@inatizzy: Here is an excerpt from an article pertaining to the “glue factory” reference:
It’s mostly used as a figure of speech these days, but yes, horses typically were sent to the glue factory or rendering plant back in the days when they were mostly used as work animals. These days, it’s more common (an undocumented source says 90% of all domestic horses) for unwanted horses to be sent to a slaughterhouse if still alive, or a rendering plant (AKA the knackers, the knackery) if deceased.
Animal (origin) glue is made from connective tissue, found in hoofs, bones, tendons, ligaments, and cartilage in vertebrate animals.
Rendering plants are the recycling links in the food chain. They take fat and bone trimmings from grocery stores, waste scraps from restaurants, and dead animals. They cook the meat and fat products together and grind it up. It’s used for animal feed and non-edible products like soap, various lubricants, and of course glue—the heavy kind that’s used to glue furniture together, for example.
But as I say, the rendering plant isn’t the only possible destination for a horse that’s outlived its usefulness. Horsemeat is considered a delicacy in some countries. Some slaughterhouses will happily accept and process horsemeat if it’s commercially feasible. I’ve read that they buy horses in the United States for about 50 cents a pound and sell the horsemeat overseas for about $15 a pound. The Bureau of Land Management periodically comes under fire for its wild horse/burro adoption program as reports of “adopted” animals going to the slaughterhouse surface. See www.cnie.org/nle/biodv-33.html for more information.
Some other sources I used in compiling this report:
www.usnews.com/usnews/issue /981109/9hors.htm
www.resolutionprod.co m/mcmurry/sec02/02-eanimals.htm (This site contains the memoirs of a man who was raised on a family farm, and I found it fascinating.)
There are many references online, this came from The Straight Dope.