Prior exposure of maiden ewes to rams enhances their behavioural interactions with rams but is not a pre-requisite to their endocrine response to the ram effect

In this study, we tested whether prior experience with rams would modify the behavioural and endocrine responses of maiden ewes to rams. During mid-anoestrus, sexually na?ve, maiden ewes were exposed to rams for 7 days (ram experienced, RE; n=61) or isolated from rams (ram na?ve, RN; n=63). All ewes were subsequently isolated from rams. In Experiment 1, RE (n=55) and RN (n=57) ewes were introduced to rams during late anoestrus. RE ewes had more total and positive interactions with rams than RN ewes (P<0.001). RE ewes showed more ram seeking behaviour and spent more time in proximity of rams than RN ewes (at least; P<0.05). In Experiment 2, RE (n=6) and RN (n=6) ewes were introduced to rams midway through a frequent blood sampling regime in late anoestrus. Ram introduction stimulated an increase in LH pulse frequency and basal LH in both RE and RN ewes (at least P<0.05). RE ewes had an increase in mean LH concentrations (P<0.01) that failed to reach significance in RN ewes (P<0.1). There was no significant effect of prior experience with rams on LH pulse frequency, amplitude or whether ewes had an LH surge. In conclusion, prior experience with rams is important in developing appropriate ewe-ram interactions but is not a pre-requisite to the endocrine response to the ram effect.     

Influence of classification levels of ram sexual activity on spring breeding ewes

Ram lambs (7–8 months old) and mature rams (19–20 months old) were used to evaluate the effect of classification levels of male sexual performance on reproductive performance of ewes during spring breeding. In Exp. 1, sexually active ram lambs with high (1.8±0.3; n=5) and low (0.9±0.2; n=5) sexual performance scores (HP and LP; mean±S.E.M.) were used in single sire breeding pens. Ewes (n=305) were stratified by age and assigned to 10 pens for 34 days starting in late March. For Exps. 2 and 3, two replicates were conducted for 2 years with sexually active mature rams in a single sire mating scheme. For Exp. 2, HP rams (n=5) averaged 3.6±0.2 ejaculations and LP rams (n=7) 1.8±0.2 ejaculations for sexual performance scores based on nine, 30 min serving capacity tests (SCT). Polypay ewes (n=152 to 153 per year) were stratified by age and assigned to pens each year for 34–38 days starting in late March for years 1 and 2. For Exp. 3, HP rams (n=6) averaged 3.7±0.1 ejaculations and LP rams (n=10) 2.3±0.1 ejaculations for sexual performance scores based on 18, 30 min SCT. Polypay ewes (n=229 in year 3 and n=244 in year 4) were stratified by age and assigned to pens each year for 34 days starting in late March. In Exp. 1, lambing rates for ewes bred to HP versus LP ram lambs did not differ (65.8 versus 53.0; P=0.20). Prolificacy tended (P=0.06) to be increased by 0.1 lambs in ewes bred by LP ram lambs. Total number of lambs born per ewe present at lambing, and lambing distribution were not altered by HP and LP ram lambs. In Exp. 2, lambing rates for fall-lambing ewes bred to mature HP or LP rams did not differ (58.1 versus 60.1; P=0.78). In Exp. 3, lambing rates for fall-lambing ewes bred to mature HP or LP rams did not differ (74.3 versus 69.0; P=0.35). There was no difference (P>0.10) between years for Exp. 2 or Exp. 3, and mature HP and LP rams did not affect the other reproductive variables monitored. Analyses of the combined data for Exps. 2 and 3 indicated only a year difference (P<0.001) in lambing rates and total lambs born. Present studies indicate that different sexual performance classifications for ram lambs and mature rams did not alter lambing rates or distribution of lambing of Polypays bred in late March to April. These results indicate that HP and LP, sexually active, Polypay rams and ram lambs with average to high quality semen can provide a source of rams for spring breeding Polypays in ambient conditions and that there was no advantage to using HP over sexually active LP ram lambs or rams.     

