Melengestrol Acetate Withdrawal Research Articles

Extending melengestrol acetate feeding from 14 to 18 days during an estrus synchronization and artificial insemination protocol for March-calving beef heifers

Our objective was to determine whether or not extending the length of melengestrol acetate (MGA) feeding beyond the recommended 14-d MGA estrus synchronization protocol would alter the proportion of heifers that expressed estrus during the estrus detection period following administration of prostaglandin F 2α (PG), and resulted in increased conception rates to AI, or both. In 2020 and 2021 March-calving heifers from 2 separate herds were allocated to pens and randomly assigned to a 14-d MGA protocol (E14) or an 18-d MGA protocol (E18, Figure 1). Timing of estrus after MGA withdrawal was recorded in the 2021 replicate only. In both years, estrus expression at AI was recorded using estrus detection aids (Estrotect) and a patch score system (1 = <25% removed, 2 = 25 to 50% removed, 3 = >50% removed, 4 = patch missing). Patch scores of 3 and 4 were considered to be expressing estrus. Pregnancy was confirmed with ultrasound. Heifers exhibiting estrus earlier after MGA withdrawal also exhibited estrus earlier after PG administration ( P < 0.01), but there was no effect of treatment on timing of estrus. Estrus after MGA withdrawal also coincided with estrus after PG administration ( P < 0.01), but there was no effect of treatment on estrus expression. Heifers exhibiting estrus were more likely to become pregnant ( P < 0.01), but conception rates to AI did not differ between treatments. A longer MGA protocol did not significantly alter timing of estrus expression or pregnancy rates resulting from AI.

Sincronização do cio e fertilidade de vacas de corte tratadas com acetato de melengestrol e cipionato de estradiol

The objective of this experiment was to evaluate estrous synchronization responses and pregnancy rates of beef cows treated with melengestrol acetate (MGA) and estradiol cypionate (ECP). Forty lactating Caracu cows (a locally adapted Bos taurus taurus breed) grazing on Panicum maximum cv. Tanzania and supplemented with corn silage received the equivalent of 0.5 mg/animal/day of MGA for 14 days. On day 5, the animals were randomly assigned to one of two groups: group 1 (n=21) received an im injection of 2.0 mg ECP and group 2 (n=19) received 2 ml saline. Animals of group 1 that exhibited estrus within 120 hours after withdrawal of MGA were inseminated (AI), while animals of group 2 were treated with cloprostenol 17 days after MGA removal and were inseminated during induced estrus. In both groups, cows without estrus detection during the day stayed with a bull at night for possible mating. Pregnancy was diagnosed by ultrasound examination 40 days after AI. Chi-square analyses were used to detect differences between groups. Eight (38.1%) animals of group 1 and 7 (36.8%) of group 2 showed estrus after withdrawal of MGA (P>0.05). The conception rates in groups 1 and 2 were, respectively, 37.5% and 50.0% for inseminated cows and 58.3% and 64.3% for inseminated cows mated by the bull at night (P>0.05). These results suggest that it is possible to use the first estrus of females treated with ECP on day 5 of a 14-day MGA treatment. Furthermore, considering the large number of cows mated by the bull at night, it is recommended to use estrus detection tools as an aid for the identification of such animals or to combine AI and bulls for breeding.

Effect of Feeding Melengestrol Acetate, Monensin, and Tylosin on Performance, Carcass Measurements, and Liver Abscesses of Feedlot Heifers

