Response of anestrous ewes to norgestomet and PMSG

A 10-day treatment regime with a subcutaneous ear implant containing 3 mg of norgestomet, accompanied by an intramuscular injection of 1.5 mg norgestomet and 0.5 mg estradiol valerate (EV) on day 1 and 750 I. U. pregnant mares serum gonadotropin (PMSG) given intravenously on day 10, proved effective in eliciting estrus in 72% of 110 anestrous ewes within 5 days of treatment. Ewes which were treated in months closer in proximity to the normal breeding scason responded with significatly increased induction of estrus, with 71, 37, 59, 74, and 97% in estrus for ewes which were treated in February through June, respectively. Comparable estrous response in nontreated, control ewes was 0, 13, 0, 10, and 24% during February through June, respectively. (Treated vs controls, P<.01). Pregnancy rate to first service of ewes in estruc was 51% in treated and 30% in control ewes (P>.10). Overall pregnancy rate for all ewes in both groups was 36% in treated and 3% in control ewes during 5 or 16 days of breeding, respectively (P<.01).     

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.     

Synchronization of estrus in beef cows with Norgestomet-Alfaprostol or Syncro-Mate B

In the spring of 1986, 506 beef cows were used to evaluate the effectiveness of two estrus synchronization systems. Cows were synchronized with either a 6-mg Norgestomet implant placed in the ear for 14 d followed by a 6-mg Alfaprostol injection given 16 d after implant removal (Norgestomet-Alfaprostol) or with Syncro-Mate B (6-mg Norgestomet implant for 9 d with an injection containing 5 mg estradiol valerate and 3 mg Norgestomet at the time of implantation). The Alfaprostol injection in the Norgestomet-Alfaprostol group was given the same day as implant removal in the Syncro-Mate B group. These treatment groups were compared to a group of untreated controls. Cows were allotted to treatments by days postpartum, age and breed. Syncro-Mate B cows had a higher estrous response within 5 d after treatment (78.6 vs 64.0%) and a shorter interval to estrus (39.2 vs 66.7 h) than did Norgestomet-Alfaprostol cows (P < 0.05). Controls had a significantly lower estrous response compared to either of the synchronized groups (27.1%). The degree of estrus synchrony was identical in both synchronization systems (72.7%). Synchronized conception rate tended to be higher (P = 0.06) in the Norgestomet-Alfaprostol cows than in the Syncro-Mate B cows (74.5 vs 62.5%). Synchronized, 21-d, 25-d and breeding season pregnancy rates were 51.2, 70.8, 76.8 and 92.9% for Norgestomet-Alfaprostol cows; 48.5, 63.0, 73.2 and 87.8% for Syncro-Mate B cows; and 15.6, 56.3, 61.3 and 86.9% for control cows. The four pregnancy rates were not different between the two synchronization treatments (P > 0.10). Controls had lower synchronized and 25-d pregnancy rates when compared to either of the synchronized groups (P < 0.05). Days postpartum had no effect on the reproductive performance of cows synchronized with Norgestomet-Alfaprostol. Our results indicate that the Norgestomet-Alfaprostol system is as effective as Syncro-Mate B in synchronizing estrus in beef cows.     

Inhibition of estrus and corpora lutea function with Norgestomet

Forty-five nonpregnant, nonlactating, Angus and Brangus cows were utilized to determine how long a Norgestomet ear implant would inhibit estrus when administered at various stages of an estrous cycle. All cows completed a nontreated estrous cycle to ensure normal cyclicity. At the second observed estrus (estrus = Day 1), cows were randomly allotted to be treated at metestrus (Day 3 or Day 4, n = 15); at diestrus (Day 9 or Day 10, n = 14); or at proestrus (Day 15 or Day 16, n = 16). All cows received a 2-ml intramuscular injection of 3 mg of Norgestomet accompanied by a 6-mg Norgestomet ear implant, which remained in situ for 21 days, or until individual cows were observed in estrus. Estrus was inhibited for a mean (+/- SEM) of 18.7 +/- 0.7, 19.9 +/- 0.8, and 17.0 +/- 0.8 days, respectively, when cows were treated at metestrus, diestrus, and proestrus (metestrus and diestrus vs proestrus; P < 0.05). Estrus was inhibited for an entire 21-day implantation period in 27, 50, and 38% of cows treated at metestrus, diestrus, and proestrus, respectively (P > 0.10). Norgestomet inhibited estrus in all cows for 11, 17, and 11 days after implantation when treatment was initiated at metestrus, diestrus, and proestrus, respectively (P > 0.10). These data indicate that a 6-mg Norgestomet ear implant effectively inhibits estrus in all cows for a maximum of 11 days, with some cows exhibiting estrus by Day 12 with the Norgestomet implant in situ.     

