Brome grass

Identification and attributes

Latin name - Bromus diandrus and B. rigidus

B. diandrus is commonly called great brome but may also be known as ripgut brome, ripgut grass, giant brome, slands grass, jabbers, Kingston grass and brome grass.

B. rigidus is known as rigid brome or sometimes ripgut brome, ripgut grass, brome grass and also great brome (which causes confusion between the two species).

Distinguishing features

Both species have erect seedlings with dull, hairy leaves that display red-purple stripes following the leaf veins. At the seedling stage brome grass may be confused with wild oats (Avena spp.) because both possess hairs on the leaves and stems.

Biology

Great brome grass usually germinates after the autumn break (optimal temperatures of 15-20°C). However, seed of some populations have developed a requirement for chilling prior to germination. As a result, these populations are germinating later in autumn/winter. Rigid brome grass seed does not all germinate after the break of season. It has a much slower release from dormancy than great brome grass.

Germination of great brome grass is more rapid and uniform when the seeds are under crop residue (as in a no tillage system) compared to bare soil. However, optimal germination occurs when seed are buried to 1cm depth. So in the no-tillage system, emergence can be stimulated by crop sowing as the seeds on the soil surface are buried. Seedlings can emerge from seeds buried up to 15cm deep, although the rate of establishment is reduced.

Rigid brome grass germination appears to be strongly inhibited by exposure to light. However, seed germination resumes upon release from innate dormancy and placement in complete darkness caused by tillage or sowing operations.

Both species will germinate at the break of season, but further cohorts will emerge in late autumn and winter. Multiple cohorts of brome grass makes it difficult to target all brome grass plants with in-crop herbicides.

Brome grass is a prolific seed producer.

Great brome grass can produce 60-3000 seeds per plant (over 30000 seeds/m²). Seed shedding is variable. Great brome grass may retain 35-75% of the seed at crop maturity. If harvest is delayed, further shedding will occur. Swathing may be necessary to retain great brome grass seed for harvest weed seed destruction.

How long do seeds persist in the soil?

Great brome grass seeds are produced at the end of the winter annual cropping season, and have an initial period of dormancy (after-ripening) over the summer fallow. A high proportion of dormant seeds survive the hot, dry summers of southern Australia, but seed viability is lost in a humid environment. As a result, great brome grass is less of a problem in northern Queensland. By the end of summer seeds have lost dormancy and most germinate with the autumn break. Some seeds on the soil surface can remain viable for 2-3 years, but little dormancy was found in great brome grass in the southern areas of Western Australia. Persistence of great brome or rigid brome grass could be prolonged on non-wetting soils, with up to 30% of the seedbank persisting from one season to the next.

Why is it a weed?

Brome grass is widespread throughout the cropping regions of southern Australia. In a national ranking of crop weeds, brome grass is number four, after annual ryegrass, wild radish and wild oats. It infests an area of 1.4 million hectares and results in a revenue loss of $22.5 million per year. The widespread herbicide resistance in brome grass populations results in an extra herbicide cost of $3.2 million per year. Over the last 10 years, brome grass has increased in importance to a greater extent than any other weed species in Australia. 25% of growers nationally now cite brome grass as their most costly weed to control.

Great brome grass is one of the most competitive grass weeds in cereal crops. A density of 0, 102, 166 and 305 seedlings/m² resulted in a wheat yield of 2.88, 2.65, 2.74 and 2.48t/ha, in Wongan Hills, WA in 2016. However, depending on seasonal conditions, wheat yield can be reduced by 30-50%. Compared to wheat, brome grass is more tolerant to drought and phosphorus deficiency and is more responsive to nitrogen. As a result, an in-crop nitrogen application can increase an existing brome grass population. Brome grass seeds contaminate grain and wool, and cause injury to livestock (by entering the eyes, mouth, feet and intestines). Left uncontrolled in fallow or pasture phases, brome grass will act as a green bridge to cereal diseases. These diseases include: ergot (Claviceps purpurea), take-all (Gaeumannomyces graminis), powdery mildew (E. rysiphe graminis), septoria glume blotch (Leptosphaeria nodorum), black stem rust (Puccinia graminis), brown rust (Puccinia recondita), barley net blotch (Pyrenophora teres), sharp eyespot (Rhizoctonia solani), bunt (Tilletia caries), cereal yellow dwarf virus, cereal cyst nematode (Heterodera avenai) and root-knot nematode (Meloidogyne sp.).

