We used to give our cows a Mu-Se shot prior to breeding at work-up time, in the fall, and prior to calving. The Mu-Se shot is excellent BUT it is only good for 30 days.
We were fortunate enough to have our cows tested for a research project. We ended up having to TRIPLE our dose of SE in our mineral program to get the cows where they need to be.
The shot will not HURT them by any means, but try to time it as close to calving as possible remembering it is only good for 30 days.