Ovarian follicular and corpus luteum changes, progesterone concentrations, estrus and ovulation following estradiol benzoate/progesterone based treatment protocol in cross-bred cows

The objectives of the present study were to investigate the effects of the stage of the estrous cycle at the start of an estradiol benzoate (EB) and progesterone (P) based treatment protocol on new follicular wave emergence, subsequent estrus and ovulation. The experiment was conducted using a crossover design with each cow (five cross-bred cows) being assigned to one of three groups at 3-month intervals within a 1-year period. Estrous cycle stage in individual cows was initially synchronized with prostaglandin F(2)alpha. After detection of estrus, each cow was injected intramuscularly (i.m.) with 2 mg EB and 200 mg P (EB/P) on day 5, 12 or 17 of the estrous cycle (estrus=day 0), followed by 1 mg EB i.m. 12 days after the EB/P treatment. Ovarian ultrasonographic examinations showed that the emergence of a new follicular wave occurred after EB/P treatment in all groups and the mean interval from EB/P treatment to wave emergence did not differ among the groups (3.2-3.8 days). All cows in each group exhibited behavioral estrus and ovulated the newly formed dominant follicle. However, cows in the day-17 group exhibited estrus 1-3 days before the second EB injection. The concentrations of progesterone showed faster reduction, during the treatment period, in the day-12 and -17 groups compared to the day-5 group. These results indicate that the EB/P treatment induces an emergence of a new follicular wave, irrespective of the estrous cycle stage at the start of treatment, but the effect of EB/P protocol on estrous/ovulation synchronization is influenced by the stage of the estrous cycle.     

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 F2alpha (PGF2alpha) 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 (> or = 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 PGF2alpha i.m. (MGA-PGF2alpha) 19 days after MGA withdrawal (Day 33 of treatment). One-half of the heifers at each location received 100 microg of GnRH i.m. 12 days after MGA withdrawal (Day 26 of treatment; MGA Select). The control group received only MGA-PGF2alpha. Heifers were observed for signs of behavioral estrus continuously during daylight hours for 7 days beginning on the day PGF2alpha 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-PGF2alpha 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-PGF2alpha 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.     

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 microg 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 microg gonadorelin (GnRH), 5 mg estradiol-17beta 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 microg 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 microg 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 microg GnRH or 2 mg EB and 50 mg progesterone in oil on day 0 and subsequently received either 100 microg 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 x 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.     

Prostaglandin-induced regression of porcine corpora lutea maintained by estrogen.

Corpora lutea were marked with suture in 24 crossbred gilts on day 7 to 9 of the estrous cycle (first day of estrus equals 0). All gilts were injected with 5 mg of estradiol benzoate (EB) daily from day 10 to 15 to extend the lifespan of corpora tutea, then the gilts were randomly assigned to two groups. On day 20, the 12 gilts of Group 1 were injected with 10 mg PGF-2ALPHA, and the 12 gilts of Group 2 were injected with saline. Ovaries were recovered 10 to 13 days after PGF-2ALPHA or saline injection. Ten gilts in Group 1 displayed estrus 5 plus or minus 0.7 days after PGF-2ALPHA injection, but only two gilts in Group 2 displayed estrus during the experimental period. In gilts that displayed estrus, all marked CL had regressed. Marked CL were still present in all 12 gilts that failed to exhibit estrus during the experimental period. These results show that in the pig, PGF-2ALPHA caused regression of CL that were maintained beyond the normal luteal phase of the estrous cycel by EB treatment.     

The regulation of precopulatory behavior by ovarian hormones in the female Mongolian gerbil

The hormonal regulation of precopulatory behavior in the female Mongolian gerbil was studied using two groups (N = 6) of sexually experienced females. A novel testing procedure was used which involved females living continuously with test males for several days. The test males showed either full sexual behavior (copulating males, C) or only precopulatory behavior (noncopulating males, NC). Experiment 1 investigated changes during the estrous cycle and following ovariectomy in females. Experiment 2 studied the effects of hormonal treatment of these ovariectomized females with 6 micrograms estradiol benzoate (EB) followed by 0.4 mg progesterone (P) or by 0.04 ml arachis oil. When tested with NC males, females displayed a greater range of precopulatory behavior. The patterns could be classified into three groups according to the manner of response to ovariectomy and hormone treatment. Group I patterns (approach, leave, and olfactory investigation of the male's head) were affected by neither ovariectomy nor EB treatment relative to Day 3 levels (Day 3, day preceding estrus; Day 4, estrus), but they were increased to estrous levels by EB and P. Group II patterns (darting, foot-stomping, and the present and piloerection postures) appeared only during estrus, did not appear after ovariectomy, and reappeared only after sequential EB and P treatment. Group III patterns (investigation of the male's anogenital area, allogrooming, ventral gland marking, and sand-rolling) were reduced relative to both estrus and Day 3 levels by ovariectomy and increased above Day 3 levels by EB alone; EB and P treatment further increased Group III patterns to the level of estrus. It is suggested that female precopulatory behavior patterns differ in their responsiveness to ovarian hormones. Estrogen appears to affect those patterns associated with the earliest stages of estrus (Group III).     

Synchronization of estrus in beef heifers using either melengesterol acetate (MGA)/prostaglandin or MGA/Select Synch

The objective of this study was to evaluate synchronization, conception, and pregnancy rates of yearling beef heifers synchronized with either the Select Synch protocol preceded by 7 days of MGA feeding (MGA/Select Synch) or the traditional MGA/PGF protocol. Heifers in the MGA/Select Synch group (n = 402) were fed MGA (0.5 mg/day/head) for 7 days, received an injection of GnRH (100 microg) the day following the last MGA feeding and an injection of PGF (25 mg) 7 days after GnRH. Heifers in the MGA/PGF group (n = 394) received MGA (0.5 mg/day/head) for 14 days, followed by an injection of PGF (25 mg) 17 days later. Synchronization rates tended (P = 0.08) to be higher for the MGA/Select Synch (82%) compared to the MGA/PGF (77%)-treated heifers. Conception and pregnancy rates to AI were similar (P > 0.10), 57 and 46% for the MGA/Select Synch heifers and 61 and 47% for the MGA/PGF heifers, respectively. Mean estrous response (h) was earlier (P < 0.05) for the MGA/Select Synch versus MGA/PGF treatment, 56 versus 61 h post-PGF treatment, respectively. In summary, short-term (7 days) MGA feeding preceding the Select Synch protocol produced similar synchronization, conception, and pregnancy rates as the traditional MGA/PGF protocol.     

Prostaglandin-induced regression of porcine corpora lutea maintained by estrogen

Corpora lutea were marked with suture in 24 crossbred gilts on day 7 to 9 of the estrous cycle (first day of estrus = 0). All gilts were injected with 5 mg of estradiol benzoate (EB) daily from day 10 to 15 to extend the lifespan of corpora lutea, then the gilts were randomly assigned to two groups. On day 20, the 12 gilts of Group 1 were injected with 10 mg PGF_2α, and the 12 gilts of Group 2 were injected with saline. Ovaries were recovered 10 to 13 days after PGF_2α or saline injection. Ten gilts in Group 1 displayed estrus 5 ± 0.7 days after PGF_2α injection, but only two gilts in Group 2 displayed estrus during the experimental period. In gilts that displayed estrus, all marked CL had regressed. Marked CL were still present in all 12 gilts that failed to exhibit estrus during the experimental period. These results show that in the pig, PGF_2α caused regression of CL that were maintained beyond the normal luteal phase of the estrous cycle by EB treatment.     

