Drought can devastate existing vegetation, removing competition for light, nutrients, moisture and space which allows 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.
Key points for managing weeds during and after droughts.
New weeds can also be introduced as herd numbers increase as part of a restocking program or when returning from agistment. Studies have shown that up to 12% of weed seeds can pass through the digestive systems of livestock and remain viable.
When bringing feed and fodder onto your property, you increase the risk of introducing new weeds found in other parts of Australia. Knowing where your feed and fodder has come from and what it is made up of can help manage any potential biosecurity risks.
If you are receiving donated fodder or feed, you can take these steps:
Other source of information
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.
Longevity of the seedbank of different weed species, assuming no replenishment.
Weed common name
Black Bindweed/climbing buckwheat
Bladder ketmia/wild cotton
Cobbler’s peg/farmer’s friend
Corn gromwell/white ironweed
False castor oil/thornapples
Citrullus and Cucumis spp.
Source: Tony Cook, NSW DPI.
Short-lived seeds (80 to 90% gone after one year, if no replenishment).
Moderate-lived seeds (50 to 80% gone after one year, if no replenishment).
Long-lived seeds (<50% gone after one year, if no replenishment).
There is further risk that contaminated fodder and grain could have herbicide resistant weed seeds – such as 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.
Feed-out areas should be restricted to one or two “sacrificial paddocks” located where regular checks can be made after each rain event for up to two years after a drought.
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. A combination of cultivation and herbicide can control fallow weeds. The weeds likely to cause most concern are:
In pastoral and tableland areas, weeds that make remarkable recovery and spread include: