Plants annual; tufted. Culms 15-120 cm, geniculate; nodes glabrous
or shortly pilose. Sheaths glabrous or hispid, margins ciliate; ligules
1-1.5 mm; blades 3-33 cm long, 5-20 mm wide, glabrous or sparsely
pilose on both surfaces, margins smooth or scabrous; collars pubescent. Panicles
5-15 cm long, 2-8 cm wide, simple, with 5-30 spikelike primary branches
in more than 2 ranks; primary branches 2-10 cm, appressed to
divergent, axils glabrous, axes 0.3-0.5 mm wide, triquetrous, scabrous
or sparsely pilose; secondary
branches usually present on the lower primary branches; pedicels scabrous
and pubescent, shorter than the spikelets. Spikelets 2-3.4 mm
long, 1.2-1.8 mm wide, obovoid, yellowish to reddish-brown or bronze-colored
at maturity, mostly paired, in 2-4 rows, appressed to the branches. Glumes scarcely
separate, rachilla internodes short, not pronounced; lower glumes 1-1.5
mm, at least 1/3 as long as the spikelets, glabrous, (1)3-5-veined; upper
glumes (2)2.2-3.1 mm, glabrous, 7-9-veined, cross venation evident
throughout;
lower florets usually staminate, sometimes sterile; lower lemmas
2-3.1 mm, usually glabrous, 7-veined, cross venation evident throughout; lower
paleas present; upper lemmas 1.8-2.9 mm long, 1.1-1.7 mm
wide, apices acute to rounded, mucronate; anthers 1-1.6 mm. Caryopses 1-1.7
mm; hila punctiform. 2n = 18, 36.
Urochloa fusca grows from the southern United States to Peru, Paraguay,
and Argentina, usually in moist, often disturbed areas at low elevations. It
frequently occurs as a weed, but is occasionally grown for forage and grain.
Plants having smaller, more compact panicles and larger (2.4-3.4
mm), mostly yellowish spikelets have been called Urochloa fusca var.
reticulata (Torr.) B.F. Hansen & Wunderlin.
This variety is mainly found in the southwestern United States, but has been
introduced into other areas, including Australia. Urochloa fusca (Sw.)
B.F. Hansen & Wunderlin var. fusca usually has larger, more open panicles
and smaller (2-2.5 mm), reddish-brown or bronze-colored spikelets. Much intergradation
is reported between the two varieties. Further investigation is needed to establish
that their recognition is warranted.