Control of the estrous cycle in guinea-pig (Cavia porcellus)

The aim of this work was to look for a simple method to obtain synchronized ovulation in guinea pigs under farming conditions while respecting animal welfare. The luteolytic activity of three different prostaglandins F2alpha (PGF2α) analogs (D-cloprostenol, D,L-cloprostenol and luprostiol) and a daily treatment with oral progestagen (altrenogest) was tested successively at different stages of the estrous cycle on the same group of females during a period of 8 mo. The estrous cycle length was not modified by the administration of PGF2α analogs, whatever the stage of the estrous cycle when the treatment was initiated. Our results led us to reject the use of PGF2α analog to induce practical synchronization of the estrus in this species. In females (n = 29), given 15 days with altrenogest (0.1 mL po once a day), ovulation occurred 4.43 ± 0.13 days after the end of the treatment. Altrenogest treatment was followed by mating. No negative impacts of the treatment on the pregnancy rates, delivery rates and litter sizes were observed. This standard method of guinea-pig estrus synchronization is less stressful for the animals compared to techniques using progesterone tubing.     

Synchronization of estrus in suckled postpartum beef cows with melengestrol acetate, 48-hour calf removal and PGF2alpha

One hundred and sixty-five suckled postpartum beef cows were utilized to evaluate the effectiveness of 2 estrus synchronization systems for the initiation and synchronization of estrus. The treatment groups consisted of 1) melengestrol acetate (MGA)-PGF2alpha (cows were given 0.5 mg MGA/head/day for 14 d with 25 mg PGF2alpha injected 17 d after the last day of MGA administration); 2) MGA-48-h calf removal (CR)-PGF2alpha (cows were given 0.5 mg MGA/head/day for 14 d with 48-h calf removal starting on the second day after completion of the MGA regimen plus 25 mg PGF2alpha administered 17 d after the last day of MGA); and 3) unsynchronized controls. Cows were assigned to treatments by the numbers of days post partum, body condition, age, and breed of sire. The cows were observed for estrus at 12-h intervals for 5 d after PGF2alpha administration and were artificially inseminated 12 to 18 h after the observed estrus. Both the MGA-PGF2alpha and MGA-CR-PGF2alpha treatments (64.8 and 61.8%) had greater (P < 0.05) 5-d estrus rates than the control treatments (34.5%). The synchronized pregnancy rate was greater (P < 0.05) for the MGA-CR-PGF2alpha than the control treatment.(52.7 vs 30.9%, respectively). The MGA-CR-PGF2alpha cows had a higher 25-d pregnancy rate than either the MGA-PGF2alpha (P < 0.05) or control cows (P < 0.08). Of the anestrous cows at the beginning of treatment, more MGA-CR-PGF2alpha (P = 0.1) and MGA-PGF2alpha cows were cyclic posttreatment than control cows (58.7 and 55.1 vs 44.7%, respectively), suggesting that treatment initiated estrous cycles in only a small number of the anestrous cows. Both MGA-PGF2alpha and MGA-CR-PGF2alpha treatments appear to be effective methods of synchronizing estrus in suckled postpartum beef cows. However, MGA-CR-PGF2alpha was more effective in establishing pregnancy earlier in the breeding season than MGA-PGF2alpha.     

Efficacy of either a single or split treatment of PGF2alpha after a 14 day melengestrol acetate treatment to synchronize estrus and induce luteolysis in Bos indicus x Bos taurus heifers

Two experiments evaluated a modified delivery of prostaglandin F2alpha (PGF2alpha) after a melengestrol acetate (MGA) treatment in Angus and Bos indicus x 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 PGF2alpha im 19 days after MGA (single) or 12.5 mg of PGF2alpha im 19 and 20 days after MGA (split). In Replication 3, heifers received the same treatments, with PGF2alpha initiated either 18 or 19 days after MGA. Estrus was detected for 72 h after PGF2alpha, 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 microg im). Heifers from Replication 2 (n = 146) had blood samples collected at the initial PGF2alpha and at timed-AI to determine corpus luteum (CL) regression by evaluating plasma progesterone concentrations. The interval from MGA withdrawal to PGF2alpha 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 PGF2alpha 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 PGF2alpha 19 days after MGA). Neither breed nor PGF2alpha 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 PGF2alpha (given on two consecutive days) improved estrous response and pregnancy rates by increasing PGF2alpha-induced luteolysis.     

