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  • Author: G. H. STABENFELDT x
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Progesterone was determined daily in the peripheral plasma of six cows for a total of seven complete oestrous cycles. Progesterone levels ranged from less than 0·5 ng/ml plasma during the follicular phase to 6·6 ng/ml plasma (6·1 to 10·2 ng) at peak luteal phase. Progesterone levels in cows with 21-day cycles increased rapidly from Day 3 to Day 8 (oestrus = 1) with a much slower rate of increase from Day 8 to Day 17. These cows showed a progesterone decrease of more than 50% from the previous day on Days 18 (two cows), 19 (one cow) and 21 (two cows). Two other cows with cycles of 22 and 23 days' duration both had a similar decline on Day 20.

A variable time interval of 1 to 5 days was observed between the decline of progesterone and the occurrence of oestrus. These data indicate that considerable variation may exist among cows as to time requirements for follicle development and maturation.

Monitoring of peripheral levels of progesterone is suggested as a means of studying corpus luteum function.

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The inoculation of viable cultures of Vibrio fetus into the uterus of cows in the second and third 3-monthly periods of pregnancy resulted in abortion. All foetuses exposed during the second 3 months were aborted 5 to 7 days post-inoculation; foetal death occurred several days before expulsion. Peripheral plasma progesterone levels declined at the time of foetal death.

Cows injected with V. fetus during the third 3 months of pregnancy aborted 9 to 20 days post-inoculation and in a majority of cases delivered live calves. The decline of progesterone levels on the day of abortion is very similar to that observed before normal parturition. Progesterone levels in the dam reflect the viability status of the foetus. The decline of progesterone associated with abortion may be due both to placental dysfunction as well as luteolysis of the cl because of the release of products from the infected foetus.

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D. P. Neely, G. H. Stabenfeldt and C. L. Sauter

Summary. Daily injections of 150 units oxytocin administered to 6 mares on Days 4, 5, 6, 7 and 8 after ovulation (Day 0 = ovulation) failed to induce luteolysis as indicated by the maintenance of normal plasma progestagen concentrations and the occurrence of normal ovulatory intervals. Three additional mares were given oestrogen injections 24 h before an injection of oxytocin on Day 7 after ovulation, but this treatment also failed to induce luteolysis since plasma progestagen concentrations were maintained in all three mares. Two mares exhibited normal ovulatory intervals, while the third developed a corpus luteum which persisted for 46 days.

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L. Zarco, G. H. Stabenfeldt, S. Basu, G. E. Bradford and H. Kindahl

Summary. Pregnant (N = 10) and non-pregnant (N = 10) ewes were bled every 2 h from Days 12 to 17 after oestrus (oestrus = Day 0). Plasma concentrations of progesterone, 15-keto-13,14-dihydro-PGF-2α and 11-ketotetranor-PGF metabolites were determined in all samples. The number of PGF-2α pulses in non-pregnant ewes was 8·2 ± 0·4 (mean ± s.e.m.) with an interpulse interval of 10·7 ± 0·7 h. Two or 3 pulses of low frequency (interpulse interval = 13·4 ± 1·6 h) occurred in most non-pregnant ewes before the onset of luteolysis; the interpulse interval then decreased to 7·9 ± 0·4 h for the 6·0 ± 0·3 pulses temporally associated with luteolysis. In contrast, the number of PGF-2α pulses in pregnant ewes was lower (2·5 ± 0·7, 0–8) and the interpulse intervals longer (18·9 ± 6·1 h). Most pulses occurred on Days 14 and 15 in the pregnant and non-pregnant ewes.

The mean concentrations of both PGF-2α metabolites in non-pregnant ewes were highest on Day 15 while basal levels of both metabolites remained constant at all times. In pregnant ewes, the mean concentrations of both metabolites were highest on Day 14; basal concentrations of both metabolites were also highest on Day 14. The mean concentrations of 15-keto-13,14-dihydro-PGF-2α were higher in pregnant than in non-pregnant ewes on Days 13 and 14 (P < 0·05) and higher in non-pregnant than pregnant ewes on Day 15 (P < 0·05). The basal concentrations of the 15-keto metabolite were higher in pregnant than non-pregnant ewes at Days 13, 14, 15, 16 and 17 (P < 0·05). Both the mean and the basal concentrations of 11-ketotetranor-PGF metabolites were higher in pregnant than in non-pregnant ewes on Day 14 (P < 0·05).

