Computer modeling of sheep reproduction: I. An algorithm for quantifying anestrous cycles in ewes

An algorithm that mathematically describes annual estrous activity in ewes is given. The function fitted as the ordinate is similar to a normal distribution function related to date as the abscissa. The data used were collected from literature on different breeds and from different locations of the world; all studies had been specifically conducted to study estrous activity in ewes in these locations. Parameters estimated in an iterative procedure from the algorithm were length of time during which estrous cycles occurred (SA), a measure of asymmetry (SD), day of mid-estrous (DE), highest incidence of estrus (MX) and lowest incidence of estrus (MN), day of lowest incidence of estrus (DN) and day of mid-anestrous season (DA). Incidence of estrus is defined as the proportion of ewes exhibiting an active estrus. These parameters were estimated for several breeds and locations. The data, as collected, were inadequately cross-classified to allow for unconfounded estimation. However, analyses of fitted parameters that describe the curves gave quantitative results about the influences of breed of sheep, location, latitude and altitude and indicated which parameters were associated with these factors. Breeds differed significantly for SA, DE, MX and DA. Locations differed for DE, MX and DA. Latitude was associated with differences in DE, DN and DA with altitude also associated with differences in DE and DA. There were no significant effects on SD even though breeds differed sizably in SD.     

Effect of cohabitation with white-faced ewes on estrous activity of Hampshire and Suffolk ewes exposed to rams in June

Two groups of 24 Hampshire and 26 Suffolk purebred ewes each were used to study effects of cohabitation with cyclic white-faced (WF) ewes on estrous activity in June. Ewes lambed in January, February and March and had been isolated from mature rams since the previous fall breeding. From June 1 to July 2, treated (T) ewes were exposed to vasectomized rams and to 65 WF ewes; control (C) ewes were exposed only to vasectomized rams. Ovulation was assessed with biweekly serum progesterone assays; crayon marks were used to detect estrus. Daily observations of ram behavior were conducted to assess sexual activity of rams joined with T and C ewes. Cohabitation with WF ewes increased (P less than .01) ovulation percentages from 46% in C (42% for Hampshires and 50% for Suffolks) to 76% in T ewes (79% for Hampshires and 73% for Suffolks). Mating percentage also was increased (P less than .05) by cohabitation with WF ewes from 14% for C ewes to 30% for T ewes. Rams with T + WF ewes spent more (P less than .05) time checking ewes for estrus than did rams with C ewes. Hence, cohabitation with cycling WF ewes increased ovulation and mating percentages. Many acyclic T ewes first ovulated after 10 or more days of teasing, possibly due to increased ram contact in the presence of WF ewes.     

Gonadotrophin releasing effects of alfaprostol in seasonally anestrous/estrous mares

Alfaprostol will induce luteolysis in mares with serum progesterone concentrations greater than 1 ng/ml. Serum LH response to alfaprostol in seasonally anestrus mares is dependent on whether the mare is in true anovulatory anestrus or transitional anestrus. The greatest LH increases were in maresin transitional anestrus. No significant LH changes occurred when just the carrier vehicle (propylene glycol) was administered.     

Uterine influences on the formation of subnormal corpora lutea in seasonally anestrous ewes

The hypothesis that subnormal luteal function after induced ovulation in anestrous ewes was the result of uterine influences exerted during the periovulatory period was tested. Crossbred ewes (n = 27) in seasonal anestrus were induced to ovulate by administration of 12 doses of 250 ng of LHRH at 2-h intervals, followed immediately by a bolus injection of LHRH (250 micrograms; d 0). Ewes were unilaterally hysterectomized on either d -3 (PRELHRH) or 2 (POSTLHRH). Daily blood samples were collected and assayed for progesterone (P4) and 13,14-dihydro-15-keto-prostaglandin F2 alpha (PGFM). All ewes were slaughtered on d 10, and corpora lutea (CL) were collected, weighed, and assayed for concentration of P4. All ewes that ovulated exclusively in the ovary ipsilateral to the remaining uterine horn had a transient increase in plasma P4 of 2 to 3 d (short luteal phase). In ewes with at least one CL in the isolated ovary, elevated plasma P4 was maintained after hysterectomy but was consistently lower (P less than .05) in POSTLHRH ewes than in PRELHRH ewes. Concentrations of PGFM did not differ between treatments. The CL ipsilateral to the remaining uterine horn weighted less (P less than .01) and contained less P4 (P less than .01) than contralateral CL. These data confirm the hypothesis that premature regression of subnormal CL is uterine-dependent in a local fashion. Presence of the uterus during the follicular and(or) early luteal phase inhibited subsequent luteal function in seasonally anestrous ewes.     