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.     

Comparison of two progestogens during out-of-season breeding in a commercial ewe flock

A comparison was made of the relative effectiveness of subcutaneous ear implants containing 2 mg Norgestomet or vagnial pessaries containing 60 mg medroxyprogesterone acetate (MAP) to induce estrus and conception in dry anestrous ewes. Groups of ewes were treated with one of the two progestogens for 14 d, and 500 IU pregnant mare serum gonadotropin (PMSG) was administered intramuscularly at the time of progestogen withdrawal. No significant differences in estrus induction, pregnancy rate or number of lambs born per ewe lambing were observed. Ewes treated with Norgestomet had 96% estrus, 60% pregnancy rate and 1.4 lambs per ewe lambing. Comparably, ewes treated with MAP had 94% estrus, 65% pregnancy rate and 1.7 lambs per ewe lambing. Norgestomet implants compared favorably with MAP pessaries for estrus induction and breeding of commercial, dry anestrous ewes.     

Peripheral progesterone levels in pregnant and non-pregnant heifers following use of HCG.

Progesterone concentration in jugular blood of bred beef heifers was determined on days 9 and 16 in two trials. Human chorionic gonadotrophin (HCG) was administered to some of the heifers in each trial in an attempt to improve pregnancy percentage.In Trial 1, 183 heifers were divided into a control group and three groups of animals which were subjected to a 9-day estrous synchronization treatment prior to breeding. The treatment consisted of an ear implant containing 17 α acetoxy-11-beta-methyl 17 nor preg 4-ene 3, 20 dione (norgestomet) left in place for 9 days and an injection of 5 mg estradiol valerate (EV) and 3 mg of norgestomet given at the time of implantation. The heifers in one group received .25 mg estradiol-17β at time of implant removal; heifers in the 2nd group received 1500 IU of HCG in 5% beeswax and 95% sesame oil at breeding time, while heifers in the 3rd group received a placebo injection containing 5% beeswax and 95% sesame oil at breeding time. No differences in serum levels of progesterone were observed (P>0.5) between treatments or between pregnant and non-pregnant heifers on day 9 or 16 (P>.05). Pregnancy percentage in heifers receiving HCG was similar to that noted in the control heifers or the placebo injected heifers while injection of estradiol 17β decreased the proportion of heifers which became pregnant.In trial 2, 58 heifers which had been bred 1 or 2 times without becoming pregnant were divided into a control group and a group in which heifers received 1000 IU of HCG 96 hr. after observed estrus. In heifers receiving HCG, serum levels of progesterone were higher (P<.01), on day 9 and 16 post estrus than in controls but no difference in serum progesterone was noted (P>.05) between pregnant and non-pregnant heifers on day 9 or 16. The proportion pregnant did not differ (P>.05) between heifers receiving HCG and control heifers.     

Effects of repetitive norgestomet treatments on pregnancy rates in cyclic and anestrous beef heifers

Sixty-eight 12- to 14-month-old crossbred beef heifers averaging 285 kg were assigned at random to treated (n = 35) and control (n = 33) groups to evaluate the use of repetitive norgestomet treatments. Treated heifers received an ear implant containing 6 mg norgestomet on two occasions 16 days apart. Injections of 3 mg norgestomet and 5 mg estradiol valerate (EV) were given intramuscularly the same day as first implantation (Syncro-Mate-B). Implants were removed after eight days. Four bulls were then placed in each of two pastures containing half of the treated and half of the control heifers for 24 days after the time of the first implant removal. Progesterone concentrations from blood samples collected prior to the first treatment were used to determine reproductive status. The overall pregnancy rate 64 days after first implant removal for treated anestrous heifers (61%; 14 23 ) was similar (p > 0.25) to untreated (73%; 11 15 ) and treated (75%; 9 12 ) cyclic heifers, but higher (p < 0.1) than for untreated anestrous heifers (33%; 6 18 ). This treatment advantage resulted from an increased (p < 0.01) pregnancy rate after the second implant removal. In summary, repetitive norgestomet treatments enhanced pregnancy rate in anestrous heifers within a 24-day breeding season.     

