The influence of photoperiod on gonadotrophin-releasing hormone stimulated luteinising hormone release in the anoestrous mare

The transition from anoestrus to oestrus in mares is controlled by photoperiod. The present study examined whether additional daylength would accelerate the mares' response to gonadotrophin-releasing-hormone (GnRH). Nine anoestrous mares were placed under ambient or artificial long lighting on 7th January. The four month experimental period was divided into a three-day sequence which was repeated at 21 day intervals. Ovaries were palpated rectally on Day 1; saline was injected (1 ml intravenously [iv]) on Day 2; GnRH was administered (0.59 microgram/kg bodyweight iv) on Day 3. Blood was taken at -60, 0, 15, 30, 60 and 120 mins relative to saline or GnRH treatment. Serum luteinising hormone (LH) was determined by a homologous equine radioimmunoassay (RIA). Several criteria were employed to define a positive response to GnRH and the results were analysed by Fisher's exact probability test. Treatment with artificial light allowed a response to GnRH within six weeks whereas the mares in ambient lighting took 12 weeks to respond to GnRH. The advancement in the time of response to GnRH under the long photoperiod could be related to changes in pituitary LH content, accelerated follicular activity or alterations in other brain-pituitary hormone levels.     

Onset of luteal activity in foaling and seasonally anoestrous mares treated with artificial light.

Sixty-four mares (27 foaling, 37 barren or maiden), mainly Finnhorses, were subjected to treatment with 14.5 h of light and 9.5 h of darkness, starting at the beginning of December. The onset of cycling in non-foaling mares was estimated by weekly serum progesterone determinations. All of the non-foaling mares cycled in the middle of March. They started to cycle on average in the middle of February, 11.1 weeks after the beginning of the trial. There were statistically significant differences in relation to breed (Finnhorses started to cycle 2 weeks later than warm blooded, p less than 0.02) and in relation to age (brood mares with mean age 10 years, started to cycle 10 days later than those 3 years old, p less than 0.03). However, there was no statistical significant relation to previous lactation, although lactating mares lactating up to the previous autumn started to cycle 7 days later than dry mares (p less than 0.15). Artificial insemination of 14 mares in the 2nd oestrus of the year, in March, resulted in 12 foalings in the subsequent year (86%). The following winter, all pregnant mares (N = 27) were exposed to the same kind of light treatment, starting on 1st December. The 1st mare foaled at the end of January. The time from foaling to 1st post partum ovulation was significantly longer (17.0 days) in foalings taking place within 10 weeks from the beginning of the light treatment period, than in foalings occurring after more than 10 weeks of lighting (12.1 days) (p less than 0.01).     

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.     

Repeated oestrus synchronisation of beef cows with progesterone implants and the effects of a gonadotrophin-releasing hormone agonist at implant insertion

A group of 97 spring-calving beef cows were initially oestrus synchronised with controlled internal drug release (CIDR) intravaginal progesterone implants inserted for nine days and a prostaglandin injection on day 7. Approximately half the cows were given 10 microg buserelin when the implants were inserted, and they all received a single fixed-time artificial insemination (AI) 56 hours after the withdrawal of the implants. The overall pregnancy rate to the first synchronised AI was 55 per cent, the buserelin-treated cows having a pregnancy rate of 63 per cent compared with 47 per cent in the untreated cows (P>0.05). Sixteen days after the first synchronised AI all the cows were re-implanted with used CIDR implants which were removed five days later, and the cows received a second synchronised AI on days 23 to 24. Cows which received the second AI were implanted with new CIDR devices 16 days later and these were removed after five days and the non-pregnant cows received a third synchronised AI. The pregnancy rates to the second and third synchronised services were 74 per cent and 75 per cent, respectively.     