Approximately 4,000 yearling heifers (initial BW = 306 ± 1.9 kg) were randomly allotted into 4 treatment groups to determine the effect of melengestrol acetate (MGA) on feedlot performance and carcass parameters. Treatments included a diet containing monensin (Rumensin) and tylosin (Tylan; RT) fed during the entire feeding period; RT plus MGA (RTM) also fed during the entire feeding period; RT withdrawn 35 d preslaughter (RTwd); and RTM withdrawn 35 d preslaughter (RTMwd). All cattle were implanted at arrival with Revalor-IH and reimplanted with Revalor-200, and were fed a standard feedlot finishing diet (that met or exceeded 1996 NRC requirements) for approximately 150 d. Final BW, ADG, hot carcass weight (HCW), DMI, and percentage of cattle grading Choice were greater (P < 0.05) for RTM than for all other treatments. Withdrawal of MGA (RTMwd) significantly (P < 0.01) decreased ADG, HCW, 12th-rib fat thickness, calculated YG, and percentage of carcasses grading Choice, but significantly increased the percentage of dark-cutting carcasses, LM area, and YG 1, 2, or 3 carcasses compared with RTM. There were no significant differences (except for dressing percentage) between RT and RTwd. Heifers continuously fed MGA increased in ADG, HCW, and carcass quality traits of economic importance compared with other treatments. Withdrawal of MGA, Rumensin, and Tylan 35 d preslaughter adversely affected carcass weight and carcass quality traits of economic importance when compared with a feeding program including MGA, Rumensin, and Tylan.

GnRH treatment at artificial insemination in beef cattle fails to increase plasma progesterone concentrations or pregnancy rates

Treatment with GnRH at the onset of standing estrus increased pregnancy percentages and circulating concentrations of progesterone in repeat breeder dairy cows. The objective of this study was to determine the effect of treatment with GnRH at AI on concentrations of progesterone and conception rates in beef cattle that exhibited estrus. Two hundred ninety-three heifers at four locations were synchronized with the Select Synch plus CIDR protocol (given GnRH and a CIDR was placed into the vagina, and 7 d later, given PGF2α and CIDR removed; n=253) or the 14–19 melengestrol acetate (MGA) protocol (MGA fed at 0.5mg/head/d for 14 d, with PGF2α 19 d after MGA withdrawal n=40) and AI was done after detection of estrus. At Location 1, blood samples were collected on Day 2, 4, 6, 10, 15, and 18 after AI (Day 0=AI). Two hundred and fifty postpartum cows at two locations were synchronized with the Select Synch plus CIDR protocol, and AI was performed after detection of estrus. At AI, cattle were alternately assigned to one of two treatments: (1) treatment with GnRH (100μg) at AI (n=127 heifers and n=108 cows); or (2) non-treated control (n=120 heifers and n=119 cows). Concentrations of progesterone tended to be greater in control heifers compared to GnRH-treated heifers on Days 6 (P=0.08), 10 (P=0.07), and 15 (P=0.11). Overall conception rates were 68% and 66% for GnRH treated and control, respectively, and were not different between treatments (P=0.72). In summary, treatment with GnRH at time of AI had no influence on conception rates in cattle that had exhibited estrus.

Synchronization of Estrus in Fat Tailed Sheep Using Melengestrol Acetate (MGA) in the Breeding Season

Gimenez Diaz, C.A., Emsen, E., Köycegiz, F., Emsen, B., Yaprak, M. and Kutluca, M. 2005. Synchronization of estrus in fat tailed sheep using melengestrol acetate (MGA) in the breeding season. J. Appl. Anim. Res., 28: 25–27. To assess the effectiveness of MGA in synchronizing oestrus sixty-eight Awassi and Redkaraman ewes were group-fed a diet either containing MGA (0.25 mg/ewe/d for 9d) or no MGA. At the end of the MGA feeding regime, ewes were exposed to one ram per group. The percentage of ewes exhibiting estrus within a 6 d period following withdrawal of MGA was significantly higher (P<0.05) for treated (80%) compared to the control ewes (45%). Number of lambs born per ewe lambing and fertility of ewes were not affected by treatment. It was concluded that MGA is effective to synchronize estrus in fat tailed Awassi and Redkaraman sheep in breeding season.

