Early Anoestrus Research Articles

Ovarian follicular dynamics during anoestrus in ewes.

The aim of the present study was to document ovarian antral follicle dynamics throughout seasonal anoestrus in sheep. Daily transrectal ultrasonography was performed during four 17 day scanning periods from March to July in Western White-faced crossbred ewes. Blood samples were collected each day with ultrasonographic scanning for measurement of serum concentrations of FSH, oestradiol and progesterone. Blood samples were also taken every 15 min for 6 h, mid-way through each period of ultrasonographic examination, to determine the patterns of secretion of gonadotrophic hormones. Hormonal data were then related to observed changes in follicular populations and the patterns of antral ovarian follicle turnover. Ultrasonography showed that the ovaries of anoestrous ewes remained active and that the largest ovarian antral follicles grew to a periovulatory size (> or = 5 mm in diameter) at all stages of anoestrus. The total number of all ovarian follicles > or = 3 mm in diameter was lower during early anoestrus compared with at mid-anoestrus because of a significantly smaller number of small (3 mm) and medium (4 mm) ovarian follicles. The largest ovarian follicles (attaining > or = 5 mm in diameter before regression) exhibited a wave-like pattern of growth; an average of three waves of follicular development were recorded in sheep during each of the four 17 day scanning periods in anoestrus, with follicular waves emerging approximately every 5 days. This rhythmic pattern of follicular emergence was found to be associated with the occurrence of fluctuations in serum FSH concentrations. The growth rate of the largest follicles of the wave increased significantly from early to late anoestrus in sheep. In addition, ovarian follicles not growing beyond 3 mm in diameter showed organized patterns of growth and regression; their numbers tended to be lower (P = 0.09) at 3 days before and on the day of follicular wave emergence. Some ewes were seen to maintain synthesis of progesterone throughout anoestrus. This submaximal progesterone secretion tended to occur at irregular intervals and was not coupled with changes in concentrations or patterns of gonadotrophin release, ovulations or detectable morphological luteinization of ovarian antral follicles. It was concluded that the growth of ovarian antral follicles to an ovulatory size was maintained throughout anoestrus in ewes, with a transient shift in the number of small and medium-sized follicles during mid-anoestrus, and that the periodic emergence of waves of large follicles (> or = 5 mm in diameter) occurred in synchrony with an endogenous rhythm of FSH secretion.

Control of luteinizing hormone secretion in ewes by endogenous opioids and the dopaminergic system during short seasonal anoestrus: rôle of plane of nutrition

AbstractThe role of endogenous opioids and the dopaminergic system on the inhibition of luteinizing hormone (LH) secretion during early and late anoestrus, together with its modulation by the plane of nutrition were investigated in ewes with a short anoestrous season. In early anoestrus (22 March; day 0), two groups of ovariectomized, oestradiol-treated adult Rasa Aragonesa ewes, maintained under natural photoperiod at 41°N, were given enough food to provide 1·4 × (high; H; no. = 6) or 0·5 × (low; L; no. = 6) energy requirements for maintenance. The effects of administration of the opiate receptor antagonist naloxone (1 mg/kg at four 1-h intervals) (day 15) and of the dopaminergic2 receptor antagonist pimozide (0·08 mg/kg) (day 21) on LH secretion were assessed. A second experiment was carried out in late anoestrus (21 June) using the same protocol. A significant increase in LH pulse frequency after naloxone treatment for both H and L groups was detected in late anoestrus. Number ofLH pulses after naloxone injections in early anoestrus also increased in H (P < 0·05) and L ewes (P = 0·08). The effect of pimozide injection on mean LH pulse frequency was greater in early than in late anoestrus, especially in ewes receiving a high plane of nutrition (P < 0·05 and P = 0·07 for H and L ewes, respectively in April and P = 0·07 for H ewes in July). A significant increase of LH pulse amplitude was also detected in early anoestrus in H ewes (P < 0·01). These results provide evidence that endogenous opioid mechanisms are involved in the inhibition ofLH pulsatile release both in early and late anoestrus in ewes with a short seasonal anoestrus. The ability of pimozide to increase LH pulse frequency in early anoestrus could be enhanced by a high plane of nutrition in the breed studied.

