Ground beef smells slightly sweet? I opened up a package of ground beef today that I had bought the day before from a reputable grocery store.  I left it out about an hour to warm up a little.  It was grass-fed, which I don't normally buy but do sometimes.  This time I immediately noticed a slightly sweetish smell--possibly like Worcestershire sauce, but hard to place.  Not really bad, but slightly stronger than ground beef normally smells.  Could it be something in the cow's diet, or have I bought meat that's too old to eat?
Grass-fed beef does certainly smell different from corn-fed beef. I haven't noticed it smelling "sweeter", but it could smell gamey, earthy, or even sulfuric if the cow got in to some wild onions or garlic. And there's plenty of variation between different types of grass and even the same grass variety from season to season. 

Corn-fed beef is not going to have that because, well, there isn't much variation in animal feed corn.

Rotten meat has a distinctive smell and it's going to be the same smell in grass-fed beef.