Induction of luteal activity and progesterone secretion in the nonpregnant one-humped camel (Camelus dromedarius)

Corpora lutea (n = 20) were detected in 5 one-humped female camels studied during a period of 4 months. Complete mating by a vasectomized male, male introduction into the pen of females without mating, or a progesterone decrease from a previous corpus luteum were followed by a similar progesterone pattern. A maximal plasma concentration of 4.5 +/- 1.5 ng progesterone/ml (2.7-8.8 ng/ml) occurred 8.55 +/- 1.32 days (6-11 days) after the inducing stimulus. Luteal regression, beginning 8.65 +/- 1.18 days after the stimulus, was completed at Day 11.55 +/- 1.05. Morphological development of ovarian structures, detected by rectal palpation, was in synchrony with the progesterone increase, but there was a prolonged period of regression. Females accepted mating up to 7 days after the ovulatory stimulus, when progesterone levels were as high as 3.5 ng/ml. This study establishes the absence of pseudopregnancy in the one-humped female camel, and offers opportunities for improving the management of reproduction. It also shows that ovulatory stimuli other than mating can be effective in these animals.     

Changes in patterns of luteinizing hormone secretion before and after the first ovulation in the postpartum mare

To determine whether luteinizing hormone (LH) secretion during the first estrous cycle postpartum is characterized by pulsatile release, circulating LH concentrations were measured in 8 postpartum mares, 4 of which had been treated with 150 mg progesterone and 10 mg estradiol daily for 20 days after foaling to delay ovulation. Blood samples were collected every 15 min for 8 h on 4 occasions: 3 times during the follicular phase (Days 2-4, 5-7, and 8-11 after either foaling or end of steroid treatment), and once during the luteal phase (Days 5-8 after ovulation). Ovulation occurred in 4 mares 13.2 +/- 0.6 days postpartum and in 3 of 4 mares 12.0 +/- 1.1 days post-treatment. Before ovulation, low-amplitude LH pulses (approximately 1 ng/ml) were observed in 3 mares; such LH pulses occurred irregularly (1-2/8 h) and were unrelated to mean circulating LH levels, which gradually increased from less than 1 ng/ml at foaling or end of steroid treatment to maximum levels (12.3 ng/ml) within 48 h after ovulation. In contrast, 1-3 high-amplitude LH pulses (3.7 +/- 0.7 ng/ml) were observed in 6 of 7 mares during an 8-h period of the luteal phase. The results suggest that in postpartum mares LH release is pulsatile during the luteal phase of the estrous cycle, whereas before ovulation LH pulses cannot be readily identified.     

Ovarian and endocrine responses in the cat after coitus

LH release leading to ovulation was induced in 17 of 29 oestrous periods. The time of ovulation after coitus was determined by histological examination or by observation at laparotomy of ovaries in situ. Histological methods revealed that ovulation was complete in most follicles (9 of 13) at 32 h post coitum and in all follicles that were involved in the ovulatory process by 36 h. When laparotomy was used, no signs of preovulatory change were noted at the first observation time, 22 h post coitum, but in 4 cycles in which the entire process of ovulation was observed, the ovulatory process occurred between 23 and 28 h (3 follicles), 23 and 27 h (2 follicles), 25 and 28 h (3 follicles), and 25 and 29 h (3 follicles) post coitum. The first ovulatory process noted was complete at 25 h post coitum. In cats, LH release continued over a 16-h period before returning to baseline (long surge), values being 616 +/- 180 ng/ml at 1/2 h and 941 +/- 154 ng/ml at 2 h post coitum. In 6 cats the LH release pattern was limited to a 4-h period (short surge), values being 537 +/- 218 ng/ml at 1/2 h and 353 +/- 245 ng/ml plasma at 2 h and basal (49 +/- 18 ng/ml) by 4 h post coitum. Decreased secretion of oestrogen by follicles in animals undergoing ovulation was first observed at 16 h post coitum. It is concluded that coitus induces LH release within minutes in the cat and that ovulation begins about 24 h later and finishes by about 32 h post coitum. Only one coital input can cause LH release for as long as 16-20 h although shorter periods of LH release (4 h or less) can result in ovulation.     

Timing of ovulation in relation to onset of estrus and LH peak in yak (Poephagus grunniens L.)