Termination of obligate anoestrus and induction of fertile ovarian cycles in dogs by administration of purified pig LH.

The potential role of LH in the initiation of the follicular phase in dogs was investigated by treating anoestrous bitches with highly purified pig LH (n = 16) or saline (n = 8) three times a day for 7 days, beginning in either early anoestrus (days 94-116 of the cycle) or mid-anoestrus (days 124-145). Treatment with LH induced pro-oestrus within 7 days (n = 16), and oestrus (n = 12) and fertile ovulations (n = 7) at 16 + 3 days, while pro-oestrus in bitches treated with saline did not occur until 46-166 days after the start of treatment. Six of the seven ovulating bitches whelped normal litters. The bitches in which pro-oestrus but not oestrus occurred were all treated in early anoestrus. During treatment, plasma oestradiol in bitches treated with LH increased from 8 + 2 pg ml-1 to 20 + 5 pg ml-1 within 1 day, and reached higher peak values (45 + 7 pg ml-1) (P < 0.05) than those observed in saline-treated controls (9 + 3 ng ml-1). These results demonstrate that LH treatment alone can terminate anoestrus by inducing a normal follicular phase in dogs. The results also suggest that, in normal cyclic bitches, anoestrus is the result of insufficient LH secretion, and that spontaneous pro-oestrus could be the result of increased LH secretion in the presence of already adequate concentrations of FSH.

Influence of previous photoperiodic exposure on the reproductive response to a specific photoperiod signal in ewes.

Two experiments were carried out to determine whether the reproductive response of ewes to a specific photoperiodic signal depends on the time of year that the signal is given, and, if so, whether this dependence can be attributed to the photoperiodic history of the ewes. The aim of experiment 1 was to expand upon previous findings that the reproductive response to a specific photoperiodic challenge in ewes previously maintained on natural photoperiod varies with time of year. Ewes were transferred at one of three times of year from natural photoperiod to photochambers and were immediately exposed to 35 long days (18L:6D) followed by continuous exposure to short days (8.5L: 15.5D); this treatment is referred to as LD-->SD. The three times of year when long days started corresponded to the beginning of the breeding season, the mid-breeding season, and early anestrus (September 21, December 21, March 21, respectively). In ewes exposed to LD-->SD beginning in September, the breeding season and subsequent anestrous season was not altered. In ewes exposed to LD-->SD beginning in December, the transition to anestrus was advanced (p < 0.05) relative to that in controls maintained in simulated natural photoperiod. Subsequently, half of these ewes resumed reproductive activity within 180 days; this occurred 131 +/- 8 days after transfer to short days. In contrast, all ewes exposed to LD-->SD beginning in March resumed reproductive activity; this began 100 +/- 3 days after transfer to short days (p < 0.05 versus December group). The purpose of experiment 2 was to assess the extent to which the difference in response to a photoperiodic challenge can be attributed to photoperiodic history. Ewes were maintained on short days from the winter solstice interrupted with 35 long days from March 21, June 21, September 21, or December 21. The majority of ewes exhibited an onset of reproductive activity after exposure to LD-->SD at the different times of year, and there was no group difference in latency to onset of reproductive activity. The duration of reproductive activity, however, was longer (p < 0.05) in ewes exposed to LD-->SD beginning in June than in the other groups. Thus we conclude that the seasonal difference in the ability of the photoperiodic challenge of long followed by short days to induce reproductive activity in ewes previously maintained outdoors can be attributed, in large measure, to photoperiodic history. Other factors, such as phase of the endogenous rhythm, however, may influence the duration of reproductive activity resulting from this photoperiodic challenge.