Induction of ovarian follicular wave emergence and ovulation in progestin-based timed artificial insemination protocols for Bos indicus cattle

The present study aimed to evaluate the efficacy of different inducers of new follicular wave emergence (FWE) and ovulation in fixed-time artificial insemination (FTAI) synchronization protocols using norgestomet ear implants (NORG) in Bos indicus cattle. In Experiment 1, the synchronization of FWE was evaluated when two different estradiol esters in different doses [2mg estradiol benzoate (EB), 2.5mg EV or 5mg estradiol valerate (EV)] were administered with NORG implant insertion in B. indicus cattle (estrous cyclic heifers and cows with suckling calves; n=10 per treatment). After estradiol treatment, ovarian ultrasonic exams were performed once daily to detect the interval between treatment and FWE. There were significant treatment-by-animal category interaction (P=0.05) on the interval from the estradiol treatment to FWE. An earlier (P<0.0001) and less variable (P=0.02) interval from estradiol treatment to FWE was observed in heifers treated with EB (2.5±0.2; mean±SE) than in those treated with 2.5mg EV (4.2±0.3) or 5mg EV (6.1±0.6). Cows treated with 5mg EV (4.0±0.5) had longer (P=0.05) interval than cows receiving EB (2.5±0.2), however, there was an intermediate interval in those cows treated with 2.5mg EV (3.1±0.4). In Experiment 2, the number of uses of the NORG implant (new; n=305 or previously used once; n=314) and three different ovulation induction hormones [0.5mg estradiol cypionate (EC) at implant removal (n=205), 1mg EB given 24h after implant removal (n=219), or 100μg gonadorelin (GnRH) given at FTAI (n=195)] were evaluated in Nelore heifers (2×3 factorial design). Similar pregnancy per AI (P/AI; 30 days after FTAI; P>0.05) were achieved using each of the three ovulation induction hormones (EB=40.6%; EC=48.3%, or GnRH=48.7%) and with a new (47.2%) or once-used NORG implant (44.3%). In Experiment 3, the effect of different ovulation induction hormones for FTAI [1mg EC at NORG implant removal (n=228), 10μg buserelin acetate at FTAI (GnRH; n=212) or both treatments (EC+GnRH; n=215)] on P/AI was evaluated in suckled beef cows treated with a once-used NORG implant and EB to synchronize the FWE. Similar P/AI (P=0.71) were obtained using GnRH (50.9%), EC (51.8%) or both treatments (54.9%) as ovulation induction hormones. Therefore, both doses of EV (2.5 or 5.0mg) with NORG implant delayed and increased the variation of the day of new FWE compared with EB in B. indicus cattle. These effects were more pronounced in B. indicus heifers than cows. Synchronization protocols for FTAI with either a new or once-used NORG implant with EB at insertion to induce a new FWE and either the use of EB, EC or GnRH as ovulation induction hormones may be successful in B. indicus heifers. Also, when a once-used NORG implant was used, either the administration of EC, GnRH or both as ovulation inducers resulted in similar P/AI in suckled B. indicus cows, showing no additive effect of the combination of both ovulation induction hormones.     

Regulation of the estrous cycle in ewes with progestin containing implants: Influence of dose and day of cycle at treatment

Implants containing Norgestomet (G. D. Searle and Co., Chicago) were inserted subcutaneously in ewes on selected days of the estrous cycle. When ewes were treated for 13 days with 2 or 3 mg Norgestomet, implantation 13 days post-estrus reduced the number of ewes in estrus within 5 days of implant removal and reduced the number of estrous ewes that lambed compared with ewes implanted 4 days post-estrus. When ewes were implanted with 3 or 6 mg Norgestomet 4 or 13 days post-estrus, no difference in estrus response was found. Conception rate was not influenced by day of treatment, but was higher in those ewes treated with 6 mg than ewes treated with 3 mg. Compared to no treatment, treatment with 3 or 6 mg Norgestomet reduced the number of uterine and oviducal sperm recovered 12 or 24 hr after insemination from ewes implanted for 12 days 2 or 12 days post-estrus. However, more sperm were recovered from ewes treated 2 days than 12 days post-estrus with the principal increase occurring in ewes treated with 6 mg of Norgestomet.     

