The timing of neuroendocrine sexual maturity in the male lamb by photoperiod

In spring-born female lambs, the long days of summer, followed by their gradual decrease, provide the seasonal cue necessary to time puberty to early autumn (approximately 30 wk of age). Male lambs begin spermatogenesis during mid-summer, some 20 wk before puberty occurs in females. Unlike young female lambs, male lambs attain puberty at the same age under a variety of photoperiodic manipulations, raising the possibility that sexual maturation in males is not affected by photoperiod. We have reinvestigated the role of photoperiod on puberty in the male lamb, using a more precise indicator of reproductive activation--the decreased sensitivity of the hypothalamo-pituitary axis to inhibitory steroid feedback leading to increased LH secretion. To test whether photoperiod can influence the onset of neuroendocrine sexual maturation in male lambs, this study compared the timing of the decrease in sensitivity to inhibitory steroid feedback in two groups of males under opposite photoperiodic conditions. Eight males were reared indoors from 2 wk of age under conditions simulating the natural increasing and decreasing day lengths around the summer solstice; an additional 7 males were exposed to a reversed simulated natural photoperiod in which the changes in day length were amplified and accelerated relative to outdoor conditions. Both groups of lambs were castrated and received s.c. implants of Silastic estradiol capsules to provide a constant steroid feedback signal. The timing of reduction in sensitivity to estradiol negative feedback, measured as a sustained increase in circulating of LH above 1.0 ng/ml, was used to define neuroendocrine sexual maturity.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of PMSG dosage on the reproductive performance of adult ewes and ewe lambs bred at a progestagen-PMSG synchronized estrus

The effect of dose of pregnant mares' serum gonadotropin (PMSG) on the reproductive performance of adult ewes and ewe lambs and lamb survival at birth after treatment with fluorogestone acetate (FGA)-impregnated intravaginal sponges and PMSG (250 IU or 500 IU) to synchronize estrus was evaluated. Ewes were exposed to rams for breeding at the synchronized and subsequent estrous cycles. The flock, comprised of three synthetic strains and two control breeds, was maintained in a controlled environment and exposed to an artificial light regimen which alternated at 4-mo intervals from 16h of light daily to 9h of light daily. Trials were conducted during January, May and September at the end of a 9-h daylength cycle. Adult ewes were bred in May and 8 mo later in January. Ewe lambs were bred in September at 6.5 to 7.5 mo of age. The overall reproductive performance of the adult ewes was similar at the two breedings: fertility approximately 90%, prolificacy approximately 2.7, fecundity approximately 240% and lambs born alive approximately 2.4. Dosage of PMSG had no effect. Reproductive performance of ewe lambs was lower and there was a strain x treatment interaction, suggesting greater variability in response. The results indicate there is no advantage to using a higher dose of PMSG in ewes with a natural relatively high fecundity. Moreover, the use of the artificial photoperiod appears to overcome the natural seasonal variation in reproductive performance.     

Importance of changing photoperiod and melatonin secretory pattern in determining the length of the breeding season in the Suffolk ewe

Three groups of ovariectomized Suffolk ewes bearing s.c. Silastic implants of oestradiol were subjected to a 90-day priming treatment of an inhibitory long photoperiod (16 h light/day; 16L:8D). On Day 0 of the experiment, they were moved to stimulatory photoperiods. One control group was transferred to 12L:12D and a second control group was transferred to 8L:16D; both groups remained in those photoperiods to determine the timing of reproductive induction and refractoriness. The experimental group was transferred to 12L:12D on Day 0 and then to 8L:16D on Day 55 to determine whether the further reduction in daylength could delay the development of refractoriness. Reproductive neuroendocrine condition was monitored by serum concentrations of LH and FSH. Both gonadotrophins remained elevated for a longer period of time in the experimental group receiving the second reduction in daylength than in either control group, indicating that the second photoperiodic drop delayed the onset of photorefractoriness. Measurement of 24-h patterns of circulating melatonin suggests that the prolonged stimulation of reproductive neuroendocrine activity in the experimental group resulted from a lengthening of the nocturnal melatonin rise. These findings indicate that refractoriness to an inductive photoperiod can be temporarily overcome by exposure to a shorter daylength, and that the change in duration of the nocturnal increase in melatonin secretion is important in photoperiodic signalling. Thus, in natural conditions, the decreasing autumnal daylength, and the resulting expansion of the nocturnal elevation in melatonin secretion, may be utilized to produce a breeding season of normal duration.     