Herbicide resistance

In Australia, there are brome grass populations that have developed resistance to:

• Group A: fops (for example, Targa® or Verdict®)
• Group B: sulfonureas (for example, Monza®) and imidazolinones (for example, Midas® or OnDuty®)
• Group C: triazines (for example, Simazine®)
• Group M: glyphosate.

Tactics for integrated weed management

Great brome grass is a major weed of cropping areas in southern Australia, but it can be effectively managed with an integrated weed management (IWM) plan. Both cultural and chemical options are required (Table 2). As the seed viability is approximately three years, the IWM plan should focus on killing all weeds, preventing seed-set (harvest weed seed destruction) and preventing introduction of seeds from external sources (that is, contaminated grain or machinery) for at least two years. A rotation of a break crop (especially herbicide tolerant canola) followed by imidazolione tolerant (Clearfield®) wheat is a good way to achieve this.

Herbicide options

Wheat

Herbicides include Midas® (MCPA/imazapic/imazapyr) in Clearfield® wheat cultivars, Monza® (sulfosulfuron), Atlantis® (mesosulfuron-methyl) and Crusader® (pyroxsulam). These herbicides have plant back restrictions (especially in low rainfall seasons), are high cost and may damage stressed crops. Further, Monza®, Atlantis® and Crusader® provide suppression rather than control. As a result, they need to be applied at the correct growth stage, and are most effective when brome grass density does not exceed 150 plants/m². Metribuzin (for example, Lexone® or Sencor®) herbicide can be used in tolerant wheat cultivars (Eagle Rock®). However, metribuzin performance can be erratic, especially in dry sown systems or when applied to non-wetting sands.

Barley

Herbicides include metribuzin, Group C herbicides or triazinone herbicides. IBS (incorporated by sowing) tank-mixes of metribuzin with either trifluralin or pendimethalin (Stomp®) have resulted in excellent control of brome grass in barley. However, metribuzin can cause crop damage following movement into furrows after rainfall. Damage can also occur in soils with low clay and/or organic matter content. Imidazolinone herbicides like Midas® can be used if Clearfield® barley cultivars are grown.

Canola and grain legume crops

A wide range of herbicides can be used in break crops, including triazines (for example, Simazine®) and Group A herbicides. Note that Group A herbicides have a high risk of resistance. Glyphosate can effectively control brome grass if Roundup ready canola (RR canola) is grown.

Pasture

Spray topping with paraquat or glyphosate can prevent seed set, but timing is critical. It can be difficult to target more than one weed species with this method as maturity dates will vary between species.

Other useful weed control techniques

Competitive crops

High seeding rates, narrow row spacing or east-west crop orientation all help to increase the competitive ability of the crop. Barley is generally more competitive than wheat and cereals are more competitive than legume crops.

Harvest weed seed collection/destruction

Seed capture at harvest is highly variable, due to the variable rate of seed shedding in brome grass populations. Once the crop is mature, the brome grass plants may retain 35-75% of their seed. Obviously further shedding will occur if harvest is delayed. If planning to use harvest seed collection/destruction, harvest fields where brome grass is a problem as soon as possible, or alternatively use swathing to delay shedding. Harvest the crop as low as possible. A study in WA found a harvest height of 10cm collected 74% of viable brome grass seed, compared to 40% when harvest height was increased to 15cm.

Delayed sowing or an autumn tickle

This will increase pre-sowing germination of brome grass, to increase the efficiency of the knockdown herbicide. These techniques obviously depend heavily on the amount and frequency of rainfall.

Crop patching

Spray glyphosate on isolated patches of the crop where brome grass infestation is heaviest.

Brome grass

The following information covers factors to consider when developing an integrated plan to manage brome grass.