Suppression of follicle wave emergence in cyclic ewes by supraphysiologic concentrations of estradiol-17beta and induction with a physiologic dose of exogenous ovine follicle-stimulating hormone

Follicle waves are preceded by follicle-stimulating hormone (FSH) peaks in ewes. The purpose of the present study was to see whether estradiol implant treatment would block FSH peaks to create a model in which the effect of the timing and mode of FSH peaks could be studied by ovine FSH (oFSH) injection. In Experiment 1, 10 ewes received estradiol-17beta implants on Day 4 after ovulation (Day 0, day of ovulation); five ewes received large implants, and five ewes received small implants. Five control ewes received empty implants. In Experiment 2, 12 ewes received large implants on Day 4. On Day 9, six ewes received oFSH twice, 8 h apart (0.5 microg/kg; s.c.). Implants were left in place for 10 days in both experiments. In both studies, ovarian ultrasonography and blood sampling was done daily. In Experiment 1, estradiol concentrations were significantly higher in ewes with large implants (10.4 +/- 0.7 pg/ml) compared with controls (3.9 +/- 0.7 pg/ml) and ewes with small implants (5.4 +/- 0.7 pg/ml; P < 0.001). A significant reduction was found in mean FSH peak concentration (31%; P < 0.05) and FSH peak amplitude (45%; P < 0.05) in ewes with large implants compared with controls. Mean and basal FSH concentrations were unaffected by the large implants. The large implants halted follicle-wave emergence between Day 0 and 8 after implant insertion. The small follicle pool (2-3 mm in diameter) was unaffected by the large implants. When oFSH was injected into ewes with large implants, a follicle wave emerged 1.5 +/- 0.5 days after injection; however, in ewes given saline alone, a follicle wave emerged 4.8 +/- 0.8 days after injection (P < 0.01). We concluded that truncation of FSH peaks by estradiol implants prevented follicle-wave emergence, but injection of physiologic concentrations of oFSH reinitiated follicle-wave emergence.     

Effects of progestagen treatments and PMSG on the induction of the preovulatory LH discharge in ewes

The characteristics of the induced preovulatory LH discharge were compared in ewes after treatment for 12 days with intravaginal sponge pessaries impregnated with 40 mg Fluorogestone Acetate or with subcutaneous ear implants containing varying quantities of Norgestomet. In Experiment 1, ewes were treated with intravaginal sponges or implants alone. In Experiment 2, ewes received similar treatments and 500 IU pregnant mares' serum gonadotropin (PMSG) i.m. at the time of sponge or implant removal. The duration of the LH discharge and an estimate of the total LH discharged were similar among treatment groups within the same experiment. Overall, the onset of LH release occurred approximately 8 h earlier in ewes treated with implants, whether or not PMSG was used. Use of PMSG, in conjunction with implant or sponge treatments, shortened the mean interval from sponge or implant removal to the onset of LH release from 41 to 28 h and doubled the estimated total LH discharged, compared with treatments using sponges or implants alone.     

Estrus synchronization and pregnancy rates in cyclic and noncyclic beef cows and heifers treated with syncro-mate B or Norgestomet and Alfaprostol

Postpartum lactating cows (N=118) and virgin heifers (N=60) were treated with subcutaneous Norgestomet implants for nine days and received either an intramuscular injection (im) of 5 mg estradiol valerate and 3 mg Norgestomet at the time of implant insertion or an im injection of 5 mg Alfaprostol 24 hr before implant removal. Animals were artificially inseminated 12 hr after detection of estrus. Of the cows and heifers, 78% and 88%, respectively, were in estrus within five days after implant removal (P<0.09). There was no difference between treatments in the proportion of animals in estrus or in the timing of estrus (P<0.85). Estrus was detected in a greater (P<0.05) proportion of animals that were cyclic prior to treatment (88%) than among those that were anestrous prior to treatment (77%). Pregnancy rates after five days were similar between heifers that were cyclic (42%) or anestrous (47%) prior to treatment; however, the five-day pregnancy rate in cows that were anestrous prior to treatment was 38% lower than that in cows that were cyclic prior to treatment (17 vs 55%, P<0.01). Although the treatments synchronized or induced estrus in both cyclic and anestrous animals, marked variability in estrous response and fertility among previously cyclic or anestrous postpartum cows limited the effectiveness of the treatments.     

