Melengestrol Acetate Research Articles

The use of a progesterone-releasing device (CIDR-B) or melengestrol acetate with GnRH, LH, or estradiol benzoate for fixed-time AI in beef heifers.

The objective of this experiment was to compare two progestins and three treatments for synchronizing follicular wave emergence and ovulation in protocols for fixed-time AI in beef heifers. On d 0 (beginning of the experiment), Angus and Angus-Simmental cross beef heifers at random stages of the estrous cycle either received a CIDR-B device (n = 257) or were started on 0.5 mg x anima(-1) x d(-1) melengestrol acetate (MGA; n = 246) and were randomly assigned to receive i.m. injections of 100 microg GnRH, 12.5 mg porcine LH (pLH), or 2 mg estradiol benzoate (EB) and 50 mg progesterone (P4). The last feeding of MGA was given on d 6 and on d 7, CIDR-B devices were removed and all heifers received 500 microg cloprostenol (PG). Consistent with their treatment groups on d 0, heifers were given either 100 microg GnRH or 12.5 mg pLH 48 h after PG (and were concurrently inseminated) or 1 mg EB 24 h after PG and were inseminated 28 h later (52 h after PGF). Estrus rate (combined for both progestins) in heifers receiving EB (92.0%) was greater (P < 0.05) than that in heifers receiving GnRH and pLH (combined) and a CIDR-B device (62.9%) or MGA (34.3%). Although the mean interval from PG treatment to estrus did not differ among groups (overall, 47.8 h; P = 0.85), it was less variable (P < 0.01) in MGA-fed heifers (SD = 2.5 h) than in CIDR-B-treated heifers (SD = 8.1 h). Pregnancy rates (determined by ultrasonography approximately 30 d after AI) did not differ (P = 0.30) among the six treatment groups (average, 58.0%; range, 52.5 to 65.0%). Although fixed-time AI was done, pregnancy rates were greater in heifers detected in estrus than in those not detected in estrus (62.6 vs 51.9%; P < 0.05). In conclusion, GnRH, pLH, or EB treatment in combination with a CIDR-B device or MGA effectively synchronized ovulation-for fixed-time AI, resulting in acceptable pregnancy rates in beef heifers.

Effect of zeranol or melengestrol acetate (MGA) on testicular and antler development and aggression in farmed fallow bucks.

Fifteen yearling fallow bucks were randomly assigned by BW to one of three treatment groups: control (C; n = 5), melengestrol acetate (MGA; n = 5), and zeranol (Z; n = 5), to evaluate effects on testicular development, aggressive behavior, antler growth, sexual activity, ADG, and BW. Zeranol-treated bucks received zeranol ear implants (36 mg) at 90-d intervals, and MGA-treated bucks received MGA in the ration (100 microg x animal(-1) x d(-1)). Bucks grazed ryegrass/Coastal bermudagrass pasture and were supplemented with 3:1 corn/soybean meal at 0.45 kg x animal(-1) x d(-1). Body weights, body condition scores (BCS), blood samples, and testis measurements were obtained at d 0 and at 14-d intervals for 229 d. As bucks reached hard antler (7/15 to 8/25), antlers were harvested and weighed, and ejaculates were collected at 14-d intervals. Aggression was evaluated using 10-min video sessions scoring body blows, avoidance, head pushes, and head bunts. Scrotal circumference (SC) and paired testis volume were affected by a day x treatment interaction (P < 0.01); testes of zeranol-treated bucks were smaller than those of control or MGA-treated bucks. First sperm in the ejaculate tended to be delayed (P < 0.10) in zeranol-treated bucks compared with control and MGA-treated bucks. Melengestrol acetate-treated bucks had a maximum sperm concentration in the ejaculate that was three times (P < 0.05) that of control bucks and nine times (P < 0.05) that of zeranol-treated bucks. Antler weight was the least (P < 0.01) for bucks receiving zeranol and greatest (P < 0.10) for MGA-treated bucks; intermediate values were recorded for the control bucks. Aggressive behavior was delayed (P < 0.05) for zeranol-treated bucks until treatment effects were overcome. Melengestrol acetate-treated bucks had decreased (P < 0.01) aggressive behavior compared with control bucks. Melengestrol acetate-treated bucks had increased (P < 0.05) serum testosterone concentrations compared with control and zeranol-treated bucks. Human chorionic gonadotropin-stimulated peak serum testosterone concentrations for zeranol-treated bucks were delayed (P < 0.01) compared with control and MGA-treated bucks. Although zeranol-treated bucks overcame treatment effects, they were never able to reach testicular measurements or sperm concentrations equal to those of the control or MGA-treated bucks. Zeranol and MGA treatments may have both positive and negative effects that can be utilized when producing slaughter bucks.

