Might've been pointed out to you but the quest that seems to be for milking Water cows actually says it's for Metal cows. Same thing for the one that seems to be saying it's for Earth cows. And it isn't really explained what Energy does. Anyway, I really like this game. Big fan of both Plants vs Zombies and similar tower defense games, and cowgirls, and breast expansions, and, well, most of the stuff this game has going on TBH.
If you're looking for suggestions, the thing I would most want is there to be a gallery implemented where you can see the sprites of all the cow types you've seen close-up. On the farm, the cows are pretty small, and in battle I don't have time to appreciate them (and they're still pretty small). Another suggestion that seems simple enough: a quest button. It'd make it a bit more obvious for those who, like me, missed that part of the description and were confused on how to get more potions. Another thing, I think it'd be cool if, later into the game, either you could choose to go into a larger, longer map in order to have more time to make money, or the map size would just grow over time. One of the things I like about PvZ, and tower defense games in general, is that you can build up your defense and see the difference in power level from beginning of the level to the end, and these short levels don't have much room for that, or much strategy at all, really.
I really like this game overall though. It might be pretty simple, but a lot of the simple mechanics, like your in-level currency being tied to the currency you use for buying new cows and (i assume) milking them, and the fact that Milking your elemental cows is required for quests but it also upgrades them too, and also also that upgrading cows isn't a straight upgrade, since it increases costs too. This game is definitely going on my (mental) list of "games to play again as soon as they update". Which includes a surprising amount of cowgirl-related games, oddly enough.