Effects of season, age and increased photoperiod on reproductive hormone concentrations and testicular diameters in thoroughbred stallions

Testicular diameters and monthly blood samples were obtained from 83 stallions aged 4 to 22 years that were maintained on Central Kentucky Thoroughbred stud farms. The effects of age, season, and exposure to increased photoperiod (16 hours light/day, December 15 to April 1) on testicular diameters and plasma concentrations of FSH, LH and testosterone were studied.The results indicated that Thoroughbred stallions show distinct seasonal and age related changes in most of the reproductive parameters studied and that exposure of such stallions to increased photoperiod produced significant alterations in these changes. Although lighting stimulated testicular growth and testosterone secretion early in the breeding season such changes were short lived. Lighted stallions appeared to become refractory to the lighting program since both testicular size and plasma testosterone concentrations were significantly reduced by June.     

Effects of age,season, and fertility status on plasma and intratesticular immunoreactive (IR) inhibin concentrations in stallions

The nature of the relationship between inhibin and reproductive function in the stallion is yet to be elucidated. Blood and testes from 51 light horse stallions ranging in age from 2 mo to 25 years were collected during the breeding and nonbreeding seasons to study the effects of testicular maturation, aging, season, and fertility status on peripheral and intratesticular concentrations of Ir inhibin and other reproductive hormones. Of the 51 stallions, 12 age-matched stallions (6 fertile, 3 subfertile, and 3 infertile) were used in the fertility study. Blood samples were taken before castration and plasma stored at −20°C for analysis of Ir inhibin, luteinizing hormone (LH), follicle stimulating hormone (FSH), testosterone (T), estradiol (E₂), and estrogen conjugates (EC) by radioimmunoassay (RIA). Testes were homogenized and testicular extracts prepared and frozen at −70°C for analysis of Ir inhibin, T, E₂, and EC by RIA. Plasma concentrations of Ir inhibin, LH, FSH, T, E₂, and EC and intratesticular concentrations of Ir inhibin, T, E₂, and EC increased with age (P < 0.01). The most dramatic effect appeared to be during testicular maturation. An aging effect was not observed in adult stallions. A seasonal effect was not detected for any of the plasma hormones, whereas for the intratesticular hormones the only change noted was an increase in T in the nonbreeding season (P < 0.05). Plasma Ir inhibin, E₂, and EC were lower (P < 0.01) and gonadotropins higher (P < 0.05) in infertile stallions. Plasma T levels did not change. Intratesticular Ir inhibin concentrations tended to be lower (P < 0.1) in subfertile stallions and significantly lower (P < 0.01) in infertile stallions, whereas intratesticular steroid levels were not different among the three groups. In conclusion, plasma and intratesticular Ir inhibin concentrations seem to be affected by testicular maturation and fertility status.     

Sexual Activity Influences the Secretion of Reproductive Hormones in the Stallion

Contents The aim of this study was to investigate the effect of sexual activity on concentrations of reproductive hormones in plasma of stallions. In the first experiment, two groups of stallions were monitored for secretion of luteinizing hormone (LH), testosterone and oestradiol from the beginning until shortly after the end of the breeding season. One group of animals were reserve stallions not used for breeding (group 1, n = 10), the other group consisted of active breeding sires (group 2, n = 8). Blood samples were withdrawn from March to August at 14-day intervals. In sexually nonactive stallions (group 1), seasonal variations in LH, testosterone and oestradiol occurred and concentrations of these hormones reached a maximum in May (p < 0.05). In the breeding stallions (group 2), no significant changes in the concentrations of these hormones were found between March and August. Concentrations of LH and testosterone were significantly lower in breeding stallions than in reserve stallions at most blood sampling times (p < 0.05). In the reserve stallions, oestradiol concentrations were significantly higher than in the breeding stallions in April and in June (p < 0.05). In a second experiment, the effect of regular sexual activity (semen collection three times per week) on the concentration of LH, testosterone and oestradiol was tested in a group of breeding stallions after a period of sexual rest for several weeks. Blood samples were taken once daily starting the day before the first semen collection was performed. Testosterone concentration significantly decreased in the first days after semen collection started (p < 0.05), while LH secretion was only transiently decreased and no effects on oestradiol concentration were found. In both experiments, semen parameters were within the normal range of fertile stallions. No correlations between the sexual drive of the stallions and concentration of reproductive hormones occurred. It can be concluded that in the stallion the secretion of reproductive hormones is influenced by sexual activity. Regular semen collection seems to inhibit testosterone release by unknown mechanisms while the effects on LH and oestradiol secretion are less pronounced.     

