I grow cover crops wherever the chickens have access.
Alfalfa, clover, barley, wheat, oats, buckwheat, peas, radish, beets, turnips (especially 7-top).
IMHO, they're not fond of grass unless it's newly sprouted. They like more succulent things like I mentioned.
Right conditions, most of them are easy. I forgot to mention sunflower as well.
Some just have different length of time for germination.
Here is a good guide for sprouting temperature. http://tomclothier.hort.net/page11.html