Synchronization of estrus in virgin beef heifers using melengestrol acetate and PGF2alpha: an efficacy comparison of cloprostenol and dinoprost tromethamine

This study compared the efficacy of two sources of PGF2alpha on the reproductive performance of virgin beef heifers, after synchronization of estrus using melengestrol acetate (MGA) and PGF2alpha. 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 PGF2alpha. 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 PGF2alpha 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 PGF2alpha 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.     

Comparison of controlled internal drug release device and melengesterol acetate as progestin sources in an estrous synchronization protocol for beef heifers

The objectives of this experiment were to compare estrous synchronization responses and AI pregnancy rates of beef heifers using protocols that included either CIDR or MGA as the progestin source. The hypotheses tested were that: (1) estrous synchronization responses after (a) progestin removal, and (b) PGF(2alpha); and, (2) AI pregnancy rates, do not differ between heifers synchronized with either progestin source. At the start of the experiment (Day 0) in both years, heifers were assigned randomly to receive, MGA supplement for 14 days (MGA-treated; n=79) or CIDR for 14 days (CIDR-treated; n=77). On Day 14 progestin was removed and heifers were observed for estrus up to and after PGF(2alpha) on Days 31 and 33 for CIDR-treated and MGA-treated heifers, respectively. Heifers that exhibited estrus within 60h after PGF(2alpha) were inseminated by AI 12h later; the remaining heifers were inseminated at 72h after PGF(2alpha) and given GnRH (100mug). More (P<0.05) CIDR-treated heifers exhibited estrus within 120h after progestin removal than MGA-treated heifers. Intervals to estrus after progestin removal were shorter (P<0.05) for CIDR-treated heifers than MGA-treated heifers. More (P<0.05) CIDR-treated heifers exhibited estrus and were inseminated within 60h after PGF(2alpha) than MGA-treated heifers. Pregnancy rates did not differ (P>0.10) between MGA-treated (66%) and CIDR-treated (62%) heifers. In conclusion, the use of CIDR as a progestin source in a 14-day progestin, PGF(2alpha), and timed AI and GnRH estrous synchronization protocol was as effective as the use of MGA to synchronize estrus and generate AI pregnancies in beef heifers.     

Comparison of two MGA-PGF2alpha systems for synchronization of estrus in beef heifers

Yearling beef heifers (n = 193) were used to evaluate reproductive performance attained with 2 MGA-PGF(2)alpha synchronization systems. These treatments were compared with an untreated control group. The 14-d MGA heifers were synchronized by feeding 0.5 mg MGA/h/d for 14 d. At 17 d after the last MGA feeding, these heifers were injected with PGF(2)alpha (25 mg, im). Heifers in the 7-d MGA treatment group were fed 0.5 mg MGA/h/d for 7 d and received a 25-mg, im injection of PGF(2)alpha on the last day of the MGA feeding period. Heifers in all 3 treatment groups were observed for estrus every 12 h for 7 d beginning 24 h after the PGF(2)alpha injection. Heifers observed in estrus during this 7-d period were artificially inseminated approximately 12 h after the onset of estrus. The percentages of heifers in estrus during the 7-d synchronized period were 75.4, 56.3 and 17.2% for the 14-d MGA, 7-d MGA and control groups, respectively. The estrous responses were significantly different in each treatment. The percentage of heifers in estrus during the peak 24-h period was higher (P < 0.05) in heifers synchronized with the 14-d MGA system than in heifers synchronized with the 7-d MGA system (75.5 vs 50.0%). The synchronized conception rate of the 14-d MGA heifers was significantly higher (65.3%) than that of both the 7-d MGA (41.7%) and control (45.4%) heifers. Synchronized conception rates were similar (P = 0.79) in the 7-d MGA and control treatments. Synchronized pregnancy rates were 55.2, 32.4 and 15.2% for the 14-d MGA, 7-d MGA and control groups, respectively. Both synchronization treatments resulted in significantly higher synchronized pregnancy rates compared with that of the controls. The synchronized pregnancy rate was higher (P < 0.05) in the 14-d MGA group than it was in the 7-d MGA group. The mean day of conception within the breeding season was 11.5 and 9.3 d shorter in the 14-d MGA heifers than in the 7-d MGA and control heifers, respectively. Our results indicate that using the 14-d MGA system to synchronize estrus in beef heifers results in better reproductive performance than that attained in heifers synchronized with the 7-d MGA system or in control heifers.     

