Honestly, I simply use the milk to supplement my mansion's food income and instead use semen and piss as my primary sources of gold income. This was a fairly decent way to earn gold, especially if you had a stud breeding your cow pretty much every night. Your farmhands would automatically extract leftover cum that had been deposited into your cows, in addition to whatever cum the cow herself provided. Unfortunately 0.9.5 kind of nerfed the crap out of the Breeder playthrough, so I've only barely dabbled with the farm so far on my other playthroughs. I'm not sure yet what the optimal gold making strategy will be overall.