33. REDROOT PIGWEED

Amaranthus retroflexus

(pigweed family)

TOXICITY RATING: High. The plant is quite common and very toxic.

ANIMALS AFFECTED: Cattle and swine are the animals most likely to be affected; goats and sheep can also be poisoned.

DANGEROUS PARTS OF PLANT: Leaves, stems, roots.

CLASS OF SIGNS: Breathing problems, trembling, weakness, abortions, coma, death.

PLANT DESCRIPTION: Redroot pigweed (fig. 33) is a large (to 5 feet tall), coarse, annual with red stems and simple, egg-shaped, wavy-margined, alternate leaves. The green, inconspicuous flowers are borne in short, compact clusters along with green spines. Seeds are small, shiny, and black. Fields, barnyards, and waste areas are the favorite habitats of this weed.

SIGNS: Pigweed contains a nephrotoxin that causes kidney failure, and also contains soluble oxalates and is capable of accumulating nitrates. Therefore, toxicity can be due to any combination of these toxicoses.

Animals need to consume pigweed in fairly significant quantities over several days before signs appear. Typically, onset of signs is 3 to 7 days from the onset of ingestion. Animals will usually avoid pigweed if there are better forages available. Common incidences of poisonings have occurred when swine have been raised in confinement and are then turned out into a pigweed-infested pasture in the late summer to early fall. Under these circumstances, the swine consume large amounts of the plant quickly, with 5-90% of the animals becoming affected, with 75% or greater mortality among the affected animals. Modern management practices have largely eliminated this type of poisoning, but it can still occur. In cattle, pigweed toxicosis resembles oak toxicosis.

In affected animals, early signs include weakness, trembling and incoordination. This progresses to an inability to stand and paralysis, yet the animals may still be alert and able to eat. Near the end of the clinical course, the affected animals may go into a coma, and have edema under the skin of the abdomen and the legs, have a bloated abdomen, and die. The course of the disease is approximately 48 hours and is primarily consistent with kidney failure. Cases where animals consume smaller amounts of plants over long time periods have not been well studied, but this is also believed to cause toxicology problems.

Treatment with herbicides may render pigweed even more palatable, therefore make sure all treated plants are dead prior to introducing animals.

FIRST AID: If pigweed is being rapidly consumed, limit further access and ingestion of the plants. A veterinarian will be able to provide supportive care for the different toxicants contained in pigweed, but the animals may still succumb to the nitrates, soluble oxalates or the kidney toxin.

SAFETY IN PREPARED FEEDS: Pigweed is not safe in hay or other prepared feeds.

PREVENTION: To prevent pigweed poisoning, do not allow animals to have access to affected pastures, especially if the animals are hungry. Spray or mow plants down, making sure they are dead before animals are on pasture. Provide for supplemental feed if pasture quality is poor, since well-fed animals are less likely to consume pigweed.