The effects of equine somatotropin on pituitary and testicular function in the stallion during the nonbreeding season

An experiment was conducted to determine the effects of equine somatotropin on the reproductive axis of the stallion during the nonbreeding season. Adult stallions were treated with equine somatotropin (20 μg/kg body weight [BW]; n = 5) or saline (n = 4) daily for 21 days starting in January. During the last week of treatment, stallions were subjected to low- and high-dose injections of luteinizing hormone (LH), as well as low- and high-dose injections of gonadotropin-releasing hormone (GnRH) and thyrotropin-releasing hormone (TRH). Two months after the onset of somatotropin treatment, semen was collected from all stallions every other day for 14 days. Treatment with equine somatotropin increased (P < .001) daily IGF-1 concentrations but had no effect (P > .1) on concentrations of LH, follicle-stimulating hormone (FSH), or testosterone. The testosterone responses to injections of LH were similar (P > .1) between treatments. Likewise, the LH, FSH, prolactin, and testosterone responses to the injections of GnRH/TRH were similar (P > .1) between groups. At seminal collections, stallions treated with somatotropin exhibited greater volumes of gel-free semen (P < .01) and gel (P < .05) and had decreased time until ejaculation (P < .05). In conclusion, somatotropin treatment for 21 days may alter the long-term accessory gland contribution to seminal volume but does not appear to alter pituitary gonadotrope function or testicular testosterone secretion.     

Influences of season and artificial photoperiod on stallions: luteinizing hormone follicle-stimulating hormone and testosterone

Influence of day length on seasonal endocrine responses were studied using stallions (seven per group). Treatments included 1) control, with natural day length; 2) 8 h light and 16 h dark (8:16) for 20 wk beginning July 16, 1982 then 16:8 from December 2, 1982 until March 5, 1984 (S-L); or 3) 8:16 from July 16, 1982 until March 5, 1984 (S-S). Blood was sampled hourly for 5 h every 4 wk; sera were pooled within horse, and luteinizing hormone (LH), follicle-stimulating hormone (FSH) and testosterone were quantified. Blood was collected every 20 min for 24 h every 8 wk and 2 wk before and after the December light shift. Samples were assayed for LH. Stallions in all groups underwent seasonal changes (P less than .05) in concentrations of LH, FSH, testosterone and basal concentrations of LH and amplitude of LH pulses. Season X treatment (P less than .05) reflected on early recrudescence of LH, FSH and testosterone concentrations in S-L stallions followed by earlier regression. Except for FSH hormone concentrations were depressed in S-S stallions. Number of LH pulses per 24 h was unaffected by season, treatment or their interaction. Mean amplitude of LH pulses was affected (P less than .05) by season X treatment; maximal values occurred in April vs February for control and S-L stallions, and minimal values occurred in December vs April. The season X treatment interaction (P less than .05) similarly affected basal concentrations of LH. Thus, seasonal changes in concentrations of LH, FSH and testosterone can be driven by photoperiod. Increased peripheral concentrations of LH during seasonal recrudescence of reproductive function apparently results from more LH secreted per discharge without an increased frequency of LH discharges.     