Timed Artificial Insemination in Postpartum Beef Cows Using Melengestrol Acetate, Prostaglandin F2α, and Gonadotropin-Releasing Hormone

This study evaluated the use of melengestrol acetate (MGA), prostaglandin F2α (PGF2α) and gonadotropin-releasing hormone (GnRH) in a timed AI (TAI) protocol in postpartum beef cows. One hundred twenty-one Angus-crossbred cows in two herds were ranked by parity, days postpartum, body condition score (BCS), and BW and then randomly assigned to one of two treatments. Cows in both treatments received 1.8kg of supplement containing 0.5 mg/d of MGA for 14 d, then received supplement without MGA for 17 d. After MGA withdrawal, cows in both groups received an injection of PGF2α. Cows receiving the MGAPGF2α (MPG) treatment were subjected to estrus detection and AI for 8 d. Cows receiving the MGA-PGF2α-GnRH (MPGG) treatment were injected with GnRH 48h after PGF2α, followed by TAI 16 to 18h after PGF2α. There was a difference (P<0.01) between herds in conception rate (CR) and pregnancy rate to AI (PR) and overall pregnancy rate (OP). The CR was not different between treatments (P≥0.24), but PR was greater in MPGG vs MPG (P≤0.08). There was no difference in OP between MPG and MPGG treatments (P≥0.25). The TAI procedure described in this study yielded results that were equal to those obtained with cows in a standard 14- to 17-d MGA-PGF2α with estrus detection and AI protocol and similar to previously described TAI procedures.

Efficacy of either a single or split treatment of PGF 2α after a 14 day melengestrol acetate treatment to synchronize estrus and induce luteolysis in Bos indicus × Bos taurus heifers

Two experiments evaluated a modified delivery of prostaglandin F 2α (PGF 2α) after a melengestrol acetate (MGA) treatment in Angus and Bos indicus × Bos taurus (BI) heifers. Experiment 1 was replicated three times with yearling BI heifers ( n = 695). Heifers received MGA (0.5 mg head −1 day −1) for 14 days. In Replications 1 and 2, heifers received either 25 mg of PGF 2α im 19 days after MGA (single) or 12.5 mg of PGF 2α im 19 and 20 days after MGA (split). In Replication 3, heifers received the same treatments, with PGF 2α initiated either 18 or 19 days after MGA. Estrus was detected for 72 h after PGF 2α, with AI commencing 8–12 h after a detected estrus. Heifers not observed in estrus by 72 h were timed-AI concomitant with GnRH (100 μg im). Heifers from Replication 2 ( n = 146) had blood samples collected at the initial PGF 2α and at timed-AI to determine corpus luteum (CL) regression by evaluating plasma progesterone concentrations. The interval from MGA withdrawal to PGF 2α did not have a significant effect on any variable in Replication 3 and there were no treatment by replication effects for any variables, therefore data were pooled. Modifying the PGF 2α treatment from a single treatment to two treatments on consecutive days increased ( P < 0.05) 72 h estrous response (43.2% versus 50.1%), timed-AI (23.9% versus 33.5%) and total-AI pregnancy rates (34.5% versus 42.5%), and CL regression (79.1% versus 92.5%), respectively. In Experiment 2, yearling Angus ( n = 66) and 2-year-old BI ( n = 68) heifers were synchronized as per Experiment 1 (with the initial PGF 2α 19 days after MGA). Neither breed nor PGF 2α treatment effected ( P > 0.05) 72 h estrous response, total-AI pregnancy rate, or CL regression rate. In conclusion, treating yearling BI heifers with split treatments of PGF 2α (given on two consecutive days) improved estrous response and pregnancy rates by increasing PGF 2α-induced luteolysis.

