In this threadseveral people pointed out that modern processes really do allow meat packers to use damn near every part of the animal.
Are there any parts of a typical cow or hog carcass that are simply unusable, even today? What stuff is so unsavory that it can’t even be turned into pet food or fertilizer?
The “moo” is about it. The various beef cuts are obvious but the manure is used as fertilizer and cow farts even contain a surprising amount of energy because of methane that is possible to recapture. Any byproducts that are left over after slaughter and butchering are sent to a rendering plant where they are used in a wide range of products. The same thing holds true for pigs except for the farts.
In a previous life, I worked in a packing house.
The only part of beef they haven’t been able to use is the last exhale of breath.
There’s “can be used” vs “actually is used”. Also what do you mean by “used”?
Technically blood and the paunch (digestive system contents) can be used. E.g., the latter can be composted and spread on fields. But given the large size of many processing plants there just isn’t anyone willing to buy up all this stuff. It has to be disposed of. Sometimes it even gets flushed into sewers. A lot of the time the plant pays to have it hauled away. That means it’s not a product.
Is injecting waste under the soil of idle land “using” it?
The hides can be turned into leather. Technically the hair can be converted into protein products and such. But, again, it may not be economical to do that so it’s trash.
William Poundstone wrote a number of fact books on topics such as these. He claimed that the ends of the very largest bones, and some of the blood that dried onto the tables couldn’t be used. As has already been said, only the ‘moo’ can’t be used. I suppose your citation has a “practically” or “damn near every” to account for an occasional bit that just gets missed by whatever slaughterhouse circumstance arises.
If you go to a Argentinian parilla like La Estancia Restaurant the only things not on the menu are the hoofs and horns.
If you’ve ever had the good fortune to be present when an Italian family kill a pig a the only thing not used is the squeal.
As pointed out by** Shagnasty **, in a meat processing facility everything gets recycled.
I had a canned “moo” toy , it would ‘moo’ when you turned it over 
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Cow brains and spines are banned in some places. Even in pet food.
What did they use the horns for?
Why couldn’t the ends of the bones be used?
I know that butchers toss the bones. In a commercial setting they may be used for something, but local butchers have an excess.
Those bones are sold as dog treats.
Geez, it finally registers for me, the symbolism of those things in Delicatessen.
You know, I don’t ever recall seeing horns.
And they slaughtered 1,200 beef a day.
Maybe they were cut off before. Or they didn’t invite the bulls?
Some beef breeds are “polled”, i.e. have been bred to be hornless. Most others destined for feedlots will have their horns cut off when they’re young, as horns and lots of cattle in close quarters can have unfortunate results.
While cow gallstones are prized for use in traditional asian medicines, the gall bladders themselves don’t get much use, IIRC. Big green wobbly bits, they are. Don’t eat 'em!
Though I wager somewhere someone’s turning them into a powder for some sort of ointment.
I thought it was the grizzly bear gall bladders that were prized as an aphrodisiac.
If they had horns, perhaps they could be used like hooves, for gelatin?
I used to live in Sioux City, Iowa, which has a number of slaughterhouses, and just south of town was the Knox Gelatin plant, where bones were processed for their gelatin.
After that, the bone residue was sent on for further processing.
OK, what do they use gut contents for?