A comparison of the uterine proteome of mares in oestrus and dioestrus

Proteomic analysis of mare uterine flush fluid provides a minimally invasive technique for studying protein changes associated with the oestrous cycle. The aim of this study was to identify differentially abundant proteins in the uterine flush fluid of mares in oestrus and dioestrus. In this study, uterine flush fluid samples were collected from eight reproductively healthy mares in either oestrus (n = 5) or dioestrus (n = 3). Proteomic analysis was performed using liquid chromatography‐tandem mass spectrometry. Of 172 proteins identified, six proteins (immunoglobulin lambda‐like polypeptide 1, haemoglobin subunit alpha, alpha‐1B‐glycoprotein, serotransferrin, apolipoprotein A‐1, and haemoglobin subunit beta) were significantly more abundant in oestrus. These proteins may contribute to the endometrial defence system through roles in inflammation, immunity or antimicrobial activity. In other species, some of these proteins have been described as immunoglobulins, negative acute phase proteins or defence agents against micro‐organisms. During dioestrus, immunoglobulin alpha‐1 chain C region‐related, complement factor I, CD 109 antigen and uterocalin, were significantly more abundant. Research in other species suggests that these four proteins contribute to the immune response through proposed immunoregulatory characteristics, complement system involvement or roles in B cell–T cell interactions. In conclusion, ten differentially abundant proteins were identified in the uterine flush fluid of mares in oestrus and dioestrus. Targeted studies on these proteins could elucidate their role in uterine defence mechanisms during the oestrous cycle in the mare.     

Factors affecting the monetary return from programmes for synchronising oestrus in dairy cows with gonadotrophin-releasing hormone and prostaglandin F2alpha

Synchronisation, using a combination of gonadotrophin-releasing hormone and prostaglandin F2alpha injections, is commonly used to control the reproductive cycle of dairy cows. A simple model that could predict the monetary return from synchronisation would be a valuable tool in determining whether it would be worthwhile. This study used data from three controlled clinical trials to investigate the effects of six factors (interval between calving and synchronisation, 21-day submission rate, number of cows synchronised, average age, pregnancy rate of non-synchronised cows and season) on the return from synchronisation. The results suggests that the herd's submission and pregnancy rates, the time between calving and synchronisation, and the number of cows synchronised can all significantly affect the return from synchronisation, but only time since calving significantly affected the return in all three trials. They also suggest that there is no simple linear relationship between these factors and the return from synchronisation.     

The effect of pulsatile gonadotropin-releasing hormone and estradiol administration on luteinizing hormone and follicle-stimulating hormone concentrations in pituitary stalk-sectioned ovariectomized pony mares

Hourly pulses of gonadotropin-releasing hormone (GnRH) or bi-daily injections of estradiol (E2) can increase luteinizing hormone (LH) secretion in ovariectomized, anestrous pony mares. However, the site (pituitary versus hypothalamus) of positive feedback of estradiol on gonadotropin secretion has not been described in mares. Thus, one of our objectives involved investigating the feedback of estradiol on the pituitary. The second objective consisted of determining if hourly pulses of GnRH could re-establish physiological LH and FSH concentrations after pituitary stalk-section (PSS), and the third objective was to describe the declining time trends of LH and FSH secretion after PSS. During summer months, ovariectomized pony mares were divided into three groups: Group 1 (control, n = 2), Group 2 (pulsatile GnRH (25 μg/hr), n = 3), and Group 3 (estradiol (5 mg/12 hr), n = 3). All mares were stalk-sectioned and treatment begun immediately after stalk-section. Blood samples were collected every 30 min for 8 h on the day before surgery (DO) and 5 d post surgery (D5) to facilitate the comparison of gonadotropin levels before and after pituitary stalk-section. Additionally, jugular blood samples were collected every 12 hr beginning the evening of surgery, allowing for evaluation of the gonadotropin secretory time trends over the 10 d of treatment. On Day 10, animals were euthanized to confirm pituitary stalk-section and to submit tissue for messenger RNA analysis (parallel study). Plasma samples were assayed for LH and FSH by RIA. Mean LH secretion decreased from Day 0 to Day 5 in Groups 1 and 3, whereas LH secretion tended (P < 0.08) to decrease in Group 2 mares. On Day 5, LH was higher (P < 0.01) in Group 2 (17.26 ± 3.68 ng/ml; LSMEANS ± SEM), than either Group 1 (2.65 ± 4.64 ng/ml) or group 3 (4.28 ± 3.68 ng/ml). Group 1 did not differ from Group 3 on Day 5 (P < 0.40). Similarly, mean FSH levels decreased in all groups after surgery, yet Group 2 mares had significantly (P < 0.001) higher FSH concentrations (17.66 ± 1.53 ng/ml) than Group 1 or Group 3 (8.34 ± 1.84 and 7.69 ± 1. 63 ng/ml, respectively). Regression analysis of bi-daily LH and FSH levels indicated that the time trends were not parallel. These findings indicate: 1) Pituitary stalk-section lowered LH and FSH to undetectable levels within 5 d after surgery, 2) pulsatile administration of GnRH (25 μg/hr) maintained LH and FSH secretion, although concentrations tended to be lower than on Day 0, and 3) E2 did not stimulate LH or FSH secretion.     

