172 John Hammond, Jr. 



levels. This treatment has the effect of shortening the cycle slightly (2, 62, 

 76), but does not appear to afTect the length of estrus (62). 



Treatment with a large dose, repeated at heat, reduced the fertility of a 

 flock allowed to lamb (62), but smaller doses have raised the lambing per- 

 centage, without at all affecting the conception rate, in a series of 1200 

 treated ewes (33). However, a small dose can seriously affect conception rate 

 (76). 



Ewes injected between the 12th and 14th day of the cycle had a high 

 conception rate, but those treated on the 10th and 1 1th day did not. Wallace 

 notes that the effect was even more marked if expressed in terms of interval 

 between treatment and heat: for intervals of over 5 days conception was very 

 poor. It should be stressed that any effect of treatment on cycle length was 

 very slight. Though the effect of treatment on the non-pregnant animals is 

 unknown it may reasonably be presumed that they did not have multiple 

 ovulations, because in those pregnant (which includes the great majority of 

 those served within 5 days) the maximal average ovulation rate, of just over 2, 

 was found in those on heat at no more than 2 or 3 days after treatment. 



Thus there is no obvious ground for assuming disturbance either of the 

 time relationship between heat and ovulation, or of fertilization or tubal 

 transport associated with multiple corpora lutea. With these excluded, 

 there remain failure of ovulation or ovulation of defective ova. If the latter 

 be accepted for the sheep, it may well apply also to the cattle results already 

 quoted (7, 25). 



AFTER PROGESTERONE TREATMENT 



In the absence of the bull, heat is not easily detected in beef cattle suckling 

 calves. Treatment by injection at a known stage of cycle is therefore not easily 

 practicable. Even when heat is detected, individual variability hampers 

 accurate prediction of the next estrus. Prolongation of the cycle by pro- 

 gesterone treatment therefore may be of advantage to ensure more precise 

 timing of the endogenous ovulatory discharge. Casida and colleagues have 

 prolonged the cycle by daily injections of progesterone in oil (sheep, 28; 

 cow, 14). With high enough dosage ovulation is inhibited, but follows 

 cessation of treatment. With lower dosage there may be formation of cystic 

 follicles. After a single treatment with crystalline suspensions there has 

 been irregularity about the time of onset of heat, silent heats, low fertility 

 after mating, and a marked tendency to formation of cystic follicles (22, 24, 

 55). Even with daily injections in oil there has been some abnormality, 

 principally increased incidence of silent heats (29, 46, 74). Clearly careful 

 control of dose level and of the decrease in blood concentration are necessary 

 to achieve normal heat and ovulation. 



Robinson (68) got very good synchronization of estrus and ovulation (90% 

 estrus within 24 hr) with daily progesterone followed by PMS. In this 