Social cues can play a role in timing onset of the breeding season of the ewe

Female Suffolk sheep were pinealectomized around the vernal equinox to eliminate the major environmental input to the reproductive system (photoperiod) and then either isolated from, or maintained with, pineal-intact gonad-intact sheep. The ewes were ovariectomized and treated with constant-release oestradiol implants and reproductive state was monitored by measuring serum LH concentrations. Pinealectomized ewes that were isolated from the normal flock showed a 2 1/2-month delay in onset of the seasonal rise in LH values compared with that of pineal-intact controls (18 November vs 5 September). On the other hand, pinealectomized ewes that were maintained with the flock showed an onset of the seasonal rise in LH that was not delayed. These results suggest a timekeeping role for social cues for timing onset of the breeding season in an animal that normally relies on photoperiodic signals for temporal regulation of the seasonal reproductive cycle.     

Serum luteinizing hormone, growth hormone and insulin-like growth factor-I after releasing hormone challenge in prepubertal ewe lambs selected for twinning

Two studies evaluated hormonal markers as indicators of the onset of puberty in Debouillet sheep selected for twinning. In Trial 1, 29 ewe lambs (50 +/- 0.5 kg, 159 to 187 d of age) were given 10 microg GnRH (i.v.) on September 15 and blood was collected at 30 min intervals after the injection for 2 h. Additional samples were taken twice weekly and progesterone (P4) was measured. The day that serum P4 was greater than 1 ng/mL for 2 consecutive sampling days was classified as the day of puberty. Average day of puberty was October 12 (average age at puberty was 199 d) and ewes with values less or greater than the average were classified as early or late, respectively. Average weight at GnRH challenge was 50 kg and ewes weighing less or more were classified as light or heavy, respectively. Early ewes weighed more (P = 0.01) and reached puberty sooner (P = 0.01) than late ewes. Heavy lambs reached puberty earlier, weighed more at GnRH challenge, and had more LH area under the curve (AUC, P < 0.05) than light ewes. In Trial 2, we gave 27 ewe lambs (54 +/- 0.9 kg, 173 to 189 d of age) a single i.v. injection of 10 microg GnRH and 10 microg GHRH on September 17. Average day of puberty was October 13, average weight was 54 kg, and average age at puberty was 208 d. Categories were designated as described for Trial 1. Early lambs reached puberty sooner (P = 0.01) and weighed more (P = 0.01) than late lambs, but the puberty groups had similar LH AUC (P = 0.64) and GH AUC (P = 0.75), whereas IGF-I was greater (P = 0.01) in early puberty ewes than in late puberty ewes. Heavy lambs reached puberty earlier (P = 0.06), weighed more (P = 0.01), and tended (P = 0.11) to have more GH AUC than light ewes. No difference was observed in LH AUC or IGF-I between weight groups (P > 0.15). Results suggest that serum LH after GnRH is not a reliable indicator of the onset of puberty in ewe lambs selected for twinning, but heavier ewes tended to produce more GH after a GHRH challenge and reach puberty earlier than lighter ewe lambs.     

Role of photorefractoriness in onset of anoestrus in Rambouillet x Dorset ewes

An experiment was conducted to determine the extent to which refractoriness to short daylength is involved in the onset of anoestrus in Rambouillet x Dorset ewes. Ovary-intact ewes (N = 36) were exposed to ambient photoperiod (C) or to a photoperiod equal to the winter solstice (S) beginning on 21 December 1986 and continuing until 21 April 1987. Samples of serum were obtained at weekly intervals and assayed for progesterone to assess ovarian activity, and for prolactin to assess response to photoperiod treatment. In addition, ovariectomized (ovx) ewes with implants containing oestradiol were housed with C(Covx) and S (Sovx) ewes (N = 4 per treatment). The concentration of LH was determined in serum collected biweekly from all ovariectomized ewes throughout the treatment period as an index of reproductive status. Time-trends for concentrations of LH differed (P less than 0.005) for ewes in Groups Covx and Sovx with LH decreasing on the order of about 8-fold in Group Covx through the treatment period, while not changing appreciably in Group Sovx. Intact ewes in both treatments began to become anoestrous by the 9th week of treatment (about the last week in February). However, fewer Group S than Group C ewes were anoestrous at 13 and 14 weeks of treatment (P = 0.09). Time-trends for concentrations of prolactin also differed for the Group C and S ewes (P less than 0.005), probably reflecting the divergence in duration of photoperiod between the two treatments by the end of the study.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence that the onset of the breeding season in the ewe may be independent of decreasing plasma prolactin concentrations

