Effect of estradiol valerate on ovarian follicles, emergence of follicular waves and circulating gonadotropins in heifers

An experiment was designed to examine the effect of estradiol valerate (EV) on the growth and regression of follicles of a wave and on the emergence of the next follicular wave. Twenty-six beef heifers were xamined daily by ultrasonography and randomly allocated to 1 of 4 treatment groups at the time of ovulation (Day 0): unterated control heifers and those that received 5 mg EV intramuscularly on Day 1, Day 3 or Day 6. Maximum diameter of the dominant follicle was greater (P<0.05) in control heifers than in heifers treated on Day 1 or Day 3. Mean day of onset of regression of the dominant follicle was later (P<0.05) in control heifers than in heifers treated on Day 1 but was not different from heifers treated on Day 3. In heifers treated on Day 6, cessation of growth, maximum diameter and onset of regression were not different from that of control heifers. The emergence of the next follicular wave was earlier (P<0.05) in heifers treated on Day 1 than in control heifers, whereas wave emergence was delayed (P<0.05) in heifers treated on Day 3 or Day 6. The mean day of maximum concentration of FSH prior to the emergence of the next wave was earlier in heifers treated with EV on Day 1 and later in heifers treated on Day 3 or Day 6 compared with that of the controls (P<0.05). Treatment on Day 1 or Day 3 resulted in a significant LH surge in 8 13 heifers, whereas no LH surges were detected in control heifers or in heifers treated on Day 6. The hypothesis that EV suppresses the growth of the dominant follicle, was supported. Estradiol valerate treatment resulted in early emergence of the next follicular wave in heifers treated on Day 1, but treatment on Day 3 or Day 6 resulted in delayed emergence of the next follicular wave.     

Gonadotropic and ovarian hormone response in dairy cows treated with norgestomet and estradiol valerate

Injection of estradiol valerate in conjunction with receiving a norgestomet ear implant (Syncro-Mate-B) reduced plasma progesterone (P<.001) and FSH (P<.02) and increased estradiol (P<.01) in postpartum cows during the treatment period. Occurrence of LH and FSH peaks (P<.05) varied between 12 and 48 hr follwing removal of SMB implants and estrous behavior was minimal. FSH was strongly correlated to LH and was inversely related to progesterone in untreated cows and to estradiol in treated cattle. Syncro-Mate-B suppressed luteal activity and peak gonadotropin release during treatment and elicited variable endocrine and behavioral response following treatment.     

Estrus after treatment with Syncro-Mate B* in ovariectomized heifers is dependent on the injected estradiol valerate

A series of experiments was conducted to determine why ovariectomized heifers exhibit estrus after they are treated with the estrus synchronization product, Syncro-Mate B(*) (SMB). In Experiment 1, 23 of 40 (58%) ovariectomized heifers exhibited estrus after treatment with SMB. The mean concentration of estradiol-17beta (E(2)) in serum was lower (P < 0.001) before treatment than after implant removal in ovariectomized heifers treated with SMB. Six of 10 heifers from which serum was collected to determine concentrations of LH exhibited estrus and 5 of 6 had a surge of LH in serum after implant removal. In Experiment 2, when no estradiol valerate (EV) was given or when the norgestomet implant period was extended from 9 to 18 d, no heifer exhibited estrus after implant removal. The mean concentration of E(2) for 3 d after implant removal was lower (P < 0.001) in ovariectomized heifers with implants for 18 d versus those with implants for 9 d and was also lower (P < 0.001) in ovariectomized heifers treated only with norgestomet compared with those receiving the standard SMB treatment. When estradiol-17beta was substituted for EV in the SMB treatment, serum E(2) was lower (P < 0.001) after implant removal than in heifers receiving the standard SMB treatment. Experiment 3 demonstrated that combining a norgestomet implant or implant plus a 3-mg injection of norgestomet with EV did not alter concentrations of E(2) in serum on the days when synchronized estrus would be expected following SMB treatment. The results indicate that the SMB-induced estrus in ovariectomized heifers is dependent upon EV in the SMB treatment. Apparently, EV elevates the concentration of E(2) in serum, and the E(2) remains sufficiently high to induce estrus after implant removal.     