Effect of photoperiod on reproductive activity and hair in mares.

The effects of photoperiod on reproductive activity and hair changes in pony mares were studied in 2 experiments. In experiment I, the effect of a fixed daily photoperiod on the onset of the breeding season was studied in 36 mares from Nov 13, 1973, to June 13, 1974. The 4 treatment groups were as follows: daily photoperiod equivalent to the normal day length (control group); constant light 24 hours a day with no dark (L24:D0 group); 16-hour daily photoperiod with 8 hours of dark (L16:D8 group); and 9-hour daily photoperiod with 15 hours of dark (L9:D15 group). The intervals from beginning of experiment to 1st ovulation of breeding season, to shedding of hair in tufts, and to appearance of a smooth coat were shorter (P less than 0.05) for L16:D8 group (107.1 +/- 11.1, 56.0 +/- 0, and 145.8 +/- 4.0 days, respectively) than for control, L24:D0, and L9:D15 groups and were shorter (P less than 0.05) for L24:D0 group (less than 156.1 +/- 12.2, 99.5 +/- 9.5, and 173.9 +/- 9.9 days, respectively) than for control group (192.1 +/- 3.3, 134.9 +/- 8.9, and 205.0 +/- 0 days, respectively) or L9:D15 group (less than 200.3 +/- 5,8, 150.6 +/- 12.9, and 201.7 +/- 3.3 days, respectively). These intervals were not significantly different between the control group and the L9:D15 group, but fewer (P less than 0.05) mares in the L9:D15 group had at least 1 ovulation by termination of the project. In experiment II, the effect of photoperiod on onset of anestrus was studied in 3 groups of 7 mares each. Mares in group A, as part of a previous experiment, were induced to enter the breeding season earlier than normal by a gradual increase in daily photoperiod beginning on Oct 13, 1972. From Feb 16, 1973, to June 22, 1973, group A mares were maintained at a fixed daily photoperiod of 15 hours 23 minutes. Mares in group B, as part of a previous experiment, were kept under environmental conditions simulating normal conditions in southern Wisconsin. On June 22, 1973 (beginning of the present experiment), the following treatments began: groups A and B were exposed to natural day length. In addition, 7 mares (group C) were allotted from a band of mares that had been exposed to natural day length and were exposed to 15-hour 23-minute daily photoperiod from the beginning of the present experiment (June 22, 1973) to the end (June 22, 1974). The interval to onset of anestrus was longer (P less than 0.05) for group C mares (234.6 +/- 35 days) than for group B mares (133.6 +/- 16.5 days). Significant difference did not exist between group A (144.0 +/- 45.9 days) and group B. A fixed daily photoperiod of 16 or 24 hours induced early onset of the breeding season and early shedding of hair, with development of a smooth coat. A photoperiod of 9 hours retarded the onset of the breeding season. Mares induced to begin the breeding season earlier than normal did not become anestrous earlier than normal. Mares kept on a long daily photoperiod in the fall became anestrous later than normal.     

