Toss it if you re-use the needle. It's the needle that's been in a calf, and then back in the bottle that will ruin it. If the bottle only sees a new sterile needle, it should be OK. Make sure you don't drop it and get 'stuff' on the little rubber top.
The bottle top needs to be absolutely uncontaminated.
Toss it if you don't keep it cold ALL THE TIME. That means out of the sun and in a cooler or better yet, the fridge, between syringe fillings.
We will use a partial bottle if we only have a couple of calves to vaccinate, but we take just the number of doses we need, put them in a syringe with a brand new needle, and leave the vaccine in the fridge in the house.
Always drive the truck so you don't have to get out and open the gate.