Genital tract smooth muscle tumors are common in zoo felids but are not associated with melengestrol acetate contraceptive treatment.

In a survey of gynecologic lesions in female zoo felids conducted to determine if the widely used progestin contraceptive melengestrol acetate (MGA) had adverse effects, numerous leiomyomas and leiomyosarcomas were detected. This current study aimed to characterize the morphologic features of these tumors, determine their prevalence, and assess if MGA was a risk factor for their genesis. Genital tracts from 219 zoo felids representing 23 species were evaluated, and leiomyomas were detected in 24% of the felids. Leiomyomas were often multiple and occurred in the myometrium, ovary, or adjacent broad ligament. The risk of developing leiomyomas increased with age, but MGA treatment or parity had no effect. Five other felids had leiomyosarcomas. Leiomyosarcomas were distinguished from poorly demarcated leiomyomas by the presence of local invasion, metastasis, and cellular atypia, but necrosis and mitotic rate were not distinguishing criteria. Four of five felids with leiomyosarcomas had been treated with MGA. These results indicate that leiomyomas are common spontaneous lesions in the genital tracts of zoo felids and their genesis is not linked to MGA exposure. Whether progression to malignancy is promoted by MGA warrants further investigation.

Melengestrol acetate blocks the preovulatory surge of luteinizing hormone, the expression of behavioral estrus, and ovulation in beef heifers.

We tested the hypothesis that melengestrol acetate (MGA), an orally active progestin, blocks estrus and the preovulatory surge of luteinizing hormone (LH) in beef heifers. Cycling yearling Angus heifers were divided randomly into two groups: MGA-treated (n = 6) and control (n = 5). All heifers received injections of prostaglandin F2alpha (PGF) on d -25, -11, and 0 to synchronize estrus. Following the last PGF injection on d 0, heifers were fed either 0.5 mg MGA in a carrier or the MGA carrier each day for 8 d. At 4-h intervals on d 1 through 6, all heifers were observed for expression of estrous behavior, and blood samples were collected and assayed for LH. Daily blood samples were collected at 0800 on d 1 through 10 and assayed for circulating progesterone concentrations. All control heifers exhibited estrus and a preovulatory surge of LH. In each case, this was followed by increases in circulating concentrations of progesterone indicative of ovulation and normal luteal function. In contrast, none of the MGA-treated heifers exhibited estrus, LH surges, or evidence of ovulation. The results of this experiment show that MGA prevents ovulation in cattle by inhibiting the preovulatory surge of LH.

Determination of acetyl gestagenic steroids in kidney fat by automated supercritical fluid extraction and liquid chromatography ion-trap mass spectrometry.

Acetyl gestagenic steroids are isolated from animal tissues such as bovine kidney fat by automated supercritical fluid extraction (SFE). After the addition of internal standards and sample pretreatment, the analytes are extracted from the matrix by supercritical CO2 and trapped directly in-line on alumina placed in the extraction vessel. The samples are analysed by liquid chromatography combined with ion-trap mass selective detection (LC-MSn). For quantification, deuterated internal standards are added and single ions of the analytes and internal standards are monitored. For confirmation of the identity of the analytes, two transition ions (one MS2 and one MS3) were monitored and the ratios between the ions were calculated and compared with those of standards. The detection capability for the multi-analyte LC-MSn analysis of megestrol acetate (MA), medroxyprogesterone acetate (MPA), chlormadinone acetate (CMA) and melengestrol acetate (MGA) is 0.5 microg kg(-1). The mean within-laboratory reproducibility ranges from 16-19% (%RSD) at a concentration level of 0.5 microg kg(-1) (n = 9). Running the SFE procedure overnight allows the analysis of 24 samples of fat per day.

