Weed competition can be affected by crop species, crop variety, weed species, crop and weed density and time of emergence of the crop relative to the weed.
Oats are more competitive with weeds than barley, wheat, canola and pulses when sown at recommended seeding rates because of its greater tillering ability. If given the right start, an oat crop has the necessary vigour to compete against weeds. Increasing crop density may improve competitiveness and ultimately impact on yield.
Cutting hay is a common method used for reducing the weed seed bank - it is important to actually cut hay as per the rotation as harvesting grain instead can result in huge weed increases the following season. But effective weed management for hay crops is also essential as weed contamination is directly related to quality. Weeds can cause downgrading or rejection of export hay as there is a weed contamination limit of 5 per cent. There is also a nil tolerance to annual ryegrass toxicity (ARGT) and prickly weeds such as doublegee.
Integrated weed management (IWM)
Preventing weeds from entering or establishing in a paddock is the best method of weed management, especially when combined with physical, agronomic and chemical options. Some of the non-chemical options available in IWM are:
- use of weed-free seed
- clean machinery when moving between paddocks and on farms
- tarp loads when moving grain
- control weeds along roadsides at the edge of paddocks
- eradicate small patches of new invading weeds
- consider weeds when importing hay
- don't import grain or products that may contain certain herbicide resistant weed seeds
- crop and pasture rotations using species with different competitive abilities, sowing dates and harvesting techniques such as swathing
- increase seeding rates to maximise crop-weed competition and yield without reducing grain size
- sow cereals in an east-west direction
- implement a tickle cultivation to stimulate germination of ryegrass and other weeds prior to seeding
- grazing with sheep or cattle.
Herbicides
There is a wide range of herbicides registered for the control of broadleaf and grass weeds. These include herbicides that are applied:
- pre-emergence
- early post-emergence
- late post-emergence.
Current herbicide recommendations should be referred to before spraying.
Varieties may differ in their reaction to the registered herbicide options for oats. Varieties are assessed for herbicide tolerance as part of the National Herbicide Tolerance project supported by Grains Research and Development Corporation. The level of tolerance will vary with the rate of herbicide, the environmental conditions when the herbicide is active in the crop, and for some herbicides, the stage of growth.
For current information on variety herbicide tolerance see the National Variety Trials website.
Reducing glyphosate resistance
Glyphosate is a key herbicide in our farming systems and responsible use is required to prolong its effective life.
IWM should be applied by growers to sustain glyphosate and reduce the incidence of resistance in weeds, particularly ryegrass.
A double knockdown technique will minimise the risk of resistance developing. Double knockdown is the sequential use of glyphosate followed by a mixture of paraquat + diquat.
Best practices for double knockdown include:
- Glyphosate followed by paraquat + diquat. This sequence provides better ryegrass, capeweed and radish control than the reverse.
- Spraying the first herbicide at the 3-6 leaf stage of ryegrass results in the best control.
- The interval between knockdowns needs to be at least one day when using the glyphosate followed by paraquat +diquat but a 2-10 day interval is more effective if seasonal conditions permit.
- Allowing a longer interval before the second spray will ensure plants emerging after the first knockdown are killed.
Broad-leaved weeds
The range of broad-leaved weeds found in crops is generally larger than in grasses, however a few such as capeweed, doublegee and wild radish are widespread. Others, such as soursob, sorrel, dock, fumitory, self sown legumes and wire weed are of significant local importance.
- Capeweed - the most common broad-leaved weed given its widespread occurrence in pastures. Cost effectively controlled by a wide range of products.
- Doublegee - infestation is generally on a lesser scale but dormancy and staggered germination pose problems with regard to optimum timing of herbicides. Seed production can begin at relatively early growth stages (four leaves) especially on stressed or later emerging plants and that if the aim of control is to reduce seed production, delays in application to ensure adequate emergence may be counter productive.
- Wild radish - some of the most widespread weeds of cereal crops. Staggered germination and dormancy make their control difficult.
Grass weeds
Annual ryegrass and wild oats are two of the most competitive weeds of cereal crops. The conditions of cereal cropping favour their germination and vigorous growth, which when combined with their high seed populations from preceding crops or pasture, can often lead to very large reductions in potential yield. Both species exhibit staggered germination which often leads to poor control.
Where these weeds emerge after seeding and before crop emergence, a knock-down treatment can be used to burn off the small grass seedlings, but it is important to ensure that no crop emergence has occurred. Do not use glyphosate based knock-downs in this way. These grasses are often a problem in emerging crops after early seeding. Control options include:
- delayed seeding
- shallow cultivation-tickle.
Annual ryegrass toxicity (ARGT)
ARGT is a disease of grazing livestock that results fro the ingestion of annual ryegrass seed heads that have been infected by the toxin forming bacteria Rathayibacter toxicus. The bacteria adhere to seed-gall nematodes (Anguina funesta) in the soil and enter the plants, with the nematodes, as they attempt to complete their life cycle. Bacteria colonise the galls, displacing the nematodes and toxicity develops as the plants hay off.
There isa zero tolerance to ARGT in export hay. In Western Australia te considered safe level of ARGT in feed is 200-300 galls per kilogram of grain and hay. The requirement for export hay is less than one gall per kilogram.
Conditions which favour the development of ARGT are:
- paddocks with moderate to high frequency of cropping
- high density of ryegrass
- short growing seasons
- spread of contaminated materials from ARGT areas
- late hay cutting.
Export hay is often grown to aid in ryegrass control, but all export hay is tested for ARGT prior to processing. To ensure the continued acceptance of WA hay into export markets it is critical that supplies remain ARGT free. Deformed heads, bacterial galls, and sometimes slime can be detected in the field but lab tests are recommended to determine the levels present.
A number of biological controls offer potential, including Safeguard and twist fungus.