164 John Hammond, Jr. 



NORMAL REPRODUCTIVE PATTERNS 



The sheep has a restricted breeding season which is photoperiodically 

 regulated. Breeds difler in the length of breeding season and also in the degree 

 of ovarian activity during anestrus. There are dilTerences in the normal inci- 

 dence of twin and multiple ovulations in different breeds. The first ovulation 

 of the breeding season is not accompanied by estrus (36), and the time of this 

 ovulation may be hastened by the introduction of a ram into the ewe flock (73). 

 So-called "silent heats", ovulation unaccompanied by estrus, may also occur 

 during the breeding season, particularly in animals growing under poor 

 nutritive conditions (38). Heat without ovulation may occur at the end of 

 the season (I). 



The cow, on the other hand, though probably affected by photoperiod, 

 breeds throughout the year. Silent heats however do occur, and even anestrus 

 has been observed, mostly in heifers under poor winter feeding conditions. 

 The incidence of twinning may be as high as 4% (30), but in general it is 

 very low, especially in beef breeds. 



Ignoring considerable seasonal and individual differences, the cycle of the 

 sheep may be said to last 16 or 17 days, with heat lasting about 30 hr and 

 ovulation occurring at about the end of heat. The cycle in the cow lasts 

 20-21 days, heat lasts about a day, and ovulation occurs 10 hr or so after 

 the end of heat (16). 



It is not unusual to speak of "follicular" and "luteal" phases of the 

 ruminant cycle, but there is no event such as menstruation to demarcate the end 

 of the luteal phase. Grant (37) has constructed an average curve for the 

 growth and regression of the sheep corpus luteum, but size does not neces- 

 sarily parallel activity. Japanese workers (56, 57) consider the corpus luteum 

 to be non-functional if injected estrogen induces estrous behavior. Their 

 results appear to indicate that estrus can be advanced by 2-3 days, possibly 

 by more. 



When the corpus luteum is removed from the ovary, follicle growth, 

 estrus and ovulation follow, and the cycle rhythm is rephased. In sheep the 

 interval is 2-4 days (52); in cattle estrus most commonly occurs 4 days later 

 but a 3-day interval is nearly as frequent. If one accepts that this represents 

 the normal rate of follicle development one may conclude that, in the cow, 

 the corpus luteum regresses, and follicle growth starts, about 3 days before the 

 onset of heat and that in the sheep the interval is probably less. Alternatively, 

 normal follicle growth is slower and the two phases of the cycle overlap. 



The chances of survival of induced multiple pregnancies seem to differ 

 in the sheep and cow. In the sheep there is a loss of fertilized ova at and before 

 the stage of implantation (11, 62). There seems to be a maternal restriction 

 upon litter size at an average figure of, in general, less than 3. With very 

 large numbers of ovulations, Casida et al. (11) noted a tendency to total loss 

 after the stage of implantation. 