Estrus synchronization in beef heifers with progestin-based protocols: I. Differences in response based on pubertal status at the initiation of treatment

Two progestin-based protocols for estrus synchronization in replacement beef heifers were compared on the basis of estrous response, interval to and synchrony of estrus, and pregnancy rate. The objective was to determine, whether addition of GnRH to a melengestrol acetate (MGA)-prostaglandin F 2α (PGF 2α) estrus synchronization protocol would improve synchrony of estrus without compromising fertility in yearling beef heifers. Heifers at two locations (Location 1, n = 60 and Location 2, n = 64) were assigned randomly to one of two treatments by breed and pubertal status. Heifers were defined as, pubertal when concentrations of progesterone in serum were elevated (≥1 ng/mL) in either one of two samples obtained 10 and 1 day prior to treatment initiation. Prior to MGA administration, 18/60 (30%) and 36/64 (56%) of the heifers at Locations 1 and 2, respectively, were pubertal. Heifers in both treatments were fed MGA (0.5 mg/head/day in 1.8 kg/head/day supplement) for 14 days followed by 25 mg of PGF 2α i.m. (MGA-PGF 2α) 19 days after MGA withdrawal (Day 33 of treatment). One-half of the heifers at each location received 100 μg of GnRH i.m. 12 days after MGA withdrawal (Day 26 of treatment; MGA ® Select). The control group received only MGA-PGF 2α. Heifers were observed for signs of behavioral estrus continuously during daylight hours for 7 days beginning on the day PGF 2α was administered. Heifers were inseminated 12 h after observed estrus. There was a treatment by location by pubertal status interaction ( P < 0.05) for interval to estrus. Compared to the respective control treatment at each location, prepubertal heifers assigned to the MGA ® Select protocol at Location 1 had longer intervals to estrus, whereas at Location 2, prepubertal heifers assigned to the MGA-PGF 2α protocol had longer intervals to estrus. The higher number of pubertal heifers at Location 2 was associated with a reduced variance in the interval to estrus among MGA ® Select treated heifers. Total estrous response and synchronized conception rates were similar between treatments at both locations. These data suggest that addition of GnRH to the MGA-PGF 2α protocol may improve synchrony of estrus, however, the degree of synchrony may be influenced by pubertal status of heifers at the time treatments are imposed. Further studies are needed to define production systems in which the MGA ® Select protocol is warranted for use in beef heifers.

Extending Interval from Seventeen to Nineteen Days in the Melengestrol Acetate - Prostaglandin Estrous Synchronization Program for Heifers11Published as paper number 13006, Journal Series, Nebraska Agricultural Research Division, University of Nebraska, Lincoln, NE 68583-0908.

<h2>Abstract</h2> Two experiments were conducted to determine whether extending the interval between removal of melengestrol acetate (MGA) from feed and injection of prostaglandin F_2α (PGF) from 17 to 19 d would affect synchronization of estrus, conception, and pregnancy rates of beef heifers. In both experiments, heifers were fed MGA for 14 d, and PGF was given at either 17 or 19 d after cessation of MGA feeding. Heifers were observed for estrus and artificially inseminated for 5 d after PGF injection. In Exp. 1, 240 yearling heifers were randomly assigned to either a 17- or a 19-d treatment group according to estrous status and day of the estrous cycle. In Exp. 2, 1409 yearling heifers on a cooperating ranch were randomly assigned to the same two treatment groups without knowledge of estrous status. The PGF injection at 19 d (Exp. 1) caused a higher (P<0.05) percentage of heifers to exhibit estrus by 72h after the injection compared with heifers receiving the injection at 17 d. A greater percentage (P<0.01) of heifers in the 19-d group were in the late luteal phase of the estrous cycle at the time of PGF injection compared with the heifers in the 17-d group, and pregnancy rates were higher for the heifers in the late luteal phase. In Exp. 2, heifers injected with PGF at 19 d after MGA had a greater (P<0.05) percentage in estrus (10%) during the 5-d breeding period, and had higher (P<0.05) pregnancy rates in 5 d (7.6%) and 50 d of breeding (5.5%), compared with heifers injected with PGF 17 d after withdrawal of MGA. These results indicate that the PGF injection given at 19 d after removal of MGA from the diet increases synchronized estrous response and results in higher pregnancy rates in heifers compared with the 17-d injection treatment.