Effects of melengestrol acetate on reproductive behavior and concentrations of LH and testosterone in bulls.

We investigated the use of an orally active progestin (melengestrol acetate; MGA) to suppress reproductive activity in yearling beef bulls. Twenty-four crossbred bull calves were given a daily dose of 0, 0.5, 1.0, or 2.0 mg MGA for 99 d. Pulsatile patterns of LH and concentrations of testosterone and MGA were characterized on d 8, 36, 63, and 92 of the experiment. Numbers of aborted mounts, mounts with intromission, total mounts, and flehmen responses were assessed on d 15, 43, 71, and 99. Plasma concentrations of MGA were proportional to dose of MGA. Melengestrol acetate did not consistently affect mounting behavior in a dose-related manner, but, on d 99, number of total mounts for MGA-treated bulls was lower (P = 0.07) than that for control bulls. On d 15, MGA suppressed (P = 0.07) numbers of flehmen responses in a dose-dependent manner, but this effect was not sustained throughout the experiment. On d 8, concentrations of testosterone in control bulls were higher (P = 0.02) than in MGA-treated bulls, but this effect was not observed at other time periods. Overall, MGA caused slight decreases in mean concentrations of LH (P = 0.09) and LH pulse frequency (P = 0.06). Scrotal circumference was not affected by MGA. None of the behavioral traits was correlated with mean concentrations of LH or LH pulse frequency. Mounting activity was not correlated with testosterone concentrations, but number of flehmen responses was positively correlated with testosterone concentrations (P = 0.01). These results fail to support the hypothesis that progestins impair male sexual behavior or fertility in males.

Effects of synthetic progestagens on the mRNA expression of androgen receptor, progesterone receptor, oestrogen receptor alpha and beta, insulin-like growth factor-1 (IGF-1) and IGF-1 receptor in heifer tissues.

Synthetic progestagens like melengestrol acetate (MGA) are widely used for oestrus synchronization and for growth promotion in cattle production. The metabolic effects exceed its primary potency as a progestagen. It is speculated that MGA stimulates follicle development and thereby endogenous oestrogen production, but inhibits ovulation. To investigate the dose-dependent effects on mRNA expression levels, six heifers were fed for 8 weeks with different levels of MGA (0.5, 1.5, 5 mg) daily and two heifers served as controls. The expression of steroid receptor mRNA [androgen receptor (AR), progesterone receptor (PR), oestrogen receptor (ER) ERalpha and ERbeta], insulin-like growth factor-1 (IGF-1) and its receptor were quantified in liver, neck (m. splenius) and shoulder muscularity (m. deltoideus). Plasma concentrations of IGF-1 were quantified by radioimmunoassay. In treated animals the MGA plasma levels were elevated over the complete treatment period, corresponding to the MGA treatment concentrations. IGF-1 concentrations of control animals were at constant levels. Plasma levels for oestradiol (E2) and IGF-1 were increased in the low MGA treatment group. Overdosed MGA decreased progesterone (P4) and E2 levels. To quantify the IGF-1 and all receptor mRNA transcripts, sensitive and reliable real-time reverse transcription-polymerase chain reaction (RT-PCR) quantification methods were developed and validated in the LightCycler. A dose-dependent relationship between increasing MGA concentration and mRNA expression was observed in liver for AR and IGF-1 receptor, and in neck muscularity for IGF-1. ERalpha in liver and neck muscle showed a trend of increasing expression.

