Breeding Season In Ewes Research Articles

Effect of different dosages of PG-600 on ovulation and pregnancy rates in ewes during the breeding season.

This study compared the reproductive effects of different dosages of PG-600 (Intervet/Merck Animal Health, Madison, NJ) during the breeding season of ewes. PG-600 is a single-dose injectable product labeled for estrous induction in swine, containing equine chorionic gonadotropin (80 IU/mL) and human chorionic gonadotropin (40 IU/mL). PG-600 is routinely used off-label for out-of-season estrous induction in sheep. However, at the most common dose administered to ewes (5 mL), PG-600 is likely to overstimulate the ovaries, resulting in reduced pregnancy rates. Following estrous synchronization with intravaginal progesterone and cloprostenol, Polypay ewes were treated with 5 mL PG-600 (T1; n = 8), 1.5 mL PG-600 (T2; n = 8), or 5 mL saline (C; n = 8) and then mated to rams. Jugular vein samples were collected prior to the PG-600 injection (0 hr) and at 2, 4, 8, 12, and 24 hr after injection. Serum estradiol-17β was determined by chemiluminescence and among groups using repeated measures analysis of covariance. Ovulation and pregnancy rates were determined by transrectal ultrasonography and compared by one-way ANOVA and chi-square, respectively. Estradiol-17β concentrations were greater in T1 compared to T2 and C (P < 0.001). Ovulation rate was greater (P < 0.001) but pregnancy rate was lower (P < 0.001) in the T1 compared to C and T2. These data confirm that a 5 mL dose of PG-600 administered to ewes during the breeding season overstimulates the ovaries, which may then reduce fertilization or embryo survival. Future research will focus on the effects of different dosages of PG-600 on pregnancy rate of ewes during the nonbreeding season.

Ovarian performance of Barbados Blackbelly and Dorper crossbred ewes in the mirror of body conditions

The aim of examination was to detect the puberty and to follow the ovarian cycle of Barbados Blackbelly×Dorper ewes in connection with their body conditions and the effects of meteorological parameters. The blood samples had been taken from 17th of November 2008 untill 08th of June 2009 for analyzing the progesteron concentrations, establishing whether they showed perpetual ovarian performance or not. The end of examinationwas timed due to releasing the ram in middle of May 2009.The daily avarage bodyweight-growth is well featured by the bodyweight tendence. The body conditions were evaluated by ultrasound tissue examination. During the full length of the examination the avarage daily temperature, the avarage humidity level and also the number of daily sunlighted hours were served by the Agrometeorological Observatory of the University of Debrecen.The bodyweight tendence and the meteorological parameters were correlated to the breeding season of ewes. In conclusion the bodyweights and their tendences at the young age and the body condition parameters measured at certain anatomical areas can influence the length of the breeding season of the ewes. However the number of daily avarage sunlighted hours, the daily avarage temperature and also the humidity did not effect the cyclic ovarian perfomance of the Barbados Blackbelly×Dorper ewes in Hungary.

Estrous response to synchronization of estrus using different progesterone treatments outside the natural breeding season in ewes

A trial was conducted to investigate the efficiency of synchronization using different progesterone treatments outside the natural breeding season. Ninety multiparous Karakul ewes were randomly assigned to one of three treatment groups ( n = 30/group). In the progesterone treated group, ewes were injected intramuscularly (i.m.) with 20 mg progesterone acetate in oil every second day for 12 days, plus an i.m. injection of 500 IU eCG on day 12. The CIDR treated group, the animals were treated with the intravaginal devices impregnated with 0.3 g of progesterone for 12 days and injected i.m. with 500 IU eCG at withdrawal while in the third group, the ewes were treated with MAP sponges impregnated with 60 mg medroxyprogesterone acetate (MAP) for 12 days and injected i.m. with 500 IU eCG at sponge withdrawal. The interval from the cessation of treatment to the onset of estrus was significantly ( p < 0.05) longer in the progesterone in oil group, compared to CIDR and MAP sponge groups (51.4 ± 10, 30.1 ± 7.6 and 29.6 ± 5.6 h, respectively). The estrous response for the sponge and device groups was higher, compared to the group injected with progesterone (93.3% for CIDR, 100% for MAP sponge and 80% for the oil treated group). The duration of the induced estrous period did not differ significantly between the treatment groups. The results of the present study show that estrous synchronization with CIDR and MAP sponge followed by an i.m. administration of eCG is superior under local conditions for fixed time AI in Karakul ewes outside the natural breeding season.

