Rennet is basically minced calf stomachs - that is the "natural" ingredient that was used for hundreds of years to produce cheese.
Yes, cows are not killed for rennet alone.
Cows are also not killed for milk alone.
People with leather sofas will tell you that vows are not killed for leather alone, either.
And I am sure there will be also people who argue that cows are not killed for meat alone.
It is the sum of all these "components" of the cow that makes killing cows a viable business.
This was basically my thinking, that cows are not killed exclusively for rennet. This is probably true. I also believed in the second line that you have there, that cows are not killed for the dairy industry alone. I no longer believe that.
I took a MOOC (Massive Open Online Course) offered by the University of Edinburgh through Coursera on Animal Behavior and Welfare. The course deliberately did not delve into animal ethics--whether it is right or wrong to use animals for meat, dairy, eggs, etc., keep them in zoos, or keep them as pets. The main concern of their field is to study the welfare of animals, basically trying to improve the quality of life for animals at each stage of the process. It was supremely scientific. Anyway, of course there was one unit on farm animals. They discussed the "production cycles" of dairy cows, egg-laying hens, broiler chickens, sheep raised for meat, and even salmon. Based on the narrative they presented, it seems like slaughter is an inherent part of any animal agriculture, necessary to the profitability of the enterprise. For example, with male calves of dairy cows, if the price of beef is very low, the farmer might just shoot the calf instead of sending to to slaughter to be used as veal.
I know that this is not really your point. Your point is that the slaughter industry is an integrated whole, and the use of a particular animal for one thing makes it more "logical" to use that animal for other things. Still, before this lesson about production cycles, I did not realize how integral killing animals is to every branch of animal agriculture. Of course, I had been told vaguely about it on forums like this one, but the ethical viewpoint and general vagueness of the information made me doubt it's complete veracity. Now I know that even on the nicest family farms, organic farms, "free-range" egg farms, killing animals is a way of life. There was also a very matter-of-fact presentation of how animals are transported to slaughterhouses and the process within slaughterhouses. The basic and factual presentation, even without any images of blood and gore, was more disturbing to me than any PETA video I'd ever seen. They were even focusing on the
best practices in slaughter and EU regulations that are meant to improve animal welfare. Maybe because I was always suspicious of videos that were blatantly trying to convince me, but also that were trying to show the worst of the worst--employees the animals, etc.. This is why I decided to vegan.
Now, what would happen if the whole world was lacto-ovo vegetarian but didn't eat any direct products of slaughter? I don't know. This seems like an absurd hypothetical of the type that we veggies hate. I imagine that as long as it's profitable, animals will be killed for whatever people will buy, even if most or all of the body is wasted. Besides, we keep breeding our companion animals who will eat whatever low-quality meat the industry produces.