Testosterone serum profile,semen characteristics and testicular biometry of Mangalarga Marchador stallions in a tropical environment

This study was conducted to characterize the daily profile of testosterone secretion and its mean concentrations in the four seasons as well as to evaluate the semen characteristics and testicular biometry of Mangalarga Marchador stallions throughout the year in a tropical region. Three stallions were submitted to semen collections and evaluation of testicular biometry every 14 days along a year. Blood samples were collected once at the middle of each season, in a 20‐min interval during 24 hr in order to evaluate the testosterone secretion profiles among seasons. Testosterone concentrations along the day were higher at the beginning of the afternoon (from 12:00 to 15:00 hr), but a circadian secretion was not clearly observed. Mean testosterone concentrations did not differ among seasons (p > .05), but a pattern of secretion along the day showed variations with higher concentrations in the afternoon during the winter. Ejaculate volume was higher during summer; however, sperm motility decreased in summer and spring. Total sperm in ejaculate, sperm morphology and testicular biometry kept constant along the year showing no differences among the seasons. The results demonstrated that in a tropical region, reproductive aspects of stallions did not show a clearly defined seasonal variation, and months of autumn and winter were not unsuitable for reproduction of the males.     

Influences of season and artificial photoperiod on stallions: pituitary and testicular responses to exogenous GnRH

Effects of season and photoperiod on the anterior pituitary gland and testes were studied by responses to exogenous GnRH. Stallions were assigned to one of three treatments: 1) control, exposed to natural day length; 2) S-L, 8 h of light and 16 h dark (8:16) for 20 wk beginning July 16, 1982 then 16:8 from December 2, 1982 until March 5, 1984; or 3) S-S, 8:16 from July 16, 1982 until March 5, 1984. Approximately every 8 wk, stallions were administered GnRH (2 micrograms/kg BW) and blood was sampled at 20-min intervals for 2 h before and 8 h after GnRH administration. Concentrations of LH, FSH and testosterone were determined. Baseline concentrations (mean of pre-GnRH samples) of all hormones fluctuated seasonally (P less than .05), but only LH and testosterone displayed seasonal changes (P less than .05) in maximum response to GnRH (highest concentration above baseline after GnRH). The FSH response to GnRH was not affected (P greater than .05) by season, photoperiod or the season X treatment interaction. Exposure of S-L stallions to 16:8 in December resulted in early recrudescence of baseline concentrations of LH, FSH and testosterone. Maximum concentration of testosterone in response to GnRH was stimulated by 16:8, but the increase in baseline LH concentrations in S-L stallions was not associated with an increase in maximum LH response to GnRH. Seasonal patterns of baseline concentrations of FSH and testosterone and maximum LH response to GnRH in S-S stallions were similar to those for control stallions.(ABSTRACT TRUNCATED AT 250 WORDS)     

Morphometric Studies on the Testis of Korean Ring-necked Pheasant (<Emphasis Type="Italic">Phasianus colchicus karpowi</Emphasis>) during the Breeding and Non-breeding Seasons

The purpose of this study was to obtain detailed quantitative information on all cell types in the testis interstitium of Korean ring-necked pheasants and to combine these data with changes in the steroidogenic function of the testis during the breeding and non-breeding seasons. For animals collected during the breeding season, their testis weights, sperm production, serum testosterone levels and leuteinizing hormone (LH)-stimulated testosterone secretion were significantly (p < 0.01) increased compared to the non-breeding season. Testes of the pheasants during the non-breeding season displayed a 98% reduction in testis volume that was associated with a decrease in the absolute volume of seminiferous tubules (98% reduction), tubular lumen (100%), interstitium (90%), blood vessels (84%), lymphatic spaces (97%), Leydig cells (79%), mesenchymal cells (51%) and myoid cells (61%) compared to the breeding season. The numbers of Leydig cells, mesenchymal cells and myoid cells per testis in the breeding season were much higher than in the non-breeding season. Although the mean volume of a Leydig cell was 74% lower in the non-breeding season, the mean volumes of myoid and mesenchymal cells remained unchanged. These results demonstrate that there are striking differences in the testicular structure of the Korean ring-necked pheasant during the breeding and non-breeding seasons. Every structural parameter of the Leydig cell was positively correlated with both testosterone serum levels and LH-stimulated testosterone secretion. The correlation of changes in hormonal status with the morphometric alterations of Leydig cells suggests that the Korean-ring necked pheasant may be used as a model to study structure-function relationships in the avian testis.     

