The ad, being published Thursday in a Calgary weekly newspaper, depicts a photo of a diaper-clad baby and a young calf side by side.
Under the photos are the words "Just three months old — Would you abuse a baby to entertain a crowd?"
The average age of a calf used in calf roping is three to four months old.
The humane society says it is calling for a ban on calf roping at all rodeos across Canada and specifically wants the Calgary Stampede — which starts Friday — to show the way by dropping the event.
It calls calf roping the "most inhumane" of all rodeo events, saying fear, pain and stress are used to coerce the animals into performing for people's entertainment.
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