A year ago we started growing comfrey. I had a few plants come up, the plant photo below came from our garden in late April. By July the plants were huge. 
These plants soon became a major part of our chickens’ diet. Now we have sheep and I’d like to grow even more comfrey to feed them as well.
Comfrey has been used for a long time to supplement animal feed. It’s protein content is over 20%, higher than most hay and grain options. It also has a number of vitamins, including B12 which is typically not found in plant matter. It’s also a good source of vitamin A. High in minerals and high in allantoin, it is said to help brighten the yolks of eggs from chickens eating some comfrey as well as help milk production in dairy animals.
If you believe as I do that the real climate change is the fact that we’ve entered into a grand solar minimum (NASA and NOAA agree that we’ve entered a GSM), we can expect issues with growing seasons. Right now in early 2021, the USDA has put out charts indicating that our available grains have dwindled significantly. Soybeans are especially expensive and stocks are declining as China keeps making huge purchases. Corn is a similar story. These are two staples in livestock feed. If we get to a point where we cannot source these legumes/grains, what will we feed our livestock?
With only two acres, our little homestead has no land to set aside for hay growing. But comfrey is so easy to grow, and, I’ve read, can produce up to 100 tons per acre (that’s plant material, not dry weight), it’s something than you may want to look into.
Note: We grow the Bocking #4 variety which is one that animals love to eat.