Yes pigweed is toxic to horses even in small amounts but even larger amounts of pigweed is much more toxic and deadly to horses.
If you have horses and have pigweed growing where they are around it you should kill the pigweed and keep it from growing.
If your horse ingested a few bites of pigweed then they likely will be okay but if the horse ate a lot of pigweed then they should be seen by a vet.
Pigweed contains toxic compounds such as nitrate which can be deadly to horses and many other animals in large enough doses.
Pigweed is sometimes referred to as Hogweed because hogs and pigs eat Pigweed sometimes.
However Hogweed is also used to describe other related plants.
Hogweed mostly refers to a biennial or perennial herb that is in the carrot family (Apiaceae) which can grow to 14 feet or more. Its hollow, ridged stems grow 2-4 inches in diameter and have dark reddish-purple blotches.
Hogweeds large compound leaves can grow up to 5 feet wide.
When it comes to pigweed the pigweed is a smaller plant that can be found and pulled from gardens and tree beds in the United States, but it's lovingly cultivated in other parts of the world.
Pigweed is also known as Amaranthus which is a cosmopolitan genus of annual or short-lived perennial plants collectively known as amaranths.
Some of the pigweed amaranth species are cultivated as leaf vegetables, pseudocereals, and ornamental plants. Most of the Amaranthus species are summer annual weeds
Pigweed and hogweed are different but sometimes they are called the same thing.