Effect of ram exposure at the end of progestagen treatment on estrus synchronisation and fertility during the breeding season in ewes

Two experiments were conducted to examine the effects of ram exposure during the breeding season, in combination with progestagen treatment on estrus synchronization, fertility the LH surge and ovulation in ewes. Experiment 1 was subdivided into experiments 1a and 1b. In all experiments cross-bred ewes were treated with an intravaginal sponge for 12–14 days and three days before sponge withdrawal ewes were divided into control (no further treatment; n=191, 103 and 50 for experiments 1a, 1b and 2, respectively) or ram exposed (three mature rams per 50 ewes were introduced; +Ram; n=187, 99 and 49 for experiments 1a, 1b and 2, respectively). At sponge withdrawal ewes in Experiments 1a and 2 received 500 IU eCG and rams were removed from all the +Ram groups. In Experiments 1a and 1b, raddled, entire rams were introduced to ewes 48 h after sponge withdrawal. The timing of mating was recorded and ewes were maintained until lambing. In Experiment 2, estrus behavior was determined every 4 h and the time of the LH surge and ovulation were determined from a subset of 10 ewes per group. In Experiment 1a, less +Ram ewes were bred by 48 h after ram introduction (control 98% versus +Ram 89%, P<0.001) and in Experiments 1a and 1b 14% fewer ( P<0.05) of the ewes bred in the first 3 h after ram introduction lambed to that service. In Experiment 1a, ram exposed ewes had a lower litter size than control ewes (1.93±0.06 versus 1.70±0.06 lambs per ewe; P<0.05). In Experiment 2, rams advanced ( P<0.05) estrus, the LH surge and ovulation by 2–6 h compared with control ewes. We speculate that exposure of ewes to rams increased LH secretion and that this in turn increased follicle development and the production of oestradiol that led to a more rapid onset of estrus, the LH surge and ovulation compared to control ewes. Unexpectedly, ewes that were bred had lower fertility in the +Ram groups than control groups.

Breeding season and ovulation rate in ewes treated with long days in spring followed by a melatonin implant and exposure to a ram

AbstractWhile the onset of the breeding season in ewes can be advanced up to 7 weeks following exposure to long days (44 days) before lambing, a melatonin implant from lambing and exposure to a ram from the summer solstice, this advance is reduced by the absence of a ram. The aim of this experiment was to determine if earlier exposure to the ram and/or exposure to more long days before lambing would further advance the onset of the breeding season. A further aim was to compare the ovulation rate and duration of the breeding season between ewes with a normal and advanced onset of the breeding season due to this treatment regime. Pregnant ewes lambing in mid March were allocated to treatments as follows: (1) controls, maintained on natural photoperiod; (2) 44 LD + mel, exposed to 44 long days (18L: 6D; LD) before lambing followed by a 700-mg melatonin implant after lambing; (3) 90 LD + mel, as in treatment 2, but exposed to 90 LD before lambing; (4) 90 LD + mel + ram, as in treatment 3, including continuous exposure to a vasectomized ram from 10 April. Reproductive state was assessed from serum progesterone concentrations in blood collected at 3- to 4-day intervals from 10 April and ovulation rate was determined during the luteal phase of the first five oestrous cycles in control ewes and ewes exposed to 90 LD + mel + ram. Ewes exposed to 90 LD + mel + ram commenced the breeding season 10 weeks earlier (median date 25 July, range 28 days;P&lt; 0·001) than controls (median date 8 October, range 68 days). Overall, the onset of the breeding season was not advanced in ewes given 44 or 90 LD + mel in the absence of a ram, but a biphasic response was evident, with 40 to 50% of ewes beginning the breeding season at the same time as ewes on 90 LD + mel + ram. Ovulation rate was similar between control ewes and ewes exposed to 90 LD + mel + ram (P &gt; 0·05). The duration of the breeding season was longer in ewes exposed to 90 LD + mel + ram (210 (s.e. 15) days;P&lt; 0·002) than in control ewes (158 (s.e. 10) days), with no difference in date of the end of the breeding season (P &gt; 0·05). Exposure of ewes to 90 LD before lambing in March, followed by a melatonin implant and exposure to the ram from 10 April advanced the onset and extended the duration of the breeding season without an overall effect on ovulation rate. The ram is an important component of this signal sequence.

