Management of brome grass

Species of brome grass in Western Australia

Great brome (Bromus diandrus var. diandrus and B. diandrus var. rigidus, previously known as B. rigidus) is a widespread grass weed (Poaceae family) throughout the wheatbelt and coastal areas of Western Australia. This species is common to agricultural fields, disturbed areas and native vegetation. There are seven other (less common) species of brome grass in WA, including B. alopecuros (twisted awn), B. catharticus (prairie grass), B. hordeaceus (soft brome), B. japonicas (Japanese brome), B. arenarius (sand brome), B. madritensis (Madrid brome) and B. rubens (red brome). Weed species within the Bromus genus are collectively known as brome grass. Other common names of brome grass are ripgut grass, spear grass, giant brome, jabbers and Kingston grass. Note that while this page focuses on great brome, the control measures discussed below can be applied to all species of brome grass in WA.

Biology and ecology of brome grass

This annual grass weed occurs in areas with >250mm rainfall and a mean July temperature <15°C. It is more common in cropping rather than pasture fields. Brome grass prefers sandy and loamy soils and can tolerate acidic or alkaline conditions. This weed is more common to paddocks that are frequently planted to cereal crops.

Great brome populations can produce 600-3000 seeds per square metre (seeds/m²) (average 1000 seeds/m²). These seeds will have an initial after-ripening dormancy, which is released during the summer period. However, by the start of the following autumn/winter growing season, 85-90% of the seed will germinate in response to adequate rainfall.

The highest rate of germination of great brome occurs if seeds are buried within 50-100mm while germination is <1% if buried at or below 150mm. Therefore, in the minimum tillage system, where seeds are generally close to the soil surface, most surviving seeds will germinate in the year after seed production. Those seeds that do not germinate will lose viability after 6-24 months in soil. A small proportion of the soil seed bank may remain viable for up to three years. The persistence of this weed is due to annual seed replenishment rather than the carryover of seed in the soil from year to year. So, seed set prevention in a single year will dramatically reduce the population.

Rigid brome grass is more common to the northern wheatbelt, as it can complete its life cycle in the shorter growing season. Some populations may emerge later in the season, allowing them to avoid herbicides at the beginning of the growing season. Further, these plants shed a larger proportion of their seed before crop harvest, making it difficult to remove/destroy seeds at harvest or kill through windrow burning.

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.

Conservation tillage

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 1 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.

Herbicide resistance

In WA, a recent survey by the Australian Herbicide Resistance Initiative has confirmed resistance to Group 2 (sulfonylureas) herbicides in six populations of brome grass and resistance to Group 1 (fops and dims) in one population collected from the wheatbelt. Group 9 (glyphosate) resistance to red brome has also been confirmed in WA. In Victoria, this weed has evolved resistance to Group 1 herbicides. In South Australia, it has evolved resistance to Group 2 and Group 9 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.

Integrated weed management

Seed is the main source of brome grass infestation, and as few seeds remain dormant, the most important aspect of brome grass management is to reduce its seed bank.

Integrated weed management (IWM) techniques are available to manage this weed in crop and pasture (Table 1).

*98% of the seed collected into the HSD will be killed, so early harvesting will have a better impact than later harvesting due to shedding of weed seed.

Cultural and physical control options

Crop choice and rotation

This remains the basis for the integrated management of brome grass, as it allows for the use of selective herbicides during the non-cereal phases. However, this weed can adapt to continued herbicide use by evolving herbicide resistance. Growing a broadleaf crop (or implementing a chemical fallow) before a cereal crop will effectively reduce brome grass density in the following cereal crop.

Herbicide tolerant crops

Brome grass can be controlled by using herbicide mixtures such as imazapic + imazapyr (for example, OnDuty®), imazapic + imazamox (for example, Intervix® ) or imazapic + imazapyr + MCPA (for example, Midas®) in Clearfield® (IT) wheat cultivars or metribuzin in Eagle Rock wheat. Brome grass can also be controlled using triazine herbicides in TT canola cultivars, glyphosate in Roundup Ready® canola, or OnDuty® herbicides in Clearfield® (IT) canola.

