Key points:
- Caltrop is a significant summer weed with harmful, persistent seeds.
- Caltrop can set seed very quickly – within a month of emergence - regular monitoring of paddocks for caltrop emergence is necessary from late winter.
- Burying seeds through tillage extends seed survival and viability.
- Caltrop has not developed herbicide resistance but can require multiple herbicide applications or a residual herbicide to prevent seed production where subsequent germinations appear.
- Use summer fallow periods to spray populations before seed set.
Caltrop (Tribulus terrestris) is a significant and persistent summer weed in most grain growing regions of Australia.
Strong vines, sharp, woody seeds and a very short growth cycle make caltrop problematic for a range of reasons.
Research by the Western Australian Department of Primary Industries and Regional Development (DPIRD), the University of Queensland and the University of Adelaide through investment by the Grains Research and Development Corporation (GRDC) has revealed useful information about caltrop ecology, seed bank persistence and susceptibility to chemical control.
A problematic weed
Caltrop grows throughout Australia, except in Tasmania.
It is also known as puncture vine, bindii, bindy-eye, cat's head and goat's head.
Caltrop will germinate most readily in temperatures between 24 degrees Celsius and 27° Celsius. Germination can occur once the average soil temperature has reached 15°C to 20°C for at least two weeks.
The plant grows quickly into prostrate, sprawling vines up to three metres in length. It can set seed within one month of seedling emergence and will continue to spread and grow seeds at its extremities, even as seeds near the centre of the plant are drying and shedding.
The seeds have sharp, woody spines that can stick in the hooves and coats of grazing animals, as well as in footwear and vehicle tyres. The spines have been known to puncture tyres.
The dried vines can also be dragged by tillage equipment, shedding seeds as they travel.
Seed germination
DPIRD researcher Dr Catherine Borger says up to 90 per cent of newly-matured caltrop seeds will have an inherent dormancy and need a ripening period of six to 12 months.
"As caltrop requires warm temperatures to germinate, those seeds will remain in the soil through winter and germinate in the following spring or summer," she says.
"Seeds without this inherent dormancy can germinate immediately if there is moisture available, allowing the population to produce several cohorts in one season."
Under warmer conditions, caltrop can germinate before harvest in Australia's southern cropping regions.
"In field trials at Wongan Hills, WA, initial emergence occurred in February 2017, November 2017 and November 2018," Dr Borger says.
"Under irrigated conditions at Northam, WA, emergence consistently occurred in October over the same seasons, although small cohorts also emerged in August 2018."
Like all summer weeds, caltrop depletes nutrient and moisture reserves in the soil.
Controlling caltrop
Dr Borger says caltrop's woody seeds, high seed production and inherent seed dormancy all contribute to a persistent seed bank in the soil.
"The viability and persistence of caltrop seeds increases when seed is buried deeper than five millimetres below the soil surface," she says.
"In our irrigated trials at Northam, seed buried at one centimetre had 56 percent emergence in the first year, 21 per cent in the second year and 10 per cent in year three.
"When seed was buried at depths of two or 10cm, seed viability remained at 10 to 11 percent after two years while the viability of seed left on the surface had reduced to three per cent."
As a result, Dr Borger says one of the most effective ways to control caltrop is to leave seeds on the soil surface during summer fallow periods.
"In no-till systems, seeds left in the first five millimetres of soil or on the surface will germinate readily if moisture is available and the plants can then be sprayed out," she says.
"A minimum tillage system will bury a higher proportion of seed, inducing dormancy.
Burying seeds with deep tillage
Deep tillage soil inversion practices used in WA, such as mouldboard ploughing, can bury caltrop seed so deeply that emerging plants will never reach the surface and germination will fail.
However, burial will induce dormancy and allow the buried seeds to remain viable for longer. These seeds will germinate if they are brought back to the surface by subsequent soil amelioration.
The cost of deep tillage limits its usefulness as a standalone control method for caltrop.
Chemical control
Currently, caltrop has not evolved resistance to herbicides so there is a range of chemical options for effective control when seedlings emerge.
Studies by University of Adelaide researchers at Roseworthy, South Australia, found treatment with glyphosate or glyphosate plus group I herbicides provided greater than 90 per cent control of caltrop.
The researchers also found that mixing glyphosate with group G herbicides could reduce the effective control of caltrop to below 50 per cent.
Because the plant only needs a month to produce viable seed, with 17 days from emergence to flowering and an average of 10 days from flowering to seed shedding, spraying may need to be repeated frequently where multiple germinations occur.
Western Australian agronomist Andrew Storrie from AGRONOMO says there are plenty of effective herbicides that are registered for control of caltrop.
"Killing the plant is not the problem," he says.
"The problem is its rapid seed set and the potential for multiple cohorts from summer rain events."
Some growers have experienced difficulty controlling caltrop with optical sprayers due to the fern-like nature of the foliage, according to Mr Storrie.
"We have now found that if optical sprayers are kept to a ground speed of 14 to 16 kilometres per hour, the cameras will detect the weed and provide effective control."
Controlling caltrop in crops
The increasing emergence of caltrop in late spring is adding to the risk of seed set before winter crops can be harvested and a summer spraying program commenced.
In some situations, a residual herbicide may be used to provide longer term control.
Mr Storrie recommends working with an adviser to identify a combination of registered residual herbicide and tolerant crop variety when planning the cropping program.
Growers need to be mindful of caltrop's short life cycle, while observing chemical residue limits and withholding periods whenever using herbicide to control caltrop before harvest.
Another option is to identify caltrop patches within a paddock and avoid sowing them until the caltrop is under control.
"As long as any risk of soil erosion can be managed, this option helps limit vehicle traffic and make timely herbicide applications possible," Mr Storrie says.
Farm hygiene
Once it is established, caltrop can be easily spread.
Mr Storrie says keeping caltrop off the property and out of clean paddocks is the most important control step growers can take.
"Vehicles need to be checked carefully and frequently, including the tyres or tracks, and in the footwells where seeds can fall from workers' boots," he says.
Ongoing management
While good farm hygiene and rigorous spraying can provide effective control, understanding caltrop's short reproductive phases and seedbank persistence are the keys to planning its management.
Dr Borger says the DPIRD Weed Seed Wizard online tool is being updated so growers can model the caltrop seedbank on their land in a range of scenarios.
"The upgraded Weed Seed Wizard will allow growers to see how their seedbank could be affected by different cropping programs, sowing and harvest times, spraying programs and even local weather conditions," she says.
Weed Seed Wizard is a freely available decision-making tool that can be downloaded via the DPIRD website.
The upgraded version with caltrop modelling is due for release in mid-2021.