Libido and scrotal circumference of rams as affected by season of the year and altered photoperiod

Two trials were conducted to evaluate the influence of season of the year and altered photoperiod on libido and scrotal circumference (SC). A 30-minute serving-capacity test was used to measure ram libido. the measures of libido were reaction time (RT), time from entry into the pen to first mount and/or service, total mounts (M), and total services (S). The serving-capacity test was conducted by placing a ram with four estrus-induced ewes and measuring RT and counting M and S. Prior to each serving-capacity test, SC was measured for each ram. Rams were tested every two weeks. In Trial I, eleven two-year-old Rambouillet rams from each of three selection lines -a high line (four rams; selected on the basis of high prolificacy), a low line (three rams; selected on the basis of low prolificacy) and a random bred control line (four rams) - were used in a one-year study. Rams were exposed to ambient conditions throughout the year. Rams were more active during the short days of fall and winter, i.e. normal breeding season, as evidenced by a greater number of total mounts and services plus a shorter reaction time. Selection line affected reproductive parameters measured, with the high line having more M and S and a shorter RT than the low line. However, SC was larger in the low line. In Trial II twelve rams were divided into two groups of six. The control group was exposed to ambient conditions from April 18 through July 24. The treated group was exposed to eight hours of light and 16 hours of darkness (8L:16D) from April 18 through July 24, simulating short days of fall and winter. Total services (S) in the 30-minute test interval were higher for rams subjected to the 8L:16D treatment (P<0.01; 2.7+/-0.2 vs 1.6+/-0.2 for 8L:16D and control, respectively). SC was 31.7+/-0.2 vs 30.2+/-0.2 for 8L:16D and control, respectively (P<0.01). Total mounts in 30 minutes were not affected by treatment (6.9+/-0.8 vs 5.7+/-0.8 for 8L:16D and control, respectively; P>0.10). No significant differences in any of the reproductive parameters between treatment groups were observed during the first 28 days. However, there were significant differences (P<0.05) observed between 8L:16D rams and control rams for SC during 42 to 84 days and S between days 42 to 70, respectively. Serving-capacity tests carried out about one month following the end of altered photoperiod trial showed no significant differences between treated and control rams, thus indicating that treatment had no carry-over effect.     

Effects of sexual experience, season, and mating stimuli on endocrine concentrations in the adult ram.

Two behavioral trials were conducted to determine the endocrine response of cortisol (C), luteinizing hormone (LH), testosterone (T), prolactin (PRL), and growth hormone (GH) in adult rams during exposure to estrous ewes during the breeding and nonbreeding seasons. One-half of the rams in each season were sexually experienced (SE) and the remainder were sexually inexperienced (SI). All SE rams (100%) achieved at least one ejaculation, but only 33% (summer) and 67% (fall) SI rams achieved ejaculation. In the fall, mean C, T, and GH concentrations were elevated (P less than .001) compared to values measured in the summer, whereas LH and PRL levels were higher (P less than .01) in the summer. Overall levels of C, LH, T, and PRL were higher (P less than .05) in SE rams than in SI rams. Mean GH concentration was higher (P less than .10) in SI than in SE rams during restricted and complete access to estrous ewes. In general, LH, PRL, and GH responses were similar during restricted and complete access to females for both SE and SI rams. Cortisol levels were higher (P less than .06) during periods of mating and T levels were higher (P less than .001) during periods where activity was limited to courtship behavior (nasogenital investigation). Correlations of hormones to reproductive behaviors indicated that mounting and intromission were associated with elevations in C and PRL, whereas elevated LH and T tended to be associated with courtship behaviors. Correlations between GH and behaviors were inconsistent. However, there was an increased coincidence between time of female exposure and hormonal response that occurred in the fall; brief exposure to estrous ewes resulted in increases in concentrations of all hormones examined. The most consistent response was observed in sexually experienced rams during restricted access to females during the breeding season. These results provide new information on the effects of season and level of sexual experience upon hormonal and behavioral characteristics of the ram during mating activity.     

