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Agricultural Institute, Grange, Dunsany, Co. Meath, Ireland

(Received 2nd May 1974)

To be ultimately successful, techniques used to control the oestrous cycle of the cow will have to embody methods which allow the time of insemination to be arranged without reference to behavioural oestrus. This is particularly important in the case of beef suckler cows as it is difficult to detect heat in these animals probably owing to the suckling stimulus. This means that treatments used to synchronize the oestrous cycle willa lso have to give precise control of the time of ovulation or that other hormones will have to be given to control ovulation time. The aim of the experiment described in this paper was to obtain hormonal control of the time of ovulation following synchronization of oestrus with implants of progesterone (Roche, 1974a). This would allow all animals to be inseminated on a fixed time basis.

Eighty-four mature

Summary.

Oestrus was synchronized in twenty-four of twenty-seven heifers with implants having a surface area of 9200 mm² and containing 4 g progesterone. Fertility following natural mating was low with only 47% of mated heifers conceiving. Neither the number of matings up to four nor the time of mating during oestrus increased the conception rate. Reducing the period of administration of progesterone from 20 to 10 days by giving 5 mg oestradiol benzoate on the day of insertion of the implants increased the conception rate to normal but resulted in a lowered oestrous response. Injection of 400 μg oestradiol benzoate 16 hr after removal of the implants following a 10-day treatment period did not increase the oestrous response but in fact lowered the conception rate compared to that of uninjected controls. Implants capable of synchronizing heat in heifers were less effective in cyclic dairy cows.

The oestrous cycle of the cow is controlled mainly by the secretion of progesterone from the CL. Effective synchronization of oestrus requires control of the functional life span of the CL. Injections of oxytocin from Days 2 to 6 of the oestrous cycle (Armstrong & Hansel, 1959), injections of oestrogen (Wiltbank, Ingalls & Rowden, 1961), the presence of foreign bodies in the uterus (Hansel & Wagner, 1960), irrigation of the uterus with iodine solutions (Nakahara, Domeki & Yamauchi, 1971) or administration of LH antibodies (Snook, Brunner, Saatman & Hansel, 1969) all cause premature luteolysis in cattle. Few of these methods of inducing CL regression, however, have been incorporated into synchronizing treatments. Administration of progestagens for the length of an oestrous cycle to suppress oestrus and ovulation, without reducing the functional life span of the CL, has been the most widely used method of synchronization. Following this method, fertility to natural

Summary.

The reasons for a reduced oestrous response in heifers following a 9-day treatment with implants of progesterone were determined. Progesterone implants were inserted into Hereford cross heifers for 9 days at different stages of the oestrous cycle. Oestradiol benzoate (5 mg) was also given at the time of insertion of implants. The oestrous response obtained was low in animals receiving implants on Days 3 or 17 but was high in those receiving implants between Days 5 to 15. Injecting 50 mg progesterone with the 5 mg oestrogen increased the oestrous response in animals receiving implants on Day 17 but the onset of oestrus was more variable following this treatment in animals receiving implants on Day 3 or between Days 5 and 15. Removing the implants after 12 days instead of 9 days, however, gave a precise onset of oestrus. The conception rate to A.I. was not different in heifers synchronized with a 9- or 12-day treatment from that of the controls but it was lower in heifers synchronized with implants of progesterone removed after 18 or 21 days. The high conception rates obtained at slaughter 30 to 40 days after the last insemination were substantiated by allowing a group of heifers to calve after synchronization with a 9-day progesterone treatment.

Summary.

The time of ovulation was determined in twenty-nine heifers by laparoscopy following treatment with progesterone. In addition, twenty-five of the animals received 100 μg Gn-RH 30 hr after the end of the progesterone treatment. Only eight of the heifers treated with Gn-RH ovulated. This contrasted with results obtained following examination of ovaries after slaughter in similar animals treated in identical fashion. Heifers which were treated with progesterone for 12 days and then slaughtered had not begun to ovulate 65 hr after the end of treatment, but five of eleven animals had ovulated 4 hr later. When Gn-RH was given, ovulation had started 60 hr after the end of progesterone treatment, 60% of heifers had ovulated at 65 hr and 90% at 69 hr. Treatment with Gn-RH can synchronize and hasten the time of ovulation in heifers given progesterone. Possible reasons for the discrepancy between the results obtained following laparoscopy and slaughter are discussed.

