seed set control was carried out it exhausted the seedbank to such an extent that a sustained period of cropping resulted in low ryegrass numbers kept in check by competitive wheat crops.

Mr Roy emphasised that the principles of weed control investigated in his study also apply to other annual weed seeds with extended seedbanks.

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