Ask yourself why did these cows lose calves in the snowstorm and other cows with similar age calves got through the snowstorms ok?

If there is some definite reason these cows lost calves (not as much shelter, etc) compared to the others then I would maybe give them another chance.

But the risk is you are creating a less hardy herd over the long run.  By keeping only those whose calves survived the snowstorms you can't help but develop a hardier, fertile herd for the future.

If it's a tossup I would sell them. jmho.

Jim at Dawn