The use of progestins in regimens for fixed-time artificial insemination in beef cattle

Four experiments were conducted to investigate modifications to gonadotropin releasing hormone (GnRH)-based fixed-time AI protocols in beef cattle. In Experiment 1, the effect of reducing the interval from GnRH treatment to prostaglandin (PGF) was examined. Lactating beef cows ( n=111) were given 100 mg gonadorelin (GnRH) on Day 0 (start of treatment) and either 500 μg cloprostenol (PGF) on Day 6 with AI and 100 μg GnRH 60 h later, or PGF on Day 7 with AI and GnRH 48 h later (6- or 7-day Co-Synch regimens). Pregnancy rates were 32/61 (53.3%) versus 26/50 (52.0%), respectively ( P=0.96). In Experiment 2, cattle ( n=196) were synchronized with a 7-day Co-Synch regimen and received either no further treatment or a CIDR-B device (Days 0–7). Pregnancy rates were 32/71 (45.1%) versus 33/77 (42.9%) in cows ( P<0.8), and 9/23 (39.1%) versus 17/25 (68.0%) in heifers ( P<0.05). In Experiment 3, 49 beef heifers were randomly assigned to receive 12.5 mg pLH on Day 0, PGF on Day 7 and 12.5 mg of pLH on Day 9 with AI 12 h later (pLH Ovsynch), or similar treatment plus a CIDR-B device from Days 0 to 7 (pLH Ovsynch+CIDR-B), or 1 mg estradiol benzoate (EB) and 100 mg progesterone on Day 0, a CIDR-B device from Days 0 to 7 (EB/P4+CIDR-B), PGF on Day 7 (at the time of CIDR-B removal) and 1 mg i.m. EB on Day 8 with AI on Day 9 (52 h after PGF). Pregnancy rate in the EB/P4+CIDR-B group (75.0%) was higher ( P<0.04) than in the pLH Ovsynch group (37.5%); the pLH Ovsynch+CIDR-B group was intermediate (64.7%). In Experiment 4, 266 non-lactating cows were allocated to a 7-day Co-Synch protocol (Co-Synch), a 7-day Co-Synch plus 0.6 mg per head per day melengestrol acetate (MGA) from Days 0 to 6 inclusive (Co-Synch+MGA) or MGA (Days 0–6) plus 2 mg EB and 50 mg progesterone on Day 0, 500 μg PGF on Day 7, 1 mg EB on Day 8 and fixed-time AI 28 h later (EB/P4+MGA). Pregnancy rates ( P<0.25) were 44.8% (39/87; Co-Synch), 47.8% (43/90; Co-Synch+MGA), and 60.7% (54/89; EB/P4+MGA). In conclusion, a 6- or 7-day interval from GnRH to PGF in a Co-Synch regimen resulted in similar pregnancy rates in cows. The addition of a progestin to a Co-Synch or Ovsynch regimen significantly improved pregnancy rates in heifers but not in cows. Progestin-based regimens that included EB consistently resulted in high pregnancy rates to fixed-time AI.

Effects of melengestrol acetate (MGA) on performance and carcass quality of feedlot heifers (2002)

Sixty yearling heifers (827 lb initial body weight) were fed finishing diets an average of 95 days. To eliminate social interaction and riding, they were fed in individual pens. Diets were formulated using steam-flaked corn and alfalfa hay. Thirty of the heifers were given 0.5 mg/head daily of MGA. Feed intakes, daily gains, and feed efficiencies were not significantly affected by MGA. However, heifers fed MGA had a greater percentage of carcasses grading USDA Prime and Choice. There also was greater incidence of USDA yield grade 3 and 4 carcasses with MGA supplementation. MGA generally increased fat deposition, but had no significant effect on feedlot performance of individually fed heifers.

