Please see the article below for an introduction to feeding sheep. The article is very inclusive, but approaches feeding sheep from the stance of grass and commodities. Commodity combinations often come up short in terms of meeting animal requirements and can have harmful effects. For example corn gluten has a high sulfur content that can kill or disrupt the microbes in the gut that are responsible for breaking down feed stuffs into forms that can be absorbed by the sheep. Commodities can be used efficiently especially in the large scale operations that produce them, but for smaller family farms it often easier to use formulated feeds that meet all of the sheep requirements set by the National Research Council (NRC). The age of the animal, stage of production, and pasture conditions determine which and how much supplemental feed the animals will need. Just to mention, TDN is a measurement of energy and is not required on feed labels by federal regulation. Comparing two feeds with the same purpose and guaranteed analysis set by the American Association of Feed Control Officials (AAFCO) can be very different in terms of TDN and rates of gain because of the ingredients used. For cool season grasses TDN is highest when grass first comes out and the value continues to decline until it heads out at which it is at low level. When grasses come back out in the fall the TDN is high again and will decline through dormancy but never as low as heading stage, this fall growth is a great source of nutrition and is the stage where people can stockpile it for the winter. Grass has a substantially higher TDN than most hay and animals will do much better on stockpiled grass than on a hay based diet.
This outstanding and broad article from the Virginia Cooperative Extension is a perfect introduction to sheep nutrition: pubs.ext.vt.edu/410/410-853/410-853.html