I sprinkle dried fish powder on their feed. It's 60% protein, and contains lots of minerals and micro-nutrients.
It is often sold in gardening stores as powdered fish fertilizer, but you can also get feed grade. (I've been told that they are the exact same thing, just a difference in labeling requirements......... and price of course). If you can't find it, your feed store should be able to order it for you.
I pay $50-some dollars for a 50-pound bag, and adding just 2 pounds of fish-meal per 50-pound bag of feed will increase your overall protein percentage by over 1%