Role of luteal regression in embryo death in cattle

Inseminated crossbred beef cows and heifers were used in 2 experiments to investigate embryo survival after prostaglandin-induced or spontaneous luteal regression. In Experiment 1, luteal regression was induced by an intramuscular (im) injection of cloprostenol (500 mug) on Day 15 (day of ovulation = Day 0). Progestagen was replaced 24 or 36 h later either by one-and-a-half Syncro-Mate-B (SMB) ear implants (9 mg of norgestomet) and norgestomet solution (2.25 mg, im) containing no estradiol in 1 replicate or by 2 SMB implants (12 mg of norgestomet) and progesterone (100 mg, im) in the second replicate. Combined for both replicates, the Day-24 pregnancy rate in an untreated control group (Group 1, 16/19; 84%) was higher (P < 0.01) than in the 24-h group (Group 2, 9/20; 45%), which also was higher (P < 0.02) than in the 36-h group (Group 3, 3/23; 13%). In Experiment 2, 15 or 16 d after breeding, cattle with a corpus luteum at least 16 x 16 mm were given either 2 SMB implants or no treatment. At 24 to 26 d after breeding, pregnancy rates (48/65, 74% versus 49/68, 72%) were not significantly different, and all but 1 of the pregnant progestagen-supplemented cattle had a functional corpus luteum.     

Comparison of Melengestrol Acetate-Prostaglandin F(2)alpha to Syncro-Mate B for estrus synchronization in beef heifers

Three hundred and ten yearling heifers of various breeds were used in five trials to compare two estrus synchronization treatments. Treatment 1 consisted of Melengestrol Acetate-Prostaglandin F(2)alpha (MGA-PGF(2)alpha). Heifers were fed 0.5 mg MGA/head/d for 14 to 16 d. Sixteen or 17 d after the final MGA feeding, heifers were injected i.m. with 25 mg PGF(2)alpha. Treatment 2 consisted of Syncro-Mate B (SMB). Heifers were given a 9-d norgestomet implant plus an injection containing 3 mg norgestomet and 5 mg estradiol valerate i.m. at implant insertion. Heifers were observed for estrus at 6-h intervals for 120 h after the end of treatments and were artificially inseminated 12 to 18 h after observed estrus. Heifers synchronized with MGA-PGF(2)alpha and SMB had a similar (P > 0.10) estrous response (83.4 vs 90.2%) and a similar (P > 0.10) degree of synchrony (71.8 vs 79.0%) following treatment. However, the synchronized conception rate (68.7 vs 40.6%) and the synchronized pregnancy rate (57.3 vs 36.6%) were higher (P < 0.01) in MGA-PGF(2)alpha than SMB heifers. Breeding season pregnancy rates were similar in both treatment groups. Heifers in both groups that were classified as cycling prior to initiation of treatment had improved reproductive performance following synchronization compared with those classified as noncycling. Based on higher synchronized conception and pregnancy rates and lower labor requirements and drug costs, the MGA-PGF(2)alpha system appears to be a better method to synchronize estrus in beef heifers than the SMB system.     

Synchronization of estrus and ovulation and associated endocrine changes in Bos indicus cows