The objective of the study was to determine the timing of ovulation in relation to onset of estrus and the preovulatory LH peak in yaks. For this purpose, a sensitive LH enzymeimmunoassay previously established in buffaloes was successfully validated for measuring the hormone in yak plasma. Plasma LH and progesterone were estimated from blood samples collected from eight non-lactating cycling yaks at 2 h intervals after estrus onset until 6 h after ovulation (ovulation was confirmed by palpation of ovaries per rectum). The mean+/-S.E.M. preovulatory plasma LH peak was 10.11+/-0.35 ng/ml with the values ranging from 8.75 to 11.51 ng/ml in individual yaks. The mean+/-S.E.M. duration of the LH surge was 7.25+/-0.55 h with a range of 6-10 h. Onset of LH surge (mean+/-S.E.M.) occurred 3.0+/-0.65 h after the onset of estrus. Mean plasma progesterone stayed low (<0.25 ng/ml) during the entire duration of sampling. Ovulation occurred 30.5+/-0.82 h (range, 28-34 h) after the onset of estrus and 20.25+/-1.03 h after the end of LH surge. The occurrence of the LH peaks within a narrow time frame of 4-8h post estrus onset in yaks could have contributed to the animals ovulating within a narrow time interval.     

Seasonal variations in the pituitary response to LHRH in the brown hare (Lepus europaeus)

In the brown hare, fertile mating takes place from the beginning of December to September. Pituitary and ovarian response to a monthly i.v. injection of 5 micrograms LHRH was studied from September 1983 to October 1984 in 2 groups of 6 hares. The basal concentrations of LH remained undetectable until the end of January, rose from 0.23 +/- 0.14 ng/ml from February to a maximum of 1.44 +/- 0.57 ng/ml in July. LHRH injection was always followed by a release of LH. Between September and December, the LH value peaked 15 min after injection and returned to basal concentrations 2 h later. From January, this pattern altered and a second peak of LH appeared 2 h after injection. Peak levels 15 min after LHRH were around 10 ng/ml between September and December, increased from 47.0 +/- 8.0 ng/ml in January to 106 +/- 33 ng/ml in July and decreased in August (69.4 +/- 10.6 ng/ml). The values of the second peak rose from 11.0 +/- 2.2 ng/ml in January to 90.6 +/- 12.4 ng/ml between March and July and decreased in August (24.5 +/- 5.1 ng/ml). The LH surge induced by LHRH was always followed by a transient rise in progesterone. During the breeding season, this progesterone secretion increased considerably. Ovulation was possible between January and August and the number of ovulating females was maximum between March and July. The amount and duration of progesterone secretion during the resulting pseudopregnancies increased during the breeding season.     

Episodic LH secretion patterns in the mare during the oestrous cycle.

Jugular blood samples were obtained from 8 mares at 5- and/or 20-min intervals for 2 to 5 days during various phases of the oestrous cycle for plasma LH determination. An episodic release pattern was observed in 1 of 3 mares sampled during the ovulatory period. One mare had one secretory burst and the other mare had several periods of fluctuating plasma LH concentration. During dioestrus, episodic secretions were observed in 2 mares sampled 11 to 13 days before and, in 1 mare, 9 days after ovulation. During the 2 to 5-day period before ovulation, episodic secretion was not observed (3 mares) but plasma LH concentrations fluctuated as much as 6 ng/ml during a period of 3--4 h. Daily plasma samples were obtained form 10 mares (1--8 oestrous cycles/mare) during which 22 single, 18 double and 2 luteal-phase ovulations occurred. Dioestrous ovulations were accompanied by small increases in plasma LH (1--4 ng/ml), but many similar increases in LH were not accompanied by ovulation. No significant differences in secretory patterns were observed between single and multiple ovulations. In one mare, 4 ovulations occurred in the presence of a prolonged luteal phase; 3 were accompanied by increasing LH concentrations and the other occurred when LH was at a low concentration.     

Molecular characterization of alpha subunit 1 of sodium pump (ATP1A1) gene in Camelus dromedarius: its differential tissue expression potentially interprets the role in osmoregulation

Molecular Biology Reports - Dromedary or one-humped camel (Camelus dromedarius) is distinctively acclimatized to survive the arid conditions of the desert environment. It has an excellent ability...     