Effect of treatment with estradiol valerate on endocrine changes and ovarian follicle populations in dairy cows

A linear-array ultrasound instrument was used to monitor the dynamics of follicular cyst formation following estradiol valerate (EV) administration in postpartum dairy cattle. Twelve cyclic cows were given two intramuscular (i.m.) injections of prostaglandin and F(2alpha) (PGF(2alpha)) 12 d apart to synchronize estrus. On Day 16 (Day 0 = day of estrus) six cows received 10 mg of EV in 1 ml sesame oil; the remaining six cows were treated with 1 ml sesame oil. The ovaries of all cows were scanned rectally each morning from Day 9 until 14 or 30 d post treatment. Plasma concentrations of luteinizing hormone (LH) and progesterone (P(4)) were also determined as objective indices of treatment effects. Day 0 to 16 ultrasound pictures of the ovaries of both control and treated cows were characterized by the presence of a corpus luteum (CL; 19 to 38 mm), several small follicles (<5 mm) and a medium-sized follicle (6 to 28 mm). Following treatment in control cows, the CL regressed gradually, and a preovulatory follicle was identifiable by Day 17 to 18, it increased in size and reached a maximum of 28 to 30 mm by Day 20 after ovulation and was identifiable throughout the rest of the cycle. Administration of 0 mg of EV resulted in a rapid reduction in the size of the CL. Growth of a large follicle was observed in all treated animals around Days 16 to 20, but having reached a maximum diameter of 12 to 24 mm it regressed without resulting in ovulation. Subsequent ultrasound pictures of EV-treated cows were characterized by the absence of a new CL and the presence of medium-sized persistent follicles. Estradiol valerate treatment induced early luteolysis (43 +/- 05 h post EV vs 101 +/- 22 h) and an LH surge (41 +/- 11 h vs 125 +/- 17 h).     

Effect of estradiol valerate on ovarian follicle dynamics and superovulatory response in progestin-treated cattle

