A few weeks ago while visiting Ryan Goodman (@AR_ranchhand, blog), I noticed a weed growing in the pastures of the Middle TN Research and Education Center. It was an unusual, yellow-flowered plant I had never seen before. It turns out it was actually a fairly common pasture weed in Tennessee and other southern states. Upon further research, I came to learn that this weed was known as bitter sneezeweed. The Toxic Plants of Texas website provides some very useful information about this weed.
Bitter sneezeweed (Helenium amarum) is an erect, upper-branching annual, 10 to 20 inches tall with narrow leaves, alternating on the stem. The flowers are noticeable in the late spring or summer and are located at the end of each branch.
Two varieties of this plant are identical except for the flower color: one is all yellow; the other is yellow with a red-brown center. Each bloom has about eight cleft ray flowers (petals) with three lobes, often bending downward at maturity.
In some years, the lower leaves are lost, new growth occurs up the stalk and new flowers appear in the fall. The entire plant has a strong odor and is bitter to the taste.
Bitter sneezeweed is found from Virginia to Florida to Texas and extending into southern parts of adjacent northern states; most abundant in coastal plain where it may be very abundant weed in pastures, roadsides and waste places.
A sesquiterpene lactone is responsible for the toxicity of bitter sneezeweed, which is greatest at time of lowering. This bitter plant is seldom consumed at a level high enough to produce clinical signs. However, it has been responsible for bitter, undrinkable milk and is suspected to be the cause of unpalatable meat from calves slaughtered off the range. The toxin is stable in plants contaminating hay.
Signs of bitter sneezeweed poisoning include: Weakness; Incoordination; Vomiting; Salivation; Diarrhea; Grinding of teeth.
Avoid cutting hay containing a large amount of bitter sneezeweed. Do not feed hay containing any of the plant to lactating dairy cows. Do not slaughter grass-fed cattle from a pasture that contains bitter sneezeweed.
Severe infestations may be controlled with broadleaf herbicides such as 2,4-D or Grazon P+D® at 0.5 to 1.0 pound a.i./acre in the spring with good growing conditions.
Additional information can be found on the Poisonous Plants of the Southern United States website.