Tragus is a genus of annual or perennial grasses in the family Poaceae (order Poales), commonly known as bur grass, burr grass, or carrot-seed grass. The genus comprises around seven accepted species and is native to Africa, Australia, and Eurasia, with additional species occurring on islands across the Atlantic, Indian, and Pacific Oceans and one species in Argentina.
Plants are monoecious and stoloniferous. The culms (stems) are ascending or low and mat-forming, glabrous, and circular in cross-section. Leaf blades are flat or folded and linear, with membranous and trichomatous ligules. Flowers are borne in narrow panicles whose primary branches are spirally arranged, each carrying 2–5 spikelets; each spikelet bears a single floret with three stamens tipped with pale yellow anthers. The caryopses (grains) are elliptical and golden-brown in colour.
Tragus species employ C4 photosynthesis, an adaptation suited to warm, high-light environments. They have been widely introduced as weeds of disturbed habitats in subtropical and tropical regions worldwide. While the genus can limit soil erosion through ground cover, it provides poor grazing value, and its abundance in a pasture is considered an indicator of overgrazing.
Notable species include Tragus racemosus, which ranges across Africa and Eurasia from France and the Canary Islands east to Kazakhstan; Tragus berteronianus, native to Africa and much of southern Asia and now naturalised in the Americas and Caribbean; and Tragus australianus, native to Australia and New Caledonia.
Etymology
The genus name Tragus derives from the Greek word for "goat" (tragos), a reference commonly applied in plant nomenclature to rough or burr-bearing plants, reflecting the spiny or bur-like spikelets characteristic of grasses in this group.
Distribution
Tragus is native to Africa, Australia, and Eurasia and occurs on islands in the Atlantic, Indian, and Pacific Oceans, with T. andicola reaching Argentina. The genus has been introduced as a weed of disturbed ground across subtropical and tropical regions globally; four species — T. australianus, T. berteronianus, T. heptaneuron, and T. racemosus — are now established in North America.
Ecology
Tragus grasses are weeds of open, disturbed, and degraded habitats. They utilise C4 photosynthesis, giving them a competitive advantage in hot, sunny conditions. The genus can contribute to soil erosion control but is of low forage value; high densities of Tragus in pasture are regarded as a sign of overgrazing. The natural chromosome counts recorded are 2n = 20 in T. berteronianus and 2n = 40 in T. racemosus.