Synchronization of estrus in virgin beef heifers using melengestrol acetate and PGF 2α: an efficacy comparison of cloprostenol and dinoprost tromethamine

This study compared the efficacy of two sources of PGF 2α on the reproductive performance of virgin beef heifers, after synchronization of estrus using melengestrol acetate (MGA) and PGF 2α. Angus-based heifers ( n=1002) in five herds were fed 0.5 mg per head per day of MGA for 14 days. Nineteen days after the last day of MGA feeding, heifers were randomly assigned to receive (i.m.) either 0.5 mg cloprostenol ( n=504; Estrumate ®, E) or 25 mg dinoprost tromethamine ( n=498; Lutalyse ®, L) as a source of exogenous PGF 2α. Heifers were observed twice daily for 5 days for signs of estrus and artificially inseminated 8–12 h later, except in herd A, wherein animals not detected in estrus by 80 h after PGF 2α were mass-mated and no longer monitored for signs of estrus. Estrumate and Lutalyse were equally ( P>0.1) effective among all response variables evaluated, including estrus response (E, 89% and L, 86%), conception rate (E, 67% and L, 67%), and synchronized pregnancy rate (E, 61% and L, 57%). Synchrony of estrus was not affected ( P>0.1) by PGF 2α source, and peak estrus response occurred 60 h post-PGF for both treatments. Conception rate to timed insemination was not different ( P>0.1) among Estrumate- and Lutalyse-treated heifers within herd A (14%, 4/28 and 7%, 2/29, respectively). Herd had a significant ( P<0.05) effect on all indicators of reproductive performance. Conception rates within herds A and D were influenced by technician ( P<0.05), however, this effect was balanced across treatments and no treatment by technician interaction was detected. In conclusion, when administered 19 days after a 14-day MGA feeding period, cloprostenol and dinoprost tromethamine are equally efficacious for synchronous induction of a fertile estrus in virgin beef heifers.

Quantitative assessment of foetal exposure to trenbolone acetate, zeranol and melengestrol acetate, following maternal dosing in rabbits

1. Residues of commonly used growth-promoting agents found in animal meat can be hormonally active and they have been implicated as possible endocrine disruptors in man. Although these compounds could be potentially detrimental to the developing foetus, it is not clear whether and to what extent they pass through placental barrier. 2. This issue was addressed using the rabbit as an animal model. Pregnant rabbits were treated with trenbolone acetate, zeranol or melengestrol acetate beginning at gestation day 14. Levels of active substances in plasma were screened by means of specific ELISA systems. The residues of parent compounds and their metabolites were quantified in maternal and foetal tissues on gestation day 27 using validated, sensitive HPLC/ELISA methods. 3. All three compounds crossed the placental barrier and were detectable in foetal tissues. The extent of tissue concentration varied depending on the compound and tissue analysed. Gender differences were observed in some instances.

Restoration of reproductive potential following expiration or removal of melengestrol acetate contraceptive implants in golden lion tamarins (Leontopithecus rosalia).

Although reversible contraception is important to successful management of small populations, there are concerns about the reversibility of melengestrol acetate (MGA), the most commonly used implant in captive animals. Female golden lion tamarins (Leontopithecus rosalia) placed in potential breeding situations after surgical MGA implant removal showed a 75% return to reproduction within 2 yr, unlike golden-headed tamarins (Leontopithecus chrysomelas), which have had a 29% return to reproduction following implant removal. This rate was indistinguishable from the breeding probability for newly formed pairs involving nonimplanted females. Litter size, stillbirth rate, and infant survival rate were not significantly different between nonimplanted and implant-removed female golden lion tamarins. However, females with implants left in (and assumed to have expired) showed higher stillbirth and infant mortality rates than did females with implants removed. For seven female golden lion tamarins for which reproductive histories before and after MGA implantation were available, litter size was unaffected by MGA implantation and subsequent removal. Infant survival rate for these females appeared to be lower after removal but was indistinguishable from rates in the nonimplanted females. Prior reproductive experience, length of time with an implant, and age of the females did not affect the probability of breeding for females after removal of the implants. Overall, breeding probability of nonimplanted females declined with age. Although the results of this study confirm the reversibility of MGA implants in golden lion tamarins, there appear to be some effects on viability of offspring, particularly offspring born to females with implants left in and presumed expired.

The fate of trenbolone acetate and melengestrol acetate after application as growth promoters in cattle: environmental studies.