Reproductive endocrinology of bitches

Domestic dogs are mono-oestrous and predominantly non-seasonal breeders with ovarian cyclicity unrelated to uterine function. During the first phase of dioestrus or pregnancy luteal function seems to be independent of gonadotrophic support and the gradual decline of progesterone concentration thereafter occurs in spite of an increased availability of prolactin and luteinising hormone (LH). Thus, the mean area under the curve (AUC) for LH (ng per 360 min ml−1) increased from 1672 in early dioestrus to 2510 in late dioestrus and from 2940 on day 20 to 5089 on day 55 of pregnancy. LH pulse frequency remained unchanged. An induced release of LH during each quarter of dioestrus had no immediate effect on progesterone concentration but production over the whole period of dioestrus was increased (P < 0.03). Results from blocking cyclooxygenase activity indicated that the rather precipitous luteolysis prior to parturition was PgF2α-mediated. During anoestrus LH pulse frequency did not change. The AUC for LH decreased from late dioestrus to early anoestrus (6332 vs. 3257 ng per 360 min ml−1) and increased from mid to the end of anoestrus (5830 ng per 360 min ml−1). Only during the terminal phase of anoestrus could resumption of ovarian activity be induced by treatment with oestradiol-17β. There was a depression of oestradiol-17β during mid anoestrus while it increased at the end of anoestrus.

Increasing sensitivity of the pituitary to GnRH from early to late anoestrus in the beagle bitch

The sensitivity of the pituitary to GnRH in early and late anoestrus and the indirect response of the ovary were investigated in six adult beagle bitches. Plasma concentrations of LH and oestradiol were determined after i.v. injection of graded doses of GnRH (0, 0.01, 0.1, 1, 10 and 100 micrograms kg-1). The responses were measured by the LH and oestradiol concentration profiles over time. The responses of LH and oestradiol were significantly dose dependent (P = 0.002 and P < 0.001, respectively). The responses of LH and oestradiol were significantly higher (P = 0.02 and P = 0.001, respectively) in late anoestrus than in early anoestrus. The responses of LH and the responses of oestradiol were positively correlated (r = 0.97, P = 0.001). It is concluded that during the course of anoestrus in the bitch pituitary sensitivity to GnRH increases while the ovary responds accordingly.

Induction of breeding in anoestrous crossbred ewes with progestagen and PMSG with or without prior immunisation against an androstenedione-protein conjugate

Results are presented of an investigation into the potential value of immunisation of anoestrous ewes with androstenedione-7-HSA, using a commercial preparation (Fecundin; PitmandashMoore, Australia) prior to treatment with intravaginal sponges impregnated with fluorogestone acetate (Cronolone; Searle) and Pregnant Mare Serum Gonadotrophin (PMSG) for out-of-season breeding. Five experiments, involving treatment of 1292 ewes in various physiological states, were conducted during one anoestrous period of 4 months. Factors introduced were: whether or not immunised; interval from booster dose to mating; and dose of PMSG. The ewes were in three stages of anoestrus, namely early, middle or late. Other associated factors were whether or not they were suckling lambs (lactating) and stage of lactation. In early anoestrus, ewes in early lactation were difficult to impregnate compared with maiden ewes (30.6% vs. 51.9% ewes lambed). In mid-anoestrus, ewes in mid-lactation were comparable with maiden ewes (60.6% lambed). In late anoestrus, by which time most lambs had been weaned, 64.3% lambed. The response to immunisation varied with the time from the booster injection to that of the induced oestrus. A 13–14-day interval was too short and had deleterious effects. At 48 days, there were no deleterious effects and a slight residual beneficial effect on lambing performance. An interval of 21 days was much more effective. It had little effect on the time of onset of oestrus, markedly increased the ovulation rate, did not reduce the percentage ewes that lambed, and resulted in a 47% increase in lambs born to ewes in late lactation or from which lambs had recently been weaned. Immunised ewes consistently shed more ova than did nondashimmunised ewes, and this difference increased with increasing doses of PMSG, raising the question of the potential value of immunised animals as donors in embryo transplant programmes. The use of immunisation in a controlled breeding programme requires evaluation of each particular circumstance. There are potential advantages in terms of increased lambs born but the programme needs to be carefully timed.

