Ask Mr. Smarty Plants

This sounds like Solanum elaeagnifolium (Silverleaf nightshade).  It is considered a noxious weed in several states in North America where it is native.   In other countries it is considered an invasive non-native plant.   Here is information from the European and Mediterranean Plant Protection Organization (EPPO) on silverleaf nightshade's geographical, biology and means of eradication.  There are several reasons to eradicate it from areas where food crops for humans and livestock are grown—it competes for moisture and nutrients of the crops you are growing for hay; it may inhibit the growth of some plants (allelopathy); and it (especially the fruits) is considered toxic for livestock so you certainly don't want it mixed in with your hay.  The EPPO admits that this plant is difficult to eradicate.  Both the EPPO and Washington State Noxious Weed Control Board report that herbicides are not very effective in controlling it.  The most benign way to get rid of the plant is to pull or dig up the plants and dispose of them.  Depending on how large your area is and how much help you have, this may or may not be feasible.  Once you have removed them, you will need to monitor for new plants.  The plants do pull up relatively easily; however, you need to wear heavy gloves to pull them because the stem has sharp spines.  Merely cutting them will not eradicate them since they can regrow from the stems and roots.