Effects of melengestrol acetate on onset of puberty, follicular growth, and patterns of luteinizing hormone secretion in beef heifers.

Our objective was to test the hypothesis that short-term (8 days) treatment of prepubertal heifers with melengestrol acetate (MGA) and subsequent steroid withdrawal would stimulate LH secretion and follicular growth. Angus heifers were divided randomly into two groups; MGA-treated (n = 8) or control (CON; n = 9). Puberty was determined by monitoring circulating concentrations of progesterone and ovarian morphology during a 14-day period following MGA withdrawal. LH secretory patterns were assessed upon initiation of MGA (Day 0), during MGA (Day 7), and 1 day after withdrawal of MGA (Day 9). All MGA-treated heifers, versus four CON heifers, exhibited corpora lutea and luteal phase concentrations of progesterone within 10 days after treatment (p = 0.01). Mean LH and LH pulse frequency increased (p = 0.005 and 0.0001, respectively) between Days 0 and 9 in MGA-treated heifers. In CON heifers, mean LH concentrations and pulse frequencies did not change. During the same period, diameter of the largest follicle increased in MGA-treated animals (p = 0.003) but did not change in the CON heifers. On the basis of these results, we suggest that MGA withdrawal enhances onset of puberty by stimulating pulsatile LH secretion that accelerates follicle growth to the preovulatory stage.

Estrus synchronization with an oral progestogen prior to superovulation of postpartum beef cows

Ovarian follicular dynamics and steroid secretion patterns were monitored in postpartum beef cows that were synchronized for estrus with melengestrol acetate (MGA) or prostaglandin F 2α (PGF) prior to superovulation. Twenty-four muhiparous Angus cows were stratified by number of days postpartum to an MGA or PGF treatment prior to superovulation. Cows in the MGA group were fed 0.5 mg MGA/d for 14 d in a grain carrier. Superstitnulatory treatments began 14 d after withdrawal of MGA from feed or 11 d after administering a single injection of 500 μg cloprostenol (PGF). Supersthnulatory treatments (FSH) were administered twice daily in decreasing doses (7.5, 5, 5, 2.5 mg) over 4 d. Sixty and 72 h after initiating the superstimulatory treatments, all cows were treated with 750 μg and 500 μg PGF, respectively Cows were inseminated at 0, 12, and 24 h from the onset of standing estrus with semen from 2 proven sires. Cows within treatment were inseminated with 1, 2 and 1 (single) or 2, 4 and 2 units (double) of semen at the designated insemination times. Blood sampling and transrectal ultrasonography of ovaries were performed daily beginning 2 d prior to the initiation of FSH treatment and were continued through embryo recovery. Ovaries were examined daily to determine the number and size of follicles. Plasma samples were analyzed for progesterone and estradiol. Follicles were counted and categorized based on a 5 to 9 mm range or ≥ 10 mm. At the end of superovulatory treatment there were more (P ≤ 0.01) follicles ≥ 10 mm among cows that were estrus synchronized with MGA (75 ± 1.2) than with PGF (3.9 ± 1.2) These differences were reflected in higher (P ≤ 0.05) subsequent concentrations of plasma progesterone, suggestive of differences in ovulation rate between treatments (MGA = 8.5 ng/ml; PGF = 5.6 ng/ml). There were no significant differences between treatments in concentrations of estradiol, total number of ova (MGA = 11; PGF = 9), fertilized embryos (MGA = 9; PGF = 7), or transferable embryos recovered (MGA = 8; PGF = 6) Furthermore, double insemination offered no significant improvement in the number of recovered fertilized ova. These data identify differences in follicular and endocrine response when cows were estrus synchronized with MGA versus PGF prior to superovulation.

Persistence of the dominant follicle during melengestrol acetate administration and its regression by exogenous estrogen treatment in beef cattle.