Recommendations for clinical GnRH challenge testing of stallions

Eight mature light-breed stallions with normal testes size, sperm output and semen quality were used to evaluate response to 3 GnRH challenge regimens in the summer in southeast Texas. Gonadotropin releasing hormone (50 μg) was administered intravenously once to each of eight stallions after three days of sexual rest (50 μg GnRH-1X). The same stallions were administered either 5μg GnRH intravenously once hourly for three injections (5 μg GnRH-3X) and 15μg GnRH intravenously once (15μg GnRH-1X) one and two weeks later. Blood samples were collected prior to and at intervals after GnRH administration. Plasma was immediately separated from blood samples and was frozen until assayed for LH, FSH, estradiol and testosterone concentrations. Percentage changes in hormone concentrations from pre-treatment values (baseline) were analyzed by paired studient'st-test to detect significant rises in hormone concentrations. Group mean percentage changes in hormone concentrations were analyzed by analysis of variance to compare responses among treatments. A computerized peak-detection algorithm (PC Pulsar) was used to detect peaks in LH and testosterone concentrations following 5 μg GnRH-3X and 15 μg GnRH-1X treatment.No differences (P>0.10) were detected in percentage change from baseline concentration for LH, FSH, or testosterone at one or two hours after administration of any of the three regimens of GnRH. When more frequent sampling intervals were analyzed for 5 μg GnRH-3X or 15 μg GnRH-1X treatments, no differences were detected in percentage change from baseline concentration for any hormone at 15, 30 or 60 minutes. Thereafter, percentage changes in concentrations of LH and FSH remained increased for 5μg GnRH-3X compared to 15 μg GnRH-1X treated stallions (P<0.05). Percentage changes in concentrations of testosterone were increased for 5μg GnRH-3X compared to 15 μg GnRH-1X treated stallions from 180–300 min (P<0.05), while no differences (P>0.10) were detected between 5 μg GnRH-3X and 15 μg GnRH-1X treated stallions for changes in concentrations of estradiol throughout the experiment.For 15 μg GnRH-1X treated stallions, maximum concentrations of LH in PC Pulsar-detected peaks occurred most commonly at 15 to 30 minutes (7/8 treatment periods) after GnRH injection. Maximum concentrations of testosterone in PC Pulsar-detected peaks occurred most commonly at 60–120 min (7/8 treatment periods) after GnRH injection.A protocol of blood sampling prior to, and 15, 30, 60 and 120 minutes after, intravenous administration of small doses of GnRH would be practical for challenge testing of stallions during the breeding season. In order to reduce cost of hormone assays, we suggest assay of the pre-challenge blood sample (baseline) could include LH, FSH, testosterone and estradiol concentrations (to assess overall hypothalamic-pituitary-testicularfunction), while only LH and testosterone concentrations need be determined after GnRH administration (to assess pituitary and testicular responsiveness). Assay for LH could be done on only the 15 and 30 minute post-GnRH samples, and assay for testosterone could be done on only the 60 and 120 minute post-GnRH samples. Failure to achieve approximately a 50% increase in LH concentration by 30 minutes after GnRH administration, and/or failure to achieve approximately a 100% increase in testosterone concentration by two hours after GnRH administration, could be further pursued either by treatment with increasing dosages of GnRH, or repeated administration of GnRH at hourly intervals, as has been suggested by other workers.     