Male effect in seasonally anovulatory lactating goats depends on the presence of sexually active bucks,but not estrous females

A study was conducted in subtropical northern Mexico (26 degrees N) to determine whether the presence of estrous females can improve the response of seasonally anovulatory goats to the introduction of bucks in the group. The induction of estrous activity was studied in three groups of anovulatory lactating goats during seasonal anestrus. These females were of the Mexican Creole breed. In the control group (sexually inactive (SI), n = 20), two control (SI) bucks exposed to normal seasonal daylength variations were used. In the second group (SI + E, n = 20 + 3), two control males were also used, but in addition, three females of the group were in estrus at the time of male introduction. In the third group (sexually active, SA + E, n = 19 + 4), anovulatory females were exposed to two bucks made sexually active by exposure to 2.5 months of long days (16L:8D) followed by two subcutaneous 18 mg melatonin implants, and four estrous females were also present when introducing the bucks. In all groups, males were introduced on 15 March and estrous detection was conducted twice daily for 15 days. The sexual activity of the bucks was observed from 08:00 to 10:00 h during the first five days of exposure to females. More females displayed estrous behavior in the first 15 days following the introduction of the males in the SA + E group (18/19) as compared with the SI or SI + E groups (2/20 and 0/20, respectively; P < 0.001). No difference was observed between the two latter groups. Thirteen females of SA + E group showed a second estrus between days 6 and 11 (short estrous cycle duration: 5.4 +/- 0.4 days). By contrast, in the SI group none showed a second estrus. The sexual behavior of the males in the SA + E group was greater as compared with that of the males in SI and SI + E groups (over 80% of the total sexual activity recorded in the three groups; P < 0.001). By contrast, no differences were found between SI and SI + E males. These results indicate that the presence of estrous females alone at the time of buck introduction is not sufficient to induce an adequate stimulation of seasonally inactive males. The use of sexually active bucks is necessary to induce reproductive activity in anovulatory females, whereas preparation of the bucks with long days followed by melatonin implants allows them to gain such a capacity.     

Comparison of two estrus synchronization and resynchronization treatments in lactating dairy cows

Reproductive performance in cows following synchronization of estrus with intravaginal progesterone releasing devices (IVD) has varied with the length of treatment, cyclic status and prolonged return to estrus intervals in some cows following first AI. The objective of this study was to compare two methods of synchronizing and resynchronizing estrus on the reproductive performance of lactating dairy cows. Cows were treated with an IVD (Day 0) for 7 days (n = 350) or 8 days (n = 350), cloprostenol (0.5 mg i.m.) at the time of device removal and estradiol benzoate (EB) at the time of device insertion (1.5mg i.m.), and again 9 days later (1.0 mg i.m.). Cows were also resynchronized starting on Days 23 and 46 by reinsertion of IVDs for either 7 or 8 days and treatment with EB (1mg i.m.) at the time of device insertion and again 9 days later. Cows were inseminated on detection of estrus for 4 days after removal of devices at each of the synchronized estrous cycles. No significant differences in reproductive performance were detected between each treatment throughout the study period. Synchrony of estrus was more precise at the first and second estrus after treatment with an IVD for 8 days compared to 7 days. Cows classified as anestrous had lower reproductive performance than cows classified as cycling and had longer intervals to estrus at the second (P < 0.001) and third estrus (P < 0.06), but not at the first estrus (P = 0.09). Mean time to onset of estrus after IVD removal was less in cows treated with an IVD for 8 days compared to 7 days at each synchronized estrus (P < 0.01). More Holstein-Friesian cows were classified as non-pregnant and not detected in estrus than crossbreed cows (15.7%, 54/343 versus 9.0%, 24/266; [P < 0.05). The results of the study suggested that the main effects of the treatments that were used to synchronize and resynchronize estrus were to alter the timing and synchrony of estrus without affecting fertility.     

