Pigweed can mean any of a number of weedy plants which may be used as pig fodder:
Amaranthus speciesAmaranthus palmeri, the 'pigweed' resistant to glyphosate in the US SoutheastAmaranthus hybridus, smooth pigweedAmaranthus blitoides, prostrate pigweedAmaranthus californicus, California pigweedAmaranthus fimbriatus, fringed pigweedAmaranthus albus, white pigweed, tumble pigweedChenopodium album white goosefootPortulaca species