166 John Hammond, Jr. 



In contrast to the frequent failure of heat to attend a second induced 

 ovulation, there is a marked tendency for heat to occur spontaneously at a 

 cycle interval after treatment (44). Furthermore, fertility to service at such 

 heats appears good; Gordon (34) had 1 1 lactating ewes which came on heat 

 in this way, and 8 of them lambed. 



Attempts have been made to obtain heat with ovulation by combined 

 treatment with estrogen and PMS. However, while estrogen alone will some- 

 times induce ovulation, given with PMS it sometimes prevents the ovulation 

 which might otherwise have been expected. Hammond (41) concluded that 

 estrogen caused a discharge of endogenous gonadotropin and produced 

 ovulation only in those animals which had already a large follicle, while 

 PMS would indirectly cause ovulaton, triggering a pituitary discharge with 

 estrogen from the ripened follicle. In animals whose follicles were small 

 at the time of treatment insufficient pituitary stores would remain to 

 ovulate the follicles when grown. Alternatively, a premature discharge may 

 render the follicles incapable of ovulating. He found cystic follicles far 

 more often with the combined treatment than with either substance given 

 separately. 



Besides interference with ovulation, there was the difficulty that heat and 

 ovulation were not synchronized; the latent period between estrogen and 

 heat was greater than that between PMS or estrogen and ovulation. Estrogen 

 induced heat more effectively in animals in which ovulation was blocked. A 

 regressing corpus luteum is normally required for estrus, but secretion from 

 newly formed corpora antagonized the estrogen. 



Robinson and colleagues (53, 64, 67, 69) have investigated estrogen 

 progesterone interaction in the spayed ewe. Progesterone pretreatment 

 lowers the threshold dose of estrogen needed to cause estrous behavior 

 and also shortens the latent period before heat is manifested; 50% of treated 

 animals came on heat within 36 hr of estrogen administration. Maximal 

 response to a dose of estrogen (which, given alone, was subthreshold) 

 required more than 6 days' pretreatment with progesterone and an interval 

 of 24-48 hr between the final dose of progesterone and administration of the 

 estrogen. It appears that progesterone produces transitory sensitization to 

 estrogen; the proprioceptor concerned is not in the uterus, as had been 

 suggested, because hysterectomy did not affect response. 



Estrus following a single injection of PMS has been obtained by pretreat- 

 ment either with testosterone (19, 61) or with progesterone (26, 63). 

 Progesterone has usually been given in oil over a period of 3-15 days, 

 followed by PMS 2 or 3 days later. In these circumstances the ovulation 

 rate has been normal (26, 34, 65). There was a tendency to multiple ovulations 

 when a single large dose of progesterone (75 mg) was followed by PMS 2 days 

 later (65). Robinson (65) reported great uniformity in the time of onset of 

 heat, but when Gordon (34) used suspensions of crystalline progesterone 



