Hormonal Augmentation of Fertility in Sheep and Cattle 167 



time of onset of heat was erratic. Those that came on heat also ovulated and 

 had a normal number of corpora lutea; but many of the others also ovulated, 

 and these had an appreciably higher average ovulation rate. 



Testosterone given at the same time as PMS, or one day earlier, tended 

 to block ovulation, and cystic or luteinized follicles were common (61). 

 Testosterone thus appears to share with progesterone the property — more 

 apparent when results in the breeding season are considered — of blocking 

 an ovulating release of endogenous gonadotropin. 



When prolonged treatment with progesterone is given, there is a distinct 

 tendency for ovulation and heat to follow cessation of treatment, even though 

 no gonadotropin is administered (23, 26, 65) — just as it may follow regression 

 of an induced corpus luteum. This might be explained by supposing that 

 progesterone, besides blocking pituitary release of hormone, also causes its 

 accumulation and hence a tendency for discharge when progesterone is 

 withdrawn. 



Almost without exception, ovulation occurs when PMS is given 2-3 days 

 after a series of progesterone injections (2, 26, 34, 48, 65). Raeside and 

 Lamond (60) got better results with this schedule than with PMS alone. The 

 incidence of estrus, as reported by these workers and by others (22, 66), was 

 generally 80-100%. Fertility, however, is more variable, and it is difficult to 

 know whether this is due to minor differences in method of treatment, 

 differences in the animals treated, or merely in quality of insemination. In 

 non-lactating sheep Dutt (26) found 50% of ova fertilized; Dauzier (22) and 

 Gordon (34) report conception rates near normal (70-80% and 70%) 

 and also definitely low (43%), while Robinson (66) obtained very few 

 pregnancies. 



There seems to be a definite difference in response of lactating and of non- 

 lactating ewes, which has been observed both with PMS twice at a cycle 

 interval (49), and also with a single PMS treatment following progesterone 

 (34). Both the incidence of heat in those treated, and the proportion of 

 those served lambing, is lowered by lactation. 



When a ewe lambs early in the breeding season there is a lactation anestrus 

 of about 6 weeks (34). When lactation is combined with seasonal anestrus it 

 seems likely there will be a greater degree of ovarian inactivity, and hence 

 possibly a greater average time interval between PMS injection and ovulation. 

 It may be that the time relationship between heat and ovulation is affected, 

 but this has not been investigated, nor have the frequencies of ovulation and 

 of fertilization been determined in the ewe treated while lactating. 



OVULATION IN THE BREEDING SEASON 



Midcycle and Pregnancy 



For a variety of reasons, induction of ovulation during the luteal phase of 

 the cycle has not been widely attempted. Russian workers had found that 

 12 



