Weeds and drought

Why do weeds thrive after drought?

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.

• Weeds can be introduced in feed and fodder
• Drought can prolong weed seed viability
• Herbicide resistant weed seed can be introduced in drought feed
• Livestock poisoning can occur after drought
• Cropping areas are susceptible to weeds after drought

Through livestock movements

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.

Feed and Fodder

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:

• Check feed and fodder
• Restrict feeding areas as much as possible
• Monitor feeding areas and water points
• Control weeds quickly after germination
• Identify any new plants

Other source of information

• Drought recovery guide
• High risk weeds
• Managing weeds and diseases on your property
• Feed and fodder - Managing biosecurity risks during dry times
• Look out for new weeds in fodder

Drought prolongs the viability of weed seeds

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.

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).

Herbicide resistant weed seed can be introduced in drought feed

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.

What can you do to reduce impact during and following drought

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.

Livestock poisoning can occur after 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:

• Panic species (causing photosensitisation, mainly in sheep)
• Rock fern (causing blood in urine and sudden death),
• Marshmallow weed (staggers and possible death),
• Green cestrum (causing sudden death),
• Amaranthus species (causing kidney failure in sheep and cattle), and
• thistles (causing nitrate poisoning in sheep and cattle).

Cropping areas are susceptible to weeds after drought

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:

• common heliotrope (Heliotropium europaeum)
• summer burrs – Bathurst and Noogoora burrs (Xanthium spp.)
• caltrops (Tribulus terrestris)
• Amaranthus species
• thistles
• Sidas (sida spp.)
• pigweed (portulaca oleracea)
• panic grasses (Panicum species)
• mintweed (Salvia reflexa)
• marshmallows (Malva parviflora)
• Johnson grass (Sorghum halepense), and
• wireweed (Polygonum aviculare).

In pastoral and tableland areas, weeds that make remarkable recovery and spread include:

• serrated tussock (Nassella trichotoma)
• marshmallows (Malva parviflora)
• pigweed (Portulaca oleracea)
• blackberry (Rubus fruiticosus species aggregate)
• blue heliotrope (Heliotropium amplexicaule)
• nodding thistle (Carduus nutans)
• Scotch thistle (Onopordum species), and
• St John’s wort (Hypericum perforatum).