
All vaccines unless stated need to be given under the skin hygienically.

I use 16g by 3/8 needles as I find there is less risk of getting it in the muscle with this slightly shorter needle than the common half inch needle.

Regularly change your needle to ensure it is clean. Don't use meths to clean the needles as it may interfere with the vaccine. Take the used needles home and boil them in water to reuse next time.

Abscesses like this occur from dirty needles and herds having the same needle used for 1000 cows!!! This is a welfare issue.
Don't vaccinate in the rain.

Most recommendations are to give them in the anterior half of the neck.

Although not recommended in my opinion it is practical to apply in the tail notch in a dairy cow. BUT YOU MUST ENSURE IT GOES UNDER THE SKIN AND NOT IN THE MUSCLE.
To ensure this use 3/8 needles. Push in through the skin. Then pull back to ensure you are not in muscle and give dose.