The steroids trenbolone acetate (TbA) and melengestrol acetate (MGA) are licensed as growth promoters for farm animals in several meat-exporting countries. Although many studies have explored their safety for both animals and consumers, little is known about their fate after excretion by the animal. Our study aimed to determine the residues and degradation of trenbolone and MGA in solid dung, liquid manure, and soil. In animal experiments lasting 8 weeks, cattle were treated with TbA and MGA. Solid dung and, in case of trenbolone, liquid manure were collected and spread on maize fields after 4.5 and 5.5 months of storage, respectively. Determination of the hormone residues in all samples included extraction, clean-up (solid-phase extraction), separation of metabolites and interfering substances by HPLC (RP-18), and quantification by sensitive enzyme immunoassay. Procedures were validated by mass spectrometry (MS) methods. During storage of liquid manure the level of trenbolone decreased from 1,700 to 1,100 pg/g (17alpha-isomer), corresponding to a half-life of 267 days. Before storage, the concentrations in the dung hill ranged from 5 to 75 ng/g TbOH and from 0.3 to 8 ng/g MGA. After storage, levels up to 10 ng/g trenbolone, and 6 ng/g MGA were detected. In the soil samples trenbolone was traceable up to 8 weeks after fertilization, and MGA was detected even until the end of the cultivation period. The results show that these substances should be investigated further concerning their potential endocrine-disrupting activity in agricultural ecosystems.

The Fate of Trenbolone Acetate and Melengestrol Acetate after Application as Growth Promoters in Cattle: Environmental Studies

The Fate of Trenbolone Acetate and Melengestrol Acetate after Application as Growth Promoters in Cattle: Environmental Studies

The use of GnRH or estradiol to facilitate fixed-time insemination in an MGA-based synchronization regimen in beef cattle

Two experiments were conducted to compare pregnancy rates when GnRH or estradiol were given to synchronize ovarian follicular wave emergence and ovulation in an MGA-based estrus synchronization program. Crossbred beef cattle were fed melengestrol acetate (MGA, 0.5 mg per day) for 7 days (designated days 0–6, without regard to stage of the estrous cycle) and given cloprostenol (PGF; 500 μg intramuscular (im)) on day 7. In Experiment 1, lactating beef cows ( n=140) and pubertal heifers ( n=40) were randomly allocated to three groups to receive 100 μg gonadorelin (GnRH), 5 mg estradiol-17β and 100 mg progesterone (E+P) in canola oil or no treatment (control) on day 0. All cattle were observed for estrus every 12 h from 36 to 96 h after PGF. Cattle in the GnRH group that were detected in estrus 36 or 48 h after PGF were inseminated 12 h later; the remainder were given 100 μg GnRH im 72 h after PGF and concurrently inseminated. Cattle in the E+P group were randomly assigned to receive either 0.5 or 1.0 mg estradiol benzoate (EB) in 2 ml canola oil im 24 h after PGF and were inseminated 30 h later. Cattle in the control group were inseminated 12 h after the first detection of estrus; if not in estrus by 72 h after PGF, they were given 100 μg GnRH im and concurrently inseminated. In the absence of significant differences, all data for heifers and for cows were combined and the 0.5 and 1.0 mg EB groups were combined into a single estradiol group. Estrus rates were 57.6, 57.4 and 60.0% for the GnRH, E+P and control groups, respectively ( P=0.95). The mean (±S.D.) interval from PGF treatment to estrus was shorter ( P<0.001) and less variable ( P<0.001) in the E+P group (49.0±6.1 h) than in either the GnRH (64.2±15.9 h) or control (66.3±13.3 h) groups. Overall pregnancy rates were higher ( P<0.005) in the GnRH (57.6%) and E+P (55.7%) groups than in the control group (30.0%) as were pregnancy rates to fixed-time AI (47.5, 55.7 and 28.3%, respectively). In Experiment 2, 122 crossbred beef heifers were given either 100 μg GnRH or 2 mg EB and 50 mg progesterone in oil on day 0 and subsequently received either 100 μg GnRH 36 h after PGF and inseminated 14 h later or 1 mg EB im 24 h after PGF and inseminated 28 h later in a 2×2 factorial design. Pregnancy rates were not significantly different among groups (41.9, 32.2, 33.3 and 36.7% in GnRH/GnRH, GnRH/EB, EB/GnRH and EB/EB groups, respectively). In conclusion, GnRH or estradiol given to synchronize ovarian follicular wave emergence and ovulation in an MGA-based synchronization regimen resulted in acceptable pregnancy rates to fixed-time insemination.

