During the last two or three years there has been an increasing trend developing in the hog belt among farmers, working with their veterinarians, to control and concentrate the breeding time of their sows. This is being accomplished with the use of the equine gonadotropin or pregnant mare serum.
"Controlled Breeding of Swine,"
Iowa State University Veterinarian: Vol. 13
, Article 5.
Available at: https://lib.dr.iastate.edu/iowastate_veterinarian/vol13/iss2/5