Ten ewes of each of two breeds, Dorset Horn (long breeding season) and Welsh Mountain (short breeding season), were given subcutaneous oestradiol-17 beta implants and then ovariectomized. Another 10 ewes of each breed were left intact. On 3 May 1982, all the ewes were housed in an artificial photoperiod of 16L:8D. After 4 weeks, half of the ewes of each breed and physiological state were abruptly exposed to a short-day (8L:16D) photoperiod while the others remained in long days (16L:8D). The time of onset of the breeding season was significantly (P less than 0.05) advanced in ewes switched to short days (12 August +/- 10 days) compared to those maintained in long days (4 September +/- 14 days). Dorset Horn ewes began to cycle (20 July +/- 7 days) significantly (P less than 0.001) earlier than Welsh Mountain ewes (19 September +/- 6 days). Disparities in the time of onset of cyclic activity in ewes of different breeds and daylength groups were echoed in disparities in the time at which plasma LH and FSH concentrations rose in oestrogen-implanted, ovariectomized ewes of the same light treatment group. Prolactin concentrations showed an immediate decrease in ewes switched to short days, but remained elevated in long-day ewes. Since the breeding season started in the presence of high prolactin concentrations in long-day ewes, it seems unlikely that prolactin is an important factor determining the timing of the onset of cyclic activity.     

Photoperiodic history and a changing melatonin pattern can determine the neuroendocrine response of the ewe to daylength

The reproductive neuroendocrine response of Suffolk ewes to the direction of daylength change was determined in animals which were ovariectomized and treated with constant release capsules of oestradiol. Two groups of animals were initially exposed to 16 or 10 h light/day for 74 days. On day zero of the study, when one group of ewes was reproductively stimulated (elevated LH concentrations) and the other reproductively inhibited (undetectable LH concentrations), half the animals from each group were transferred to an intermediate daylength of 13 h light/day. The remaining ewes were maintained on their respective solstice photoperiods to control for photorefractoriness. LH concentrations rose in animals experiencing a 3 h decrease in daylength from 16L:8D to 13L:11D while LH concentrations fell to undetectable values in those that experienced a 3 h increase in daylength from 10L:14D to 13L:11D. The photoperiodic response of the Suffolk ewe, therefore, depends on her daylength history. Such a result could be explained if the 24-h secretory pattern of melatonin secretion, known to transduce photoperiodic information to the reproductive axis, was influenced by the direction of change of daylength. Hourly samples for melatonin were collected for 24 h 17 days before and three times after transfer to 13L:11D. The melatonin secretory profile always conformed to daylength. Therefore, the mechanism by which the same photoperiod can produce opposite neuroendocrine responses must lie downstream from the pineal gland in the processing of the melatonin signal.     

Variation in the timing of the reproductive season among breeds of sheep in relation to differences in photoperiodic synchronization of an endogenous rhythm.