Three experiments evaluated the effects of estradiol valerate (EV) on ovarian follicular and CL dynamics, intervals to estrus and ovulation, and superovulatory response in cattle. Experiment 1 compared the efficacy of two norgestomet ear implants (Crestar and Syncro-Mate B; SMB) for 9 d (with PGF at implant removal), combined with either 5 mg estradiol-17beta and 100 mg progesterone (EP) or 5 mg EV and 3mg norgestomet (EN) im at the time of implant insertion on CL diameter and follicular wave dynamics. Ovaries were monitored by ultrasonography. There was no effect of norgestomet implant. Diameter of the CL decreased following EN treatment (P < 0.01). Mean (+/- S.D.) day of follicular wave emergence (FWE) was earlier (P < 0.0001) and less variable (P < 0.0001) in EP- (3.6 +/- 0.5 d) than in EN- (5.7 +/- 1.5 d) treated heifers. Intervals from implant removal to estrus (P < 0.001) and ovulation (P < 0.01) were shorter in EN- (45.7 +/- 11.7 and 74.3 +/- 12.6 h, respectively) than in EP- (56.4 +/- 14.1 and 83.3 +/- 17.0 h, respectively) treated heifers. Experiment 2 compared the efficacy of EP versus EN in synchronizing FWE for superovulation in SMB-implanted cows. At random stages of the estrous cycle, Holstein cows (n = 78) received two SMB implants (Day 0) and were randomly assigned to receive EN on Day 0 or EP on Day 1. Folltropin-V treatments were initiated on the evening of Day 5, with PGF in the morning and evening of Day 8, when SMB were removed. Cows were inseminated after the onset of estrus and embryos were recovered 7 d later. Non-lactating cows had more CL (16.7 +/- 11.3 versus 8.3 +/- 4.9) and total ova/embryos (14.7 +/- 9.5 versus 7.9 +/- 4.6) than lactating cows (P < 0.05). EP-treated cows tended (P = 0.09) to yield more transferable embryos (5.6 +/- 5.2) than EN-treated cows (4.0 +/- 3.7). Experiment 3 compared the effect of dose of EV on ovarian follicle and CL growth profiles and synchrony of estrus and ovulation in CIDR-treated beef cows (n = 43). At random stages of the estrous cycle (Day 0), cows received a CIDR and no further treatment (Control), or an injection of 1, 2, or 5 mg im of EV. On Day 7, CIDR were removed and cows received PGF. Follicular wave emergence occurred within 7 d in 7/10 Control cows and 31/32 EV-treated cows (P < 0.05). In responding cows, interval from treatment to FWE was longer (P < 0.05) in those treated with 5 mg EV (4.8 +/- 1.2 d) than in those treated with 1 mg (3.2 +/- 0.9 d) or 2 mg (3.4 +/- 0.8 d) EV, while Control cows were intermediate (3.8 +/- 2.0 d). Diameter of the dominant follicle was smaller (P < 0.05) at CIDR removal and tended (P = 0.08) to be smaller just prior to ovulation in the 5 mg EV group (8.5 +/- 2.2 and 13.2 +/- 0.6 mm, respectively) than in the Control (11.8 +/- 4.6 and 15.5 +/- 2.9 mm, respectively) or 1mg EV (11.7 +/- 2.5 and 15.1 +/- 2.2 mm, respectively) groups, with the 2mg EV group (10.7 +/- 1.5 and 14.3 +/- 1.7 mm, respectively) intermediate. Diameter of the dominant follicle at CIDR removal was less variable (P < 0.01) in the 2 and 5mg EV groups than in the Control group, and intermediate in the 1mg EV group. In summary, treatment with 5mg EV resulted in a longer and more variable interval to follicular wave emergence than treatment with 5mg estradiol-17beta, which affected preovulatory dominant follicle size following progestin removal, and may have also affected superstimulatory response in Holstein cows. Additionally, 5 mg EV appeared to induce luteolysis in heifers, reducing the interval to ovulation following norgestomet removal. Conversely, intervals to, and synchrony of, follicular wave emergence, estrus and ovulation following treatment with 1 or 2 mg EV suggested that reduced doses of EV may be more useful for the synchronization of follicular wave emergence in progestogen-treated cattle.     

Neonatal exposure to estradiol valerate programs ovarian sympathetic innervation and follicular development in the adult rat

A single injection of estradiol valerate (EV) to 14-day-old rats (when the ovarian follicle population has been already established) disrupts cyclicity, increases the activity of key enzymes of androgen biosynthesis, and develops polycystic ovary by a causally related increase in ovarian noradrenaline (NA). The current study examined an early window of ovarian development to look for a specific stage of development at which estradiol can induce such changes in sympathetic activity and follicular development. A single dose of EV applied to rats before the first 12 h of life rapidly increases (after 24 h) the ovarian expression of nerve growth factor (Ngfb) and p75 low-affinity neurotrophic receptor (Ngfr) mRNAs. When adults, rats presented early vaginal opening, disrupted cyclicity, appearance of follicular cyst, absence of corpus luteum, and infertility. Total follicles decreased, mainly due to a reduced number of primordial follicles, suggesting that estradiol acts in the first stages of folliculogenesis, when primordial follicles are organizing. These changes paralleled a 6-fold increase in NA concentration. No changes in NA content were found in the celiac ganglia, suggesting a local, non-centrally mediated effect of estradiol. Surgical section of the superior ovarian nerve (the main source of sympathetic nerves to the ovary) to rats neonatally treated with EV decreased intraovarian NA, delayed vaginal opening, and blocked the development of follicular cyst and that of preovulatory follicles. Therefore, we can conclude that early exposure to estradiol permanently modifies ovarian sympathetic activity and causes profound changes in follicular development, leading to the polycystic ovary condition.     

