Two common weeds in Western alfalfa pack a punch so powerful that even small quantities can be deadly to cattle and horses.
"Common groundsel and fiddleneck contain pyrrolizidine alkaloids (PAs)," says Arthur Craigmill, a University of California-Davis toxicologist. "The consumption of even small quantities of PAs causes severe liver damage."
"PA poisoning isn't a widespread problem. However, it does show up in different degrees every year," says Mick Canevari, a California farm advisor.
The alkaloids are resistant to heat and also maintain their toxicity in dry hay.
There are no cures or antidotes for PA poisoning, so preventing growth of PA-containing weeds in hay and pasture crops is crucial.
Fiddleneck is usually a big problem in new alfalfa stands and less of a problem in established ones. The opposite is true of common groundsel, Canevari says.
To control common groundsel in new seedings, Canevari recommends Buctril, Gramoxone or 2,4-DB; in established stands, apply Velpar. To control fiddleneck in new seedings, he recommends Buctril; in established stands, Gramoxone with Velpar or Karmex.
The alkaloids are typically concentrated in the weeds' small shoots and in the seeds at maturity. But the concentration varies from year to year. Craigmill says consumption of large amounts of groundsel or fiddleneck may kill animals in a few weeks or less.
"Other times, an animal can eat the weeds for several months before signs of poisoning actually become visible. When symptoms suddenly appear, it looks like something just happened. But it's actually a result of months of intake of the compound."
If a diagnosis is made in time, animals may recover, he says.
In cattle, common signs of PA poisoning include dull haircoat, dry muzzle, lack of conditioning, diarrhea and depression. Horses lose condition, stop eating, become depressed and wander aimlessly.
"There's a definite hierarchy in sensitivity among species to this particular toxin," points out Craigmill. "Horses are the most sensitive, followed by cattle, hogs and chickens, which are less sensitive, and sheep, goats and turkeys, which are the least sensitive. The young of all species are more sensitive than adults.
"If hay with PAs is fed to dairy cattle, the alkaloids are then excreted in milk. Humans are susceptible to PA poisoning."
Most animals find the toxic weeds unpalatable and won't eat them if better forage is available. Of course, animals can't select toxic plants out of cubes or pellets, says Craigmill.