Definition of "pigweed"

Any of various weedy plants sometimes used as pig fodder  quotations

1. Amaranthus species  examples

2. Chenopodium album, white goosefoot  examples

3. Portulaca bicolor, of Australia  examples

4. Polygonum aviculare (common knotgrass)  examples

5. (uncommon, dialectal) Heracleum sphondylium  examples