What length and size of needle should be used to vaccinate a calf?

It depends on what vaccine or treatment you are giving and the mode of administration and weight of the animal. As always, follow the directions that are on the label and how it is to be administered, whether in the muscle (intramuscular, I.M.), under the skin (subcutaneous, sub Q), or in the vein (intravenous, I.V.). The length depends on the route of administration. For an I.M. injection consider a needle length of 1 inch; for I.V., consider 1.5-inch length; and for sub Q. consider a 0.5- to 0.75-inch needle. Depending on how fluid the material is, you may consider a 16 to 18 gauge needle. Follow Beef Quality Assurance practices which states that all injections are to be given in the neck, and if given I.M. or sub Q, inject no more than 10 mls (cc) in any one site.