When giving injections to calves, always use subcutaneous administration if possible and the smallest gauge needle you can, advises Jim Reynolds, service chief of dairy production medicine at the Veterinary Medicine Teaching and
Smaller needles translate into less pain for the animal and less tissue damage. In calves, he recommends using an 18-gauge needle and replacing it after three or four injections. Needles develop a barb on the tip after just a few uses. This barb makes the injection more painful and also results in more blood, which, in turn, leads to more flies and potential problems.