When she's in the barn being a cow, she loses Ego and Temper every day unless she's S+ trained. Are you suggesting frequently pulling the cow out of the barn to try and train Ego & Temper back up?
B+ cow "skill", with a farmhand, is the minimum to have a cow not incur negative mood impacts from staying overnight in the barn. There's no reason not to continue training the cow up to S+, and until that time, you don't
need to leave her overnight in the barn. Expectation is that you will take the cow out for training, as needed, yes. The fact that the cow each day has a full energy bar and spending a night in the barn doesn't have severe negative effects when the cow is trained means that the cow is really an extra "active" slave that you can train, who also happens to be a cow. At S+ the only negative of overnights in the barn is intellect drain. Intellect is not a factor in milk production nor in the value calculation when selling a cow. So, you can ignore that, or you can take her out periodically and give her lessons to build up her intellect again (clerical, rhetoric, etc.) if you want to keep a smart cow. If you take her out at the end of every decade and have her take care of the accounting, that alone will make up half of the intellect drain.
Impregnating the cow yourself has similar ego and temper decreases.
If I recall correctly, assuming S+ master penetration skill, you only need to impregnate her once every couple of days to maintain optimal milk production. So, -5 ego and temper per decade. She can gain +10 ego and temper by winning a battle in the arena once per decade. Being mature and chubby doesn't preclude arena victories. You may find it helpful to equip her with some good armor for the battles. Of course, if she gets injured, that's another cost, but you can use healing balm from alchemy or let her heal naturally (farmhand medical skill contributes). Scarring doesn't reduce the value of a cow.
I also believe that the activities that raise Ego & Temper undo Cow training (e.g. Gladiatrix).
Yes, non-cow lessons will erode her cow training, but you can also train that back up. It's a balancing act. The other skills do not degrade. She won't lose her gladiatrix skill after learning it. Cow training isn't "forgetting other skills", it is "acceptance of being treated like a cow and living like a cow".
So unless you can get super lucky by purchasing a slave who already has enough cow training that you can get to S+ before she becomes mindbroken, it looks to me that having a sane S+ trained cow is impossible in practice... And impregnating her yourself will eventually break her.
Even if you skip arena battles (which allow you to enter multiple slaves each decade, with high enough reputation, so it's practically expected that you will take advantage of the opportunity to have your cow compete), there are other ways to build up nature (ego) and temperament. Orgasms raise temperament, for example, and presumably she'll have some when you inseminate her. Assuming she orgasms at least once every time you inseminate her, you should break even on temperament that way.
As for nature (ego), again assuming you are skipping arena battles (why would you?), sending her to the golden cage as a reward three times per decade, or giving her five martial arts lessons, will make up for five inseminations per decade. Or you can take her to spar (and win) at the colosseum five times per decade, which will raise both temperament and nature. In short: if you simply take the cow out for arena battles and not otherwise, that's enough to keep up everything except intellect; if you dislike arena battles, you can still keep everything up by taking her out of the barn now and then. Also note that low nature
improves the value of a cow, so if you plan to sell the cow and you're skipping arena battles, you can just let it go down. She won't mindbreak if her temperament stays high, even if her nature drops all the way to F-.
As far as my gameplay testing has gone the theoretical maximum milk production you assume does not seem to be achievable in-game. It also relies on a fiend and fiend-milker, which require additional investment and upkeep. Do you have a save with a profitable cow?
Fiend doesn't require much upkeep and lowers costs for other slaves too, if you're feeding them from it; it more than pays for itself in cost savings. It also gives you alchemical ingredients that further lower your costs and allow you to win consistently in Fogs battles, which gives you another source of income.
Selling the fully trained cow does not make sense to me. From what I understand, the point is to get her to produce more money than she costs and thus eventually earn back the large initial investment and make money from milk (or net saving if you avoid buying it). If you sell her you just have to start over with another initial investment of slave purchase/training/cow gear/etc.
The point is to make money from milk, yes, but you can also make money by selling cows. You can wait to sell her until you have another slave trained up enough to take her place. At the upper end, a cow can be worth more than 5000 sparks, which is equivalent to 33 decades of milk profit assuming my 150/decade profitability figure is accurate. Clearly you can make more sparks faster by selling a great cow than by keeping her for milk. Of course, a great cow could be even more valuable in the regular slave market, depending on other skills etc., in which case you might make even more sparks selling her at auction or directly to a client. Or you can just keep her, probably make less sparks in the long run by only having the profits from her milk, but save
yourself the effort of training another cow. In the end, as long as you can make more sparks per decade from her milk than it costs to keep the cow, you will eventually bring in more than whatever amount you invested up front. If you take advantage of the arena too, the break-even point will happen sooner.
