[superuser2]

If that were the case we should expect health problems from horses that go too long between pregnancies, yes? Otherwise the nutrients are not "vital."

[jessaustin]

Whence do you suppose the nutrients in a placenta originate?

Actually everything in this thread is a just-so story. It's pretty silly to pick out one specific rare behavior as even relevant to evolutionary fitness. How would any of this be tested?

[peejaybee]

I dunno, if you have been gestating a foal, you might be a little undernourished at the end of the process, so a placenta's worth of nutrients might then be "vital."