THE NECESSITY FOR PROGESTERONE WITH ESTROGEN FOR THE INDUCTION OF RECURRENT ESTRUS IN THE OVARIECTOMIZED EWE

Abstract

Introduction WHILE considerable attention has been directed to the possible role of progesterone in the mating behaviour of animals, emphasis has been placed on the effects either of simultaneous estrogen-progesterone influence, or of progesterone following estrogen. The possibility that progesterone, produced by the corpus luteum of a preceding cycle, can condition an animal to respond to subsequent estrogen stimulation has received little attention. This is rather surprising, since in some species, notably the sheep and cow, ovulation without estrus is a common phenomenon (Grant, 1934; Weber, 1911; Hammond, 1946). At the onset of the breeding season in the ewe, the first ovulation is unaccompanied by estrus (Grant, 1934 and others). If ovulation is induced in the anestrous ewe, heat does not normally occur. If a second ovulation is induced 16 days later, estrus is exhibited by a high proportion of ewes which ovulate (Cole and Miller, 1953 and many others). Hammond Jr., Hammond and Parkes (1942), Hammond Jr., (1945) and Robinson (1950) have shown that the ovulation-estms response to gonadotrophin injection in anestrum, is markedly influenced by the presence and age of a corpus luteum in the ovaries at the time of injection. In 1945 Cole, Hart and Miller reported estrus in 39 of 48 ewes in which injection, in anestrum, with pregnant mare serum gonadotrophin (PMS) was preceded by 1 to 3 days by 40–100 mg. testosterone. More recently Robinson (1952, 1954a, 1954b) and Dutt (1952, 1953) have shown that when progesterone in injected for several days prior to PMS injection, estrus regularly accompanies induced ovulation.