It is concluded that uterine production of PGF-2α peaks at Days 14–15 after oestrus in pregnant and non-pregnant ewes. Patterns of release differ, however, in that non-pregnant ewes have a pulsatile PGF-2α pattern superimposed on a constant baseline, while pregnant ewes have an increasing basal secretory pattern which is more nearly continuous, i.e. not pulsatile in form. Modification of pulsatile PGF-2α synthesis and release is therefore a key aspect of prolongation of luteal function at the beginning of pregnancy in the ewe.

Keywords: PGF-2α; sheep; pregnancy

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L. Zarco, G. H. Stabenfeldt, J. F. Quirke, H. Kindahl and G. E. Bradford

Summary. Ewes (N = 32) were bled every 2 h from 5 days before expected oestrus until the end of oestrus. Plasma concentrations were determined for progesterone to monitor luteal activity and for the prostaglandin F-2α (PGF-2α) metabolites, 15-keto-13,14-dihydro-PGF-2α and 11-ketotetranor-PGF to determine uterine synthesis and release of PGF-2α. Most of the variation in cycle length was associated with the time of onset of luteolysis, the timing of events after luteolysis being constant and not related to cycle length. The time of occurrence of the first PGF-2α pulse and the interval between this pulse and the start of luteolysis were the two main determinants responsible for oestrous cycle length. Several PGF-2α pulses with interpulse intervals of 15·9 h occurred before the onset of functional luteolysis compared with 7·7 h for pulses associated with luteolysis. The numbers of PGF-2α pulses and interpulse intervals were similar for oestrous cycles of different lengths. While a gradual decline in progesterone concentrations was observed before functional luteolysis in the ewes with longer cycles, this did not appear to be an integral part of the stimulus which initiates the pulse frequency of PGF-2α required for luteolysis. We therefore suggest that differences in oestrous cycle length in the ewe are determined by the time of the onset of PGF-2α pulsatile release, and especially by the time of increased pulse frequency.

Keywords: PGF-2α; luteolysis; sheep; oestrous cycle

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R. H. BonDurant, B. J. Darien, C. J. Munro, G. H. Stabenfeldt and P. Wang

Summary. Oestrus and ovulation were induced in a group of 19 yearling dairy goats which had been maintained for 70 days on a 19 h/day photoperiod regimen. Six yearling females, raised under natural photoperiod, served as controls. An intact, light-treated male was introduced to each group 42 days after termination of the artificial lighting. Behavioural, endocrine and kidding observations indicated that 15 of the experimental females (79%) ovulated within 21–30 days after introduction of the male, that 12 (63%) conceived at the induced ovulation, and that 10 (53%) gave birth to live kids, while none of the controls ovulated during this time. The breeding season was advanced between 60 and 80 days.

In experimental and control nannies there was a brief, small surge (320 ± 42 pg/ml) of plasma progesterone which occurred 19·5 days after introduction of the male and which closely preceded oestrus in the nannies that ovulated and at 25 days in control females.

Ovulatory surges of LH (to 70 ng/ml plasma) were closely associated with oestrus, and remained above basal levels for 9·0 ± 0·75 h, in 7 experimental females. Two of 6 control nannies also showed LH surges but they did not ovulate.

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M. B. Wheeler, G. B. Anderson, C. J. Munro and G. H. Stabenfeldt

Summary. Blood was collected, at 5-min intervals for 3 h, via jugular cannulation every 14 days during the first 4 months post partum from beef cows and heifers, 4 suckling 2 calves and 4 suckling 1 calf. Calves were isolated from the dams to prevent suckling for approximately 4 h before and 2½ h during sampling to obtain baseline values for prolactin, measured by radioimmunoassay. During the last 30 min of sampling, calves were allowed to suck. Milk samples were also collected at 28-day intervals from 60 females, 30 with twins and 30 with single calves, beginning 2 weeks after calving and continuing until calves were weaned at 180 days of age. No differences were observed between dams with 1 and 2 calves for baseline plasma prolactin level or for prolactin response to the suckling stimulus by 1 or 2 calves. However, milk prolactin concentration was significantly higher (P < 0·01) for dams with 2 calves, probably reflecting the more frequent suckling that occurs with twins. Milk prolactin value in this study was not highly correlated with the post-partum interval to first ovulation from another study on these animals. It is concluded that prolactin is not the primary factor controlling the longer post-partum interval to first ovulation in beef cattle with twins.

