Left to right, the plants are youngest to oldest. You can see the one on the left is just coming in. When the time comes, I'll remove the plant on the right and move the other two to the right. I'll cut the grass off of the old one and rework its soil. Then I'll plant a new "crop," and put the container at the left on the dresser. I recently amended the soil in all three containers with slow release nitrogen: Osmocote http://www.osmocotegarden.com/
This is what I'm growing now, but that's because that's what the local grocery store sells:
They were carrying a different brand for years and then suddenly switched to this. This stuff seems to grow more quickly than the old stuff. In any case, if you google cat grass or kitty grass, you'll see that there are many, many different products out there. Oats seem to be the common thread amongst them.
Charlie really gobbles this stuff up. At least once a day, he's at the plants, eating.