Experiments were conducted to test the hypothesis that estrogen treatment will regress a persistent dominant follicle developed during melengestrol acetate (MGA) treatment in the absence of a functional corpus luteum (CL) with normal fertility following development and ovulation of a newly recruited follicle. In Exp. 1, nonlactating beef cows (n = 31) were administered .5 mg.cow-1.d-1 of MGA (d 0) for 14 d with 25 mg of prostaglandin F2 alpha (PGF2 alpha) administered on d 6 and 8 to regress the CL. On d 11 of treatment, approximately half the MGA-treated cows received 5 mg of estradiol valerate (EV) i.m. (MGAEV, n = 14) and the remainder were maintained on MGA (n = 17). Ovaries were checked with ultrasound on d 8, 10, 12, and 14 of MGA treatment and every day until ovulation. A persistent dominant follicle developed in 90% of the MGA-treated cows by d 10 of treatment. Most of the MGA-treated cows ovulated the persistent dominant follicle (n = 13/17), whereas EV treatment regressed the persistent dominant follicle (n = 10/14) with the recruitment of a new follicle that ovulated (n = 8/10). Diameter of the ovulatory follicle was larger (P < .05) for the MGA (19.8 +/- .6 mm) than for the control (15.1 +/- .8 mm) and MGAEV (14.8 +/- .7 mm) cows. In Exp. 2, nonlactating, multiparous beef cows (n = 97) and yearling heifers (n = 38) were equally allotted to either a control, MGA alone, or MGA + estradiol-17 beta (MGAE) group with the same dose of MGA as administered in Exp. 1. The 1st d of MGA feeding was the 1st d of treatment. On d 10 of treatment half the MGA-treated animals were injected i.m. with 5 mg of estradiol-17 beta. In controls, behavioral estrus was detected and animals were artificially inseminated (AI) for 5 d (d 10 to 14 of experiment). All controls not exhibiting estrus by d 15 of experiment were injected with 25 mg of PGF2 alpha. The remaining controls and all MGA cows were observed for behavioral estrus and AI commenced for 7 d following withdrawal of MGA (d 15 to 21 of experiment). More (P < .05) controls (90.3%) than MGA (84.8%) or MGAE (63.6%) cows showed estrus within 7 d after MGA withdrawal. The percentage of animals conceiving to the synchronized estrus did not differ (P > .05) among treatments. The data support our hypothesis that a persistent dominant follicle developed and can be regressed with exogenous estrogen treatment followed by the recruitment and ovulation of a new follicle after MGA withdrawal and fertility of that estrus does not seem to be significantly compromised.

Change in morphology of corpora lutea, central luteal cavities and steroid secretion patterns of postpartum suckled beef cows after melengestrol acetate with or without prostaglandin F 2α

Change in morphology of the corpus luteum (CL) and patterns of progesterone and estradiol secretion after treatment with melengestrol acetate (MGA) were monitored in postpartum beef cows. Twenty Angus cows were randomly assigned to MGA or MGA + prostaglandin F 2α (PGF) treatments. All cows were fed 0.5 mg of MGA per cow per day for 14 d. The MGA-treated cows (n = 10) were allowed to return to estrus spontaneously at the second estrus after withdrawal of MGA from the feed. The MGA + PGF-treated cows (n = 10) received an injection containing 25 mg of PGF 2α 17 d after the last feeding of MGA. Cycle 1 was defined as the first luteal phase after MGA feeding and Cycle 2 represented the subsequent cycle or luteal phase after PGF. Blood sampling and transrectal ultrasonography of the ovaries was done daily through the completion of 2 estrous cycles upon removal of MGA from the feed. Blood samples were analyzed for plasma progesterone and estradiol concentrations. Area of CL and fluid-filled cavities within each CL were determined by ultrasonography. Concentrations of progesterone and area of CL were similar between cycles and treatments. Estradiol concentrations were higher (P < 0.05) in Cycle 2 than in Cycle 1. Fluid-filled cavities were larger (P < 0.001) in Cycle 1 than in Cycle 2 for both mid-luteal (Days 5 to 9) and late-luteal (Days 10 to 14) phases. Multiple CL (2 or more during 1 cycle) were observed in 5 cows. Progesterone concentrations and total area of luteal tissue did not change with respect to treatment or cycle, but CL morphology was altered in the first cycle after MGA treatment. Of the 19 cows that ovulated after withdrawal of MGA, 3 experienced a short luteal phase. These data characterize changes that occur among cows that are fed melengestrol acetate during the postpartum period and enhance observations from prior studies regarding MGA use.