Pharmacokinetics and biotransformation of estradiol valerate in ovariectomized women

Estradiol valerate is well suited for treatment of the characteristic symptoms accompanying menopause in women. The pharmacokinetics and biotransformation of estradiol valerate were studied in women following intravenous, intramuscular and oral administration. Intravenously given, estradiol valerate is split by enzymatic hydrolysis into 17 beta-estradiol and the fatty acid. The estrogen arising in vivo from the steroid ester is subject to intermediate metabolism. The metabolites are eliminated at a nearly constant rate mainly in urine. The biotransformation of estradiol valerate was not different following intravenous or intramuscular injection. Orally given estradiol valerate is subject to an extensive first pass metabolism. Daily repeated oral administration does not result in accumulation of 17 beta-estradiol and its metabolites.     

Ultrastructural study of cholestasis induced by longterm treatment with estradiol valerate. II. Gap junctional analysis

Treatment of male Wistar rats with estradiol valerate induced alterations in hepatic gap junctions as visualized by the freeze-fracture technique. The alterations involved the spacing, and regularity of packing of the membrane particles of the P face (PF) and complementary pits on the E face (EF), as well as internalization and changes in the number, size and shape of the junctional domains. In approximately 20% of the PF's of the lateral membrane of treated animals the nonjunctional IMPs were aggregated, while the bile canalicular membrane was never involved, maintaining its random distribution of particles. It is proposed that the changes in junctional area and the more general arrangement of the junctional particles may indicate a decrease in coupling between hepatocytes. The invaginations of gap junctions may represent a means for removing gap junctional membrane from the surface or may be an expression of a higher turnover of gap junctions. We assume that the alterations observed here are due to the specific effects of estrogen. This study addresses in detail a number of possible sites of activity and modes of action for estrogen.     

Ultrastructural study of cholestasis induced by longterm treatment with estradiol valerate. I. Tight junctional analysis and tracer experiments

Over a period of 20 weeks estradiol valerate (1.5 mg/kg body weight/week) was administered subcutaneously to male Wistar rats from which the livers were examined at four week intervals employing a freeze-fracture technique and colloidal lanthanum tracer studies. In connection with intrahepatic cholestasis, distinct alterations in the tight junctions were observed, consisting of disorganization, rarification and proliferation. Disruption of the tight junctions was not seen and colloidal lanthanum did not penetrate into the bile canalicular lumen. Holding the view that the term "leakiness" of tight junctions should be defined with reference to the tracer employed, we conclude that in the liver one tight junctional strand is sufficient to prevent the escape of larger bile constituents such as bile acids and that a back diffusion of bile acids over the tight junctional barrier does not play a role in the pathogenesis of the estrogen-induced cholestasis. Interruptions of tight junctions, as described by other authors, are interpreted as a secondary mechanical effect. On the other hand, we consider an increased permeability of the tight junctions to water and small solute molecules as probable; possibly this increased permeability is brought about by alterations in the microfilaments. A model for the pathogenesis of the estrogen-induced intrahepatic cholestasis is proposed.     

Exogenous estradiol reduces inhibition of luteinizing hormone by estradiol in prepubertal heifers

The hypothesis that high levels of exogenous estradiol administered to heifers during the prepubertal period would decrease subsequent negative feedback of estradiol on luteinizing hormone (LH) secretion was tested. Fourteen prepubertal heifers were ovariectomized on Day 0. Ovariectomized heifers received either no further treatment (OVX, n = 4), a single estradiol implant on Day 0 (OVXE, n = 5), or the single implant on Day 0 and two additional implants between Days 16 and 30 (OVXE+ E, n = 5). Ten ovary-intact heifers received either no treatment (INT, n = 5) or were administered the two estradiol implants between Days 16 and 30 (INT+ 5, n = 5). Comparison of LH secretion in OVXE to OVXE+E, and in INT to INT+E resulted in significant time-by-treatment interactions (p less than 0.05 for both). As pubertal age approached, mean concentration of LH (p less than 0.05) and pulse frequency (p less than 0.05) increased more rapidly in OVXE+E and INT+E than in OVXE and INT, respectively. Amplitude of LH pulses was unaffected by treatment. When data were standardized to day of puberty in INT and INT+E heifers, mean LH concentration and LH pulse frequency increased as puberty approached in both groups. These data confirm earlier reports indicating that secretion of LH increases gradually as puberty approaches in heifers. It was concluded that administration of estradiol during the prepubertal period hastened the decline in the subsequent negative feedback of estradiol. Precocious puberty was not induced in ovary-intact females.     

