Michael Pollan, in his Omnivore's Dilemma, talks of corn as some sinister evil out to take over the world. While I can admit that corn is likely overused (especially in people food), it is not the big evil Pollan makes it out to be. There is a time and a place for high protein energy food in an animal's diet. Sometimes that can be corn. We also use oats on occasion, depending on what we have available.
If you look closely at the background of the above picture, you'll see what our sheep eat most of the year. They are either out on pasture, eating grass, or in the barn, eating hay. Hay is just grass (and other green plants) harvested at the peak of nutrition, dried thoroughly (so as not to spoil) and baled for easy storage. Sheep were designed by God to eat grass and other green plants. Their digestive system works best when their diet is primarily grass-based.
There are times, though, when they need a little extra. Pregnancy and lactation are times when we consider using grain as a supplement to the grass-based diet, to provide adequate nutrition for both ewe and lamb(s). It is POSSIBLE to do this completely grass-based. But it is hard. And you run the risk of damaging the health of the ewe if you don't provide enough nutrition.
And after all, corn and oats and other grains are just the seeds that form naturally on "grass". It's still the product of green plants--just in a different form. So we use it carefully, along with the hay, to supplement.