Perilla Mint is a wild mint that has a square stem.

It will kill a cow, a horse, or a goat in a heartbeat.

I have a neighbor that had it all over his pasture and he lost 5 brood cows to it last summer.

Cattle were autopsied at Mississippi State and Perilla Mint was named as the cause of death.

Kills through the respiratory system.

Animals develop pneumonia like symptoms.

Looks like to me it would harm deer but maybe not.

Never have researched that aspect of it.