Lifespan of corpora lutea induced in estrous-synchronized cycling and anestrous ewes

The effect of pretreatment with flurogestone acetate (FA) on the lifespan of corpora lutea induced with pregnant mare serum gonadotropin (PMS) was examined in cycling and anestrous ewes. Cycling ewes received one of three treatments: 750 IU PMS 2 d before expected estrus (P), FA-impregnated vaginal sponges for 16 d (F), and FA sponges for 16 d and 750 IU PMS 2 d before sponge removal (FP). A fourth group served as controls (C). When compared with d 12 means within treatment, plasma progesterone means were lower (P less than .05) on d 16 in control ewes, on d 15 in P and F ewes, and on d 14 in FP ewes. Only 44% of ewes receiving FA treatment alone exhibited estrus (P less than .05) compared with 100% of untreated ewes. The FP treatment increased ovulation rate compared with controls (P less than .01). The decrease in luteal lifespan observed in cycling ewes suggests a possibility of asynchrony between the uterus and embryo, which could result in failure of an embryo to prevent luteal regression, thus resulting in reduced fertility. None of the seasonally anestrous ewes that received PMS alone and only 55% of those treated with FA sponges for 8 d before PMS injection exhibited estrus. Ewes pretreated with FA exhibited higher plasma progesterone concentrations on d 10 through 16 after PMS injection. There were no differences in luteal lifespan as measured by peripheral plasma progesterone patterns. Although FA treatment did not alter luteal lifespan in anestrous ewes, the increased plasma progesterone concentrations observed with FA treatment suggest that progestogen pretreatment may be essential for optimal luteal function.     

Concentration of manganese in the tissues of cycling and anestrous ewes.

A wide range of variation in concentration of manganese was found in the blood of 51 cycling ewes although no significant differences were evident (P less than 0.05) between days 4, 11 and 15 (day 0 = day of estrus) of the estrous cycle. In addition, no differences (P less than 0.05) were found in the manganese concentrations of various soft tissues of 15 ewes, killed on day 4 (five animals), day 11 (five animals) or during anestrus (five animals). The highest concentration of manganese was present in the liver, pancreas and kidney cortex, tissues which are rich in mitochondria. Among the tissues of the reproductive tract, the corpus luteum showed the highest manganese concentration and the level increased significantly (P less than 0.01) between days 4 and 11 suggesting that this trace element may be closely related to the metabolic and possible functional characteristics of this endocrine gland. Uterine horn and caruncles contained greater manganese concentrations than other components of the reproductive tract. The significance of these findings in relation to the effect of manganese intake on ewe fertility remains to be assessed.     

Diurnal changes in plasma melatonin and the timing of reproductive onset in anestrous sheep fed melatonin.

Seasonally anestrous Suffolk ewes (n = 28) were randomly divided into 5 groups and treated with varying doses of melatonin as follows; Groups C (n = 4), M1 (n = 6), M2 (n = 6), M3 (n = 6), M4 (n = 6) of ewes were fed pellets containing 0, 1, 2, 3 and 4 mg melatonin, respectively, daily for 60 days from May 17 (Day 0). Following feeding of the pellets at 13.00 hr plasma levels of melatonin rapidly increased reaching the peak values within 30 min, which ranged from 92.0 to 292.7 pg/ml and were highly correlated with the dose of melatonin administered (r = 0.986, P less than 0.01). Maximum dose of melatonin (4 mg) produced an increase of plasma melatonin similar in magnitude to nocturnal peaks of endogenous secretion. The onset of ovulatory cyclicity, assessed from plasma progesterone profiles, was advanced by melatonin administration. The mean +/- SEM intervals from the commencement of melatonin treatment until the onset of ovulatory cyclicity were 53.0 +/- 5.8, 53.6 +/- 2.5, 42.0 +/- 5.6 and 44.3 +/- 4.3 days for the Groups M1, M2, M3 and M4, respectively, which were shorter (M1, P less than 0.05; M2, M3 & M4, P less than 0.01) than that for the Group C (72.5 +/- 1.4 days). The melatonin treatment also suppressed, in a dose related manner, the rise in plasma prolactin under the lengthening photoperiod. We conclude that the dose-related efficacy of melatonin could be ascribed to the difference in the diurnal profiles of circulating melatonin.     

