Setaria pumila

Setaria pumila is a species of grass known by many common names, including yellow foxtail, yellow bristle-grass, pigeon grass, and cattail grass. It is native to Europe, but it is known throughout the world as a common weed. It grows in lawns, sidewalks, roadsides, cultivated fields, and many other places. This annual grass grows 20 centimetres (8 inches) to well over 1 metre (1 yard) in height, its mostly hairless stems ranging from green to purple-tinged in color. The leaf blades are hairless on the upper surfaces, twisting, and up to 30 centimetres (12 inches) long. The inflorescence is a stiff, cylindrical bundle of spikelets 2 to 15 centimetres (3?4 to 6 inches) long with short, blunt bristles. The panicle may appear yellow or yellow-tinged. In New Zealand S. pumila can cover 20 C40% of otherwise productive dairy farming pasture causing a loss in milk production.

from Wikipedia