Phase pastures against herbicide-resistant weeds

New pasture systems are under investigation by Agriculture Western Australia in the search for management tools to control herbicide-resistant weeds.

There are now several research projects supported by growers through the GRDC that focus on the use of 'phase' pastures. One is located at the Centre for Cropping Systems in Northam.

'Phase' pasture is the term now used to describe a short period of pasture (1-4 years) which breaks up extended periods of cropping.

Such a change to the farming system has a number of advantages. It can restore soil fertility (organic matter and soil nitrogen) that may have declined due to frequent cropping and it can provide an opportunity to control herbicide-resistant weeds when combined with the use of grazing management, non-selective herbicides and cultural management practices (such as green manuring).

Continual re-sowing is a feature of phase pastures which involve new pasture legume cultivars such as Cadiz serradella. Cadiz is very soft-seeded and can be easily harvested with conventional cereal-harvesting machinery.

Field trial

A large field trial was established in 1998 to look at the impact of length of pasture phase, pasture species and various herbicide and cultural strategies on populations of ryegrass.

The project has highlighted the difficulties of dealing with annual ryegrass, including its spread of germination time and a capacity for seed carryover from year to year. Despite that, there are some promising indications for future weed management strategies.

Early results

The last year of the pasture phase is important. Growers can consider techniques such as hay-freezing, mowing or green manuring in that year to control herbicide-resistant weeds, without concern for the sown pasture. Other work is examining establishment systems for phase pastures that include time of sowing and herbicide treatments.

Autumn cultivation appears important to encourage full ryegrass germination and reduce the carryover of seeds from year to year. Delayed sowing of the pasture legume has the potential to reduce grass populations but the performance of the legume is likely to be substantially compromised and may limit the usefulness of the strategy.

Trifluralin-based herbicides appear to be safe options when establishing serradella pasture and green-manuring and hay-freezing techniques have been very effective in preventing ryegrass seedset.

The first wheat crops in the rotation experiment were sown in 1999 and their performance will be influenced by the various pasture management strategies.

Programs 3.5.3, 3.3.3

Contact: Dr Clinton Revell 08 9690 2000