Induction of ovulation in the acyclic postpartum ewe following continuous, low-dose subcutaneous infusion of GnRH

Pituitary and ovarian responses to subcutaneous infusion of GnRH were investigated in acyclic, lactating Mule ewes during the breeding season. Thirty postpartum ewes were split into 3 equal groups; Group G received GnRH (250 ng/h) for 96 h; Group P + G was primed with progestagen for 10 d then received GnRH (250 ng/h) for 96 h; and Group P received progestagen priming and saline vehicle only. The infusions were delivered via osmotic minipumps inserted 26.6 +/- 0.45 d post partum (Day 0 of the study). Blood samples were collected for LH analysis every 15 min from 12 h before until 8 h after minipump insertion, then every 2 h for a further 112 h. Daily blood samples were collected for progesterone analysis on Days 1 to 10 following minipump insertion, then every third day for a further 25 d. In addition, the reproductive tract was examined by laparoscopy on Day -5 and Day +7 and estrous behavior was monitored between Day -4 and Day +7. Progestagen priming suppressed (P < 0.05) plasma LH levels (0.27 +/- 0.03 vs 0.46 +/- 0.06 ng/ml) during the preinfusion period, but the GnRH-induced LH release was similar for Group G and Group P + G. The LH surge began significantly (P < 0.05) earlier (32.0 +/- 3.0 vs 56.3 +/- 4.1 h) and was of greater magnitude (32.15 +/- 3.56 vs 18.84 +/- 4.13 ng/ml) in the unprimed than the primed ewes. None of the ewes infused with saline produced a preovulatory LH surge. The GnRH infusion induced ovulation in 10/10 unprimed and 7/9 progestagen-primed ewes, with no significant difference in ovulation rate (1.78 +/- 0.15 and 1.33 +/- 0.21, respectively). Ovulation was followed by normal luteal function in 4/10 Group-G ewes, while the remaining 6 ewes had short luteal phases. In contrast, each of the 7 Group-P + G ewes that ovulated secreted progesterone for at least 10 d, although elevated plasma progesterone levels were maintained in 3/7 unmated ewes for >35 d. Throughout the study only 2 ewes (both from Group P + G) displayed estrus. These data demonstrate that although a low dose, continuous infusion of GnRH can increase tonic LH concentrations sufficient to promote a preovulatory LH surge and induce ovulation, behavioral estrus and normal luteal function do not consistently follow ovulation in the progestagen-primed, postpartum ewe.     

Luteinizing hormone requirements for ovulation in the rat.

Luteinizing hormone requirements for ovulation induction were studied in proestrous rats through detailed observation of the preovulatory surge, through various forms of LH injection under sodium pentobarbital blockade, and through estimation of LH uptake by the ovary. Blood LH levels in individual proestrous rats were obtained every 30 min and grouped according to their peak time (designated 0 h); mean LH levels higher than 7 and 5 ng/ml continued for 30 min and 2.5 h, respectively, the pituitary LH contents at 1400 and 2000 h on the day of proestrus were 2.1 and 0.7 micrograms, respectively, indicating that the amount of LH secreted during the surge was at least 1.4 micrograms. Single intravenous injections of 2 micrograms and 1 micrograms of pure rat LH (NIDDK-rLH-I-7; FSH and prolactin contaminations: 0.02% and less than 0.01%, respectively) to sodium pentobarbital-blocked rats induced ovulation in 4 out of 4 rats and 4 out of 6 rats, respectively, while 500 ng failed to induce ovulation in any (out of 7) rats. Two injections of 300 ng each with an interval of 20 min induced ovulation in 3 out of 8 rats, but if the interval was prolonged to between 30 and 120 min, 100% ovulation was obtained. Blood LH levels in these experiments indicated that a lower long-lasting LH level (about 5 ng/ml blood) is more important than a short, high level for ovulation induction. It was also shown that this level of LH could be given in separate doses if the interval was 30-120 min.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of progesterone on the responses of Merino ewes to the introduction of rams during anoestrus