Effects of PMSG and ram contact on the reproductive performance of progestagen-treated ewes during breeding and anestrous seasons

The objective of this study was to evaluate the efficacy of combinations of PMSG treatment and ram contact on the reproductive performance of progestagen-treated ewes during three different times of the year, Febraury (early anestrus), July (late anestrus) and October (breeding season). A total of 109 multiparous Dorset ewes was used. Ewes were treated with intravaginal progestagen pessaries for 12 days, injected with 500 IU PMSG at pessary removal and either isolated from rams prior to mating ( n = 12, February; n = 12, July; n = 8, October) or exposed to rams during pessary treatment ( n = 17, February; n = 12, July; n = 8, October). A third treatment group ( n = 18, February; n = 6, July; n = 8, October) received pessaries and ram exposure but no PMSG. An additional treatment of progestagen pessaries alone was included in October ( n = 8). There were no differences among treatments in their ability to induce estrus at different times of the year, but incidence of estrus tended ( P < 0.10) to be lower for PMSG treatment during the July breeding. During February, the use of pessaries with PMSG treatment increased ( P < 0.05) conception and lambing rates, whereas ram contact was without any beneficial effects. The trend was reversed during July breeding, when ram contact increased ( P < 0.05) fertility of progestagen-treated ewes compared with other treatment combinations. Pessaries alone were sufficient to attain acceptable levels of fertility and fecundity in October.

Ovarian stimulation following repeated injections of LH or LH + FSH in Ile-de-France sheep in early and mid-seasonal anoestrus

Ile-de-France ewes were treated either 32 ± 9 days (early anoestrus, n = 12) or 80 ± 10 days (mid-anoestrus, n = 8) after the beginning of seasonal anoestrus (4 April ± 14 days). At each treatment time all animals received intravenous injections of 10 μg ovarian luteinising hormone (oLH) every 2 h for 48 h. In addition to the oLH injections, half the animals from each group received 50 μg ovarian follicle-stimulating hormone (oFSH) every 2 h for the first 24 h of treatment. Jugular and ovarian venous blood samples were taken every 20 or 60 min on the day preceding the commencement of treatment and throughout the entire treatment period. The plasma concentrations of luteinising hormone (LH), follicle-stimulating hormone (FSH) and oestradiol-17β (E 217β) were determined in these samples using radioimmunoassays. Jugular blood samples were taken daily for 15 days after the last injection in order to study the pattern of progesterone secretion. Follicular populations were studied in 12 ewes at three different times: early anoestrus ( n = 4), mid-anoestrus ( n = 4) and at resumption of cyclicity ( n = 4). The number of LH pulses observed in ewes prior to the commencement of treatment on Days 32 and 80 was 0.73 ± 0.9 pulses 6 h −1 and 0.75 ± 0.5 pulses 6 h −1, respectively. Mean FSH levels prior to treatment on Days 32 and 80 were 3.7 ± 0.49 ng ml −1 and 2.6 ± 0.19 ng ml −1 ( P < 0.001), respectively. In ewes treated with LH only six out of six of the ewes in early anoestrus ovulated compared with only one out of four ewes in mid-anoestrus. All the ewes receiving the combined LH and FSH treatment ovulated, regardless of the stage of anoestrus. Of all the ewes that ovulated, short luteal phases were observed in two out of seven and one out of ten ewes receiving the LH and the combined LH-FSH treatment, respectively. During mid-anoestrus significant decreases in the total number of follicles in class C1 (170–863 μm) and class C2 (863–3569 μm) were observed and there were fewer normal follicles in both classes ( P < 0.05). We distinguished two types of anoestrus: (1) a slight anoestrus characterised by high FSH plasma levels and presence of normal follicles. In this case, ovulation was induced by repeated injections of LH only; (2) a deep anoestrus characterised by low FSH plasma levels and a severe reduction of the total number of antral follicles. In this case, the induction of ovulation required LH and FSH.