Endocrine and Ovarian Changes in Response to the Ram Effect in Medroxyprogesterone Acetate-primed Corriedale Ewes During the Breeding and Nonbreeding Season

Two experiments were performed to determine the endocrine and ovarian changes in medroxyprogesterone acetate (MAP)-primed ewes after ram introduction. Experiment 1 was performed during the mid-breeding season with 71 ewes primed with an intravaginal MAP sponge for 12 days. While the control (C) ewes (n = 35) were in permanent contact with rams, the ram effect (RE) ewes (n = 36) were isolated for 34 days prior to contact with rams. At sponge withdrawal, all ewes were joined with eight sexually experienced marking Corriedale rams and estrus was recorded over the next 4 days. The ovaries were observed by laparoscopy 4–6 days after estrus. Four weeks later, pregnancy was determined by transrectal ultrasonography. In eight ewes from each group, ovaries were ultrasonographically scanned; FSH, LH, and estradiol-17β were measured every 12 hours until ovulation or 96 hours after estrus. The response to the rams was not affected by the fact that ewes had been kept or not in close contact with males before teasing. No differences were found in FSH, LH, estradiol-17β concentrations, growth of the ovulatory follicle, onset of estrus, ovulation rate, or pregnancy rate. Experiment 2 was performed with 14 ewes during the nonbreeding season. Ewes were isolated from rams for 1 month, and received a 6-day MAP priming. Ovaries were ultrasonographically scanned every 12 hours, and FSH, LH, estradiol-17β, and progesterone were measured. Ewes that ovulated and came into estrus had higher FSH and estradiol-17β levels before introduction of the rams than did ewes that had a silent ovulation. The endocrine pattern of the induced follicular phase of ewes that came into estrus was more similar to a normal follicular phase, than in ewes that had a silent ovulation. The follicle that finally ovulated tended to emerge earlier and in a more synchronized fashion in those ewes that did come into estrus. All ewes that ovulated had an LH surge and reached higher maximum FSH levels than ewes that did not ovulate, none of which had an LH surge. We conclude that (a) the effect of ram introduction in cyclic ewes treated with MAP may vary depending on the time of the breeding season at which teasing is performed; (b) patterns of FSH, and estradiol-17β concentrations, as indicators of activity of the reproductive axis, may be used to classify depth of anestrus; and (c) the endocrine pattern of the induced follicular phase, which is related to the depth of anestrus, may be reflected in the behavioral responses to MAP priming and the ram effect.     

Effects of sexual stimulation on the sexual performance of rams

The sexual performance of bulls and male goats is improved if they are allowed to view the hetero-sexual behavior of other males as a prelude to mating. The purpose of the following study was to determine whether sexual stimulation enhances the sexual performance of rams. In Experiment 1, 11 sexually experienced ram lambs ( 9 months of age) and 18 sexually experienced yearling and 2-year-old rams were individually exposed to 4 unrestrained, hormone-induced estrous ewes for 60 min after viewing the courtship and mounting behaviors of a male conspecific for 20 min (two tests) and in the absence of stimulator animals (two tests). In contrast to the results with bulls and bucks, the rams were hardly influenced by the sexual stimulation treatment. Latencies for first mount and first ejaculation were shorter for sexually stimulated ram lambs; otherwise, treatment differences were negligible.

A similar follow-up experiment was administered to 12 mature rams using restrained females in the sexual performance tests. Again, treatment differences were minor.

It was concluded that sexual stimulation does not functionally enhance the sexual performance of rams. Species differences in response to sexual stimulation are discussed in terms of female sexual behaviors that may result in a selective (competitive) advantage to males that are stimulated to locate and mate with females early in the estrous period.     

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.     

