Drought conditions impact heavily on primary production and an unfortunate side effect of a drought breaking is an increase in the presence of new and existing weeds on a property.
Drought creates dry soil conditions that prolong the viability of weed seeds. The fungi and bacteria that break seeds down need moisture to function. In dry soil, weed seeds do not break down, and remain completely viable, as if they have been kept in a paper bag in the cupboard.
|Weed common name||Botanical name||Seed life|
|Annual ryegrass||Lolium rigidum||Short|
|Barnyard grass||Echinochloa spp.||Short|
|Bathurst burr||Xanthium spinosum||Medium|
|Barley grass||Hordeum leporinum||Short|
|Black Bindweed/climbing buckwheat||Fallopia convolvulus||Medium|
|Bladder ketmia/wild cotton||Hibiscus trionum||Medium|
|Cobbler’s peg/farmer’s friend||Bidens pilosa||Short|
|Common heliotrope||Heliotropium europaeum||Long|
|Corn gromwell/white ironweed||Lithospermum arvense||Medium|
|Dwarf amaranth||Amaranthus macrocarpus||Short|
|False castor oil/thornapples||Datura spp.||Medium|
|Khaki weed||Alternanthera pungens||Short|
|Liverseed grass||Urochloa panicoides||Short|
|Melons||Citrullus and Cucumis spp.||Medium|
|Mexican poppy||Argemone mexicana||Medium|
|Noogoora burr||Xanthium pungens||Medium|
|Paradoxa grass/phalaris||Pharlaris paradoxa||Short|
|Paterson’s curse||Echium plantagineum||Long|
|Prickly lettuce||Lactuca serriola||Short|
|Saffron thistle||Carthamus lanatus||Long|
|Silverleaf nightshade||Solanum elaeagnifolium||Long|
|Skeleton weed||Chondrilla juncea||Short|
|Sowthistle/milk thistle||Sonchus oleraceus||Short|
|Turnip weed||Rapistrum rugosum||Long|
|Variegated thistle||Silybum marianum||Long|
|Wild oats||Avena spp.||Short|
|Wild radish||Rhaphanus rhaphanistrum||Long|
Drought can devastate existing vegetation, removing all competition for light, nutrients, moisture and space and affording quick establishment of weeds when conditions become favourable. Weeds will germinate from the seed bank immediately after rain occurs.
Drought also causes mineralisation of nitrogen in the soil, and newly germinated weeds take advantage of these nutrients.
After a drought, weeds that are already on a property may spread to new areas, and weed densities can increase. As well as this, new weeds may have been introduced.
Drought often results in the importation of fodder and grain, and this can bring new weeds onto a property. The following weeds are most likely to cause concern:
Some weeds of major concern are introduced when drought feed is brought in from neighbouring states. These include parthenium weed (Parthenium hysterophorus) from Queensland, and bifora (Bifora testiculata) and bedstraw/cleavers (Galium tricornutum) from South Australia (both widespread in the South Australian wheat belt). If grain is purchased from interstate it is important to check for any occurrences of these major weeds, especially in areas where stock have been fed.
If grain is imported from overseas there is a very high risk of introducing serious new weeds, such as kochia (Kochia scoparia) from the United States. By following the mitigation strategies below, this risk can be managed.
Other likely weed contaminants of both grain and fodder are silverleaf nightshade, wild radish (Raphanus raphanistrum) and spiny emex (Emex australis). These species are currently major weeds in NSW, but will spread as a consequence of drought feeding.
There is further risk that contaminated fodder and grain could contain herbicide resistant weed seeds - mainly annual ryegrass. Herbicide resistance is widespread in the grain belts of South Australia and Western Australia – and large quantities of grain are often obtained from these areas when there are drought conditions in NSW. Once again, by following the mitigation strategies below, this risk can be managed.
New weeds can also be introduced when animals are brought in, either in a restocking program or returning from agistment. Studies have shown that up to 12% of weed seeds can pass through the guts of animals and remain viable.
Weeds can also be introduced to cropping land in contaminated seed at sowing time, and when weed seeds are spread on contract machinery – especially harvesters.
After rainfall following drought conditions there can be an increase in livestock poisoning from weeds. Stock losses are attributed to direct plant poisoning and photosensitisation, with the main problems caused by:
In cropping areas the immediate problem is fallow weeds, which quickly rob the soil of both valuable nutrients and moisture. Fallow weeds can be controlled by a combination of cultivation and herbicide. The weeds likely to cause most concern are:
In pastoral and tableland areas, weeds that make remarkable recovery and spread include:
Weeds provide an additional financial burden in both lost production and cost of control after a long period of low farm productivity. Drought conditions will often result in serious setbacks for long-term planning and for controlling pastoral and cropping weeds. It is important to maintain vigilance so that weeds do not become a serious threat to drought recovery on a property. An accurate assessment of the situation must form the basis of an effective weed control program after drought.
Obtain as much detail as possible about the source of the fodder or grain that is being brought onto the property. Fodder and grain from interstate and overseas pose greatest risks of introducing new weeds onto a property and into New South Wales.
Carefully consider where the grain and fodder are to be fed. Feeding out areas should be restricted to make weeds easier to find and control. A small ‘sacrifice paddock’ may be the best option, located where regular checks can be made after each rain event. Flat, arable areas will allow easy access for most types of weed control (mechanical, chemical, biological or grazing management). When considering herbicide resistance, it is best to avoid feeding in or near cropping paddocks.
Pay careful attention to all feeding areas. Closely monitor all feeding areas for two years after a drought. Identify all weed outbreaks – even if it is only one plant.
Native and feral animals may also be accessing livestock feed, so water points should also be monitored for two years after a drought.
Livestock are excellent distributors of weed seeds. It is recommended to quarantine livestock for at least 2 weeks. This includes holding livestock returning from agistment. Check holding paddocks for the presence of weeds germinating.
When rain occurs and weeds have germinated, they need to be identified and controlled as soon as possible, and well before they are able to set seed. If possible, delay sowing crops for up to a week to allow the first germination of weeds to be fully controlled. Be aware that the vegetative parts of perennial weeds, such as silverleaf nightshade and blue heliotrope, need to be controlled as well.
The most important aspect of controlling newly introduced weeds is early detection and identification. Identification and control of weeds are important for two years after a drought . Keep a close watch for unknown plants and have them identified as soon as possible. Don’t let them establish and set seed. Contact your local council weeds officer for advice and assistance with identifying potential weeds.
For agronomy advice contact Local Land Services.
For weed identification and advice contact your local council weeds officer.
Authors: Bob Tounce, James Dellow, Andrew Storrie, Tony Cook
Editor: Elissa van Oosterhout
Primefact 430 (2007) What impact does drought have on weeds?, NSW Department of Primary Industries, Orange.
Primefact 372 (2007) Weed strategies following drought, fire and flood, NSW Department of Primary Industries, Orange.
Primefact 365 (2007) Weeds – a threat to drought recovery, NSW Department of Primary Industries, Orange.