Here's an idea for a new feature: Sacred Cows. Or maybe just one would be enough, Perhaps it could be one of the final achievements on the Ascension screen.
A Sacred Cow produces... what else but... Milk! Milking the cow could be done by spending a sugar lump, sacrificing buildings or some other thing that involves a penalty. There would have to be a minimum time restriction between successive milkings, because the cow takes time to produce more milk. This milk would go into the reservoir along with the milk gained from achievements, and would therefore add to the power of the kittens.
While on the topic of milk, I think it would make more sense to show the amount of milk in litres rather than %. I find the % figure meaningless. % of what?
I can think of further possibilities: enhancements/upgrades to the cow to increase milk production, such as Grazing Fields of Elysium, or Pampered Menial.