Synchronization of estrus in suckled postpartum beef cows with melengestrol acetate and PGF 2α

A total of 402 two suckled postpartum beef cows at 2 locations (CSU and ECRC) were utilized in 3 trials to evaluate the effectiveness of a combination melengestrol acetate (MGA)-PGF 2α estrus synchronization system in spring-calving cows. The cows were allocatted by days post partum, body condition score (1 = emaciated;9 = obese), sire breed, and dam age at the beginning of treatment to 1 of 2 treatments within location and trial: MGA-PGF 2α (0.5 mg MGA/head/d for 14 d with 25 mg of PGF 2α injected 17 d after MGA withdrawal), and unsynchrpnized controls. All cows were observed for estrus at 12-h intervals for at least 5 d post injection. Cows observed in standing estrus were inseminated 12 to 18 h later. There was a location effect on response to treatment that was attributed to differences in body condition score between locations so data were analyzed within a location. Body condition score at the CSU location was 5.7 compared with 4.0 at the ECRC location. The CSU MGA-PGF 2α treated cows had higher (P < 0.05) 5-day estrus and 5-d pregnancy rates (78.6 and 61.0%, respectively) than the CSU controls (11.1 and 6.9%, respectively). Similar results for 5-d estrus and pregnancy rates but of decreased magnitude were also observed for the ECRC MGA-PGF 2α treated (31.6 and 21.4%, respectively) cows compared with that of the ECRC controls (11.9 and 8.5%, respectively). The CSU MGA-PGF 2α treated cows had higher (P < 0.05) 25- and 60-d pregnancy rates (82.5 and 94.8%) than the CSU controls (65.3 and 87.5%). The 25- and 60-d pregnancy rates were similar between the ECRC MGA-PGF 2α treated cows and ECRC control cows. The MGA-PGF 2α estrus synchronization system appears to contribute to pregnancy early in the breeding season in postpartum beef cows, although its effectiveness is limited by cow body condition.

Fertility and embryo survival after administration of melengestrol acetate in rabbits.

SummaryFertility of vaginal inseminations in rabbits treated with melengestrol acetate (MGA) was significantly improved when phenobarbital was administered at MGA withdrawal. Oxytocin administration after vaginal insemination in MGA-treated rabbits resulted in partial restoration of normal fertility. Ova transferred from superovulated donors to synchronous MGA-treated recipients failed to survive, but ova transferred to MGA-treated recipients in which pseudopregnancy was one day advanced in relation to the donors resulted in normal pregnancy.

Luteinizing Hormone, Prolactin, and Interval to Ovulation After Melengestrol Acetate in Cattle

Nine Holstein heifers were fed 1 milligram melengestrol acetate daily for 18 days. After melengestrol acetate withdrawal, duration and magnitude of ovulatory surge of luteinizing hormone resembled that reported for control estrus, and serum prolactin was elevated the day before or on the day of luteinizing hormone surge. In another experiment 25 Holsteins lactating 60 to 90 days were fed 1 milligram melengestrol acetate daily for 14 days and injected with 2,500 international units of human chorionic gonadotropin on day 17. Based upon twice-a-day rectal palpations, 22 cows ovulated at an average of 36±3 hours after injection.