Dose and duration effects of estradiol valerate on serum and lipoprotein lipids

Non-alkylated estrogens, like estradiol valerate (F2V), are widely used in the treatment of the postmenopausal hormonal deficiency syndrome. Their effects on serum and lipoprotein lipids are characterized by an increase in the lipid constituents of high density lipoproteins (HDL) and, usually, a decrease in low density lipoproteins (LDL). These effects are considered beneficial as regards atherogenesis and the risk for cardiovascular diseases. Unlike the effects of alkylated estrogens, no concomitant increase in triglycerides (TG) in serum and very low density lipoproteins (VLDL) - adverse effects - are seen in doses of up to 2 mg E2V. In order to compare the effects of 2 and 4 mg of E2V on serum and lipoprotein lipids, 19 bilaterally oophorectomized women participated in a cross-over study after a 4 week long wash-out period. To evaluate the influence of the time factor, 10 of the women continued taking 2 mg and 9 taking 4 mg of E2V respectively for an additional period of 12 weeks, resulting in a total treatment period of 24 weeks per group. The serum lipoproteins were separated by preparative ultracentrifugation, the serum and lipoprotein lipids being assessed using commercially available kits. In the cross-over part of the study, total (TC) and free cholesterol (FC) and phospholipids (PL) increased in HDL and decreased in LDL. Neither dose increased TG in serum or VLDL. These changes in the lipoprotein pattern persisted at the end of the entire study. Consequently, within the range of commonly used doses (2 and 4 mg) E2V seems to have a constant and, in terms of cardiovascular disease, favourable influence on lipoprotein metabolism irrespective of doses and periods studied.     

The effect of estradiol valerate on follicular dynamics and superovulatory response in cows with Syncro-Mate-B implants

Two experiments were designed to evaluate the effect of estradiol valerate on follicular dynamics and superovulatory response in cows with Syncro-Mate-B (SMB)implants. In Experiment 1, 5 mg estradiol valerate (E(2)), injected at the same time as superstimulation treatments were initiated, resulted in fewer corpora lutea (CL), ova/embryos collected and fertilized ova (P<0.05) than if E(2) was administered with the SMB implant 7 days earlier. In Experiment 2, 31 beef cows and 26 Holstein cows were placed in one of four treatment groups. Group I (control) cows were superstimulated on Day 9 (estrus=Day 0). On Day 2, cows in Groups II, III, and IV received SMB and cows in Group III received E(2). On Day 9, cows in Group IV received E(2), and all cows were superstimulated with Folltropin. The number of CL did not differ (P>0.19) among groups. However, there were more follicles < 10 mm and fewer fertilized ova and transferable embryos (P<0.02) in Group IV cows. Ovarian ultrasonography revealed that the diameter of the largest follicle in Group III cows declined from Day 2 to Day 7 and subsequently increased until Day 13. In contrast, Groups I, II and IV were characterized by apparently linear growth between Days 2 and 13. Differences (P<0.05) were detected between Days 5 and 9. Mean diameter of the largest follicle was smaller for cows in Group III than for the remaining groups on Day 9. It was concluded that SMB did not adversely affect superovulatory response and that E(2) administration resulted in atresia of the antral follicles in the cows with SMB implants.     

Twenty-four-hour secretory patterns of cortisol,progesterone and estradiol in heifers during the follicular and luteal phases of the ovarian cycle