Serum anti-eCG improves luteal function and increases ova/embryos recovery in eCG-superovulated ewes

Superovulatory response to an eCG treatment (1200 IU) alone (Group eCG, n= 10) or with anti-eCG serum (Group AS, In = 10) given 12 h after the onset of estrus was studied during the breeding season in ewes. Blood samples were taken for LH and progesterone determination and on Day 5 post-estrus all ewes were flushed and ova/embryos recovered. Mean overall interval to first signs of standing estrus for all ewes was 23.6 ± 1.8 h (mean ± SEM) and peak of LH surge ranged between −11 h to 9 h after onset of estrus (2.0 ± 1.6 h). Mean number of CL (8.8 ± 1.5 vs. 10.6 ± 0.9 for eCG and AS groups, respectively; P = NS) and large anovulatory follicles (2.8 ± 2.0 vs. 0.9 ± 0.4, P = NS) were not different. A higher ova/embryo recovery was obtained in the AS group (4.1 ± 1.0 vs. 0.9 ± 0.5, P < 0.05) and this was related to a lower proportion of ewes that had small (4.0 ± 0.1 mm in diameter, n = 75), pale and non-protruding corpora lutea at time of embryo collection ( 2 10 vs. 5 10 , P = 0.14). Pooled data of all ewes that displayed this CL ( n = 7) show that this condition: (1) is related to the endocrine status of the ewes at time of eCG treatment (low endogenous levels of progesterone); (2) is characterised by a premature luteinisation (increase of progesterone at the onset of oestrus) and a subsequent premature luteolysis (decrease of progesterone levels at Day 3 to 5 post-estrus) and (3) is associated with a lower number of horns that yielded some ovum/embryo ( 4 14 vs. 16 24 , P ≤ 0.01). We conclude that the administration of anti-eCG serum at 12 h after the onset of estrus increases the ova/embryos recovery rate because of lower number of ewes that displayed prematurely regressing corpora lutea in the eCG-superovulated ewes.

Effect of the presence of male and female flockmates on reproductive activity in ewes.

Four experiments were carried out to determine the effect of the presence of ewes and rams on the reproductive state of ewes. In Expt 1, the breeding season of ewes kept with a vasectomized ram ended later (April 18 +/- 8 days; mean +/- SEM) than that of ewes isolated from rams (6 March +/- 7 days; P < 0.01). In Expt 2, the end of the breeding season was later (5 May +/- 6 days; P < 0.05) and the onset of the next breeding season earlier (29 September +/- 2 days; P < 0.001) in ewes maintained with rams, compared with ewes isolated from rams (14 April +/- 7 days and 1 November +/- 2 days, respectively). There was no difference in the timing of, or variation in, reproductive transitions between ewes maintained either as individuals or in groups. In Expt 3, all ewes exposed to artificial short days from the date of the winter solstice and interrupted with 35 long days in spring resumed cyclicity (median date, 7 September; range, 59 days). Most ewes (seven of nine) exposed to short days from the date of the winter solstice and isolated from other ewes did not resume cyclicity in the following 11 months. In contrast, all ewes resumed cyclicity (median date, 19 October; range, 144 days) when exposed to short days but housed in social contact with other ewes that became reproductively active in early September; however, the onset of cyclicity was later than in ewes exposed to long days (P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of supplementary long days, a subcutaneous melatonin implant and exposure to a ram on the onset of the breeding season in ewes