The effects of 4 estrus synchronization treatments on intervals to and synchrony of estrus and ovulation, on timing of the preovulatory LH surge and associated changes in plasma progesterone, LH, FSH, and 17beta-estradiol (E(2)) were investigated in 48 Bos indicus cows. Treatment 1 consisted of 2 injections of PGF(2alpha) 14 d apart (n = 12); Treatment 2 of a subcutaneous 3-mg norgestomet implant and an intramuscular injection of 3 mg of norgestomet and 5 mg estradiol valerate, with the implant removed 10 d later (n = 12; norgestomet-estradiol); Treatment 3 of norgestomet-estradiol, with a subcutaneous injection of PMSG given at time of implant removal (Day 10; n = 12); and Treatment 4 of norgestomet implant (as for Treatments 2 and 3) inserted for 10 d, with an intramuscular injection of PGF(2alpha) given at the time of implant removal (n = 12). The experiment was conducted in 2 replicates (24 cows/replicate, 6 cows/group). Estrus, ovulation and timing of the preovulatory surge of LH varied less in cows treated with norgestomet-estradiol and PMSG than in cows in Treatments 1 and 4 (P < 0.008). Treatment with PMSG reduced variation in ovulation times and timing of the LH surge in cows treated with norgestomet-estradiol (P < 0.02). Concentrations of E(2) were higher in cows in Treatments 2 and 3 on the final day of treatment and at about 6 h post ovulation compared with cows in Treatments 1 and 4 (P < 0.05). Different methods for synchronizing estrus did not alter sequential endocrine and behavioral changes in relation to the timing of the LH peak, and the results were consistent with current recommendations for insemination times in Bos taurus cattle.     

Dose determination and efficacy of remotely delivered norgestomet implants on contraception of white-tailed deer

Management of overabundant wildlife populations using contraceptives is being considered with increasing frequency in many localities. A wide array of effective contraceptives is needed to meet a variety of management objectives. Therefore, we evaluated the synthetic progestin norgestomet for its efficacy and its minimum effective dose in free-ranging white-tailed deer (Odocoileus virginianus). We evaluated two doses of norgestomet implants (14 and 42 mg) at a site in southern Connecticut during 1992–1995. Four doses (14, 21, 28, 42 mg) of norgestomet implants were tested at a site in northern Indiana during 1993–1996. The effectiveness of norgestomet implants in preventing pregnancy was similar for the 42 mg (92%), 28 mg (100%), and 21 mg (100%) doses. There was a significant decline in efficacy using the 14 mg (48%) dose. It appears that 21 mg is approximately the lowest dose that consistently prevents reproduction in adult white-tailed deer of various sizes and ages. Norgestomet implants show promise as a highly effective contraceptive agent that is safe to treated animals and secondary consumers and simple to deliver remotely. Zoo Biol 16:31–37, 1997. © 1997 Wiley-Liss, Inc.     

Use of short-term progestin treatment to resynchronize the second estrus following synchronized breeding in beef heifers

Two trials were conducted to evaluate the efficacy of short-term progestin administration to resynchronize the second estrus after artificial insemination in yearling beef heifers. In Trial 1 crossbred yearling heifers (n = 208) were synchronized with Syncro-Mate-B (SMB) and artificially inseminated (AI) between 48 and 54 h following implant removal. Implant removal is defined as Day 1. Following AI, the heifers were randomly assigned to 1 of 2 experimental groups. Group 1 heifers were fed melengestrol acetate (MGA) daily from Day 17 to 21 at a rate of 0.5 mg/head, while Group 2 control received no exogenous progestin during this period. Synchrony of estrus was defined as the 3-d period in which the highest number of heifers expressed behavioral estrus in each group. There was no difference (P < 0.05) in the pregnancy rate during the second estrus due to MGA supplementation. More MGA-treated heifers (P < 0.01) expressed estrus in a 3-d period than the controls. In Trial 2, yearling heifers (n = 108) were synchronized with 2 injections of PGF(2alpha) (second PGF(2alpha) injection is designated as Day 1) administered 14 d apart with AI 12 h after the onset of behavioral estrus. The heifers were then randomly assigned to 1 of the following 3 treatment groups after initial AI: 1) MGA fed at 0.5 mg/head daily from Days 17 to 21; 2) norgestomet administered in 6.0-mg implants from Days 17 to 21; 3) untreated control heifers. Blood samples were collected on Day 21 and analyzed for progesterone (P(4)). Elevated P(4) (> 1 ng/ml) on Day 21 indicated pregnancy to the first insemination. Synchrony among the 3 groups of heifers was similar (P > 0.10); however, the second estrus was less (P < 0.05) variable in the MGA and norgestomet treated heifers. During the resynchronized second estrus, conception rates were not affected by progestin treatment (MGA 40%, norgestomet 64%, and control 62%; P > 0.10). However, a proportion of heifers treated MGA 10% 4 36 and norgestomet 3% 1 36 expressed behavioral estrus during second estrus even though they were diagnosed as pregnant from first service by elevated P(4) levels on Day 21. We conclude that short-term use of progestin from Days 17 to 21 following AI causes closer synchrony of estrus; however, inseminating pregnant heifers that exhibit behavioral estrus may cause abortion.     