Daily variations of plasma cortisol, progesterone and estradiol concentrations were measured by radioimmunoassay in six different normally cycling heifers during estrus (day 1 of the cycle) and diestrus (days 12–15 of the cycle). Each animal was fitted with an indwelling jugular catheter, and blood was withdrawn at 30-min intervals over a 24-h period. Statistical evaluation of the hormonal profiles using time series analysis revealed that all three steroids are secreted episodically with secretory episodes varying in number, magnitude and timing among different heifers. After dividing the 24-h into three 8-h time periods (I, 09.00–17.00 h; II. 17.00–01.00h; III, 01.00–09.00 h) a prominent circadian rhythm was found for cortisol during estrus and diestrus. Diurnal periodicity similar to that of cortisol was noticed for plasma progesterone during estrus but not diestrus when a functional corpus luteum was present. Estradiol secretion during the follicular and luteal phase of the estrous cycle was characterized by intermittent sustained elevations lasting about 9–15 h and marked by a graded rise and fall of hormone levels unrelated to photoperiod.From our results obtained in cycling heifers we conclude the following: (1) Plasma cortisol exhibits a distinct circadian rhythm during estrus and diestrus which is highly correlated with the light–dark cycle. (2) Plasma progesterone during estrus demonstrates a diurnal pattern which is absent in diestrous heifers bearing a corpus luteum. (3) Plasma estradiol lacks circadian rhythmicity but shows a distinct pattern different from that of progesterone, indicating that both steroids are secreted independently and not controlled by a circadian pacemaker.     

Ultrasonography for detection of pregnancy and study of embryonic development in heifers

Real-time B-mode ultrasonography was used to evaluate uterine changes associated with the estrous cycle in 22 ovulatory periods in 12 nulliparous heifers. Irregular, nonechogenic (black) areas were seen on the images of uterine horns during the periovulatory period. These nonechogenic areas were presumably due to intraluminal fluids since they coincided with the discharge of clear, viscous mucus preceding ovulation and blood-tinged mucus after ovulation. Eight heifers were bred until five pregnant heifers were obtained for study of the ultrasonic morphology of the conceptus. Ultrasound examinations were done daily to day 50 of pregnancy. Discrete, nonechogenic areas were first visible within the uterus between days 12 and 14, when they were approximately 2 mm in diameter. These discrete nonechogenic structures were identified as the embryonic vesicle, since they were observed only in heifers later confirmed to be pregnant and were always in the uterine horn ipsilateral to the corpus luteum. The presence of an embryo within the embryonic vesicle was confirmed by observing an echogenic (white) area with rhythmic pulsations (heartbeat). The embryonic vesicle gradually increased in length from the day of first observation until day 26 when it extended past the curvature of the horn and began to encroach into the contralateral horn. In all heifers, by day 32 the vesicle extended to the tip of the contralateral horn. The embryo was first visible between days 26 and 29 when the mean length was 10 mm. The embryo increased in length an average of 1.1 mm per day. A heartbeat was detectable in the embryo on the first day observed. In one superovulated heifer, five vesicles were visible in the uterine horns by day 14 and by day 33 seven embryos were observed; two of the seven embryos apparently resorbed by day 43.     

Determination of estradiol valerate in pharmaceutical preparations and human serum by flow injection chemiluminescence

A novel method for the detection of trace estradiol valerate (EV) in pharmaceutical preparations and human serum was developed by inhibition of luminol chemiluminescence (CL) by estradiol valerate on the zinc deuteroporphyrin (ZnDP)‐enhanced luminol‐K₃Fe(CN)₆ chemiluminescence system. Under optimized experimental conditions, CL intensity and concentration of estradiol valerate had a good linear relationship in the ranges of 8.0 × 10^‐8 to 1.0 × 10^‐5 g/mL. Detection limit (3σ) was estimated to be 3.5 × 10^‐8 g/mL. The proposed method was applied successfully for the determination of estradiol valerate in pharmaceutical preparations and human serum and recoveries were 97.0‐105.0% and 95.5‐106.0%, respectively. The possible mechanism of the CL system is discussed. Copyright © 2012 John Wiley & Sons, Ltd.     