The effects of progesterone on the responses of Merino ewes to the introduction of rams during anoestrus were investigated in two experiments. In the first experiment, the introduction of rams induced an increase in the levels of LH in entire ewes. The mean levels increased from 0.68 +/- 0.04 ng/ml (mean +/- s.e.m.) to 4.49 +/- 1.32 ng/ml within 20 min in ewes not treated with progesterone (n = 10). In ewes bearing progesterone implants that provided a peripheral concentration of about 1.5 ng progesterone per millilitre plasma, the LH response to the introduction of rams was not prevented, but was reduced in size so that the concentration was 1.38 +/- 0.15 ng/ml after 20 min (n = 5). Progesterone treatment begun either 2 days before or 6 h after the introduction of rams and maintained for 4 days prevented ovulation. In the second experiment ovariectomized ewes were used to investigate further the mechanism by which the ram evoked increases in tonic LH secretion. In ovariectomized ewes treated with oestradiol implants, the introduction of rams increased the frequency of the LH pulses and the basal level of LH. In the absence of oestradiol there was no significant change in pulse frequency but a small increase in basal levels. Progesterone again did not prevent but reduced the responses in ewes treated with oestradiol. It is suggested that following the withdrawal of progesterone treatment, the secretion of LH pulses in response to the ram effect would be dampened. This effect could be a component of the reported long delay between the introduction of rams and the preovulatory surge of LH in ewes treated with progesterone. Continued progesterone treatment prevented ovulation, probably by blocking positive feedback by oestradiol.     

Hormone concentrations before and after semen-induced ovulation in the bactrian camel (Camelus bactrianus)

During the follicular phase of bactrian camels, basal concentrations of LH were 2.7 +/- 1.2 ng/ml. By 4 h after insemination peak values of 6.9 +/- 1.0 ng/ml occurred. In addition, a smaller LH peak (5.4 +/- 2.5 ng/ml) appeared 1 day before regression of the follicle began in unmated camels. During the follicular phase peripheral plasma progesterone values were low (0.36 +/- 0.28 ng/ml), but values increased to reach 1.73 +/- 0.74 ng/ml at 3 days and 2.4 +/- 0.86 ng/ml at 7 days after ovulation. Plasma oestradiol-17 beta concentrations were 26.8 +/- 9.0 pg/ml during the follicular phase and 30.8 +/- 5.1 pg/ml when the follicle was maximum size. Values fell after ovulation but rose to 29.8 +/- 6.5 pg/ml 3 days later.     

Plasma concentrations of luteinizing hormone and progesterone during superovulation of dairy cows using follicle stimulating hormone or pregnant mare serum gonadotrophin

Eighteen lactating Holstein cows were randomly divided into three groups of equal size. Six cows were not superovulated; the remaining cows were superovulated using either FSH-P or PMSG beginning on Day 12 of the estrous cycle (day of ovulation = Day 0). Animals treated with FSH-P were injected intramuscularly (i.m.) with 4 mg FSH-P every 12 h for 5 d. PMSG was administered i.m. as a single injection of 2350 IU. Cloprostenol (PG, 500 ug) was injected i.m. 56 and 72 h after commencement of treatment and at the same time in the cycle of controls. All cows were inseminated 56, 68 and 80 h after the first PG injection. Blood samples (5 ml) were collected daily and every 15 min for a period of 9 h on Days -1, 0, 2, 8 and 10, with continuous blood sampling at 15-min intervals during Days 3 to 6. Ovulation rate was 27.7 +/- 8.22 in animals treated with PMSG, and 8.0 +/- 3.2 embryos per donor were recovered. In the FSH group, ovulation rate was 8.3 +/- 1.48 and 3.0 +/- 1.1 embryos per donor were recovered. Progesterone concentrations were similar in all three groups until the onset of the LH surge, when progesterone concentrations were greater (P<0.05) in animals of the PMSG group. After the preovulatory LH surge, concentrations of progesterone started increasing earlier (44 h) in cows treated with PMSG, followed by FSH-treated cows (76 h) and controls (99 h). The LH surge occurred earlier (P<0.05) in PMSG-treated cows (37 h after first PG treatment), than in animals treated with FSH-P (52 h) or controls (82 h). In animals treated with FSH-P, the magnitude of the preovulatory LH surge (24.2 +/- 1.02 ng/ml) was higher (P<0.05) than in the other two groups (PMSG = 17.1 +/- 2.04 ng/ml; control, 16.7 +/- 1.24 ng/ml). Superovulation with FSH-P or PMSG did not affect either mean basal LH concentration, frequency or amplitude of LH pulses during Days -1, 0, 2, 3, presurge periods, or Days 8 and 10 post-treatment. At ovariectomy, 8 d post-estrus, more follicles > 10 mm diam. were observed in the ovaries after treatment with PMSG (8.5 +/- 5.66) than after treatment with FSH-P (0.7 +/- 0.42) (P<0.05). Maximum concentrations of PMSG were measured 24 h after administration. Following this peak, PMSG levels declined with two slopes, with half-lives of 36 h and 370 h.     