An automated method for the evaluation of ram libido in real mating conditions

We investigated if sexual behaviour of rams can be assessed with an electronic Alpha-Detector (AD) which automatically records mounts of mating rams. To evaluate the rams’ libido (i.e. all sexual activities), we used six intact and six vasectomised rams in pen tests in three different seasons (late spring, autumn and early spring). The pen tests consisted of 30-min visual observations of each ram placed in a group of six Merino ewes (three ewes in oestrus and three ewes not in oestrus). In the pen tests, sexual behaviour was recorded and divided into two categories: pre-copulatory and copulatory. For validation purposes, during the pen tests the 12 rams were equipped with the AD and the number of times the 18 oestrous ewes were mounted were counted over a period of 3 days. Of the 1191 mounts visually identified in the six 30-min sessions, 1026 were recorded automatically by the AD (i.e. 94%). The paddock test is an automated method consisting of the same rams wearing an AD and placed in a flock of ~250 Merino ewes on two occasions (late spring (spring 1) and early spring of the following year (spring 2)), their copulatory activities were automatically recorded over a 5-day period. The results of the pen tests in the three seasons revealed no difference between the two types of rams (breeding v. detecting rams). Based on live observations high correlations (r=+0.81, P<0.003 for breeding and r=+0.76, P<0.02 for detecting rams) were found between pre-copulatory and copulatory behaviours. The libido of the two types of rams measured in pen tests showed high repeatability across the three seasons (83 and 75%, P<0.05 for copulatory and pre-copulatory behaviours, respectively). When measured automatically in paddock tests over two consecutive springs, even higher repeatability was observed in both breeding (94%; P<0.01) and detecting rams (97%; P<0.004) in the number of mounts. In addition, high correlations (+0.89<r<+0.94) between copulatory behaviours, as measured by live observations, and those measured by the AD were obtained. The automatic measurement of ram libido in paddock tests appears to be more reliable than pen tests and far less time consuming. We therefore recommend this automated method to estimate the libido of rams. In addition, this method can be used at any season of the year provided that ewes in oestrus are present in the flock.     

A note on ram preference by oestrous ewes: Influence of rams’ age and sexual performance

Ewes in oestrus, besides being receptive to rams, develop an active search for them. We performed two experiments to determine if oestrous ewes prefer: (1) high or low sexual-performance rams, and (2) 2-year-old or aged (>8-years-old) rams. In the first experiment, the preference of ewes for rams with high or low sexual performances was compared in 16 tests. In the second experiment, 17 tests were performed comparing ewes’ preference for aged rams (>8-years-old) or young rams (2-years-old). In each test, two rams were tied at equal distances from the point of entrance of the ewe, with a maximum mobility for each ram of 2 m. The first approach to a ram, the quantity of approaches toward each ram (less than one body length), and the total time that the ewe remained near each ram were registered during 5 min. Oestrous ewes did not display any preference according to sexual performance of rams, but spent significantly more time (P = 0.02), and did it in more tests (P = 0.025), with young than with aged rams. Ewes also tended to approach first (P = 0.07), and more times in more tests (P = 0.06), to young than to aged rams. We conclude that ram's sexual performance seems not to influence oestrous ewes preference, and that ewes’ prefer young (2-years-old) rather than aged rams (>8-years-old).     

Effect of melatonin implants on secretion of luteinizing hormone in intact and castrated rams

Rams were treated with melatonin implants in 2 experiments designed to examine the control of reproductive seasonality. In Exp. 1, rams (n = 12) were allocated to 3 treatment groups: 2 groups were treated with 2 melatonin implants per ram for 4 months from 11 November (N) and 9 December (D) and the remaining group was untreated (C). The seasonal increase in luteinizing hormone (LH) pulse frequency and testes size was advanced in Groups N and D. A second seasonal cycle in LH secretion and testes size occurred in Groups N and D after melatonin implants became exhausted. In Exp. 2, rams (n = 20) were allocated to 4 treatment groups: 10 rams were castrated on 6 October and 1 group of entire rams (EM) and one group of castrated rams (CM) were treated with 2 melatonin implants per ram each month from 3 November until 8 January. The other group of entire rams (EC) and castrated rams (CC) was untreated. An increase in LH pulse frequency occurred after castration. Melatonin treatment increased LH pulse frequency in entire rams and reduced LH pulse frequency in castrated rams. The results demonstrated that the advanced reproductive development as a result of treatment with melatonin implants was due to an effect of melatonin on the hypothalamic pulse generator to increase LH pulse frequency. The ability of melatonin to influence LH pulse frequency in entire and castrated rams indicated that an effect of melatonin on the hypothalamic pulse generator is independent of testicular steroids.     

