If you can only grow one Fodder plant for your small livestock, I would recommend comfrey. A long lived, easy to grow perennial, comfrey is one of the few plants that offer a source of vitamin B12 and is a great source of vitamin A. It’s strong, extensive root system mines below the soil level for essential minerals and concentrates them in it’s fuzzy, sage green leaves.
Dairy animals especially benefit from plants with high mineral content, such as comfrey, as the act of producing milk is highly stressful on the metabolism. Most animals seem to find comfrey highly palatable and will eat it readily. If this isn’t enough, comfrey can contain up to 30% protein, although around 22% is more common.
Comfrey is most easily planted from roots or divided plants. It is not picky about soil, in fact, it grows eagerly on my rocky, poor hillside. Once the plant is established, very little is needed to keep it happy. You may need to water it during times of extreme drought, but that is pretty much the extent of care it requires.
Comfrey is also an attractive plant with lovely pink or purple flower stalks that attract bees and butterflies. This is especially nice in an urban area since it can be planted as part of a landscape arrangement and it won’t draw too much attention to the fact that you aren’t doing something ordinary with your little slice of urban heaven.