Alterations in the onset of ovulatory failure and gonadotropin secretion following steroid administration to lightly androgenized female rats

The present studies were designed to characterize the gonadotropin response to exogenous steroids in neonatally androgenized female rats in various states of reproductive decline. Female rats were androgenized by the administration of a single injection of testosterone propionate (TP) (10 or 100 micrograms) at 5 days of age. Control rats received sesame oil. Treatment with 100 micrograms TP resulted in persistent vaginal estrus (PVE) from the onset of vaginal introitus. Treatment with 10 micrograms TP resulted in a period of regular estrous cyclicity followed by PVE. In the first experiment, all animals were ovariectomized between the ages of 60-85 days and the gonadotropin response to exogenously administered estradiol benzoate (EB) (10 micrograms/100 g BW) and progesterone (P) (2 mg/animal) was determined. When testing began 3 days following ovariectomy, control females exhibited significant (P less than 0.01) afternoon elevations of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) following EB, which were further amplified following P. When ovariectomy occurred prior to the onset of PVE (PRE PVE), lightly androgenized females (10 micrograms TP) showed no significant afternoon gonadotropin increase following EB. Following P, phasic LH secretion was present but significantly (P less than 0.01) decreased in amplitude and delayed in onset versus that of control females. When ovariectomy occurred 3 to 4 wk following the onset of PVE, lightly androgenized females (PVE group) as well as fully androgenized females (FAS) (100 micrograms TP) showed no gonadotropin response to steroid priming.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evaluation of a melengestrol acetate and prostaglandin F(2)alpha system for the synchronization of estrus in beef heifers

This study was conducted to determine the efficacy of feeding melengestrol acetate (MGA) for 14 days and administering prostaglandin F(2)alpha (PGF) 17 days after MGA to synchronize or induce estrus in yearling beef heifers. The study involved 56 Angus (n = 19), Hereford (n = 15) and Simmental (n = 22) heifers that were assigned by breed and pubertal status to either MGA+PGF or to control groups. Heifers in the synchronized group were fed 0.5 mg MGA per head per day for 14 days from a grain carrier and were injected with 25 mg, i.m. PGF 17 days after the last daily feeding of MGA. Control heifers were fed from a grain carrier without MGA and were not treated with PGF. Heifers were classified as pubertal when concentrations of progesterono in the serum exceeded 1 ng/ml in 1 of 2 samples collected prior to the initiation of treatments. Blood samples were collected 7 days before and on the day that treatment with MGA or carrier began and 7 days before and on the day that PGF was administered. Progesterone concentrations in the serum were elevated ( > 1 ng/ml) in 61% (17 28 ) of the MGA+PGF-treated heifers and in 61% (17 28 ) of the control heifers prior to feeding MGA. However, concentrations of progesterone in the serum at the time PGF was administered differed (P<0.05) between MGA+PGF and control groups. Concentrations of progesterone in the serum exceeded 1 ng/ml in 100% (28 28 ) of the MGA+PGF-treated heifers and in 71% (20 28 ) of control heifers at the time PGF was administered (P<0.05). All heifers were inseminated 12 hours after the first detected estrus. Twenty-two of 28 (79%) of the MGA+PGF-treated heifers exhibited estrus within 6 days after PGF compared with 9 of 28 (32%) of control heifers (P<0.05). The conception rate at first service did not differ between MGA+PGF and control groups (64% and 67%, respectively). Synchronized pregnancy rates were higher (P<0.05) for MGA+PGF-treated heifers than for control heifers (14 28 , 50% vs 6 28 , 21%). Increased concentrations of progesterone in serum at the time PGF was administered and higher pregnancy rates during the synchronized period among MGA+PGF-treated heifers demonstrate the efficacy of this treatment for use in estrus synchronization. Moreover, this treatment may have a potential effect on inducing puberty in breeding age heifers.     