Effect of early and late exposure to estrual ewes on ram sexual performance classifications

This study was conducted to determine whether exposure of ram lambs to estrual ewes during their first autumn and again as adults just before serving capacity tests (SCT) affected the outcome of the sexual performance tests. Treatments were either early exposure of Polypay ram lambs (i.e., 7-8-mo-old rams with ewes for 17 d [n=30] or no early exposure [n=30]), and late exposure (i.e., 16-19-mo-old rams with estrual ewes for 3 d) or no exposure to estrual ewes in a 2x2 factorial arrangement. Three serving capacity tests were conducted immediately after the early exposure period for individual ram lambs that were exposed to ewes early. Three sham sexual performance tests (i.e., four ram lambs placed in test pens for 30-min without ewes) were conducted with ram lambs that were not exposed to ewes early. All rams were evaluated during nine 30-min serving capacity tests over a 2-mo period at 16-19 mo of age to determine sexual performance. Prior to serving capacity tests, one half of the rams from each early exposure treatment were exposed to estrual-induced ewes for 3 d. Specific sexual behaviors (e.g., sniffs, flehmens, foreleg kicks, vocalizations, mount attempts, mounts, and ejaculations) were recorded during serving capacity tests. Number of sniffs, flehmens, foreleg kicks, vocalizations, and mount attempts were summed without estimating the value of importance and analyzed as courtship behaviors. Sexual performance data were analyzed with Mixed model procedures for repeated measures. During serving capacity tests, the early exposed rams exhibited more courtships (40.3+/-8.0 versus 23.4+/-4.6; P<0.05; LSM+/-estimated SE), mounts (11.3+/-1.0 versus 7.7+/-0.9; P<0.01), and ejaculations (3.3+/-0.2 or 2.4+/-0.2; P<0.01) than rams not exposed to ewes as ram lambs, respectively. We conclude that early exposure of 7-8-mo-old ram lambs to estrual ewes improves sexual performance in serving capacity tests at 16-19 mo of age in most rams whereas, late exposure to estrual ewes for 3 d prior to serving capacity tests did not improve sexual performance scores.     

Plasma LH and testosterone in Zebu crossbred bulls after exposure to an estrous cow and injection of synthetic GnRH

Plasma hormone levels were examined in 4 mature Zebu bulls of normal libido (HL) and 4 which were sexually inactive (LL). When used in an artificial insemination programme the 8 bulls had similar fertility. Basal levels of LH and testosterone (T) estimated from 8 sequential blood samples at 30 minute intervals were not different in HL and LL bulls. Exposure of the animals to an estrous cow did not stimulate LH release. Following sexual stimulation plasma T levels actually decreased by an average (±S.E) of 2.9 (±1.9) ng/ml in the HL group and increased by 3.9 (±1.6) ng/ml in the LL group. An injection of 1 mg GnRH (Hoechst) caused LH release of similar magnitude in HL and LL bulls. The elevation of plasma T which followed GnRH injection was significantly larger in HL bulls.Low libido was not associated with a deficiency of basal LH or T, nor with the ability of the pituitary to respond to GnRH.     

