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