A few days ago this chart made the rounds in the blogosphere. It shows, among other things, that eating lamb is much worse for the environment than is eating beef. A key part of the problem is that a greater percentage of the cow ends up being used for food, compared to the sheep. You therefore might be tempted to apply a heftier carbon tax, or “personal guilt tax,” to the lamb, but not so fast.
To the extent that farmers feed a whole big lamb and get a little squib of meat in return, the price of lamb is already correspondingly higher than the price of beef (where the tripe is sold to Italy, the cheese is sold to Kraft, etc.). Consumers are already internalizing this relative price difference between cows and sheep.
The correct response is to eat less meat of all kinds. It’s not obvious you need to apply a special tax (fiscal or conscience tax) to lamb, above and beyond the general meat tax which is called for.
You may reject a constant returns to scale or proportionality assumption and view the proper tax as a fixed mark-up on both beef and lamb. That still will lower the relative price of the costlier item.
In terms of animal welfare, a sheep is probably free range with greater probability than is a cow, which somewhat favors lamb consumption again in relative terms only.
You can make other assumptions and get other results, naturally. Still, in relative terms there is no prima facie presumption against lamb compared to beef.