Luteinizing hormone release in intact and castrate rams is altered with immunoneutralization of endogenous estradiol

Changes in the dynamics of luteinizing hormone (LH) release in the adult ram following immunoneutralization of endogenous estradiol were investigated. Castrate rams were actively immunized against estradiol-6-bovine serum albumin for 7 months and then their patterns of episodic LH release and LH response to multiple injections of gonadotropin-releasing hormone (GnRH, two 5-micrograms doses given iv 2 h apart) were assessed (April). In comparison with control rams immunized against rabbit gamma globulin, estradiol-immunized rams (antibody titre approximately 1:5000) exhibited more frequent LH releases (11.7 +/- 0.3 vs. 9.3 +/- 0.8 pulses/8 h, P less than 0.05) and a greater LH response to the first GnRH injection (peak delta value 190 +/- 8 vs. 130 +/- 25 ng/mL, P less than 0.01). Estradiol antiserum collected from the castrate rams was used in the passive immunization of intact rams (antibody titre approximately 1:200) for 1 month (beginning mid-July). Although episodic LH release was always similar for control and immunized rams, testosterone levels in the latter group increased approximately 150%. In contrast with the castrate ram response, GnRH treatment (two 5-micrograms doses given iv 80 min apart) produced a "self-priming" effect on LH release in the intact rams, an effect that was dampened with estradiol immunoneutralization. Consequently, peak 2:peak 1 ratios for delta value and 80-min mean incremental increase were much smaller (P less than 0.01) for the immunized rams (approximately 2:1 vs. 4:1 for the control rams).(ABSTRACT TRUNCATED AT 250 WORDS)     

Can audio–visual or visual stimuli from a prospective mate stimulate a reproductive neuroendocrine response in sheep?

Stimuli from a prospective mate increase the secretion of luteinising hormone (LH) in sheep. This 'male effect' in ewes and 'female effect' effect in rams is predominantly mediated by olfactory signals, though it is thought that non-olfactory signals play synergistic or substitutive roles. In this study, we tested whether exposure to visual or audio-visual stimuli from a prospective mate would stimulate an increase in LH secretion in ewes (Experiment 1) and rams (Experiment 2). In Experiment 1, groups of eight Merino ewes were exposed to one of three stimuli midway through a frequent blood-sampling regimen: full ram contact, still images of rams, a video of ewes and rams mating. Control ewes (n = 8) were completely isolated from rams. Exposure to still images of rams appeared to stimulate an increase in mean LH concentrations (P < 0.05) and tended to increase LH pulse frequency (P < 0.1), but the response was significantly smaller than that observed in ewes exposed to rams (P < 0.01). Audio-visual stimuli had no effect on any parameters of LH secretion (P > 0.1). In Experiment 2, Merino rams were allocated to either an Exposure (n = 7) or a Control (n = 7) group. Exposure rams underwent two exposure periods midway through a frequent blood-sampling regimen; exposure to still images of ewes and audio recorded during mating of ewes and rams (audio-visual exposure); exposure to oestrous ewes (ewe exposure). Control rams were sampled at the same frequency but remained isolated from ewe stimuli. Exposure of rams to the audio-visual stimuli did not affect any parameters of LH secretion (P > 0.1). In contrast, exposure to oestrous ewes increased LH pulse frequency (P < 0.05) and advanced the onset of the next LH pulse (P < 0.05). In conclusion, visual signals appear to be involved in eliciting the neuroendocrine response of ewes to rams and are of greater importance to this phenomenon in ewes (male effect) than rams (female effect). However, overall the visual and audio-visual signals used in this study were far less effective than stimulus animals, suggesting that these stimuli are less important than olfactory signals, or a combination of olfactory and audio-visual signals.     

Active immunization of boars against gonadotropin releasing hormone. II. Effects on libido and response to testosterone propionate

Of 20 sexually mature Duroc boars showing normal libido, 10 were actively immunized against gonadotropin releasing hormone (GnRH). After immunization against GnRH, boars showed minimal sexual interest in an estrous female, while untreated boars showed normal libido. Eight of the boars actively immunized against GnRH were randomly assigned to treatment (T) or control (C) groups. Boars in the T and C groups were given testosterone propionate or vehicle, respectively, on Days 0, 5, 10, and 15. Boars in both groups were observed for libido in the presence of an estrous female every 4 d for 28 d. Mean libido score for T boars increased gradually until all boars displayed maximum libido on Day 20, but libido returned to low levels on Day 28. In contrast, C boars remained sexually inactive throughout the study. The results of this study indicate that active immunization of sexually mature boars eliminates sexual behavior and that sexual behavior can be restored quickly by administering testosterone propionate.     

