The alfalfa thing isn't to avoid ketosis, it's to avoid Milk Fever, or hypocalcemia. There are differing opinions on this, but I'll tell ya mine.
Most dairy producers (cow) feed low calcium diets through gestation to make the cow learn to mobilize calcium from her own body. This helps stop milk fever from happening during parturition, because when cows get that huge milk let down, they need a LOT of calcium. If they don't have enough, it causes milk fever. Calcium is needed for proper muscle function, so when the cow lacks enough calcium, she won't be able to walk/stand/eat/etc. The calcium goes to the milk first, and to the cow's muscles second. If a cow has been fed a low calcium diet prior to parturition, she'll be able to properly mobilize calcium from her bones and body, and not rely on calcium in the diet (which may not be enough to support her milk let down). After the cow freshens, THEN you add alfalfa (or other high calcium forages) to the diet to support her milk production.
I feed my animals this way, and every farm I've worked on (sheep and dairy and beef) feeds this way as well. There are differing opinions saying that you should feed alfalfa, but I don't agree.
I've also never seen milk fever in goats or sheep, but I have treated many a cow with milk fever. I just don't think the average small ruminant produces enough milk to cause issues, usually it's the high producing cows that have issues.
Just my $.02.