GnRH-stimulated LH and FSH release by perifused anoestrous red fox pituitary cells: gonadal steroid modulation

In this study, gonadotropin releasing hormone (GnRH)-stimulated luteinizing hormone (LH) and follicle stimulating hormone (FSH) secretions in the red fox (Vulpes vulpes L.) have been studied during anoestrus using perifused dispersed pituitary cells. The objective was to compare LH and FSH responses to pulses of GnRH in lactating and non-lactating females and to examine the feedback of gonadal steroids. Our results indicate that the pituitary sensitivity to GnRH was greatly reduced in lactating females compared with non-lactating females. Treatment in vitro of cells during 24 h (short-term effect) by progesterone (P) or oestradiol (E2) induced an increase of LH and FSH release, indicating that even during anoestrus, pituitary cells are able to respond positively to gonadal steroids. In contrast, administration of P or E2 in vivo for several days by silastic capsules (long-term effect) induced a reduction of LH and FSH releases by pituitary cells in vitro. Thus, steroids act directly on the pituitary but their action is time- and dose-dependent. During anoestrus in the fox, particularly during lactation, P and E2 might exert a potent negative feedback control on pituitary secretion. In the red fox (Vulpes vulpes L.), the fact that GnRH-stimulated LH and FSH secretions are reduced in lactating females more than in non-lactating females, suggests that P (secreted in vivo during lactation, probably as a consequence of a luteotrophic action of prolactin) and E2 secreted jointly during episodic phases of oestrogenic activity, may be responsible for decreased pituitary responsiveness observed during early anoestrus. This action of gonadal steroids might occur in association with a specific role of prolactin or a suppressive effect of suckling.

Postpubertal Maturation of Endogenous Opioid Regulation of Luteinizing Hormone Secretion in the Female Sheep1

This study investigated whether the role of endogenous opioid peptides in the suppression of LH secretion during seasonal anestrus in the sheep changes with age. The experimental approach was to determine the effect of blockade of opioid receptors with naloxone on LH secretion at different times of year within the anestrous season, and to compare responses between seasonally anestrous sheep of different ages. Sheep, all past the normal age of puberty, were ovariectomized before the study and treated s.c. with estradiol implants to provide a fixed estradiol feedback signal. One-year-old females responded to naloxone with a rapid increase in LH pulse frequency in the early (April) and late (August) phases of their first anestrous season. This response was similar to that previously found in prepubertal female sheep. Only 5 of the 8 females responded to the same naloxone challenge in mid anestrus (June), suggesting that the contribution of opioid pathways to the inhibition of LH secretion at this time of year is not necessarily the same as that in early and late anestrus. None of the older anestrous sheep (greater than or equal to 2 yr) responded to naloxone in June, indicating age-related changes in the role of endogenous opioid mechanisms in the inhibition of LH secretion. Ovary-intact mature sheep did not respond to naloxone, in contrast to our previous observations in intact prepubertal females. We infer that the neural mechanisms underlying the superficially similar hypogonadotropic states that occur during the prepubertal period, first anestrous season, and later anestrous seasons are not identical.(ABSTRACT TRUNCATED AT 250 WORDS)

Efficacy of intermittent or continuous administration of GnRH in inducing ovulation in early and late seasonal anoestrus in the Pere David's deer hind (Elaphurus davidianus)