Agricultural Institute, Grange, Dunsany, Co. Meath, Ireland

The widespread application of control of oestrus in cattle using progesterone requires a simple practical method of administering the steroid. The retention rate of intravaginal pessaries in cows and heifers has been variable (Carrick & Shelton, 1967; Wishart & Hoskin, 1968; Scanlon, Sreenan & Gordon, 1972; Sreenan, 1974) and is, therefore, an unacceptable method at present for routine administration. Since the demonstration that steroids can be eluted from silastic rubber (Dziuk & Cook, 1966), silastic implants containing progesterone or progestagens have been used to control oestrus in cattle (Scanlon & Burgess, 1971; Roche & Crowley, 1973; Roche, 1974), but the large surface area required to release sufficient progesterone to block oestrus in cattle does not favour their widespread use. However, silastic vaginal rings containing a progestagen have been shown to suppress the mid-cycle peak of LH and ovulation in women (Mishell, Talas, Parlow

Summary.

Two injections each of 500 μg Cloprostenol (ICI-80,996), a synthetic analogue of PGF-2α, 11 or 12 days apart or pretreatment for 7 days with progesterone from an intravaginal silastic coil and one injection of 500 μg Cloprostenol were both effective in synchronizing oestrus in heifers or nursing beef cows. After two inseminations at 72 and 96 hr after the end of treatment, the calving rate for cows observed in oestrus after both treatments or for cows that had oestrous-like mucus after the progesterone+Cloprostenol treatment did not differ from that of control cows, but was significantly (P<0·025) lower for cows diagnosed as ready for insemination by the characteristics of mucus after the two injections of Cloprostenol. Treated cows diagnosed per rectum as having inactive ovaries had a significantly (P<0·005) lower calving rate than those diagnosed as having active ovaries at the start of treatment. Significantly (P<0·005) more treated than control cows were inseminated and became pregnant in the first 15 days of the treatment period, but the overall level of reproductive efficiency was low.

Summary. Heifers between Days 6 and 10 of the cycle were allocated at random to groups of 8 and treated with (i) a 4% progesterone-releasing intravaginal device (PRID) + oestrogen capsule for 12 days; (ii) 4% PRID for 12 days; (iii) 20% PRID for 12 days; (iv) 4% PRID for 14 days; or (v) 20% PRID for 14 days. Blood was obtained daily during treatment and at 2- or 4-h intervals for 72 h after removal of PRIDs. Some animals were sampled every 20 min for 4·67 h on the 3rd day after PRID insertion, and 1 day before and 36 h after removal of the PRID. During progesterone treatment there was: (i) no correlation between concentrations of progesterone and LH within days; (ii) a significant negative correlation between progesterone and days (P < 0·01) and also between progesterone and LH over days (P < 0·01); (iii) the overall correlation co-efficient between LH and days was positive (P < 0·05). The amplitude of LH or FSH episodes was not affected as progesterone concentrations declined during PRID treatment, but the number of LH (but not FSH) episodes was increased (P < 0·01). After PRID removal, the amplitude of both LH and FSH episodes increased (P < 0·01). We suggest that progesterone is part of a negative feedback complex on LH secretion in cattle and that this effect is apparently mediated through frequency of episodic LH release.

Summary.

Oestrus and the time of ovulation were controlled in eighty-seven mature non-pregnant heifers by using silastic implants of MGA followed by an injection of HCG 48 hr after removal of the implants. The heifers were allocated at random to three groups and inseminated with frozen semen before the estimated time of ovulation. Animals in Groups 1 and 2 were inseminated once at 14 and 24 hr, respectively, after HCG while animals in Group 3 were inseminated twice 14 and 24 hr after HCG. All heats observed after insemination were recorded until the animals were killed 55 days later, when 23% of them were found to be pregnant. The conception rate was not affected by changing the time or the frequency of insemination. Possible reasons for the low fertility are discussed.

Oestrus and ovulation can be controlled in ewes during the breeding season by administering progestagens (Lamond, 1964). However, fertility at the controlled oestrus (40 to 60%) is generally low, regardless of the particular progestagen or the mode of administration (Robinson, 1968). The physiological basis for this subfertility at the controlled oestrus is not clear at present.

Fertilized eggs transferred to ewes previously treated with progesterone developed normally (Schmidt, 1961; Shelton & Moore, 1966), indicating that the capability of such uteri to sustain normal growth and development of the embryo was not adversely affected. Quinlivan & Robinson (1969) found that the pattern of distribution and survival rate of spermatozoa became progressively altered at the first oestrus following progestagen withdrawal when compared with the pattern in non-treated control ewes. A significant decline in the cervical population of spermatozoa occurred