Blood protein polymorphism in the one-humped camel (Camelus dromedarius) in the Sudan

The blood protein polymorphism of serum albumin, haptoglobin, transferrin, ceruloplasmin and haemoglobin have been studied in 135 samples from one-humped Arabian camel (Camelus dromedarius) of the Sudan by starch gel electrophoresis. Only the serum albumin and haptoglobin systems exhibited polymorphism with the estimated frequencies of 0.0222, 0.2227 and 0.7773 for Alb^v, Hp¹Hp⁰respectively. The frequency of Hp was 0.0325. No electrophoretic variant was observed at transferrin, ceruloplasmin and haemoglobin loci in the camel. The activity of the ceruloplasmin of the camel sera was weak.     

Self-limiting reflex luteinizing hormone release and sexual behavior during extended periods of unrestricted copulatory activity in estrous domestic cats

Estrous female cats (queens) were permitted 36-h periods of unrestricted mating activity; they then were injected with various doses of gonadotropin-releasing hormone (GnRH) at 36 h and allowed single copulations at 48 or 72 h of study. Serum luteinizing hormone (LH) levels were determined in samples collected prior to and 2 h after the initial copulation, before and 30 min after selected copulations during the next 10 h, and before and 30 min after copulations occurring at 20-24, 36, and 48-72 h, as well as 0, 15, and 30 min after the GnRH injections (0.3-3.0 micrograms/kg) at 36 h of study. Copulations occurred 14-20 times in 12 h and 20-36 times in 36 h. Copulation frequency (mean +/- SEM) decreased (p less than 0.05) from 5.5 +/- 0.6/2 h initially to 1.5 +/- 0.6/2 h during the subsequent 2-h period, and was 1.4 +/- 0.2/2 h at 12-36 h of study. Intromissions lasted 1-27 (8 +/- 0.3) s. Variation in durations of mounting by males (1.7 +/- 0.1 min; range, 0.3-10 min) or of the postcoital behavioral reactions displayed by the queens (2.5 +/- 0.1 min; range, 1-17 min) could not be related to animals or time of study. Peak serum LH levels (11-280 ng/ml; mean, 112 +/- 30 ng/ml) were observed at 2-4 h after the first mating. Mean LH steadily declined thereafter, reached basal values (less than or equal to 3 ng/ml) by 20-24 h, and remained low at 36 h.(ABSTRACT TRUNCATED AT 250 WORDS)     

FSH and LH variations in beef cows during the postpartum period

To study the plasma gonadotrophin profiles of 9 cows after parturition, blood samples were obtained every 20 min for 12 hrs on three occasions between 5 and 50 days postpartum and analysed by RIA techniques. The time of the first ovulation, as judged by plasma progesterone levels, varied from 30 to more than 60 days postpartum. Variations in mean levels of FSH and LH were not significantly correlated with the postpartum interval. However, the mean levels of plasma FSH and number of LH pulses were lower in females which had not ovulated than in those which had. The cows could be classified into four groups: group 1 with less than 4 LH pulses in 12 hrs and a mean plasma FSH level less than 138 ng/ml; group 2 with more than 4 LH pulses in 12 hrs and varying plasma FSH levels; group 3 with less than 4 LH pulses in 12 hrs and a mean plasma FSH level greater than 138 ng/ml; group 4 which had ovulated. This classification indicated that the LH and FSH levels progressed significantly (2.46 to 3.56 ng/ml, P less than 0.05; 120 to 159 ng/ml, P less than 0.01, respectively) from groups 1 to 3, and that they decreased in the females which had ovulated (group 4). Since the time of the first ovulation after parturition varied, it was not possible to demonstrate any relationship between that interval and the mean plasma gonadotrophin profiles. However, when ovulation was considered as time zero there was a clear increase in plasma gonadotrophin before ovulation.     

