This is from the Merck Veterinary Manual.
Oxytocin is used to promote milk letdown, to treat agalactia, as an adjunctive treatment of mastitis, and to cause contraction of the uterus either to induce (or supplement) labor or to enhance postpartum uterine contraction and expulsion of uterine fluid or fetal membranes. It is administered parenterally (IV, IM, or SC). Oxytocin may be given intranasally but absorption can be erratic. Uterine relaxation is caused by ?2 -mimetic agents, such as clenbuterol. Such agents have been used for postponing parturition (to reduce obstetrical complications in heifers) and for facilitating obstetrical manipulations in large domestic animals. Clenbuterol use in animals is illegal in the USA.