Dose‐dependent effects of melengestrol acetate (MGA) on plasma levels of estradiol, progesterone and luteinizing hormone in cycling heifers and influences on oestrogen residues in edible tissues

Melengestrol acetat (MGA) is widely used as a growth promoting feed additive in cattle breeding in the USA and several other non‐European countries. To explore the physiological effects of MGA four heifers were fed during 8 weeks with 0.5 mg MGA daily as registered in the USA and two heifers each received 0, 1.5 or 5 mg/day, respectively. Plasma samples were collected twice a week and concentrations of MGA, progesterone (P4) and estradiol‐17β (E2‐17β) were quantified. The pulsatile secretion of luteinizing hormone (LH) was investigated in 6‐hour profiles before and during treatment. After slaughter the reproductive organs were examined and oestrogen residues in edible tissues were measured. Four days after the beginning of MGA feeding MGA concentrations in plasma reached levels of 30 and 100–400 pg/mL depending on the dose received. Three weeks after the beginning of MGA feeding P4 plasma concentrations had dropped to base levels below 0.3 ng/mL in all three treatment groups. Mean plasma E2‐17β levels increased in physiological range from 1 to 5 pg/mL during 0.5 mg MGA/day feeding with many acyclic peaks. Overdosed MGA decreased E2 levels and suppressed cyclic peaks. Number and size of ovarian follicles were not altered by any treatment. Mean LH levels and pulse frequencies increased significantly during labelled treatment (0.5 mg/day), while higher doses had reducing effects. The development of corpus luteum was suppressed. E2‐17β residues in fat increased about 300% following labelled MGA treatment.

Breeding ewes out-of-season using melengestrol acetate, one injection of progesterone, or a controlled internal drug releasing device

A series of studies was designed to identify methods of improving out-of-season breeding success in ewes. In Experiment 1, 190 mature ewes were assigned to receive in April, either: (A) a control ration of 0.3 kg corn twice daily for 8 d before ram introduction (control ewes n=49), (B) the control ration containing 0.125 mg of melengestrol acetate (MGA) in 0.3 kg corn (MGA8a ewes n=46), (C) the control ration for 7.5 d followed by 1 feeding of 0.5 mg MGA in 0.3 kg corn (MGA 1 ewes n=48), or (D) the control ration plus a 20 mg im injection of progesterone on D 8 (P ewes n=47). Ewes were exposed to rams for 21 d. A greater percentage of MGA8a and P ewes lambed than did control ewes (P < 0.04). The lambing rate was greatest among MGA8a (P < 0.02 vs. control), intermediate among P ewes (P < 0.19 vs. control) and least among MGA1 and control ewes (P > 0.79). In Experiment 2, 70 mature ewes were assigned to receive in June, either: (A) a control ration of 0.3 kg of corn twice daily for 8 d before to ram introduction (control ewes n=25, (B) the control ration containing 0.125 mg of MGA per 0.3 kg corn (MGA8b ewes n=21), or (C) the control ration and simultaneous treatment of ewes with a progesterone-containing controlled internal-drug releasing device (CIDR ewes n=24). Ewes were exposed to rams for 21 d. Both CIDR and MGA8b ewes exhibited estrus earlier than did control ewes (P < 0.01). The CIDR ewes exhibited estrus earlier than did the MGA8b ewes (P < 0.05). A greater percentage of ewes treated with CIDR or MGA8b lambed than did control ewes (P < 0.01), with more CIDR ewes lambing than MGA8b ewes (P < 0.01). The lambing rate was greater in CIDR ewes than in control ewes (P < 0.04). These data provide evidence that several options exist to improve pregnancy success in ewes bred out of season and that success varies with method of treatment.