Père David's deer hinds were treated with GnRH, administered as intermittent i.v. injections (2.0 micrograms/injection at 2-h intervals) for 4 days, or as a continuous s.c. infusion (1.0 micrograms/h) for 14 days. These treatments were given early (February-March) and late (May-June) in the period of seasonal anoestrus. The administration of repeated injections of GnRH increased mean LH concentrations from pretreatment values of 0.54 +/- 0.09 to 2.10 +/- 0.25 ng/ml over the first 8 h of treatment in early anoestrus, and from 0.62 +/- 0.11 to 2.73 +/- 0.49 ng/ml in late anoestrus. The mean amplitude of GnRH-induced LH episodes was greater (P less than 0.01) in late (4.03 +/- 0.28 ng/ml) than in early (3.12 +/- 0.26 ng/ml) anoestrus, but within each replicate (early or late anoestrus), neither mean LH episode amplitude nor mean plasma LH concentrations differed significantly between the four periods of intensive blood sampling. On the basis of their progesterone profiles, 6/12 hinds had ovulated in response to treatment with injections of GnRH (1/6 in early anoestrus and 5/6 in late anoestrus), and oestrus and a preovulatory LH surge were recorded in all of these animals. Oestrus and a preovulatory LH surge were also recorded in one other animal treated in early anoestrus in which progesterone concentrations remained low. The mean times of onset of oestrus (91.0 +/- 1.00 and 62.4 +/- 0.98 h) and of the preovulatory LH surge (85.8 +/- 3.76 and 59.4 +/- 0.25 h) both occurred significantly earlier (P less than 0.001) in animals treated in late anoestrus. Continuous infusion of GnRH to seasonally anoestrous hinds resulted in an increase in mean plasma LH concentrations, but this response did not differ significantly between early (2.15 +/- 0.28 ng/ml) and late (2.48 +/- 0.26 ng/ml) anoestrus. Ovulation, based on progesterone profiles, occurred in 2/7 hinds in early anoestrus and in 4/6 hinds in late anoestrus. Oestrus was detected in all except one of these hinds. The mean time of onset of oestrus occurred earlier in animals treated in late anoestrus (66.2 +/- 0.32 h and 46.7 +/- 0.67 h, P less than 0.01). The administration of GnRH, given either intermittently or continuously, will induce ovulation in a proportion of seasonally anoestrous Père David's deer. However, more animals ovulate in response to this treatment in late than in early anoestrus (75% compared with 23%).

LH secretion and response to GnRH during seasonal anoestrus of the Pere David's deer hind (Elaphurus davidianus)

The pattern of LH secretion and response to exogenous GnRH was determined on 5 occasions during seasonal anoestrus of the Père David's deer hind. LH pulse frequency was low (3.3 +/- 0.6 pulses/18 h) in early anoestrus (February), increased significantly in mid-anoestrus (April; 8.4 +/- 1.4 pulses/18 h) and thereafter declined slightly in late anoestrus (June; 6.3 +/- 0.25 pulses/18 h). Mean LH concentrations also showed significant changes during anoestrus with higher levels in mid-anoestrus (April; 0.85 +/- 0.12 ng/ml) when compared with other times (0.53 +/- 0.05, 0.60 +/- 0.10, 0.68 +/- 0.06 and 0.71 +/- 0.05 ng/ml for February, March, May and June, respectively). LH pulse amplitude showed no significant changes during the study. The LH response to intravenous injections of 2 micrograms GnRH was lowest in early anoestrus (February), increased significantly in mid-anoestrus (April) and remained high through late anoestrus. The response during the luteal phase was similar to that seen during late anoestrus. These results indicate that seasonal anoestrus in the Père David's deer hind is not a uniform state but is characterized by an early period of 'deep' anoestrus.

Basal and gonadotropin releasing hormone stimulated gonadotropin levels in the female red fox (Vulpes vulpes L.). Negative feedback of ovarian hormones during anoestrus

In the red fox, Vulpes vulpes L., an inhibition of gonadotropic function is observed in early anoestrus, particularly during lactation. During this period, secretion of progesterone as a result of the persistent corpora lutea after parturition and episodic releases of estradiol signify ovarian activity, suggesting involvement of these hormones in the modulation of pituitary hormones (luteinizing hormone (LH), follicle-stimulating hormone (FSH)). Effects of ovariectomy and (or) progesterone or estradiol treatments in vivo upon basal and gonadotropin releasing hormone (GnRH)-stimulated LH and FSH were observed. After ovariectomy, a great increase in the basal level of both gonadotropins and in GnRH-stimulated LH release, but not GnRH-stimulated FSH release, were observed. Progesterone treatment induced a decrease in GnRH-stimulated LH and FSH secretions and a decrease in basal LH and FSH levels in ovariectomized females. Estradiol treatment abolished basal secretions and GnRH responses for both hormones. These results suggest a negative feedback of both ovarian steroids at the hypothalamopituitary level on LH and FSH secretions during early anoestrus.

