Pigweed actually is not harmful nor poisonous to humans and actually humans can eat pigweed although I would not want too.
Pigweed is mostly just toxic to livestock such as cows, sheep and swine and can potentially cause death in the livestock if they ingest enough of it.
Most often though when a cow or other livestock ingests pigweed they will just get sick and not die unless the cow or other livestock ate a lot of the pigweed.
Then it could be a real problem.
If you have pigweed in a field where cows or livestock are grazing it's best to get the pigweed removed or remove it somehow to prevent the livestock from ingesting it as they graze.