Cocklebur

One of the pigs ate a cocklebur plant and died.

No human poisonings from cocklebur are known but seeds and seedlings are toxic and potentially fatal to animals.

Fig. 1.—Spiculated pollens of ragweeds (ambrosias) low in protein. […] In the cocklebur (Xanthium americanum) and the rough wild elder (Iva ciliata), the spicules are shorter, being 0.7 and 0.5 microns, and the reaction is proportionately less active than with the ragweeds (ambrosias). […] While the grass pollens have so light a coat that they are frequently crushed in the ordinary process of mounting, the ragweed (ambrosias) pollen grains resist pressure between two glass slides carried to the point of crushing the glass.

These burrs stuck to the tails of the cows as they walked by, and were brought to the cowpen, the cowtails greatly enlarged by the huge amount of cockleburs stuck on them.