Please do NOT feed this plant (Apocynum cannabinum) to any livestock! It is a known toxic plant and deaths have been attributed to a chemical in its sap that affects the heart. It isn’t particularly appealing to livestock and they tend to avoid it. However, if it is cut and baled along with hay, they may indeed ingest large quantities of it and indeed experience problems.
This article http://pubs.aic.ca/doi/abs/10.4141/CJPS08103 discusses control of the weed. You could also contact your county Extension educator and ask for advice on control.
Thanks for sending in this question and asking about hemp dogbane before cutting and baling it. You are a wise livestock caretaker!