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.
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.
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.
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
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.
Spray glyphosate on isolated patches of the crop where brome grass infestation is heaviest.
The following information covers factors to consider when developing an integrated plan to manage brome grass.
|Tactic name||Most likely % control (range)||Comments on use|
|Improving crop competition||-|| |
Optimum sowing rates essential.
Row spacing >250mm will reduce crop competitiveness.Sow early where possible, especially for populations that emerge in May.
|Burning residues||70 (60-80)||Sufficient crop residues are needed.|
|Autumn tickle||50 (20-60)||Depends on seasonal break. Seed burial through shallow cultivation enhances seed depletion through germination, especially in B. diandrus with its shorter dormancy and faster germination.|
|Delayed sowing||70 (30-90)||Depends on seasonal break.|
|Knockdown (non-selective) herbicides for fallow and pre-sowing control||80 (30-99)||If possible delay spraying until full emergence and youngest plants have two leaves.|
|Pre-emergent herbicides||80 (40-90)||Follow label directions, especially on incorporation requirements of some herbicides. Use triazines and trifluralin mainly in pulses.|
|Selective post-emergent herbicides||90 (75-99)||Apply when weeds have 2-6 leaves and are actively growing.|
|Pasture spray-topping||75 (50-90)||Timing is critical. Respray or graze survivors.|
|Silage and hay - crops and pastures||60 (40-80)||Hay freezing works well. Silage is better than hay. Graze or spray regrowth.|
|Grazing - actively managing weeds in pastures||50 (20-80)||Graze infested areas heavily and continuously during winter and spring.|
|Weed seed collection at harvest||55 (10-75)||Low harvest height (10cm) will capture more seeds. Works best on early harvested crops before weeds shed their seeds. Consider swathing to retain seed.|