The effect of season and technique on synchronized and induced estrus and the induction of lambing in the ewe in a commercial setting

At 40 day intervals, groups of 87 to 142 commercial ewes of mixed breeding were subjected to a 5 day breeding period, following one of three estrus inducing or synchronizing treatments (intravaginal sponges containing 60 mg of medroxyprogesterone acetate) for 12 days with (progestin P.M.S.G. group) or without pregnant mares serum gonadotropin (P.M.S.G.) (Progestin group) at sponge removal, or a single injection of prostaglandin F 2α (breeding season and early anestrus only) (PGF 2α group)). Mean pregnancy rates (ewes lambing of those treated) and lambing percentages (lambs born per 100 ewes lambing) were 31 ± 4%, 169 ± 6%, 20 ± 5%, 105 ± 2% and 18 ± 8%, 118 ± 3%, respectively, for the three treatment groups above. Pregnancy rates for the progestin, P.M.S.G. group were 52% in late July, prior to the breeding season, 52% in September and declined to 14% in March (early anestrus). The time from the first of two daily injections of estradiol benzoate to lambing was 34.5 ± 4.4 hours in ewes 142 to 146 days pregnant.

Effectiveness of a novel prostaglandin analog in postpartum mares and mares with early season anestrus

Contents Fifty thoroughbred mares, located at Central-Southern Brazil near Sao Paulo (13 anestrous maiden, 18 anestrous barren mares and 19 postpartum mares) were treated with one to three injections, 48 hrs apart, of 2 mg of the novel PGF analog K 11941*, during the early part of the breeding season. Early season anestrus in these problem mares was compounded by the adverse effects of a drought. Plasma progesterone determinations revealed unexpectedly elevated plasma progesterone levels in 61.5% of the maiden mares, in 77.8% of the barren mares and in 68.4% of the postpartum mares (20 to 25 days postpartum). Treatments with K 11941 initiated luteolysis, heat and ovulations in 91.4, 45.5 an 63.3% of these mares, respectively, but initiated also heat and ovulation in 13% and 33% of mares with baseline progesterone levels. Treatments initiated covert and overt cyclic functions in 17 and 12 of those animals which did not conceive immediately. Fifteen animals with recurrent anestrus received further treatment with K 11941. Ten mares cycling silently were force bred when palpations indicated ovulatory follicles, and 7 conceived. Of the 50 mares treated, 38 became pregnant (76%: maiden mares: 76.9%; barren mares: 77.8% and postpartum mares: 73.7%) with breeding indices of 1.9, 1.36 und 1.57, respectively; and 37 mares gave birth to a live foal (74%), At the same stud, 138 control mares had a foaling rate of 82.6%.

Androgens in peripheral blood of the red fox ( Vulpes vulpes L.) during the reproductive season and the anoestrus

Androstenedione (A) and testosterone (T) were measured by radioimmunoassay after chromatography on celite micro-columns throughout the year in peripheral plasma of five vixens. There was a typical pattern in the fluctuations of A and T concentrations around the onset of oestrus. The A concentration rose from a basal level of 196.4 ± 120.4 pg/ml to 1543 ± 458 pg/ml (mean ± SD) within the 3 days preceding the increase of the progesterone (P) level. The concentration of T varied to a lesser degree. During the reproductive season, there was a significant positive correlation between these 2 steroids and estradiol (E 2) levels. During the anoestrus, the A level was low (97.4 ± 43.7 pg/ml) and rose from October when corpora lutea had regressed and when follicles began to develop. A peak of T was detected in the early anoestrus and thereafter there appeared no characteristic change in the T level (62.0 ± 71.1 pg/ml). There was no A or T peak that might be associated to the episodic E 2 release during the anoestrus.