The hazards associated with the use of intrauterine glass balls to suppress oestrus in mares

Placing a sterile glass ball or marble into the uterine lumen is a popular method to suppress unwanted oestrous behaviour in mares. This is in spite of the original report of Nie et al. (2003), which detailed the lack of efficacy of the technique. Recent reports in the literature have described a number of untoward problems associated with the technique. This paper describes two further cases, one involving fragmentation of marbles in the uterine lumen and a second describing the laparoscopic removal of a marble after it had tracked through the endometrium and became pedunculated between the myometrium and serosa. The inefficiency of suppressing oestrus with glass balls, combined with the potential problems their placement may cause, argue strongly that this practice should be considered unethical.     

Qualitative analysis and functional classification of the uterine proteome of mares in oestrus and dioestrus

Quantitative analysis of the uterine flush fluid proteome of mares in oestrus and dioestrus has been previously reported. The objectives of this study were to: a) evaluate qualitative differences in the uterine flush fluid proteome between mares in oestrus and mares in dioestrus and b) perform a functional classification of proteins either unique to each stage or common between the two stages. Uterine flush fluid samples were collected from 8 light breed mares in either oestrus (n = 5) or dioestrus (n = 3). Proteomic analysis of the samples was conducted using liquid chromatography–tandem mass spectrometry. Proteins exclusively detected in oestrus or dioestrus and those common to both stages were identified using the Scaffold software (version 4.4.8, Proteome Software Inc., Portland, OR). The identified proteins were classified into gene ontology (GO) categories (cellular component [CC], molecular function [MF] and biological process [BP]) using the PANTHER ( www.pantherdb.org ) classification system version 14.0. Of 172 proteins identified, 51 and 28 were exclusively detected in mares in oestrus and dioestrus, respectively, and 93 proteins were common to both stages. The most represented terms in various GO categories were similar among the three subsets of proteins. The most represented CC terms were extracellular region and cell, the most represented MF terms were catalytic activity and binding, and the most represented BP terms were metabolic process and cellular process. In conclusion, proteomic analysis of the uterine flush fluid enabled the identification of subsets of proteins unique to oestrus or dioestrus, or common to both stages. The results of this study can serve as a baseline for future research focused on finding stage-specific protein markers or evaluating differences in the uterine flush fluid proteome between normal mares and those with uterine disease.     

Comparative efficacy of bromocriptine,cabergoline and thyroxine in inducing oestrus in bitches

Forty bitches in anoestrus for more than six months from the last heat, with a serum progesterone level less than 1 ng/ml were subjected to oestrus induction trials using anti-prolactin drugs and levothyroxine, once daily orally for 20 consecutive days. The mean serum progesterone level among them was found to be 0.57 ± 0.03 ng/ml. Out of 10 animals treated in each group, five (50%) in Group I (bromocriptine @ 50 μg/kg body weight), nine (90%) in Group II (cabergoline @ 5 μg/kg body weight), eight (80%) in Group III (thyroxine @10 μg/kg body weight) and seven (70%) in Group IV (thyroxine @ 5 μg/kg body weight) responded by evincing proestrual bleeding. The mean (±SEM) time taken from initiation of treatment to onset of proestrual bleeding in Groups I, II, III and IV was 28 ± 3.39, 13.44 ± 3.12 (P < 0.05), 24.50 ± 3.18 and 33 ± 2.21 days respectively. The mean (±SEM) duration of proestrus and oestrus in the treatment groups was 9.80 ± 0.86, 10.11 ± 0.68, 11.25 ± 0.88 and 10.71 ± 0.68 days and 7.60 ± 0.24, 8 ± 0.29, 8.5 ± 0.63 and 7.85 ± 0.46 days respectively. The conception rate in relation to the number of animals responding to oestrus induction in the treatment groups was 80%, 78%, 63% and 57%, respectively. The mean (±SEM) gestation length calculated from the last breeding date and litter size in the treatment groups varied from 60.50 ± 1.55 to 64.00 ± 0.82 days and 5.14 ± 0.34 to 6.40 ± 0.40 respectively.     