Variable progesterone response and estradiol secretion in prepubertal beef heifers following treatment with norgestomet implants

Two experiments were conducted to determine the effects of norgestomet ear implants on progesterone response and estradiol secretion in prepubertal beef heifers. In the first experiment, 47 beef heifers were treated with norgestomet. The implants were implanted subcutaneously for 9 d. After implant removal, blood samples were taken from heifers 2 to 4 d per week for 40 d. Following progesterone determination in jugular venous plasma, heifers were classified according to their progesterone response: 1) no response (Group 1); no rise in progesterone above 1 ng/ml throughout the sampling period; 2) one cycle (Group 2); one increase in progesterone above 1 ng/ml for at least 2 d followed by no further increase in progesterone during the sampling period; and 3) two cycles (Group 3); a rise in progesterone above 1 ng/ml for at least 2 d followed by another cycle of normal duration. Heifers treated with norgestomet were classified as 23 with no response, 9 with 1 cycle and 15 with 2 cycles. Concentrations of estradiol were measured in jugular venous samples on Day 2 after implant removal. Mean concentrations of estradiol were greater in Group 3 than in Group 1 (P < or = 0.01). In Experiment 2, 29 prepubertal beef heifers were assigned randomly to either a 9-d treatment with norgestomet (n = 14) or to serve as untreated controls (n = 15). Blood plasma samples were collected daily from Days 0 to 44 after implant removal. After progesterone determination, heifers were classified as 8 with no response, 4 with 1 cycle and 3 with 2 cycles in the control group, and 5 with no response, 3 with 1 cycle and 6 with 2 cycles in the norgestomet group (frequencies did not differ; P > 0.1). Jugular venous blood plasma was also collected at 4-h intervals from 0 h to 96 h after implant removal and concentrations of estradiol were measured. Patterns of estradiol secretion differed (P < or = 0.05) and overall mean concentrations of estradiol over the first 96 h following implant removal were greater (P < or = 0.01) in norgestomet-treated heifers versus the controls. We conclude that norgestomet can produce a variable progesterone response with heifers with 2 cycles secreting more estradiol. Implants of norgestomet also causes more acute secretion of estradiol in prepubertal beef heifers.     

Estrous behavior and ovarian activity in peripuberal heifers.

Estrous behavior and ovarian activity were investigated in peripuberal heifers. Reproductive tracts of 37 Holstein heifers were examined per rectum twice weekly beginning at least 30 days before the 1st ovulation and continuing until 10 days after the 7th ovulation. The signs of 1st estrus occurred at an average of 279 days of age and the 1st corpus luteum was not detected until 30 days later (p .01). The mean interestrual interval was 20 days for the 1st 7 cycles (p .05). The differences in interestrual intervals for cycles accompanied by silent, nonstanding, and standing estrous behavior, and by different ovarian conditions were not significant (p .05). Silent, nonstanding, and standing estrus occurred during 7, 25, and 68% of 245 estrous cycles, respectively. The occurrence of standing estrus increased from the 1st to 7th cycle (p .01). Normal corpora lutea occurred during 61% of 245 cycles, cystic corpora lutea during 14%, anovulaton during 14%, and cystic follicles during 11%. The occurrence of anovulation decreased from 1st to 7th estrus while the occurrence of corpora lutea increased during the same period (p .01).     

Estrus and ovulation in beef cows following use of progesterone-releasing devices, progesterone and estradiol valerate

Two hundred nonsuckling beef cows were treated with either 1) a progesterone-releasing intravaginal device (PRID) for 12 days; 2) PRID plus an IM injection of 200 mg progesterone (PRID-P); 3) PRID plus 5-mg IM injection of estradiol valerate (PRID-EV); or 4) PRID-EV-P. Cows were started on treatment on one of the first eight days of the estrous cycle. The number of cows which had P levels above 1 ng/ml one day after PRID removal was 12 to 50% lower in PRID-EV and PRID-EV-P groups than in PRID and PRID-P groups (P < 0.05). The proportion of cows showing estrus by 96 hours after PRID removal was 38, 36, 77, and 88% (P < 0.05) for the PRID, PRID-P, PRID-EV and PRID-EV-P groups, respectively. Thirty-one percent fewer cows treated with PRID on days 5 through 8 of the estrous cycle showed estrus by four days after PRID removal than those treated on days 1 through 4. In addition, 18 to 22% more cows had P levels above 1 ng/ml among cows treated with PRID or PRID-P on days 5 through 8 than among cows treated similarly on days 1 through 4. It was concluded that effective synchronization of estrus is achieved only when estrogen is used in conjunction with PRID in cows treated for twelve days during the first eight days of an estrous cycle.