Competitive crop and pasture species

Barley is a more competitive crop species than wheat and suffers a lower yield penalty from great brome infestations. Within the same crop, some crop cultivars are more competitive than others. A healthy crop, free of disease and insect damage, will be more competitive against weeds than a stressed crop.

Seeding rate and row spacing

Higher seeding rates and narrow row spacing that result in a higher number of crop plants/m² can suppress brome grass and reduce its seed production.

Fertiliser placement

Brome grass is tolerant to phosphorous deficiency and more responsive to added nitrogen than wheat. Banding fertiliser under the wheat rows at sowing enables the wheat seedlings to access the nutrients before the weed seedlings. Fertiliser banding can result in a lower yield loss than other methods of fertiliser application.

Delayed seeding

This allows greater weed kill with knockdown herbicides prior to sowing. However, delayed sowing may cause a yield penalty in those seasons with an early or average start.

Controlled traffic

Controlled traffic may allow for optimal timing of herbicide applications and better soil conditions for crop growth, leading to better suppression of brome grass in some situations.

Cultivation

Full-cut tillage increases emergence of brome grass compared to knife point and disc sowing. However, deep cultivation such as mouldboard ploughing may reduce the brome grass population substantially by burying brome grass seed deep into the soil.

Autumn tickle

An autumn tickle (shallow cultivation) with light harrows will encourage greater pre-sowing emergence, which can be effectively controlled with follow-up knockdown herbicides (caution: effectiveness depends heavily on the amount and frequency of rains).

Green and brown manuring

In case of a high density of uncontrolled brome grass in a poorly established crop, green or brown manuring should effectively control seed set of this weed.

Crop patching

Crop patching with glyphosate is done by spraying localised areas of the paddock where brome infestations are the heaviest (crop sacrifice) to reduce the soil seed bank.

Early hay cut

Early hay cut will result in a reduction of the seed production of brome grass.

Residue burning

Windrow burning should effectively kill the brome grass seed present in the windrows. Narrow windrow burning will kill most seed when the dry biomass in the windrows is about 15t/ha. However, many seeds will shed prior to harvest and so will not be present in the windrow.

Grazing

Brome grass is only palatable at the vegetative stage so grazing is a poor but opportunistic management tool.

Chemical control

Even though some brome grass populations in WA have evolved resistance to some Group 1, 2 and 9 herbicides, a range of pre-seeding and in-crop selective herbicides are now available to control this weed (Table 2). Brome grass is more effectively controlled in broadleaf crops or in pastures than in cereal crops. On the average, brome grass seed bank life is about three years. In a non-wetting soil, seed of brome grass will survive from a break crop (canola or lupins) to subsequent cereal crop despite all control measures in the break crop.

Crop topping by glyphosate in wheat, chickpea, lentils, faba beans and field peas may reduce brome grass seed set. Timing is the main issue to consider when thinking about crop-topping, particularly if brome grass is the target. Brome grass matures a lot quicker than some crops and will set seed before the crop can be legally sprayed. Lentils, field peas and lupins generally mature early enough to kill many grass weeds by crop topping. To reduce brome grass seed set by crop-topping, it is essential that an early maturing crop variety is selected and the crop is sown early so that weeds can be targeted before they have an opportunity to set seed and withholding periods are avoided.

While trifluralin can suppress brome grass, herbicides such as prosulfocarb + s-metolachlor (for example, Boxer Gold®) at pre-sowing and pyroxsulam (for example, Crusader®) at post-emergence may be more effective on brome grass in wheat crops.

An untreated wheat plot with 156 plants/m2 of brome grass (Bromus diandrus)

Wheat plot treated with prosulfocarb s-metolachlor with less than 40 plants/m2.

An untreated wheat plot is shown in the top image and a plot treated with prosulfocarb is shown in the bottom image.