The effect of progesterone impregnated sponges on fertility in anoestrous ewes

Progestagen impregnated sponges have been widely used for the induction and synchronization of oestrus in the ewe. We report here a study of the use of the natural hormone, progesterone, administered on sponges in order to induce oestrus in anoestrous ewes. A linear increase in fertility was observed when the progesterone dose was increased from 500 to 1000 mg. A similar quantity of progesterone (250 mg) was absorbed by the ewes regardless of the dose administered in the range 500 to 1200 mg. Sixty eight per cent lambing of treated ewes was achieved using sponges impregnated with 1000 mg progesterone. Optimum fertility appeared to be governed by adequate absorption of the hormone at a rapid initial rate, and this was ensured at the higher doses of progesterone.

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Kurzfassung Progestagen-getränkte Schwämme wurden zur Herbeiführung und Synchronisierung der Brunst bei Schafen weitgehend benutzt. Eine derartige Behandlung der Tiere kann zu einer geringeren Fruchtbarkeit, als sie unter natürlichen Zuchtbedingungen im Herbst besteht, führen. Die Wirkung von zunehmenden Dosen der verabreichten synthetisch hergestellten Progestagene auf die anschliessende Fruchtbarkeit der Schafe ist gut dokumentiert. Mehrere Verfasser haben den Restgehalt an SC-9880 gemessen, der auf den Schwämmen nach ihrer Entfernung von den Schafen, die zur Herbeiführung und Synchronisierung der Brunst behandelt worden waren, verblieb. Sie stellten eine lineare Zunahme der Frunchtbarkeit mit der verabreichten Dosis bei SC-9980 fest; bei höheren Dosen (30 bis 50 mg) wurde eine verlangsamte Progestagen-absorption von den Schwämmen beobachtet. Die Forscher berichteten, dass die Frucktbarkeit nicht-brünstiger Schafe bei Dosen von SC-9880 von über 30 mg bei 38 bis 67% Lammen liegen kann.Hier wird eine Untersuchung des natürlich vorkommenden Hormons Progesteron und seiner Verabreichung auf Schwämmen zur Herbeiführung der Fruchtbarkeit bei nicht-brünstigen Schafen beschrieben. Auch bei dieser Untersuchung wurde eine lineare Zunahme der Frucktbarkeit bei zunehmenden Dosen des verabreichten Progesterons beobachted. Ein Schwellenwert von Progesteron (250 mg) wurde von den Schafen unabhängig von der verabreichten Dosis im Bereich von 500 bis 1200 mg absorbiert. Bei den behandelten Schafen (mit 1000 mg imprägnierte Schwämme) wurde ein Ergebnis von 68% Lammen erzielt. Eine optimale Fruchtbarkeit wird anscheinend bei einer anfänglich schnellen, angemessenen Absorption des Hormons erzielt und diese Faktoren waren bei höheren Dosen von Progesteron gegeben.

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Resume On a largement utilisé la technique des éponges impregnées de progestagène pour induire et synchroniser l'oestrus chez les brebis. Un tel traitement peut entraîner un taux de fertilité inférieur à celui obtenu dans des conditions normales de reproduction en automne. L'effet de l'augmentation des doses de progestagène synthetique administrées sur la fertilité ultérieure des brebis a fait l'objet de nombreuses études. Plusieurs auteurs ont etudié les niveaux résiduels de SC-9880 sur les éponges après leur retrait des brebis qui avaient été traitées en vue de l'induction et de la synchronisation de l'oestrus. Ils ont constaté une progression linéaire de la fertilité en fonction des doses de SC-9880 administrées et un ralentissement de l'absorption du progestagène contenu les éponges aux doses les plus elevées utilisées (30 a 50 mg). Ces chercheurs ont signalé que la fertilité des brebis pendant l'anoestrus pouvait aller de 38 a 67% d'agnelage aux doses de SC-9880 supérieures à 30 mg.Le présent rapport a pour objet l'étude de l'hormone naturelle, la progestérone, administrée au moyen d'éponges pour induire l'oestrus chez les brebis en période l'anoestrus. Dans la présente étude, on a également observé une augmentation linéaire de la fertilité avec l'augmentation de la dose de progestérone administrée. On a constaté chez les brebis une valeur limite d'absorption de progestérone (250 mg), quelle que soit la dose administrée allant de 500 a 1200 mg. On a obtenu 68% d'agnelage chez les brebis traitées au moyen d'éponges impregnées de 1000 mg de progestérone. La fertilité optimale semble dépendre de l'absorption adéquate de l'hormone à un taux initial élevé, condition qui se trouve remplie en administrant des doses élevées de progestérone.