Reproductive performance of synchronized beef cows as affected by inhibition of suckling with nose tags or temporary calf removal

A study was conducted to determine whether presence of the calf during suckling inhibition influences the response to estrus synchronization in beef cows. Angus or Hereford cows (n=89) were administered Syncro-Mate-B (SMB), which consisted of a 6-mg norgestomet ear implant (in situ 9 d) in conjunction with 5 mg of estradiol valerate and 3 mg (im) of norgestomet. Cows were allotted by breed, body condition, stage of the estrous cycle, parity and date of parturition to 1 of 3 treatments: 48-h calf removal; ad libitum suckling; or inhibition of suckling with a nose tag for 48 h. Calves were weighed at time of SMB implant removal, 48 h later and at weaning. Cows were mated via AI approximately 12 h after detection of estrus during a 30-d period after implant removal followed by a natural service period of 35 d. At 48 h after implant removal, calf removal and nose tag calves had lost an average of 3.6 and 0.9 kg, respectively, while the suckled calves gained 1.8 kg (P < 0.01). Mean calf weight at weaning did not differ among treatments. Synchronized estrous response (within 5 d of implant removal) was not different among treatments. Pregnancy rate for cows exhibiting a synchronized estrus (5 d AI) for calf removal, nose tag and suckled cows was 76, 48 and 48%, respectively (P>0.10). Treatment did not affect the 30-d AI or overall 65-d pregnancy rate. In this study, there were no differences observed in the percentage of synchronized or pregnant cows following suckling inhibition by either a nose tag or calf removal. Transient reductions of calf body weight during the 48-h calf removal period did occur in both the nose tag and calf removal groups.     

Equine chorionic gonadotropin and gonadotropin-releasing hormone enhance fertility in a norgestomet-based, timed artificial insemination protocol in suckled Nelore (Bos indicus) cows

Two experiments were conducted to investigate the effects of equine chorionic gonadotropin (eCG) at progestin removal and gonadotropin-releasing hormone (GnRH) at timed artificial insemination (TAI) on ovarian follicular dynamics (Experiment 1) and pregnancy rates (Experiment 2) in suckled Nelore (Bos indicus) cows. Both experiments were 2 × 2 factorials (eCG or No eCG, and GnRH or No GnRH), with identical treatments. In Experiment 1, 50 anestrous cows, 134.5 ± 2.3 d postpartum, received a 3 mg norgestomet ear implant sc, plus 3 mg norgestomet and 5 mg estradiol valerate im on Day 0. The implant was removed on Day 9, with TAI 54 h later. Cows received 400 IU eCG or no further treatment on Day 9 and GnRH (100 μg gonadorelin) or no further treatment at TAI. Treatment with eCG increased the growth rate of the largest follicle from Days 9 to 11 (means ± SEM, 1.53 ± 0.1 vs. 0.48 ± 0.1 mm/d; P < 0.0001), its diameter on Day 11 (11.4 ± 0.6 vs. 9.3 ± 0.7 mm; P = 0.03), as well as ovulation rate (80.8% vs. 50.0%, P = 0.02), whereas GnRH improved the synchrony of ovulation (72.0 ± 1.1 vs. 71.1 ± 2.0 h). In Experiment 2 (n = 599 cows, 40 to 120 d postpartum), pregnancy rates differed (P = 0.004) among groups (27.6%, 40.1%, 47.7%, and 55.7% for Control, GnRH, eCG, and eCG + GnRH groups). Both eCG and GnRH improved pregnancy rates (51.7% vs. 33.8%, P = 0.002; and 48.0% vs 37.6%, P = 0.02, respectively), although their effects were not additive (no significant interaction). In conclusion, eCG at norgestomet implant removal increased the growth rate of the largest follicle (LF) from implant removal to TAI, the diameter of the LF at TAI, and rates of ovulation and pregnancy rates. Furthermore, GnRH at TAI improved the synchrony of ovulations and pregnancy rates in postpartum Nelore cows treated with a norgestomet-based TAI protocol.