I experienced this nasty sucker living over in Wagga, where the burs (shaped like cat heads - hence the name) could puncture bicycle tyres without a thought, go through shoes, and had even brought the occasional car tyre to a flat end. The name "Caltop" means "spiked weapon" just to further the nastiness of this one.
A couple turned up in the yard Pepper stayed in for her first 18hrs, but that was expected as the paddocks she came from were littered with this weed. What I didn't expect was to find them in paddocks I've had closed off since before Pepper arrived, in nearby paddocks, and even in my back yard (a fenced section I haven't been in since pepper arrived). That's further than pep can be held responsible for!
but as a heads up - if you can grab this weed before it sets seed, DO IT or it will be horrible to control. Don't spray it, rip the thing up tap-root and all. Oh and wear gloves, even the juvenile developing seeds can puncture skin.
anyway, on to the weed:
Caltrop Tribulus terrestris
Alternative Name(s): Cat-head, Bindii.
Origin: Native at least of the Mediterranean, now cosmopolitan. One Australian species is probably included under this name.
Flowers/Seedhead: Solitary in the leaf axils, 5-petalled. Flowers summer and autumn in southern Australia.
Description: Prostrate annual with stems to 2 m long. Leaves with 4–8 pairs of oblong leaflets, each leaflet to 12 mm long, the upper surface dark green and often with hairy margins, the lower surface paler and hairy. Fruit 11–20 mm wide (including spines), comprising a cluster of 5 segments each with 2 larger divergent spines above and 2 smaller downward projecting spines below. Each segment with 1–5 seeds.
Distinguishing features: Distinguished by petals 2–10 mm long; style shorter than to slightly longer than length of stigma and fruit with 2 larger and 2 smaller spines per fruit segment.
Dispersal: Spines of fruit segments ensure rapid dissemination of seeds.
Confused With: Other small-fruited spiny Tribulus species.
Notes: A troublesome weed of wasteland, pastoral land, cropping, vineyards and recreation areas. Sharp spines on dry fruit hamper stock handling, are a nuisance in recreation areas and fruit may contaminate drying grapes. Photosensitisation, staggers and nitrate poisoning are also caused by stock grazing Caltrop. Young sheep are especially sensitive. A native insect and mite damage plants and overseas biological control has been used to reduce problems associated with this species.