Amaranthus retroflexas L —Pigweed; redroot
FAMILY: Amaranthaceae—the Amaranth Family
The Amaranthaceae is a widely distributed family of herbs. Flowers: small, often unisexual, subtended by dry scales, frequently in showy cones; fruit: a utricle. Many species are weedy; some are grown as ornamentals.
PHENOLOGY: Pigweeds produce flowers in mid- to late summer.
DISTRIBUTION: Amaranthus retroflexus is a weed in Pennsylvania, as is the more common closely related smooth pigweed (A. hybridus L.). The weedy amaranths, native to tropical America, are distributed in Pennsylvania in gardens, cultivated fields, pastures, roadsides, waste places, and fields.
PLANT CHARACTERISTICS: Amaranthus retroflexus isa tall annual plant, to 2 m; leaves: longpetioled, ovate or rhombic-ovate to 1 dm; inflorescence: a terminal panicle of densely crowded spikes, 5-20 cm long.
POISONOUS PARTS: The foliage is poisonous.
SYMPTOMS: Losses have occurred to livestock, especially sheep. Cattle and horses are relatively resistant. Postmortem: gross lesions: perirenal edema(pigs, growing calves) possibly containing blood, affected kidneys pale yet normal in size; rectal and abdominal wall edema; distended pleural and peritoneal cavities caused by straw-colored fluid; kidneys may have ecchymotic hemorrhages in the cortex. Histological lesions: interstitialedema in renal cortex, tubular nephrosis, necrosis and dilation of the convoluted and collecting tubules with proteincasts. Hydrothorax and hydroperitoneum is more pronounced in calves. In ruminants methemoglobinemia may appear. Blood and body tissue appear chocolate brown. Nitrite and ammonia ions may cause the stomach mucosal surface to be congested.
POISONOUS PRINCIPLES: The nephrotoxic agent(s) is not known. Oxalates, which are found infrequently as crystals in histological studies, may account for some symptoms. Toxic concentrations of nitrates also are responsible for toxicity.
CONFUSED TAXA: Among Amaranthus spp. occuring in the Commonwealth, the two most common are A. retroflexus, described above, and the related A. hybridus. The floral bracts are rigid, tapering to a point, 4-8 mm long in the former, whereas they are stout-tipped, 2-4 mm long in the latter. All species of Amaranthus should be considered dangerous to livestock.
SPECIES OF ANIMALS AFFECTED: Sheep, hogs, and young calves are more susceptible than adult cattle and horses.
OF INTEREST: Lambsquarter (Chenopodiaceae) is a distantly related weed that contains at least one poisonous species, Chenopodium ambrosioides L (Mexican tea, wormseed). It is found in gardens, roadsides, and wasteplaces. Symptoms include nausea, vomiting, abdominal pain, headache, dizziness, impaired vision, and depression. Oil of chenopodium contains ascaridol, an anthelmintic used in treating internal parasitic worms.