The effects of nutrition on the reproductive performance of Finn x Dorset ewes. II. Post-partum ovarian activity, conception and the plasma concentration of progesterone and LH.

After lambing forty-five ewes were allocated to three groups, two of sixteen and one of thirteen ewes. The lambs of the two groups of sixteen ewes were weaned on Day 1 after lambing and the ewes were fed a diet of 100% (Group H) or 50% (Group R) of maintenance energy requirements. The thirteen ewes in the third group (Group L) suckled twin lambs and were fed freely. During the first 3 weeks after lambing, oestrus was observed for 11/16 (Group H) and 8/16 (Group R) ewes; of the ewes which had shown oestrus in the two groups, ovulation occurred in 5/8 and 5/7 respectively. Only 1/13 Group-L ewes showed oestrus and ovulated during the same period. The mean plasma concentrations of progesterone and LH were unaffected by the treatments and were around 0-4 and 1-5 ng/ml, respectively. Restricted feeding had no effect on oestrus, ovulation or the hormone levels during the oestrus cycle following synchronization. The onset of oestrus and the start of the preovulatory discharge of LH were 3 and 6 hr later, respectively, in the lactating ewes (Group L) than in those in Groups H and R. Ewes in Group L also had a higher ovulation rate, 2-8 +/- 0-2 versus 2-1 +/- 0-2 (P less than 0-05). Restricted feeding reduced the number of ewes lambing; only 1/11 ewes in Group R, considered to have conceived because of the presence of high progesterone levels 17 days after mating, subsequently lambed compared with 6/12 in Group H and 5/9 in Group L.     

Preovulatory biosynthesis and granulosa cell secretion of immunoreactive oxytocin by goat ovaries

Oestrous cycles of goats were synchronized hormonally. Immunoreactive oxytocin was undetectable (less than 0.1 ng/mg protein) in media from granulosa cells isolated before the LH surge for small (1-2 mm), medium (3-5 mm) and large (greater than 5 mm diameter) follicles when cultured for 24 h without or with added hormones. Granulosa cells from large and medium, but not small, follicles isolated 6-12 h after spontaneous preovulatory LH surges secreted high concentrations of oxytocin (4-12 ng/mg protein). Addition of PGE-2 (1 microgram/ml) caused a further significant (P less than 0.05) increase in oxytocin secretion by cultured granulosa cells, whereas PGF-2 alpha, FSH and LH were ineffective when added to culture media. Ovarian venous blood and granulosa cells were collected at 0, 6, 12 or 18 h after GnRH injection in hormonally synchronized goats. Peripheral serum LH values were increased significantly in all but 2 of 22 goats within 2 h of GnRH injection. At the earliest sampling time after GnRH (6 h), ovarian venous levels of oxytocin were increased significantly from basal levels of 0.4 pg/ml to 2.4 pg/ml. Oxytocin concentrations in follicular fluid increased from a basal value of 67 pg/ml to 155 pg/ml by 6 h and to 372 pg/ml by 18 h after GnRH injection. Oxytocin secretion by cultured granulosa cells was not increased significantly by 6 h (0.1 ng/mg protein) but rose to 1.4 and 3.5 ng/mg protein at 12 and 18 h, respectively. Approximately parallel increases occurred in progesterone in ovarian venous blood and granulosa cell culture media over the same time period. (ABSTRACT TRUNCATED AT 250 WORDS)     

The acute effect of bull presence on plasma profiles of luteinizing hormone in postpartum, anoestrous dairy cows

The objective of this study was to investigate whether bull exposure affects LH profiles in postpartum, anoestrous dairy cows. Eight cows between 10 and 17 days after parturition were used. On Day 1, blood samples were taken at 10 min intervals for 8 h. On Day 2, blood sampling continued at 10 min intervals and after 2 h a bull was introduced behind a fence, and blood sampling continued for another 8 h. Time of resumption of luteal activity was between 25 and more than 80 days after parturition for these animals and was not related (P>0.1) with frequency of LH pulses, amplitude of pulses and basal LH concentration on either Day 1 or Day 2. In 6 of the 8 cows, average and basal LH concentration were greater (P<0.001) during the 8 h of bull presence (0.56 +/- 0.33 and 0.39 +/- 0.26 ng/ml, respectively) compared to the 8 h without a bull (0.50 +/- 0.30 and 0.35 +/- 0.24 ng/ml, respectively). Pulse amplitude did not differ (P=0.85) between Day 2 (0.45 +/- 0.24 ng/ml) or Day 1 (0.45 +/- 0.14 ng/ml). LH pulse frequency was greater (P<0.1) on Day 2 (5.3 pulses/8h) compared to the Day 1 (4.6 pulses/8h). In conclusion, fenceline bull exposure early postpartum seems to have an acute effect on LH-release in anoestrous dairy cows. Whether sustained bull exposure can hasten first ovulation after calving through an effect on LH release in dairy cows is an interesting area of research.     

