You can find many experiments regarding fertility using sponges. Regarding the milk production, sorry I don't know the precise english verbiage. Here is a paper in French regarding the “extended lactation” practice. It's from a professional body, with real-life testimonies: https://vienne.chambre-agriculture.fr/fileadmin/user_upload/...
> Keep in mind the other questions as well, if you please. How long do they live, compared to a life in nature? Don't guess, actually ask him.
Probably much longer, given that they don't have to face predators, and the humans provide food in winter and medications for them.
> Giving birth is indeed a part of life, but forcible insemination is not. Do you think most milk doesn't come from forcible inseminations?
It does come from insemination – extended lactation allows you to reduce insemination. However, goat milk is not an invention of the modern time – you can also let males do their thing with the goats.
> I doubt you've spent much time on real farms, instead of hobby farms.
Thanks to a long experience in cheese making, which sells at a premium, the French can make a living out of small herds. I grew up chasing the semi-wild Charolais beef living the best of their lives in the Morvan's pastures. I happen to have a few friends who make a living out of cattle farming, so it's definitely not a "hobby" for them. Sorry if it's not the case for you, maybe it's a time to rethink your business model?