Lepto or Vibrio can both cause these, as can Neospora Caninum, <A HREF="http://www.Jefferslivestock.com" TARGET="_blank">www.Jefferslivestock.com</A> sells vacine for all of these. The best way to tell what you have is to draw blood on the female soon after abortion is seen and have your vet send it off to the lab.
The Neospora Caninum is one I have had to deal with, it is about $3.00 per head per vaccine, compared to calf loss it is cheap. But really you want to identify the cause, "Cattle Master 4+ VL5" covers all of your lepto vibrio, as well as "Stay bred", but the Neospora Caninum is a separate vaccine.
Find out from your vet what diseases are prevalent in your region.
Have you changed their pasture lately? Sometimes there are poisonous weeds on the property. Also, I wonder if you took them from a dry pasture to a rich, lush green pasture. I don't understand grass tetany, but it may figure in this.
Our pastures are nearly weed free, very clean Coastal, Burmuda, Tifton 85, and some Bahaia. Grass Tetany, can occur on Rye or Wheat and happens after a frost, if cattle are removed from pasture to allow stress to move out of grass the cattle will not be affected; but if they are not I think you can treat with an IV infusion of CalDex, a calcium dextrose soln. But must be done ASAP or they die, simmilar to milk fever, same treatment. I think, consult a vet! Any way I use the lab to identify our problems, and we have Neospora Caninum, not common but found in some areas to be a big problem. We are in a heavily forrested wild part of Texas, with tons of wild life, and many Cyotes, the culprits for passing the problem. Central Texas, best I can figure, but its not exact.