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Riassunto Le spugne impregnate di progesterone sono comunemente utilizzate per indurre e sincronizzare l'estro della pecora. Un simile procedimento rischia tuttavia di rendere l'animale meno fecondo rispetto alle condizioni di riproduzione naturale nella stagione autunnale. Gli effetti che dosi crescenti di preparati progestogeni producono sulla fecondità delle pecore sono del resto ampiamente documentati. Analizzando i residui di SC-9880 che restano sulle spugne rimosse dal corpo della pecora, diversi autori sono giunti alla conclusione che esiste un incremento lineare della fecondità se si somministrano le dosi sotto forma di SC-9880. La stessa indagine evidenzia inoltre un minore assorbimento di sostanza progestagena dalle spugne se si utilizzano dosi massime comprese fra 30 e 50 mg. I ricercatori affermano che le pecore in anestro hanno parti in una percentuale compresa fra il 38 e il 67% se ricevono dosi di SC-9880 superiori ai 30 mg.La presente ricerca riguarda la somministrazione mediante spugne di progesterone naturale per indurre l'estro nelle pecore in anestro. Essa dimostra che dosi crescenti di progesterone conducono ad un incremento lineare della fecondità. Esiste un tetto massimo di assorbimento del progesterone (250 mg) anche con dosi di somministrazione comprese fra i 500 e i 1200 mg. Il 68% dei parti delle pecore trattate si ha con spugne impregnate con 1000 mg di progesterone. La fecondità ottimale sembra dipendere da un adeguato ed inizialmente rapido assorbimento dell' ormone, ottenibile con dosi massime di somministrazione.

     

The effect of oestrus on milk production in cows

The rectal temperatures, pulse rates and milk yields of 44 Ayrshire and 38 Friesian cows were recorded during oestrus and compared with values obtained after oestrus had ceased. The fat, solids-non-fat (SNF) and protein percentages and the cell counts of milk samples were also compared. Forty of the cows were housed in cubicles and 42 were kept loose in yards. During oestrus rectal temperatures and pulse rates were significantly raised and milk yields were lowered. Milk had, on average, significantly higher fat contents and cell counts and lower SNF and protein contents than post-oestrous milk. The differences due to oestrus between the cows kept in cubicles and yards were minimal, but the Ayrshire cows tended to show more significant effects than the Friesians, except in the milk cell counts, where the difference was higher in the Friesians.     

Induction of luteolysis and oestrus in mares with a synthetic prostaglandin analogue (ICI 81008)

Extract

Cyclical regression of the corpus luteum is induced by an uterine luteolysin which has been shown in several species to be prostaglandin F_2a (PGF_2a , although it is not yet known how it achieves its effect.     

The effect of artificial photoperiod at the end of the breeding season on plasma testosterone concentrations in stallions

Testosterone concentrations in stallions showed a seasonal trend with peak concentrations in the spring (April and May in Britain) and lowest concentrations in the period from December to February. The effect on this pattern of changing the length of the photoperiod at the end of the normal breeding season (mid-summer's day) was studied in 2 experiments. In the first experiment artificial illumination was organised from 21 June to mimic the effect of transfer to a southern hemisphere spring and summer, that is short days becoming longer. The stallions had low concentrations of testosterone in February and high concentrations in April. Concentrations in July, August and September were extremely low with a return to high values in late November/early December. In the second experiment, illumination was maintained at the equivalent of a 16 1/2-hour day from 21 June to mid December. These stallions had high testosterone concentrations in April, after which they fell until August, later rising to a maximum in October. These results are discussed in relation to transfer of stallions between the northern and southern hemispheres.