Seasonality of breeding in sheep occurs as a consequence of exposure to photoperiodic, social and other environmental stimuli. The objective of this experiment was to determine if the onset of the breeding season in ewes could be advanced by giving supplementary long days during pregnancy, followed by a subcutaneous melatonin implant at lambing. The additional effect of exposing ewes treated in this way to a ram was also investigated. Twin bearing ewes (mean±SEM lambing date 7 March± 2 days) maintained on natural photoperiod were randomly allocated within breed to one of the following treatments: 1. (1) control, ewes maintained on natural photoperiod throughout ( n=22); 2. (2) melatonin, ewes maintained on natural photoperiod throughout and given a subcutaneous melatonin implant at lambing ( n=23); 3. (3) long days plus melatonin, ewes were exposed to 18 h light: 6 h dark for an average of 44 days before lambing, given a melatonin implant at lambing and then maintained on natural photoperiod ( n=21); 4. (4) long days plus melatonin plus ram, as Treatment 3, followed by continuous exposure to a ram from the summer solstice ( n=22). Reproductive activity was assessed from serum progesterone concentrations in blood collected once weekly from 1 June. Control ewes began their breeding season in September (median date 21 September, range 50 days). Ewes given melatonin alone became reproductively active around the same time (median date 13 September, range 77 days; P > 0.05). Ewes exposed to long days during pregnancy followed by a melatonin implant at lambing began their breeding season earlier than controls (median date 6 September, range 93 days; P<0.05). Exposure to a ram from the summer solstice in addition to treatment with long days and melatonin resulted in an additional advancement in the onset of the breeding season (median date 29 July, range 77 days) compared with ewes exposed to long days and given melatonin ( P<0.01). Thus, the combined treatment of long days during pregnancy followed by a melatonin implant at lambing can advance the breeding season of ewes. Additional exposure to a ram in such ewes can further advance the onset of the breeding season.

Effect of exogenous thyroxine on timing of seasonal reproductive transitions in ewes.

The thyroid gland is necessary for the transition from the breeding season to anestrus in the ewe, but the importance of seasonal changes in thyroxine in this process is not known. The objective of this experiment was to determine the effects of exogenous thyroxine during the breeding season on the time of the transition to anestrus and on the onset of the subsequent breeding season in ewes maintained on natural photoperiod. Cyclic Galway ewes were blocked on the basis of body weight and randomly allocated to the following groups: 1) untreated controls (n = 11), 2) placebo-injected controls (n = 9), and 3) ewes injected daily with thyroxine from the middle of the breeding season until the onset of anestrus (n = 10). Reproductive state was assessed from serum progesterone concentrations determined twice weekly until the breeding season had ended in placebo-injected controls, and until the onset of the subsequent breeding season in untreated and thyroxine-injected ewes. The mean (+/- SEM) date of onset of anestrus did not differ between untreated ewes (April 19 +/- 9 days) and placebo-injected controls (April 15 +/- 9 days; p > 0.05), but occurred earlier in ewes injected with thyroxine (February 28 +/- 6 days; p < 0.001). Thyroxine treatment during the breeding season, however, did not affect the onset of the subsequent breeding season. These data indicate that supplementary thyroxine during the breeding season can advance anestrus and thus shorten the duration of the breeding season.(ABSTRACT TRUNCATED AT 250 WORDS)

3 beta-hydroxysteroid dehydrogenase inhibitor reduces ovarian steroid production but increases ovulation rate in the ewe: interactions with gonadotrophins and inhibin.

Two experiments were carried out during the breeding season in ewes, first to investigate the effects of oral administration of a 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) inhibitor (epostane) on the number of corpora lutea, and secondly to investigate the mechanism through which epostane acts. In the first experiment Dorset Horn ewes were treated orally with 25, 50, 100 or 200 mg epostane twice daily between days 10 and 15 of the oestrous cycle. All doses of epostane resulted in an increase in the number of corpora lutea per ewe, although the response was curvilinear, with the 25 mg dose showing the largest response and the 200 mg group the smallest response. Although there was no difference between groups in the number of ewes showing oestrus, the higher doses of epostane had a detrimental effect on fertility. In the second experiment Welsh Mountain ewes were treated twice daily with 25 mg epostane from day 10 of the oestrous cycle and the ovaries were removed for analysis during either the luteal or the follicular phases. Treatment significantly increased the number of follicles greater than 6 mm in diameter, but significantly reduced in-vitro follicular oestradiol and testosterone production. Despite a marked increase in peripheral inhibin concentrations there was no effect on in-vitro inhibin production. Epostane treatment also caused a significant reduction in peripheral FSH concentrations and an increase in mean LH concentration. The latter was due to an increase in LH pulse frequency during the luteal phase and LH pulse amplitude during the follicular phase. These results confirm that treatment of ewes with epostane orally has a significant effect on follicular steroidogenesis and causes a significant increase in the number of corpora lutea per ewe. This effect on ovulation rate is not via an increase in peripheral FSH concentration, but may be caused by a reduction in follicular steroid activity either directly on the ovary or via an alteration in the pattern of LH secretion.