Characteristics of Cervical Mucus from Progestagen-Treated Cattle

SUMMARY Five characteristics of cervical mucus were investigated to determine if abnormalities might be responsible for the lowered fertility observed in cattle following progestagen administration. Mucus from animals fed 1 mg of melengestrol acetate (MGA) daily for 3 and 14 days did not differ significantly from that of controls in quantity, crystallization patterns or alkaline phosphatase content. However, on the last day of MGA feeding (day 14) treated animals had thicker mucus (P Based on quantity, consistency and alkaline phosphatase content of mucus from treated animals on day 4 after MGA withdrawal and from control animals on the day of oestrus, there appears to be no reason to indict cervical mucus as the cause of lowered fertility in MGA-treated animals. However, the altered crystallization patterns and sperm migration might indicate changes of significance in relation to feritlity.

Managing Reproduction in Dairy Cattle: I. Progestogens for Control of Estrus in Dairy Cows

Two experiments studied the effectiveness of medroxyprogesterone acetate (MAP) and melengestrol acetate (MGA) for control of estrus in normal and anestrous dairy cows. A third experiment investigated the use of MGA for controlling estrus during 17 successive 3-week breeding periods. In Experiment 1, 73 anestrous cows received MAP (180 mg/day) or a placebo for 10 days. Conception rate for first service, total conception rate, and services per conception were 59, 88,’ 1.7 and 54, 74, 1.8 for MAP and control groups. In Experiment 2, 232 cows in four herds were classified normal or anestrous according to estrous detection during the first 7 weeks postpartum. Beginning on Day 48 postpartum, some cows received MGA (1 mg/day) for 10 days while others served as controls. The mean interval from calving to first service, average days open for cows conceiving, and services per conception were 76, 107, 2.4; 76, 123, 2.5; 74, 98, 1.9 and 75, 107, 2.0 for MGA-normal, MGA-anestrous, control-normal and control-anestrous groups. Conception rates for first service were between 50 and 53% for normal cows and between 35 and 39% for anestrous cows irrespective of hormone treatment.In Experiment 3, 268 cows which calved in 17 successive 3-week periods in three herds were assigned to MGA or control groups. Some MGA cows received a single 14-day treatment (MGA-lX) while others received a double sequence (MGA-2X). Eighty-six per cent of the MGA cows and 60% of the controls were in estrus within 30 days following MGA withdrawal. Ninety-two per cent of the MGA cows were synchronized in two periods of 7 and 5 days. Cows which received the double sequence of MGA were synchronized to a greater degree than MGA-1X animals. The interval from calving to first service, average days open for cows conceiving, and services per conception were 80, 102, 1.8; 72, 102, 2.1 and 72, 97 and 1.9 for control, MGA-1 X and MGA-2X groups. The MGA reduced the variation among herds in interval from calving to first service and average days open. Progestogen administration did not affect milk yield. The system for controlling estrus in groups of cows could be beneficial as part of a total system to manage for a high reproductive rate in large herds.

Effect of melengestrol acetate on the growth and reproductive physiology of fattening heifers.

Crossbred beef-breed heifers, when supplied with 0.3 mg. melengestrol acetate (MGA) per head per day in the feed for 140 days as the agent for control of estrus, gained significantly (P<.01) faster (21%) and made 11% more efficient use of their feed than heifers receiving the same feed without MGA. Seven instances of estrus occurred in the 32 MGA-treated animals. One estrus occurred during the final two-day MGA withdrawal period. The control heifers continued to cycle during the experiment. Carcass quality, as reflected in the marbling and grade scores, was not influenced by MGA. External and internal carcass fat did not appear to differ between the treated and untreated animals. Dressing percent was not affected. MGA-treated heifers had heavier ovaries, but the total number of Graafian follicles was not significantly increased over controls. Follicles 12 mm. and larger were more numerous (P<.01) in the MGA groups. Sixteen of the 32 treated heifers had from two to five follicles each which were 12 mm. or larger.