Effect of immunization against melatonin on prolactin concentrations and the timing of reproductive transitions in ewes

The objective of this experiment was to develop a procedure for immunizing ewes against melatonin that would alter the effects of changing photoperiod on seasonal reproduction and prolactin secretion. Ewes were immunized against human serum albumin (HSA) as controls (n = 9) or a melatonin-human serum albumin conjugate (0.25 mg; n = 10) on December 14th (Day 0) and boosted 9 times. They were maintained on natural photoperiod and then transferred indoors and exposed to long days for 35 d, followed by short days for 146 d, long days for 93 d, and short days for a further 123 d. Antibody titers to melatonin (at a serum dilution of 1:1,250) were significantly higher in immunized ewes (27.3 ± 6.6%) than controls (0.7 ± 0.1%; P < 0.001). At the end of the experiment, antibody titers in immunized ewes (at dilution of 1:50) were higher in blood (43.7 ± 8.2%) than in cerebrospinal fluid (10.8 ± 3.9%; P < 0.05), and highly correlated (r² = 0.746). Onset of the breeding season was advanced slightly after the second transfer from long to short days in immunized ewes (April 12 ± 3 d) compared with controls (April 25 ± 3 d; P < 0.05). Mean serum prolactin concentrations were lower (P < 0.05) in melatonin-immunized ewes compared with controls on natural photoperiod, after transfer from long to short days, during long days, and after the second transfer from long to short days. In conclusion, despite melatonin-immunization increasing antibody titers in blood and cerebrospinal fluid, and decreasing prolactin concentrations over much of the experiment, minimal effects on the timing of reproductive transitions in the ewes were evident. This discrepancy between the response of the prolactin and reproductive axes to melatonin immunization supports the hypothesis of a dual site of action of melatonin, with melatonin acting in the pituitary gland to mediate the effects of photoperiod on prolactin secretion and in the mediobasal hypothalamus to affect reproductive responses.     

Effects of interrupted photoperiods on the induction of ovulation in anestrous mares

The ability of interrupted photoperiods to induce early estrus and ovulation was examined. Horse mares were exposed to long (16 h light) or short (10 h light), noninterrupted photoperiods, ambient light, or various interrupted photoperiod treatments from December 1 to April 15 (135 d). Follicular development was assessed by rectal palpation and estrous behavior was determined by teasing with a stallion. Serum concentrations of progesterone were used as an indicator of corpus luteum function. Differences among the light treatment groups were compared for the following behavioral and ovarian characteristics: days to first detectable 3-cm follicle, days to first estrous behavior, days to first ovulation, the number of mares ovulating within the treatment period, and the number of ovulations within the treatment period per mare. Compared with the ambient and 10L:14D (L = h of light and D = h of darkness) photoperiod treatments, ovulation was advanced to the greatest extent by a photoperiod of 16L:8D and the interrupted photoperiod 10L:8D:2L:4D. These two stimulatory photoperiod treatments were characterized by the presence of light 8 to 10 h after dusk. Therefore, the present data are consistent with an external coincidence model for the induction of seasonal breeding in horses, with the photoinducible phase occurring within the period 8 to 10 h after dusk.     

Endocrine changes during restoration of estrous cycles following induction of anestrus by restricted nutrient intake in beef heifers

The working hypotheses in this experiment were: that ovarian estradiol would inhibit luteinizing hormone (LH) secretion in heifers that were anestrus as a result of restricted dietary energy intake and the responsiveness of LH secretion to estradiol negative feedback would decrease during the period when restoration of estrous cycles occurred following feeding of diets adequate in energy. Fifteen heifers weighing 341 +/- 12 (mean +/- SE) kg were fed a diet containing 50% of the energy required for maintenance until 40 to 50 d following cessation of estrous cycles. Heifers were assigned to intact control (C, n = 5), ovariectomized (OVX, n = 5) or ovariectomized-estradiol-17 beta-implanted (OVX + E2, n = 5) treatments. Heifers were subsequently provided a high-energy (HE) diet until termination of the study. Progesterone concentrations indicating cessation of corpus luteum function were detected after heifers had lost 71 +/- 8 kg body weight over 186 +/- 28 d. Control heifers re-initiated estrous cycles as indicated by increased progesterone concentrations in serum at 49 +/- 9 d after initiation of feeding the HE diet (360 +/- 18 kg body weight). Initiation of pulsatile LH secretion was observed in heifers by d 12 following OVX. Estradiol suppressed LH secretion in OVX + E2 heifers during the period of nutritional anestrus in C heifers. Suppressive effects of E2 on LH secretion continued in OVX heifers after C heifers had initiated corpus luteum function. Therefore, the working hypothesis that LH secretion is inhibited by E2 in the nutritionally anestrous heifer is accepted but responsiveness to estradiol does not subside with re-initiation of estrous cycles, thus this working hypothesis is rejected.