Photoperiod may regulate seasonal reproduction either by providing the primary driving force for the reproductive transitions or by synchronizing an endogenous reproductive rhythm. This study evaluated whether breed differences in timing of the reproductive seasons of Finnish Landrace (Finn) and Galway ewes are due to differences in photoperiodic drive of the reproductive transitions or to differences in photoperiodic synchronization of the endogenous rhythm of reproductive activity. The importance of decreasing photoperiod after the summer solstice in determining the onset and duration of the breeding season was tested by housing ewes from the summer solstice in either a simulated natural photoperiod or a fixed summer-solstice photoperiod (18 h light:6 h dark; summer-solstice hold). Onset of the breeding season within each breed did not differ between these photoperiodic treatments, but Galway ewes began and ended their breeding season earlier than Finn ewes. The duration of the breeding season was shorter in Galway ewes on summer-solstice hold than on simulated natural photoperiod; duration did not differ between photoperiodic treatments in Finn ewes. The requirement for increasing photoperiod after the winter solstice for initiation of anoestrus was tested by exposing ewes from the winter solstice to either a simulated natural photoperiod or a winter-solstice hold photoperiod (8.5 h light:15.5 h dark). Onset of anoestrus within each breed did not differ between these photoperiodic treatments, but the time of this transition differed between breeds. These observations suggest that genetic differences in timing of the breeding season in Galway and Finn ewes do not reflect differences in the extent to which photoperiod drives the reproductive transitions, because neither breed requires shortening days to enter the breeding season or lengthening days to end it at appropriate times. These findings are consistent with the hypothesis that photoperiod synchronizes an endogenous rhythm of reproductive activity in both breeds and that genetic differences in timing of the breeding season reflect differences in photoperiodic synchronization of this rhythm.     

Timing of puberty and synchronization of seasonal rhythms by simulated natural photoperiods in female Siberian hamsters

The timing of puberty is a critical life history trait of short-lived species; spring-born individuals mature rapidly and breed in the season of birth, whereas young born in mid- to late summer delay puberty until the next spring. The cues that govern the transition from rapid to delayed maturation in natural populations remain unknown. To identify ecologically relevant photoperiod cues that control timing of puberty, we monitored nine cohorts of female Siberian hamsters (Phodopus sungorus) born every 2 wk from 4 wk before to 12 wk after the summer solstice in a simulated natural photoperiod (SNP). Hamsters born by the summer solstice underwent rapid somatic growth and achieved puberty that summer; among females born 2-4 wk after the solstice, some delayed puberty by many weeks, whereas others manifested early puberty. Hamsters born 6 or more weeks after the solstice generally delayed puberty until the following spring. The transition from accelerated to delayed pubertal development in the SNP occurred at day lengths that induce early puberty when presented as static photoperiods. Despite differences in timing of birth and timing of puberty, fall and subsequent spring seasonal events occurred at similar calendar dates in all cohorts. We found no evidence that prenatal photoperiod history influenced postnatal development of female hamsters. Considered together with a parallel study on males, the present findings point to sex differences in responsiveness to natural photoperiod variations. In both sexes, incrementally changing photoperiods exert a strong organizing effect on seasonal rhythms.     

Reproductive potential and endocrinological responses of sheep kept under controlled lighting. II. Pituitary and gonadal responses of ewes and rams to a six-monthly light cycle

Forty entire ewes of mixed breeds were kept in environmentally-controlled rooms with a 6-monthly light cycle. Six mature spayed Border Leicester × Merino ewes and four mature intact Poll Dorset rams were kept under the same conditions.Over a period of 2 years (four light cycles) estimates were made of ovarian, testicular and pituitary activity in response to the artificial light regime. Non-pregnant ewes were bled twice weekly: peripheral plasma progesterone levels > 1 ng/ml were taken as indicative of ovarian activity. Testicular activity was estimated by weekly tests for peripheral plasma testosterone and scrotal sac volume. Pituitary activity was estimated monthly by the response to the injection (i.v.) of 75 μg gonadotrophin releasing hormone (GnRH) to rams and of 18.75, 37.5 or 75 μg to ewes, using peripheral plasma luteinising hormone (LH) estimations from the time of GnRH injection.Data for ovarian and testicular measurements were classed into categories according to week and for pituitary response to month of the light cycle.Cubic regression analyses were conducted on the percentage of ewes with progesterone levels > 1 ng/ml and on ram testosterone and scrotal measurements.Despite the irregularity of the curve for the light cycle, the ovarian and testicular responses of rams and ewes followed an alternating curve with peaks and troughs of activity separated by approximately 13 weeks in the 26-week cycle. The peak of ovarian activity occurred during the long daylength period which followed a 22-week period of decreasing daylength and was preceded by 1 month by peak ram testosterone and scrotal sac volume.The pattern of pituitary response was related to that of the actual light cycle and the data were subjected to time-lag regression. This econometrical technique revealed that there was a delay in pituitary response to daylength changes of 2 months for rams, and between 2 and 3 months for the spayed ewes. The peak pituitary response to the GnRH test occurred one month earlier for rams than for the spayed ewes, and coincided with the corresponding troughs of gonadal activity of each sex.The results showed that the breeding season of sheep can be compressed into 6 months and that the pattern of pituitary response follows the daylength pattern more closely than do measurements of gonadal activity. Peak reproductive activity in rams, as measured by pituitary and gonadal activity, precedes that of ewes by approximately 1 month.     