Sexual performance and production traits in white-faced yearling rams

Poor mating behavior results in increased ram costs, an extended lambing season, and decreased genetic progress from sires with desired production traits. Producers have recognized the importance of ram libido, but constraints regarding time, labor, and facilities generally limit the routine evaluation of ram breeding behavior. Easily measurable production traits would be an usable selection tool if correlated to reproductive performance. To determine the discriminating value of production traits as indicators of reproductive performance, white-faced rams (n = 82; 10–12 months of age) consigned to the University of Wyoming ram test were fed using the Grow-Safe^® feeding system. Rams were categorized based on sexual behavior as low, intermediate or mounting. Body weight gain performance, feed intake and feeding behavior were analyzed. Overall production performance was determined using a calculated index ratio based on body weight gain adjusted for wool characteristics. Sexual behavior classification did not influence the index ratio or feed consumed per day. However, numbers of no-intake feeding episodes (P < 0.01) differed according to ram sexual behavior. Rams expressing sexual mounting behavior visited the feed bunk more often than the intermediate performing rams (P < 0.01) and tended to differ from low sexually performing rams. The numbers of visits to the feed bunk in which feed was consumed did not differ according to ram sexual behavior. Average daily gain was not correlated (r ≤ 0.18; P = 0.1) to sexual behavior. These easily attainable measures of performance did not correlate strongly with any measure of sexual behavior. This study provides evidence that rams that exhibit a high level of sexual behavior most likely investigate their environment more thoroughly, which could facilitate the identification of ewes in estrus.     

Female effect in sheep. II. Role of volatile substances from the sexually receptive female; implication of the sense of smell

The importance of olfactory cues in inducing luteinizing hormone and testosterone secretion as a response to stimulation by sexually receptive ewes has been tested. In sexually experienced rams, olfactory stimulation with urine, wool and vaginal secretions from sexually receptive females placed in a mask did not induce an endocrine response. The female-induced secretion of LH and testosterone was similar in anosmic, sham-operated and in control rams. These results show that olfactory cues are not necessary in the mediation of interindividual stimulation of endocrine response in the sexually experienced ram.     

Luteinizing hormone, testosterone and cortisol responses in rams upon presentation of estrous females in the nonbreeding season

The physiological responses of luteinizing hormone, testosterone and cortisol in sexually experienced Ile de France rams to the introduction of estrous females were studied during the nonbreeding season. Blood sampling were collected from males for 7 h at 20-min intervals, starting 3 h before stimulation by estrous females. The differences in hormonal secretions were tested by comparisons between pretreatment and treatment Periods in 45 stimulated rams. Comparisons were conducted between rams that had increased LH pulse frequency and those that did not, between rams that ejaculated and those that did not, and between rams that were in direct physical contact and those that were kept at a distance of 30 cm from estrous females. Twenty-five rams (55% of the total) showed significant increases in LH pulse frequency (range, 0.80 to 4.00 peaks/ram/6 h, P<0.05), in basal and mean LH levels (1.5- and 2.5-fold, respectively), and in mean testosterone levels (3.5-fold). More frequent LH pulses had been found during the pretreatment period in 20 rams without increased LH pulse frequency. Eight ejaculating rams showed higher cortisol and mean, basal, and peak LH amplitude levels. Deprivation of physical contact with estrous females was associated with an absence of endocrine response. These results suggest that olfactory and/or tactile cues may be involved in the female effect on hormone levels.     