Estradiol-induced blockade of ovulation in the cow: effects on luteinizing hormone release and follicular fluid steroids

Administration of 10 mg estradiol valerate (EV) to nonlactating Holstein cows on Days 16 of the estrous cycle prevented ovulation in 7 of 8 cows for 14 days post-injection. In these 7 cows, the timing of luteolysis and the luteinizing hormone (LH) surge was variable but within the normal range. At 14 days post-treatment, each of these cows had a large (greater than 10 mm) follicle, with 558 +/- 98 ng/ml estradiol-17 beta, 120 +/- 31 ng/ml testosterone, and 31 +/- 2 ng/ml progesterone in follicular fluid (means +/- SE). A second group of animals was then either treated with EV as before (n = 22), or not injected (control, n = 17) and ovariectomized on either Day 17, Day 18.5, Day 20, or Day 21.5 (24, 60, 96, or 132 h post-EV). Treatment with EV did not influence the timing of luteolysis, but surges of LH occurred earlier (59 +/- 8 h post-EV vs. 100 +/- 11 h in controls). The interval from luteolysis to LH peak was reduced from 44 +/- 6 h (controls) to 6.9 +/- 1.5 h (treated). Histologically, the largest follicle in controls tended to be atretic before luteolysis, but nonatretic afterwards, whereas the largest follicle in treated animals always tended to be atretic. Nonatretic follicles contained high concentrations of estradiol (408 +/- 59 ng/ml) and moderate amounts of testosterone (107 +/- 33 ng/ml) and progesterone (101 +/- 21 ng/ml), whereas atretic follicles contained low concentrations of estradiol (8 +/- 4 ng/ml) and testosterone (12 +/- 4 ng/ml), and either low (56 +/- 24 ng/ml) or very high (602 +/- 344 ng/ml) concentrations of progesterone. This study suggests that EV prevents ovulation by inducing atresia of the potential preovulatory follicle, which is replaced by a healthy large follicle by 14 days post-treatment.     

Effect of zearalenone and estradiol benzoate on serum concentrations of LH, FSH and prolactin in ovariectomized gilts

Six ovariectomized gilts were given zearalenone (Z), estradiol benzoate (EB) or vehicle in a replicated 3 x 3 Latin square design. Zearalenone was added to 2.3 kg of a corn-soybean ration at a dose of 1 mg Z/kg body weight; EB was given intramuscularly at 0.1 mg EB/kg body weight. Control gilts received vehicle solvent for both Z and EB. Blood samples were collected from indwelling jugular cannulas at 6-h intervals for 48 h before Z, EB or vehicle was given. After treatment, blood samples were drawn at 6-h intervals for an additional 84 h. Serum concentrations of luteinizing hormone (LH) decreased (P<0.001) from 4.67 ng/ml to 0.29 ng/ml within 6 h of EB. From 54 to 84 h after EB, serum concentrations of LH rose to 15.60 ng/ml (P<0.001). Serum concentrations of LH were reduced (P<0.001) in a similar pattern after Z (3.70 ng/ml to 0.49 ng/ml), but a rise in serum LH was not observed 54 to 84 h after Z (1.30 ng/ml). Serum concentrations of LH remained unchanged (P=0.55) in gilts given vehicle. Serum concentrations of follicle stimulating hormone (FSH) were suppressed (P<0.03) at 6 h in EB (19.10 vs 11.35 ng/ml) and Z gilts (16.16 vs 11.41 ng/ml) but remained unchanged in vehicle gilts. Serum concentrations of FSH did not change in EB or Z gilts during the next 36 h. These data indicate that the suppressive action of Z on serum concentrations of LH and FSH was similar to that of EB, while the biphasic stimulatory effect of EB for LH was not manifested by Z.