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Herbicides for weed control in barley
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Herbicide tolerance of barley
The wide range of herbicides that are registered for the control of broadleaf and grass weeds are summarised in the Department of Agriculture and Food Bulletin No. 4675 'Cereal spraying charts: 2006/2007', available from Julie Roche, Centre for Cropping Systems, Northam 08 9690 2148
Basically herbicides can be divided into four phases depending upon the time when they are applied:
Herbicide tolerance of barley
The tolerance of varieties to herbicides varies with the rate of herbicide application, the environmental conditions when the herbicide is active in the crop and possibly the stages of crop growth.
Guiding Principles for good herbicide use
- Use recommended label rates
- Correctly identify target weeds, weed growth stage and crop growth stage (Zadok)
- Do not apply to waterlogged soils
- Ensure tank mixtures are proven and registered
- 1/3 to fill tank with water before adding products
- Use wetting agents as required
This herbicide works in a similar manner to simazine and as such it requires adequate soil moisture to ensure it is active. However, it is very water soluble and on sandy soils may be leached from the root zone before it can be taken up by weeds.
Trifluralin and Oryzalin are both dinitroaniline herbicides. Unlike trifluralin, oryzalin is not volatile but when the trifluralin is present this herbicide must be treated in the same way as other trifluralin based products.
Like oryzalin, pendimethalin has lower volatility and water solubility than trifluralin. Uniform incorporation in the top 2.5 cm of soil is needed for good weed control. Like trifluralin, crop damage will occur if seed is sown into the treated band.
Trifluralin is relatively cheap and can be tank mixed with knockdown herbicides. Points to note are:
- Trifluralin is volatile and must be incorporated in the soil within four hours. Losses from volatilisation are greatest at high temperatures.
- For best results with trifluralin, the paddock should be either pre-worked or stubble residue must not cover more than 25 per cent of the soil surface. Excessive amounts of stubble will make good incorporation very difficult, if not impossible, unless culti-trash seeders are used.
- Trifluralin should be incorporated evenly in the top 2.5 cm of soil. The barley must be sown under the trifluralin layer otherwise it will either fail to emerge or if it emerges it may have stunted root systems, poor subsequent growth, or lodge later in the season. As a safety margin increase seeding rates by 10 per cent if trifluralin is used.
- As an alternative to incorporation by sowing, trifluralin can be applied immediately after sowing and incorporated by prickle harrows. Losses by volatilisation will be greater than with a full incorporation.
- Trifluralin is registered for application and incorporation one or two weeks before sowing. However, many farmers have successfully incorporated trifluralin at sowing, by maintaining good depth control and adequate seeding rates.
- The level of control of ryegrass will vary but, generally speaking, 75 per cent control could be expected. This makes trifluralin an option where ryegrass densities are likely to be low to medium.
The development of diclofop resistance in wild oats has resulted in renewed interest in tri-allate. Losses of tri-allate from a dry soil surface are expected to be low and a delay in incorporation of up to 24 hours is unlikely to significantly reduce herbicide activity. This will enable more flexibility in the seeding operation.
In considering early post-emergence (until 5 leaves on main stem (Z15)) treatments to barley it is important to bear in mind timeliness of application, rate of herbicide and crop effects. Herbicides are available for most broad-leaved weeds in-crop but of the grasses, only wild oats and annual ryegrass have herbicides registered for in-crop weed control.
Late Post-emergence Herbicides
To avoid yield losses from competition, weeds must be controlled early in the season. However, weeds may appear late in the crop because of:
- Late germination of weeds;
- Failure of earlier herbicide treatments;
- Lack of a control method.
It is important that these weeds are prevented from maturing and setting seeds within the crop. Harvested grain contaminated by weed seeds and other plant fragments may be unsaleable. An integrated approach to weed control will ensure that low numbers of seeds are carried over.
While a small infestation of weeds late in the season may not reduce grain yields, the weed seed bank may be substantially increased. Weed seeds returned to the field may create a problem for many years, particularly when the seeds have a high level of dormancy. Late post emergence chemical control will reduce weed seed contamination of the harvested grain sample and reduce weed carryover into the following season.
The phenoxy herbicides (2,4-D and MCPA) are commonly applied as late post-emergence treatments and to reduce the seed set of wild radish, wild mustard, wild turnip and lupins. These herbicides must be applied before booting and preferably no later than full flag leaf emergence otherwise serious yield losses may occur. Combinations of 2,4-D Ester and Metsulfuron-Methyl have caused large yield reductions and should not be used. Herbicides registered for the post-emergence control of the major broad-leaved weeds in cereal crops in Western Australia are listed in the Department of Agriculture and Food Bulletin No. 4675 'Cereal spraying charts: 2006/2007', available from Julie Roche, Centre for Cropping Systems, Northam 08 9690 2148
Page updated: June 2006