Progesterone ensures full-length luteal phases during the breeding season in ewes

To test the hypothesis that each luteal-phase increase in the serum concentration of progesterone throughout the breeding season prevents a short luteal phase in the next cycle, 22 ewes were treated with an i.v. injection of 10 micrograms gonadotrophin-releasing hormone (GnRH) agonist every 12 h for 33 days beginning on day 12 of a cycle synchronized with prostaglandin F2 alpha. Six days after the last injection of GnRH agonist, ten of the ewes were treated s.c. for 14 days with progesterone-containing silicone elastomer implants to generate luteal-phase serum concentrations. Twenty ewes stopped cycling during GnRH agonist treatment and 16 of these, eight controls and eight treated with progesterone, resumed cycling after the end of treatment. In the control ewes, oestrous cycles began 25.0 +/- 7.5 (S.E.M.) days after the end of GnRH agonist administration, a short luteal phase preceding initiation of cycles in six ewes. In contrast, all eight progesterone-treated ewes resumed cycling synchronously 22.0 +/- 0.2 days after the end of GnRH agonist treatment and all began with full-length luteal phases. These results support the hypothesis that each luteal-phase increment in the serum concentration of progesterone throughout the breeding season prevents a short luteal phase in the next cycle.

Response of suffolk and texel rams to implantation with melatonin

There is currently much interest in overcoming the seasonality of sheep meat supplies in the UK and improving carcass quality. Towards that end, much research effort has been devoted to the development of techniques to induce early or out-of-season breeding in the ewe, but little attention has been paid to the male. Rams of most breeds show marked seasonal fluctuations In reproductive performance, with a depression in testis size, libido and semen quantity and quality during the spring and summer months and a period of high fertility in the autumn/winter, coinciding with the breeding season of ewes of the same breed. Problems of subfertllity which are frequently encountered following the use of various techniques to induce out-of-season breeding in the ewe may therefore be attributable, at least in part, to the ram. The choice of terminal sire for early lambing flocks is further compromised because some breeds of ram show later testicular recrudescence than others.

Seasonal Modification of Ovine Pineal Function

Pineal beta-adrenoceptor density and affinity in ewes are modified in a season-dependent manner by gonadal steroids and by the sympathetic innervation of the gland. The present study was undertaken to relate the steroidal effects on the receptors to post receptor endocrine events, and to investigate the influence of the sympathetic innervation of the pineal gland on these events. Plasma melatonin and prolactin profiles were determined during anestrus and during the normal breeding season in ewes subjected to sympathetic denervation of the pineal and/or a range of steroid-related treatments. Wherever valid comparisons could be drawn between effects of the treatments on beta-adrenoceptor variables and on circulating hormone levels, similar effects were noted. Further, ganglionectomy influenced hormone profiles similarly to estradiol under all conditions tested. It appears that gonadal steroids (estradiol) and the sympathetic neurotransmitter noradrenaline have opposing actions on prolactin levels, just as they have on pineal beta-adrenoceptor binding affinity. These findings suggest that steroid-mediated changes in receptor number and affinity are reflected in post receptor endocrine events. In addition, other factors (e.g. photoperiodic information transmitted via the sympathetic innervation) also play important roles in the regulation of the observed hormonal profiles. A steroid-mediated feedback regulation of pituitary prolactin release, partly direct and partly via pineal melatonin release, is suggested.

The use of vaginally administered melatonin in the manipulation of the breeding season in ewes

Melatonin has been shown to be effective in advancing the breeding season of the ewe when given either orally or by subcutaneous implantation. We have demonstrated that it is also effective when given as an intravaginal implant containing melatonin in silastic tubing. Such implants have the advantage that they are much easier to insert and remove than subcutaneous implants. Rate of implant loss is extremely low. The present abstract summarises the result of several experiments which have examined the effectiveness of melatonin implants when given at different times of year. Animals used were either Mule ewes or their Suffolk cross offspring. All were kept outside under normal management conditions. Groups of in ewes were given implants on either 12 April, 13 May, 18 July, or 21 December. Control ewes received empty implants. Blood samples were taken twice Weekkly for progesterone analysis to estimate the tire of onset of ovarian activity.

