With the onset of rain, winter weeds will emerge and grow in crops, pasture and fallow land. Good weed control is vital to keeping weed numbers low and reducing their impact on production levels.
The most common weeds usually found on a small landholding include grass species — brome grass, barley grass, silver grass, Guildford grass or onion grass and wild oats — and broadleaf species, which include one or two-leaf cape tulip, saffron thistle, spear thistle, cotula (funnel weed), crassula, doublegee, prickly lettuce, matricaria, dandelion, Paterson’s curse, soursob, toad rush, erodium, Mexican poppy, mallows, wild radish, wild mustard, wild turnip and fleabanes.
The intensive and uninterrupted use of a particular herbicide, or herbicide group, can lead to weeds becoming herbicide resistant.
In order to manage resistance, all herbicides sold in Australia are grouped by mode of action (the way they work). The mode of action labelling is based on the resistance risk of each group and is indicated by a letter on the product label.
For each group, there are often a number of sub-groups, as indicated in the table, right.
Grass weeds, in a broadleaf crop such as lupins or canola, can be controlled by some herbicides from Group A (sub groups ‘Fop’ and ‘Dim’) and Group B following the instructions on the label, without killing the crop.
For cereal crops, herbicides from sub-group ‘Fop’ (Group A) and some herbicides from sub-group ‘Sulfonylurea’ (Group B) may be sprayed to control weeds.
Broadleaf weeds in a cereal crop can be controlled by some herbicides from Group B, C and I. Some Group C and I herbicides can be safely used to control both grass weeds and broadleaf weeds in some herbicide-tolerant crops such as Clearfield crops, TT canola and Eagle Rock wheat.
Weed control in fallow situation
Some weeds, such as annual ryegrass, clover, medics and capeweed, are important plants in an established pasture — but they can be aggressive weeds in crops.
If there is no stock on the farm to eat the pasture plants in the year before planting a crop, it may be worthwhile to fallow or hay freeze the pasture.
Fallow involves killing the weeds in late winter or spring by either ploughing the field (green manure) or spraying a non-selective herbicide such as glyphosate (hay freeze or brown manure). Stock will readily eat pasture that has been hay frozen, as the nutrients and sugars remain in the plant. Therefore, farmers must be careful not to overgraze these paddocks.
Glyphosate (for example, Roundup 450) is commonly used at 800 to 2000mL per hectare to control a range of weeds. It is a non-selective (knockdown) herbicide that will kill almost any plant when sprayed, including crop and weed species. Low rates should be used when the weeds are small or less dense and higher rates when weeds are dense or plants are larger.
This herbicide needs to be used when weed plants are present, because it does not have any soil activity. If this herbicide is sprayed on bare ground with a hope that weeds will die before or after germination, it would be a waste of time and money.
When sprayed according to the label, this herbicide has a low drift risk.
Paraquat (for example, Spray.Seed 250) is another common non-selective herbicide, similar to glyphosate, used at 600 to 3200mL/ha to control a wide range of weeds.
Spray.seed is very toxic to humans. Do not use this chemical unless you have extensive experience with herbicides and the safety procedures needed for these chemicals.
Spray drift is the movement of herbicide molecules in fine droplets away from the target site, either by erratic spraying methods or due to high wind speed. Spray drifts can be controlled by using a nozzle that produces a coarse droplet size, reducing the pressure at spraying, keeping spray boom height low and avoiding spraying when wind speeds are high or in absence of wind.
Herbicides can be an effective way of managing weeds. However, before spraying any herbicide, always read the label, choose the right herbicide for the right weeds species, use appropriate safety gear, analyse the environmental conditions at spraying time and ensure that you use the correct herbicide rate (particularly when applying herbicides to crops).
It can be difficult to calculate the rate, particularly if you are using a hand sprayer or backpack sprayer. However, you can consult an expert for advice on all issues of herbicide application.
To identify weeds, the Department of Agriculture and Food publishes ‘Southern Weeds and Their Control’ and ‘Western Weeds’. To purchase one, or for more information, phone the Small Landholder Information Service on 9733 7777
Abul Hashem is from the Department of Agriculture and Food, Northam
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