I found this neat site while surfing the web, trying to find something to trigger this old mind to remember "vetch." It shows pictures of all the different types of weeds that might show up in a field of hay. Its quite helpful because there are pictures. Once you have a picture, you can put a name to your plant, then look it up on GOOGLE and see if its toxic.
http://harnett.ces.ncsu.edu/files/library/43/weed id 7-20-09.pdf
http://harnett.ces.ncsu.edu/files/library/43/weed id 7-20-09.pdf