Advancement of the breeding season of ewes and lambs by intravaginal application of melatonin

Advancement of the breeding season of ewes and lambs by intravaginal application of melatonin

Field Trials in Fixed Time Sheep A.I. in Ireland

Studies reported from this laboratory in the mid seventies showed that it was possible to achieve a conception rate of 73% to a single fixed-time insemination in the autumn breeding season in ewes previously treated with intravaginal progestagen and pregnant mare serum gonadotrophin (PMSG). The present work was aimed principally at an examination of the AI technique in commercial flocks when applied over a period of three years (1979–81) and when applied in sheep of different breeds (Galways and Suffolk-cross).In the course of three successive breeding seasons, a total of 2,704 cyclic ewes in 87 farm flocks were bred to Texel or Suffolk rams by A.I. using a sperm dose of 400 million in 0.2 ml volume of a skim milk diluent; only semen samples showing a wave motion rating of 3.5 or higher were used and accepted samples were pooled. A further 525 ewes in 20 farm flocks were bred by natural service (ratio of 1 ram: 10 ewes) in one area covered by the AI work. Control of oestrus and ovulation was achieved by a 12-day treatment using commercially available progestagen sponges (Chronogest, Intervet. Ltd.; Veramix, Upjohn Ltd.); at sponge withdrawal a single intramuscular injection of 500 i.u. PMSG (Intervet. Ltd.) was administered.

Melatonin can induce early onset of the breeding season in ewes.

Patterns of plasma melatonin, similar in the duration of high levels to those found in winter, were induced in Suffolk-cross ewes kept in summer light (16 h light: 8 h darkness) by daily oral administration of melatonin (3 mg/13 mumol). The onset of oestrous cycles in these sheep occurred in August, 2-8 weeks before the onset of oestrous cycles in untreated ewes kept in natural light. The onset of oestrous cycles in a further group of ewes kept in winter light (8 h light: 16 h darkness) from mid-June was indistinguishable from that of the melatonin-treated ewes. Rams were excluded from the premises. These data indicate that melatonin alone in physiological quantities is sufficient to induce early onset of the breeding season in the ewe, and provide strong evidence for a hormonal role of melatonin in a short-day breeder.

Ovarian morphology and the concentration of steroids, and of gonadotrophins during the breeding season in ewes actively immunized against oestradiol-17 beta or oestrone.

Ewes were actively immunized against oestrone-6-(O-carboxymethyl)-oxime-bovine serum albumin, 17 beta-oestradiol-6-(O-carboxymethyl)oxime-bovine serum albumin or bovine serum albumin (controls). All 4 control ewes, 1 of 5 oestradiol-immunized ewes and 1 of 5 oestrone-immunized ewes had regular oestrous cycles. The other animals displayed oestrus irregularly or remained anoestrous. The plasma concentrations of LH and, to a lesser degree, FSH were increased relative to those in control ewes on Days 11-12 after oestrus or a similar total period after progestagen treatment in ewes not showing oestrus. The ovaries were examined and jugular venous blood, ovarian venous blood and follicular fluid were collected at laparotomy on Days 9-10 of the oestrous cycle. The ovaries of immunized ewes were heavier than those of control ewes. There were no CL in 5 of the immunized ewes but in the other 5 there were more CL than in the control ewes. Ovaries from 4 of 5 oestrone-immunized ewes contained luteinized follicles, while ovaries from 4 of 5 oestradiol-immunized ewes contained very large follicles with a degenerated granulosa and a hyperplastic theca interna. Both types of follicles produced progesterone, detectable in ovarian venous plasma and production of other steroids, particularly androstenedione, was also increased. The steroid-binding capacity of plasma was increased in the immunized ewes. The binding capacity of follicular fluid for oestradiol-17 beta and oestrone was similar to that of jugular venous plasma from the same ewes. These results suggest that immunization against oestrogens disrupts reproductive function by interfering with the feedback mechanisms controlling gonadotrophin secretion.

Changes in the dilatability of the ovine vagina in relation to oestrus and anoestrus

Changes in the dilatability of the ovine vagina have been measured during the breeding season and during anoestrus. An increase in apparent vaginal volume occurs at the time of oestrus. There is little change in volume over the remainder of the oestrous cycle. In anoestrus, the dilatability is smaller than at any time during the breeding season. It is suggested that these changes in the dilatability of the ovine vagina could be used in studies of the breeding season of ewes in which the use of the ram is not desirable.

Variations in the Length of Estrus Cycles and the Breeding Season in Ewes

Variations in the Length of Estrus Cycles and the Breeding Season in Ewes