This a great web site: http://www.fao.org/DOCREP/005/Y2808E/y2 ... m#Contents
It has sections on using the prickly pear as forage in Brazil, Mexico, the US, etc. As you can tell by the table of contents, it's a comprehensive view of the subject that covers quite a bit of territory.
Opuntia as fodder- a worldwide study
Wow, that is detailed for sure. I heard of a guy in Texas that supposedly worked really hard breeding a spine/glochid free Opuntia and tried to make a go of it some 50-60 years ago, but it was just too different and it never caught on. Don't know if that is true or not...
All Cacti are succulents, but not all succulents are Cacti