Localization of aromatase in Equine Leydig cells

Stallion testes secrete large amounts of estrogens, but the cellular location of the enzyme that converts androgens to estrogens, cytochrome P₄₅₀ aromatase, has not been determined. The goal of the present study was to immunocytochemically localize stallion testicular aromatase using a polyclonal antibody generated against human placental cytochrome P₄₅₀ aromatase. Testes were obtained from 12 stallions from 2 to 23 years of age, during both the breeding and non-breeding seasons. Immunoreactivity was confined to the Leydig cells in all testes examined. No immunostaining was observed in the Sertoli or germ cells. Heterogeneity in the level of immunostaining among individual Leydig cells was observed. The results of this study indicate that in postpubertal, adult, and aged stallions, testicular aromatase is located in Leydig cells.     

Effects of Intratesticular Injection of 70% Glycerin on Stallions

The removal of endogenous germ cells of recipient stallions is a key step to produce donor germ cell-derived sperm using the germ cell transplantation technique. Six Thoroughbred stallions were divided into a treatment (n = 3) and a control group (n = 3), and 70% glycerin (1, 2, 3-trihydroxypropane, 40 mL per testis) or phosphate-buffered saline, respectively, was locally injected into testes. General semen evaluation, libido, and testicular volume were performed weekly from 3 weeks before to 10 weeks after treatment. The number of round germ cells in the ejaculate was counted using a hemocytometer. The hematoxylin and eosin staining was performed on the cross sections of testicular tissue obtained 11th week of treatment. Plasma testosterone levels in blood collected weekly were measured using a colorimetric competitive enzyme immunoassay kit. The sperm number was significantly lower than that of the control group at 5 and 10 weeks after glycerin injection. No differences in the status of spermatogenesis in the cross sections of seminiferous tubules and testicular volume were found between the two groups. The 70% glycerin-treated stallions had reduced total and progressively motile sperm and exhibited a significantly higher population of round germ cells in the ejaculate. Testosterone levels, testicular volumes, and libido of stallions were not significantly different between the groups. In conclusion, although intratesticular injection of 70% glycerin may have caused disassociation of some germ cells in the seminiferous tubules for several weeks, it did not significantly ablate germ cells in the tubules at 11 week in stallions.     

Patterns of secretion of luteinizing hormone, follicle stimulating hormone and testosterone in stallions during the summer and winter

Samples of jugular blood were drawn from each of five stallions every 15 min for 12 h during the summer and winter to determine the short-term fluctuations in plasma concentrations of luteinizing hormone (LH), follicle stimulating hormone (FSH) and testosterone. Concentrations of LH and FSH were generally not pulsatile, although one stallion exhibited three distinct pulses in these hormones during the winter. In general, patterns of secretion of all three hormones were similar in both seasons and the number of significant rises in hormonal concentrations did not differ between seasons. Concentrations of LH and FSH were positively correlated (P less than .05) for eight of the ten sampling periods, indicating a close relationship between the secretion rates of these two gonadotropins. Testosterone concentrations varied in an episodic manner during the 12-h period, and all stallions exhibited at least one episode of high testosterone secretion regardless of the pattern of LH concentrations. The response in testosterone concentrations to the three LH pulses exhibited by the one stallion in winter was not the same for each pulse. The correlations between a single random sample and mean concentrations over the 12-h period were high (r between .88 and .99) for all three hormones, indicating that a single sample of blood would be representative of overall concentrations. It appears that the stallion differs from males of other domestic species in that concentrations of gonadotropins and testosterone vary in a much less pulsatile manner.     

Immunolocalization of steroidogenic enzymes and their expression during the breeding season in the testes of wild raccoon dogs (Nyctereutes procyonoides)