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J. F. Quirke, H. H. Meyer, A. Lahlou-Kassi, J. P. Hanrahan, G. E. Bradfords and G. H. Stabenfeldt

Summary. Ovulation rate, in mixed-age groups of prolific and non-prolific ewe breed types, after administration of a range of doses of PMSG (0, 375, 750 and 1500 i.u.) during the follicular phase of the oestrous cycle, were compared in Ireland, Morocco and New Zealand. The ewes in Ireland and Morocco were from the Finnish Landrace and Galway, and D'Man and Timhadite breeds, respectively. In New Zealand Booroola Merino × Romney ewes which had been previously identified as heterozygous carriers (F +) of the Booroola high fecundity gene and purebred Romneys were used to represent the prolific and non-prolific genotypes respectively; in addition a group of Booroola Merino × Romney non-carriers (++) of the major gene were also included for comparison. Ovulation rate at the oestrus which preceded stimulation with PMSG was also measured in all animals.

In all 3 locations the ewes of the prolific genotype had a greater ovulation rate after PMSG stimulation than did the non-prolific controls. However, this association between prolificacy and response to PMSG was removed when ovulation rate after PMSG was transformed by dividing by the ovulation rate observed before PMSG administration. Despite the differences in the genetic basis of their high prolificacy the pattern of response to PMSG over the range of dosages used was similar in Finnish Landrace, D'Man and Booroola Merino × Romney (F +) ewes and all breeds had means of about 10 ovulations in response to 1500 i.u. PMSG. Amongst the non-prolific breeds, the Timhadite was the most responsive to PMSG although it had the lowest natural ovulation rate.

Information on fertility and litter size was available for all breeds in Ireland and New Zealand. In all breeds fertility was depressed at the highest dose of PMSG. Litter size at birth increased in all non-prolific breeds as the dose of PMSG was raised. For the prolific breeds, increasing the dose of PMSG from 750 to 1500 i.u. resulted in a substantial reduction in the number of lambs born.

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Progesterone levels were determined in the peripheral plasma of four gilts on each day of the oestrous cycle. An initial rise was observed at Day 3 or 4 of the cycle (oestrus = Day 1) followed by a very rapid increase up to Day 7 or 8 and a slower rate of increase until Day 14 or 15 of a 20-day cycle. The highest levels of progesterone during the luteal phase were about 35 ng/ml plasma and the average concentration during Days 10 to 15 was approximately 27 ng/ml plasma. The decline in progesterone levels after Day 15 was precipitous, a thirty-fold decrease occurring in most cases within 48 hr. Progesterone levels remained low (about 0·5 ng/ml) for about 7 days during the phase of follicle growth and ovulation.

A rather consistent time interval (7 days) was observed between the decline in progesterone concentration and the onset of overt oestrus. The high levels of circulating progesterone in the gilt may suppress follicle development sufficiently during the luteal phase so that approximately 1 week is required between corpus luteum regression and ovulation.

The concentration of plasma progesterone in nine castrated boars was 0·9 ng/ml suggesting an extra-gonadal source of progesterone, possibly adrenal in origin.

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P. F. Daels, S. Shideler, B. L. Lasley, J. P. Hughes and G. H. Stabenfeldt

Summary. Oestrogen secretion was determined by oestrogen conjugate (EC) analysis of urine in three groups of pregnant mares: Group I (N = 6), animals ovariectomized on Day 18–19 of gestation with pregnancy maintained by daily administration of an oral progestagen, altrenogest; Group II (N = 9), untreated, pregnant mares; Group III (N = 5) intact, pregnant mares treated daily with altrenogest.

The mean EC concentrations in the ovariectomized mares in Group I increased in a constant linear manner from 17 ng/mg Cr on Day 20 to 291 ng/mg Cr on Day 70, with no apparent surge in oestrogen secretion around Day 39. Mean EC concentrations on Days 33, 39 and 44 were respectively 41, 48, and 73 ng/mg Cr. In the intact mares in Groups II and III (shown in parentheses), the mean urinary EC concentrations were 201 (171) ng/mg Cr between Days 20 and 33 of gestation, increased rapidly from 172 (77) ng/mg Cr on Day 33 to a peak of 1066 (895) ng/mg Cr on Day 39, followed by a decline to 637 (719) ng/mg Cr on Day 44. After Day 44, EC concentrations continued to increase in a linear manner to 1191 (842) ng/mg Cr on Day 70. The mean EC concentrations between Days 20 and 70 in Group I were significantly (P < 0·05) lower than in mares in Groups II and III. EC concentrations in Group III mares were significantly lower (P < 0·05) than in Group II mares between Days 28 and 34.

We suggest that the ovary is a major contributor to the oestrogen conjugate concentrations measured between Days 20 and 70 and that the rapid increase between Days 33 and 39 is the result of a change in the rate of ovarian oestrogen synthesis.

Keywords: oestrogen; pregnancy; ovary; horse