Thiis is a situation the just plain sucks and we have been thru it. If there is any mold in the hay or grain you are feeding this can cause the slipped calf. Our Vet suggested we hit the cows with a high level of CTC in their feed for a week, yes this gets pretty spendy, but so is losing some top end calves. Like one of the other posters said there are MANY reasons that a cow will slip a pregnancy,,,a fall on the ice, at bump at the hay feeder,,or nature just decided that there was a problem and deciced to end it. When we make the choice to have livestock we better be ready to loose some,,,good management helps but sometime we just have to put it down to "stuff happens"