Marion County is in central Florida. Here's the initial news coverage from FoxNews, and a more detailed story in The Gainsville Sun. Both stories note the deaths are suspected to have been caused by bad hay, but toxicology isn't showing anything definite yet.
EquiTransfer LLC is an embryo transfer facility standing Paso Fino and Gypsy Vanner stallions. According to their website they have over 900 mares (two farms, 37 and 80 acres respectively), and have accomplished over 1,000 successful embryo transfers in the past two years. They also offer breeding management, frozen semen, insemination, and semen evaluation. The site gives stats back to 2002, and since the vet in charge "performed his first embryo transfer in 2001," I'm guessing they've been in business about 6 years.
Scary and horrible.