More than 20 years ago, when we were first starting out, we visited Little King Farm. Marianne Eberth told us about a year when they first starte in which they had lost a lot of foals for no apparent reason. What they determined was that the mares kidneys were stressed due to being fed alfalfa, which is very high in protein, and a high protein feed. Perhaps your mares were stressed in this way as well. It might also go along with the thickened placenta. Since your mares were getting more alfalfa this year, particularly in late pregnancy, maybe it was a factor. Just a thought to consider.