Oestrous cycle dynamics in peri-pubertal and mature Javanese thin-tail sheep

Duration of oestrus, time of ovulation and hormone profiles for progesterone and LH in prepubertal, pubertal and mature Javanese thin-tail sheep were studied at synchronized oestrus following progestagen-PMSG treatment and at the first natural oestrus after synchronization.The ewe lambs responded to progestagen-PMSG treatment by showing earlier onset of oestrus and an earlier and higher peak of LH concentration than mature ewes. For pre-pubertal, pubertal and mature ewes the mean LH peaks were 49.9, 43.9 and 37.9 ng/ml (P>0.05) at mean intervals of 7.5, 8.4 and 16.5 h (P < 0.05), respectively, after onset of oestrus. Duration of oestrus was 41.2 h in pubertal lambs and averaged 37.5 h in the other two groups (P>0.05). Except in one mature ewe, ovulation occurred between 24 and 36 h after onset of oestrus and the majority ovulated at around the end of oestrus. The corpora lutea developed normally, as indicated by plasma-progesterone changes. The patterns of plasma-progesterone changes were similar in all three groups, though the concentrations were lower in the ewe lambs.At the first natural oestrus after synchronization, mature ewes showed longer (P>0.05) oestrus (31.5 vs. 24.3 h), longer time interval from onset of oestrus to the LH peak (16.0 vs. 12.0 h) and from the LH peak to ovulation (21.0 vs. 19.6 h) than peri-pubertal lambs. Six of eight pre-pubertal lambs did not ovulate at their first natural oestrus, resulting in a conception rate of 11% for that group, while in pubertal lambs and mature ewes conception rates were 70% and 100%, respectively.     

Absence of an increase in gonad-independent drive to pulsatile luteinizing hormone secretion during photoperiod-induced puberty

This study was conducted to determine if photoperiod can influence the pattern of luteinizing hormone (LH) secretion in the absence of the ovaries in the developing female sheep. Lambs were raised in a photoperiod sequence (short, long, short days) known to induce puberty between 30 and 35 wk of age, or in a photoperiod (only short days) that prevents puberty during the first year. Their ovaries were removed at 10 wk of age, and the detailed pattern of LH was assessed (samples at 12-min intervals for 4 h) each 3- to 5-wk period between 9 and 45 wk of age. Rapid LH pulses (40- to 50-min interpulse interval) were evident within a few weeks after ovariectomy in both groups of females. Those exposed to the artificial photoperiod sequence that induces normal sexual maturity did not increase their pulse frequency further during the pubertal period. Moreover, their LH pulse frequencies were not greater than those in agonadal females exposed to the photoperiod that delays puberty. These findings indicate that photoperiodic induction of puberty in the sheep does not require steroid-independent modulation of pulsatile LH secretion.     

Melatonin and the pituitary response to GnRH in prepubertal and adult ewes

GnRH-induced LH-release was studied in female lambs from 5 weeks of age until puberty and in adult anoestrous ewes. In pre-pubertal animals LH was released rapidly after GnRH treatment but after puberty the responses became slower and more sustained although the peak concentration did not change. In neither pre-pubertal nor adult sheep did prior treatment with melatonin influence LH release after GnRH treatment.     

Ovulatory activity and plasma prolactin concentrations in wild and domestic ewes exposed to artificial photoperiods between the winter and summer solstices