Comparison of timed insemination with insemination at estrus following synchronization of estrus with a MGA-prostaglandin system in beef heifers

Three trials utilizing 231 beef heifers were conducted in 1993 to determine if a timed insemination would result in similar synchronized pregnancy rates as insemination by estrus following synchronization of estrus using the 14-d MGA-prostaglandin system. All heifers were fed 0.5 mg MGA/h/d fof 14 d and given a 25 mg injection of PGF(2)alpha im 17 d after the final day of MGA feeding. Heifers in Group 1 (timed AI treatment) were inseminated at 72 h after the prostaglandin injection independent of whether or not they were observed in estrus. Heifers in Group 2 (AI by estrus) were inseminated 12 to 18 h after the onset of estrus. Since the trial was a significant source of variation for synchronized pregnancy rate, the effect of treatment on pregnancy rate was analyzed for each trial. Synchronized pregnancy rates in Trials 2 and 3 were similar in both treatment groups; 37 vs 35% and 61 vs 58% for the timed AI vs AI by estrus (Groups 1 and 2) in Trials 2 and 3, respectively. In both of these trials the degree of estrous synchrony was high. In Trial 1, the synchronized pregnancy rate in heifers that were time-inseminated was significantly lower than that of heifers that were inseminated by estrus (29 vs 57%). The lower synchronized pregnancy rate of Group 1 (timed AI) heifers in Trial 1 appeared to be due to the low degree of estrous synchrony in this trial. Our results indicate that using timed insemination with the 14-d MGA-prostaglandin system will give similar synchronized pregnancy rates as inseminating by estrus in groups of beef heifers where the degree of synchrony is high. However, in heifers where the degree of estrous synchrony is low, a timed insemination reduces synchronized pregnancy rates.     

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.     

Reproductive performance of lactating dairy cows and heifers resynchronized for a second insemination with an intravaginal progesterone-releasing device for 7 or 8d with estradiol benzoate injected at the time of device insertion and 24h after removal

One aim of this study was to compare the reproductive performance of cows and heifers when resynchronizing returns to estrus for a second insemination by treating with an intravaginal progesterone-releasing device (IVD) for 7 or 8d when estradiol benzoate (EB) was administered at the start of treatment and again 24h after device removal. An additional aim was to document the pattern of onset and characteristics of estrus with each resynchrony treatment. Lactating cows in three herds were synchronized for a first estrus and AI by treatment with an IVD for 8d, starting on Day 0, cloprostenol (0.5 mg im) at device removal and EB at device insertion (2.0 mg im) and 24h after removal (1.0 mg im). Cows were resynchronized for a second estrus starting on Day 23 by reinsertion of IVDs for 7 (IVD-7-EB; n=449) or 8d (IVD-8-EB; n=445) with EB (1.0 mg im) administered at device insertion and 24h after removal. Cows were resynchronized for a third estrus by administration of EB (1.0 mg im) on Day 46, but subsequent treatments (no further treatment, reinsertion of CIDR or administration of EB on Day 55) varied among herds as part of separate studies. Maiden heifers (7-Day, n=68; 8-Day, n=69) were similarly treated as cows in a separate herd, but doses of EB were always 1.0 mg im at device insertion and 0.75 mg im 24h after removal. Heifers were not resynchronized for a third estrus. Cattle were inseminated on detection of estrus at each synchronized estrus. Cumulative pregnancy rates 4 week (66.0%, 276/418 versus 59.1%, 247/418) and 7 week (72.7%, 304/418 versus 67.7%, 283/418) after the start of AI were greater (P<0.05) in the IVD-7-EB cows compared to the IVD-8-EB cows, respectively; this was associated with a 9% increase in conception rates at the second estrus (P=0.051) in the IVD-7-EB cows. Treatment did not significantly affect reproductive performance in heifers. Characteristics of estrus measured with radiotelemetry did not differ significantly between the two treatment groups, but more cows were detected in estrus 36 h after removal of IVDs in the IVD-8-EB cows compared to the IVD-7-EB cows (P<0.05). We concluded that reproductive performance in resynchronized dairy cows but not heifers was greater following resynchronization of estrous cycles after AI with an IVD for 7 compared to 8d when EB was injected at the start of treatment and 24h after device removal.