The objective of this study was to investigate immunolocalization of steroidogenic enzymes 3βHSD, P450c17 and P450arom and their expression during the breeding season in wild male raccoon dogs. The testicular weight, size and seminiferous tubule diameters were measured, and histological and immunohistochemical observations of testes were performed. The messenger RNA expression (mRNA) of 3βHSD, P450c17 and P450arom was measured in the testes during the breeding season. 3βHSD was found in Leydig cells during the breeding and non‐breeding seasons with more intense staining in the breeding season. P450c17 was identified in Leydig cells and spermatids in the breeding season, whereas it was present only in Leydig cells in the non‐breeding season. The localization of P450arom changed seasonally: no immunostaining in the non‐breeding season; more extensive immunostaining in Leydig cells, Sertoli cells and elongating spermatids in the breeding season. In addition, 3βHSD, P450c17 and P450arom mRNA were also expressed in the testes during the breeding season. These results suggested that seasonal changes in testicular weight, size and seminiferous tubule diameter in the wild raccoon dog were correlated with spermatogenesis and immunoreactivity of steroidogenic enzymes and that steroidogenic enzymes may play an important role in the spermatogenesis and testicular recrudescence and regression process.     

Seasonal changes in spermatogenesis and immunolocalization of inhibin/activin subunits in the wild male ground squirrel (Citellus dauricus Brandt)

The objective of this study was to investigate the seasonal changes in spermatogenesis and the immunolocalization of the inhibin alpha and inhibin/activin (betaA and betaB) subunits during the breeding and non-breeding seasons in the wild male ground squirrel. The testicular weight and size and seminiferous tubule diameter were measured, and histological observations of testes were performed. The sections of the testes were immunostained by the avidin-biotin-peroxidase complex method (ABC) using polyclonal antisera raised against porcine inhibin alpha, inhibin/activin betaA and inhibin/activin betaB during the breeding and non-breeding seasons. There were marked variations in testicular weight and size and seminiferous tubule diameter between the breeding and non-breeding seasons, and all types of spermatogenic cells, including spermatozoa, were found in the breeding season. In addition, immunoreactivity was also detected for the inhibin alpha, betaA and betaB subunits in Sertoli and Leydig cells during the breeding season, but immunostaining was only present for the inhibin alpha and inhibin/activin betaB subunits in Sertoli cells during the non-breeding season. These results suggest that seasonal changes in testicular weight and size and seminiferous tubule diameter of wild ground squirrels are correlated with changes in spermatogenesis, and the cellular localization of the inhibin/activin subunits showed season related changes in the breeding and non-breeding seasons.     

Annual Changes in Day-length,Temperature, and Circulating Reproductive Hormones in Thoroughbred Stallions and Geldings

Changes in follicle-stimulating hormone (FSH), luteinizing hormone (LH), prolactin, immunoreactive(ir)-inhibin, testosterone, estradiol-17β, and insulin-like growth factor (IGF)-I in Thoroughbred stallions along with changes in prolactin secretion in geldings were studied. The correlations of day-length with changes in the concentrations of these hormones were also studied. Five stallions and thirteen geldings were employed to draw blood samples in monthly basis and radioimmunoassay was performed to measure these hormones. All hormones showed a seasonal pattern, the levels being highest during the breeding season and lowest during the winter months. Most of the hormones were at their highest concentration during the month of April, the mid of spring in northern hemisphere. The concentration of circulating IGF-I also demonstrated seasonality, the peak lying on the month of April. The plasma concentration of prolactin also increased during the breeding season. This phenomenon was similar both in stallions and geldings although geldings had lower concentration than that of stallions. The changes in concentration of prolactin in stallions and geldings correlated more towards the day-length than towards the temperature. These results clearly indicate the seasonality of pituitary and gonadal hormones of Thoroughbred stallions, the activity being highest during the month of April and May of the breeding season.     

Endocrine abnormalities and hormonal therapy.

Routine measurement of estrogens, testosterone, T4, insulin, FSH, and LH at least four times per year (e.g., during each of the four seasons) may improve the efficiency of stallion management. Benefits may not be realized in the short term but will provide valuable historical data on individual stallions that, when added to other data, will improve ability of management personnel to initiate early treatment and delay or slow declining fertility. This ability will be greatly improved as more data and products become available. There appears to be a relationship between low total estrogen concentration/high FSH concentration and subfertility. This condition is associated with high average breedings per pregnancy. A decrease in concentration of estrogen and an increase in FSH concentration often precede a decline in fertility associated with oligospermia. Hypogonadotropic stallions have not been reported. This condition is not likely to be a cause of declining fertility in stallions and greatly limits the potential efficacy of GnRH therapy in subfertile stallions. Much research must be done to elucidate the etiology of testicular degeneration associated with increased FSH concentrations and decreased estrogen concentrations in stallions. At present, no reliable hormonal therapeutic protocols exist that will improve fertility in subfertile stallions.     