Mouflon and domestic Manchega sheep differ in the timing of their reproductive season under natural photoperiod (NP) conditions. This study examines whether they also differ in their reproductive responses to artificial photoperiod cues. For this, mouflons (n=24) and ewes (n=24) were exposed between the winter and summer solstices to artificial long days (LD: 16 h light/day), to short days (SD) simulated via the use of melatonin implants, or to NP conditions (controls), and their ovulatory activity monitored. The effects of these treatments on annual changes in prolactin concentration were also recorded. In the LD mouflon ewes, the offset and onset (7 March ± 5 and 2 October ± 4, respectively) of cyclic ovulatory activity occurred earlier (P<0.001) than in the NP animals (26 April ± 6 and 20 October ± 2, respectively), but no differences were seen (P>0.05) between the SD and NP mouflon ewes in either the onset of anoestrus (12 May ± 6 and 26 April ± 6, respectively) or the onset of subsequent ovulatory activity (13 October ± 8 and 20 October ± 2, respectively); however the duration of the anoestrus period was significantly reduced in the SD. In LD Manchega ewes, the onset of anoestrus was advanced (2 February ± 5 vs 15 March ± 11), but ovulatory activity started at the same time as in NP Manchega ewes (16 July ± 4 vs 5 July ± 8). In the SD Manchega ewes, two animals showed continuous cyclic ovulatory activity over the course of the experiment while the remainder entered anoestrus two months later (16 May ± 6, P<0.001) than their NP counterparts. In these SD ewes, the onset of cyclic ovarian activity was very variable. An annual rhythm of plasma prolactin concentration was seen in both the mouflon and Manchega ewes under all three photoperiod conditions. However, the amplitudes of the changes seen in prolactin concentration were smaller in both the LD and SD animals than in the corresponding NP animals (P<0.001). In conclusion, the results show that these two types of Mediterranean sheep differ in their ovulatory response when subjected to artificial photoperiods. The results also suggest that refractoriness to SDs may be the most important physiological mechanism regulating the onset of anoestrus in highly seasonal breeds, but not in less seasonal breeds.     

Reproductive activity in Finnish Landrace x Rambouillet ewes exposed to various daylength cycles applied over periods of six months

Twenty-six Finnish Landrace x Rambouillet ewes were exposed to a series of artificial daylength cycles over periods of 6 mo. The cycles in general consisted of 3 mo. of long or increasing daylength, followed by 3 mo. of short or decreasing daylength. Increasing daylength to 22 hr.; 17.5 hr. or 15.75 hr. but not 14 hr. resulted in the onset of anestrus in all ewes. Only four of eight ewes in the 14 hr. cycle became anestrus. Abrupt decreases in daylength resulted in a more rapid return to reproductive cyclicity than a gradual decline. In addition, there appeared to be an inverse relationship between the length of the longest day of a treatment cycle and the time taken for ewes to return to reproductive cyclicity, following a subsequent abrupt decrease in daylength. Reducing daylength form 22 hr. to 17.5 hr. did not induce reproductive cyclicity. Reducing daylength from 17.5 hr. to 15 hr.; 12.75 hr. or 7.5 hr. did result in the onset of reproductive cyclicity. Finally, extending the period of long day exposure in a daylength cycle decreased the response time to a subsequent reduction in daylength. It was concluded that reproductive seasonality may be initiated by the interaction of darkness with a daily photosensitive phase in the ewe.     

Effects of season of birth on the prepubertal pattern of gonadotropin secretion and age at puberty in beef heifers

There is an early transient rise in gonadotropin secretion in spring-born prepubertal heifers and there is an indication that this pattern is different in autumn-born heifers. The effect of season of birth on age and weight at puberty is equivocal. This study was designed to compare the temporal patterns of LH and FSH secretion between spring- and autumn-born heifers and to determine the effects of season of birth on age and weight at puberty. Blood samples from 2 groups of heifer calves born in spring (last week of March, n = 5) or autumn (last week of October, n = 5) were collected every other week from birth to puberty and every 15 min for 10 h at 6, 12, 18, 24 and 32 wk of age. Timing of puberty was determined by measuring progesterone in plasma samples collected every 2 to 3 d starting at 42 wk of age. Age and weight at onset of puberty did not differ between the 2 groups of heifers (P > 0.05); however, the autumn-born heifers tended to mature in a wider range of ages and weights. Based on the 10-h sampling periods, mean serum concentrations of LH and LH pulse frequency and amplitude were higher in spring-born heifers at 18 wk of age than in autumn-born heifers (P < 0.05). In spring-born heifers, LH pulse frequency increased over time to 32 wk of age, and LH pulse amplitude was higher at 12 and 18 wk than at 32 wk of age (P < 0.05). Autumn-born heifers had higher LH pulse frequency at 6 wk and showed a decrease in mean concentrations of LH at 12 and 18 wk of age (P < 0.05). The FSH pulse frequency of spring-born heifers was higher at 12 wk of age than in autumn-born heifers (P < 0.05), FSH pulse amplitude in autumn-born heifers decreased from 6 to 32 wk of age. It was concluded that although the mean age and weight at puberty did not differ between spring- and autumn-born heifers, the range in age and weight at puberty was wider in the autumn-born heifers. The patterns of LH secretion differed between spring- and autumn-born prepubertal heifers, with spring-born calves exhibiting an early rise in LH secretion, while mean serum concentrations of LH decreased during this period in autumn-born heifers.     

