Why is brome grass a weed?
There are several reasons why brome grass is a problematic weed.
Highly competitive with crops
This weed is highly competitive in all crops but is most problematic in wheat. Brome grass is highly competitive because it is tolerant to drought and phosphorus deficiency, can respond rapidly to utilise nitrogen fertiliser applied in crop and can produce a large number of seeds. It is a more aggressive weed than annual ryegrass, barley grass or silver grass. The presence of 100 brome grass plants/m2 can reduce wheat yield by 30% in WA.
Host for crop pests
This weed can serve as a suitable host for nematodes and cereal diseases such as ergot, take-all, barley net blotch and bunt.
Contamination and stock injury
The seed of brome grass is a contaminant to wool and can cause damage to the hides and meat of animals, as well as causing injury to the eyes and mouth. If the seeds are ingested they may puncture the intestine, leading to death of animals.
Why is brome grass persistent?
The continued persistence of this weed could perhaps be attributed to changes in a number of management factors in the last two decades.
Brome grass is favoured by the minimum or no-tillage system and can be a severe problem when a paddock with a high seed bank is sown immediately after opening rains without an application of knockdown herbicide. Reduced tillage systems maintain brome grass seeds at or near the soil surface. This allows maximum germination, which occurs when seeds are buried at 50-100mm.
Lack of selective herbicides
In cereal crops there are few selective herbicides to control brome grass, although more selective herbicide options are available in broadleaf crops. The use of some Group A and B herbicides that are able to control brome grass have declined, as other weed species (for example, annual ryegrass) have developed widespread resistance to these herbicides.
In WA, a recent survey by the Australian Herbicide Resistance Initiative has confirmed resistance to Group B (sulfonylureas) herbicides in six populations of brome grass and resistance to Group A (fops and dims) in one population collected from the wheatbelt. Group M (glyphosate) resistance to red brome has also been confirmed in WA. In Victoria, this weed has evolved resistance to Group A herbicides. In South Australia, it has evolved resistance to Group B and Group M herbicides.
Decline in system diversity
Changes in climate and grain price have resulted in more cereal cropping in WA, less broadleaf crops and fewer sheep. It is easier to control brome grass in broadleaf crops (more herbicide options) or through grazing (sheep will graze brome grass prior to seed head production) than it is to control this weed in minimum tillage cereal cropping systems.
Protracted germination and establishment
Most brome grass seeds will germinate rapidly at the season break. However, some late emergence may occur and these late emerging seedlings are likely to escape in-crop weed control measures.
Non-wetting sandy soil
On sandy soils, the rapid spread and establishment of brome grass is partly due to its protracted and patchy emergence resulting from non-wetting sandy soils. On the average, brome grass seed bank life is about three years. So, in a non-wetting soil seed of brome grass will survive from a broadleaf crop to subsequent cereal crop despite all control measures in the break crop.