Genotoxic potential of xenobiotic growth promoters and their metabolites

This paper reviews data reported in the literature as well as recent and unpublished studies from our laboratory on the metabolism and genotoxicity of the xenobiotic growth promoters 17β‐trenbolone, melengestrol acetate and zeranol. In our metabolic study, the oxidative in vitro metabolites generated by hepatic microsomes from rats, bovine and humans were analyzed by HPLC and GC/MS. 17β‐Trenbolone gave rise to at least 13 monohydroxylated products, whereas 12 mono‐ and dihydroxylated metabolites were obtained with melengestrol acetate and at least 5 with zeranol. The genotoxic potential of the parent compounds was studied using the following endpoints: induction of HPRT mutations in cultured V79 cells and of lacI mutations in E. coli; induction of micronuclei in V79 cells; and formation of DNA adducts in cultured primary rat hepatocytes. Negative results were obtained in most of these assay systems. Only the micronucleus induction was marginally positive with 17β‐trenbolone and zeranol at near‐cytotoxic concentrations. Commercial melengestrol acetate was found to contain an impurity causing apoptosis in V79 cells. The genotoxic potential of the numerous oxidative metabolites of the xenobiotic growth promoters remains to be studied.

Assessment of oestrogenic potency of chemicals used as growth promoter by in‐vitro methods

Three in‐vitro bioassays were used to compare the oestrogenic potency of chemicals used as growth promoter in beef cattle in certain non‐European Union countries (17β‐oestradiol, α‐zearalanol, testosterone, trenbolone, trenbolone acetate, melengestrol acetate) or found as food contaminant such as the mycotoxin zearalenone and some of their metabolites (17α‐oestradiol, oestrone, 17α‐epitestosterone, 19‐nortestosterone, androstendione, zearalanone, α‐zearalanol, β‐zearalanol, α‐zearalenol, β‐zearalenol). The strong oestrogens 17α‐ethinyl oestradiol and diethylstilboestrol were used as standards. The first bioassay was based on the activation of a reporter gene by oestrogens in recombinant yeast expressing human or rainbow trout oestrogen receptor. In the second bioassay, the vitellogenin gene induction of rainbow trout hepatocyte cultures was used as a biomarker for the exposure to oestrogens. The third bioassay was based on the alkaline phosphatase gene induction by oestrogens in the human endometrial Ishikawa cell line. The assessment of oestrogenic potency of these chemicals clearly demonstrates the strong oestrogenicity of the mycotoxin zearalenone and its metabolites and particularly α‐zearalenol which was as potent as ethinyl oestradiol and diethylstilboestrol in the human endometrial Ishikawa cell line.

A sensitive enzyme immunoassay (EIA) for the determination of melengestrol acetate (MGA) in adipose and muscle tissues

The development of a sensitive screening method of MGA residues in bovine perirenal fat and muscle based on a competitive microtitration plate enzyme immunoassay is described. The samples were extracted with petroleum ether and purified with octadecyl-silica-cartridges. The detection limit for fat was 0.4ng/g and for muscle tissue 0.05ng/g, much lower than required for reliable detection of positive samples. The mean recovery rates of fortified samples amount to 75%, the mean intraassay variations to 7% and the interassay variation to 13%. Determination limits were validated for fat at 2ng/g and for muscle at 0.1ng/g. The efficiency of the new screening method was successfully demonstrated by the direct comparison to GC-MS and LC-MS methods performed at natural positive samples originating from an animal experiment in which the labelled dose (0.5mg per animal and day) with and without a 48h withdrawal period or 3-fold or 10-fold the amount of MGA, respectively, was fed to Holstein Frisian heifers. In conclusion, this new screening method can be used for sensitive determination of MGA residues in adipose tissues even after low treatment doses or longer withdrawal periods.