Pasture thistles are a group of plants that are considered a serious threat to rangelands. Like most weeds they propagate very quickly and choke out any grasses that are growing there.
The combination of their sharp spines on their leaves and being unpalatable means they will reduce the grazing area on your pasture. Animals will not travel near them so they will seek out other areas to go. This can lead to overgrazing and increased traffic to other areas.
Once they have fully established in a field it is very hard to get rid of them. It will take many years as well as the combination of different methods to really get them under control.