Influence of season of birth on growth and reproductive development of Brahman bulls

Seasonal effects on reproduction are more dramatic in Bos indicus than Bos taurus cattle. This experiment evaluated reproductive development of fall- (n=7) versus spring- (n = 10) born Brahman bulls to determine if season of birth affects reproductive development. Measurements of growth and reproductive development began after weaning and continued at bi-weekly intervals until each bull reached sexual maturity. Different stages of sexual development were classified according to characteristics of the ejaculate and included first sperm in the ejaculate, puberty (> 50 x 10(6) sperm/ejaculate), and sexual maturity (two ejaculates with > 500 = 10(6) sperm/ejaculate). Average daily increases in all measured traits were similar in fall- and spring-born bulls and there were no differences in age, body weight, scrotal circumference, or paired testis volume between groups at first sperm or puberty. However, fall-born bulls were older (P < 0.05) than spring-born bulls at sexual maturity (553 days versus 481 days, respectively) as the interval between puberty and sexual maturity was longer (P < 0.05) in fall- than in spring-born bulls (82 days versus 54 days, respectively). The prolonged interval between puberty and sexual maturity in fall-born calves coincided with a short photoperiod (winter) whereas the short interval between puberty and sexual maturity in spring-born calves coincided with a long photoperiod (summer). In conclusion, season of birth affected sexual development; photoperiod might be involved in regulating testicular function immediately after puberty in Brahman bulls.     

The influence of artificial photoperiod on the growth,appetite and reproductive status of male red deer and sheep

Young male red deer and Suffolk X (Finn X Dorset) sheep were kept on an artificial photoperiod such that two cycles of daylength occurred during one calendar year. They were penned separately, fed to appetite, weighed weekly and measured tri-weekly.Both species showed two cycles of intake, growth and gonadal activity in response to the daylength when only one would have been shown on natural photoperiod, although in the sheep these cycles were of lower amplitude than in the deer. The deer grew two sets of antlers during the study. A lag of some 3–4 months occurred between an event such as peak food intake and the time it would have been expected to occur relative to the daylength cycle. It is considered that although daylength controls these cycles, there is an endogenous rhythm which photoperiod cannot completely suppress.     

The effect of experience on twin-care in New Zealand Romney sheep

Behaviour of ewes at lambing was observed in a flock of N.Z. Romney sheep composed of selected (or “elite”) ewes that had produced litters of 2 or more lambs at least twice previously, and 2-year-old ewes that had produced 0, 1 or 2 lambs at 12 months of age. The observations concentrated on twin-bearing ewes.Only 6% of twin-bearing elite ewes became separated from a twin compared with 16% of primiparous ewes. Conflicts between elite and younger ewes over ownership of twin lambs were won by elite ewes in 6 of 7 cases, and elite ewes gathered their twin lambs together more quickly after artificial separation than primiparous ewes.Irrespective of litter size, primiparous ewes remained within 20 m of their birth-site for a mean of 11 h compared with 7 h for elite ewes, and the incidence of separation from a twin in primiparous ewes was higher with ewes that moved from the birth-site within 6 h of birth than in those that remained for more than 6 h. Also, primiparous ewes appeared more reluctant than elite ewes to leave one twin to recover the other that had been deliberately separated.The behaviour towards twins in the small number of 2-year-old ewes that had produced twins at 12 months of age was similar to that of the elite ewes in several respects, but contrasted with behaviour of primiparous mothers of the same age. This indicates that twin care improved with experience rather than age. The behaviour towards twins in 2-year-old ewes that had previously produced a single only tended to fall between that of primiparous ewes and ewes that had produced twins when 12 months old.