COLLECTION OF SEMEN FROM STALLIONS AT STUD

Semen was collected from 222 stallions of 13 breed or colour types in southern Queensland and northern New South Wales. A total of 648 collection attempts were made, using an artificial vagina, during 4 consecutive breeding seasons (1974/5 to 1977/8). Modifications were made to the techniques used by previous workers because collections were made at commercial studs using minimal animal restraint.
Of all collection attempts, 621 (96%) were successful, while at least one semen sample was collected from each of 216 stallions (97%). There were no significant relationships between stallion collection failures and breeding season, time of year or age and breed of stallion. Time of year (seasons and months) was the only factor having a significant relationship with collection failures; highest failure rates occurring in autumn and winter.
The techniques described are applicable for breeding soundness examinations of untrained stallions and for collection of semen for artificial insemination.     

Increased germ cell loss rates and poor semen quality in stallions with idiopathic testicular degeneration

Five mature stallions with poor semen quality were tentatively diagnosed as having testicular degeration of unknown cause. Testis samples from these five stallions, and from three mature stallions with normal semen quality, were obtained by castration and prepared for histomorphometry. Increased germ cell loss rates during late meiosis and spermiogenesis occurred in the stallions with idiopathic testicular degeneration. Poor semen quality, represented by a low percentage of morphologically normal, progressively motile sperm in ejaculates, appeared to be a good predictor of testicular degeneration in these stallions.     

Reproductive hormone concentrations in stallions with breeding problems: Case studies

Plasma concentrations of LH, FSH and testosterone are reported in stallions exhibiting a variety of reproductive problems. Stallions with poor libido were found to have low LH and FSH concentrations, while testosterone concentrations appeared normal. Stallions with good libido but experiencing ejaculatory disorders had normal concentrations of LH, FSH and testosterone. Older stallions experiencing a marked reduction in fertility had elevated FSH concentrations which were accompanied by increased LH concentrations in some cases, however, testosterone concentrations appeared normal in such stallions. Two young stallions which had experienced poor fertility (40 to 60% conception rates) from the beginning of their stud careers were found to have normal FSH and testosterone concentrations while LH concentrations were consistently low in one and normal in the other.     

Concentrations of prolactin, luteinizing hormone and follicle stimulating hormone in pituitary and serum of horses: effect of sex, season and reproductive state

Pituitary and serum from 86 male or female horses of various reproductive states were collected in the normal breeding season (summer) and in the nonbreeding season (winter) at a commercial slaughterhouse. Concentrations of prolactin (PRL), luteinizing hormone (LH) and follicle stimulating hormone (FSH) were measured by radioimmunoassay. Concentrations of pregnant mare serum gonadotropin and reproductive steroids in serum and gross appearance of the reproductive tract and gonads were used to catagorize reproductive state. Concentrations of PRL were higher (P less than .01) in summer than in winter in pituitary and serum of mares, stallions and geldings. In summer, mares had higher (P less than .01) concentrations of PRL in serum than stallions. In mares, concentrations of LH in pituitary were higher (P less than .05) in summer than in winter. Concentrations of LH in serum were higher (P less than .01) in summer than in winter in mares and geldings, higher (P less than .01) in mares than in stallions in summer, higher (P less than .01) in geldings than in stallions in summer and higher (P less than .01) in mares with low serum progesterone (P) concentrations than in mares with high P concentrations in summer. Concentrations of FSH in pituitary and serum did not differ between summer and winter for any type of horse.(ABSTRACT TRUNCATED AT 250 WORDS)