Reproductive response and LH secretion in ewes treated with melatonin implants and induced to ovulate with the ram effect

Two experiments were conducted to examine the effects of treating seasonally anoestrous ewes with melatonin before ram introduction on reproductive response, and on LH secretion in anoestrous ewes induced to ovulate by rams.In Experiment 1, a total of 667 ewes from three flocks involving Merino (Flock 1, N = 149), Merino entrefino (Flock 2, N = 325) and Rasa Aragonesa (Flock 3, N = 203) breeds were used. Within each flock, ewes isolated from rams since the previous lambing were assigned at random to receive melatonin implants of Regulin (75, 175 and 105 in Merino, Merino entrefino and Rasa Aragonesa flocks, respectively) or to serve as untreated controls (74 in Merino, 150 in Merino entrefino and 98 in Rasa Aragonesa flocks). Fertile rams were introduced into all flocks 5 weeks after implantation in March (Flocks 1 and 2) or April (Flock 3), and remained with the ewes for a 50 day mating period. Percentage of ewes with luteal activity at ram introduction did not differ between melatonin treated and control ewes in any flock. There were no significant differences in either the mean interval from ram introduction to lambing or the distribution of lambing. Implantation with melatonin resulted in an improvement of prolificacy in all three flocks, although this only reached statistical significance in the Merino flock (1.15 vs. 1.03 in treated and control ewes, respectively, P < 0.05). Fertility was increased significantly (P < 0.05) in the Merino entrefino flock (64.5% in treated vs. 51.3% in control ewes).In Experiment 2, two trials were undertaken utilizing a total of 63 ewes. Trial 1 involved 24 mature Manchega ewes and Trial 2 involved 39 Merino ewe lambs. Half of the animals in each trial received a Regulin implant on 28 February (Trial 1) or 12 March (Trial 2) and the remaining half acted as controls. Rams were introduced 5 weeks after implantation and remained with the ewes for a 25 day period. In both trials, anoestrous ewes at ram introduction were bled at 20 min intervals for 3 h before and 5 h after ram introduction and then at 3 h intervals over the next 24 h for assessment of plasma concentrations of LH. Secretion of LH before or following introduction of rams was not affected by melatonin. Both treated and control anoestrous ewes in each trial responded to introduction of rams with an increase in the frequency of the LH pulses (P < 0.05), but no significant changes were detected in pulse amplitude or mean levels of LH. A preovulatory surge of LH was detected between 8 and 26 h after ram introduction, but neither mean interval from ram introduction to the peak of LH surge, nor the magnitude of the LH peak, was influenced by melatonin treatment.Results from this study show that: (1) melatonin implants administered during early seasonal anoestrus have the potential to improve reproductive performance in Spanish breeds of sheep, but the response is conditioned by breed, management system and environmental factors; (2) melatonin did not modify the secretion of LH in anoestrous ewes induced to ovulate by the ram effect under our experimental conditions.     

The introduction of rams induces an increase in pulsatile LH secretion in cyclic ewes during the breeding season

Application of the ram effect during the breeding season has been previously disregarded because the ewe reproductive axis is powerfully inhibited by luteal phase progesterone concentrations. However, anovulatory ewes treated with exogenous progestagens respond to ram introduction with an increase in LH concentrations. We therefore tested whether cyclic ewes would respond to ram introduction with an increase in pulsatile LH secretion at all stages of the estrous cycle. We did two experiments using genotypes native to temperate or Mediterranean regions. In Experiment 1 (UK), 12 randomly cycling, North of England Mule ewes were introduced to rams midway through a frequent blood-sampling regime. Ewes in the early (EL; n=3) [corrected] and late luteal (LL; n=6) phase responded to ram introduction with an increase in LH pulse frequency and mean and basal concentration [corrected] of LH (at least P<0.05). In Experiment 2 (Australia), the cycles of 32 Merino ewes were synchronised using intravaginal progestagen pessaries. Pessary insertion was staggered to produce eight ewes at each stage of the estrous cycle: follicular (F), early luteal (EL), mid-luteal (ML) and late luteal (LL). In all stages of the cycle, ewes responded to ram introduction with an increase in LH pulse frequency (P<0.01); EL, ML and LL ewes also had an increase in mean LH concentration (P<0.05). In conclusion, ram introduction to cyclic ewes stimulated an increase in pulsatile LH secretion, independent of ewe genotype or stage of the estrous cycle.