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.     

Prolonging the MGA-prostaglandin F2 alpha interval from 17 to 19 days in an estrus synchronization system for heifers

Our objective was to determine whether extending the interval from 17 to 19 d between removal of melengestrol acetate (MGA) feed and administration of PGF2 alpha would alter conception rates, pregnancy rates and the degree of synchrony in replacement beef heifers. A commercial heifer operation in north-central Kansas purchased 591 Angus x Hereford heifers from 12 sources. Prior to the spring breeding season, 14% of the heifers were culled. The remaining heifers were assigned randomly to 2 MGA-PGF2 alpha synchronization systems. All heifers were fed MGA (0.5 mg/head/d) for 14 d, and PGF2 alpha was administered either 17 or 19 d after the completion of MGA feeding. Heifers were inseminated artificially for 30 d followed by 30 d of natural mating. Based on each source, first-service conception rates ranged from 66 to 90%, whereas overall pregnancy rates ranged from 91 to 100%. Heifers given PGF2 alpha on Day 17 after MGA had first-service conception rates of 75.9% compared with 81.4% for heifers receiving PGF2 alpha on Day 19. In response to the PGF2 alpha injection, 99% of the Day 19 heifers that were detected in estrus were inseminated artificially by 72 h after the PGF2 alpha injection, whereas 74% of the heifers in the Day 17 treatment were inseminated by that time. Average interval to artificial insemination (AI) after PGF2 alpha was greater (P < 0.01) for the Day 17 heifers (73.1 +/- 1.1 h) than for the Day 19 heifers (56.2 +/- 1.1 h). No differences in conception rates or overall pregnancy rates occurred; however, heifers receiving PGF2 alpha on Day 19 after MGA had shorter intervals to estrus, and a greater proportion was inseminated within 72 h after PGF2 alpha, thus possibly facilitating successful timed insemination of the remaining heifers not yet inseminated by that time.     

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.     

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.     

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

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

Synchronization of estrus in suckled postpartum beef cows with melengestrol acetate and PGF2alpha

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)-PGF2alpha estrus synchronization system in spring-calving cows. The cows were allocated 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-PGF2alpha (0.5 mg MGA/head/d for 14 d with 25 mg of PGF2alpha injected 17 d after MGA withdrawal), and unsynchronized 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-PGF2alpha 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-PGF2alpha treated (31.6 and 21.4%, respectively) cows compared with that of the ECRC controls (11.9 and 8.5%, respectively). The CSU MGA-PGF2alpha 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-PGF2alpha treated cows and ECRC control cows. The MGA-PGF2alpha 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.     

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

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

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.     

Factors influencing estrus and conception in dairy heifers after prostaglandin F2-alpha

The influence of interval between insemination (AI) and estrus on subsequent fertility of PGF(2alpha)-treated (two injections of 25 mg, 11 days apart) heifers was assessed in two experiments. In Experiment I, 240 heifers were allotted to Control (AI 8 to 16 hr after estrus detection), PGF(2alpha)-E (AI 8 to 16 hr after estrus within five days of second PGF(2alpha)) or PGF(2alpha)-T (AI 80 hr after second PGF(2alpha)). In Experiment II, 130 heifers were assigned to control (AI as before) or PGF(2alpha) (AI 72 or 80 hr after second PGF(2alpha)) with half the PGF(2alpha) heifers receiving 100 mug GnRH 72 hr after first PGF(2alpha). Heifers of both experiments that were bred at a predetermined time were arrayed by interval from AI to estrus. Conception rates of heifers detected in estrus from 32 hr before AI to 24 hr after AI did not differ (x(2)=3.35, df=5, P>0.5). The percentage of GnRH-treated heifers in estrus within five days (81.8%) was not (P>0.75) greater than those not receiving GnRH (77.3%) but they had higher (P<0.05) serum progesterone (P(4)) concentration at second PGF(2alpha) (3.17 vs 2.41 ng/ml). When P(4) values were arrayed for both groups at 1 ng intervals, the percentage of heifers exhibiting estrus increased with increasing P(4) level (P<0.05).     

Synchronization of estrus and fertility in zebu beef heifers treated with three estrus synchronization protocols

The effects on estrus and fertility of 3 estrus synchronization protocols were studied in Brahman beef heifers. In Treatment 1 (PGF protocol; n=234), heifers received 7.5 mg, i.m. prostianol on Day 0 and were inseminated after observed estrus until Day 5. Treatment 2 (10-d NOR protocol; n = 220) consisted of norgestomet (NOR; 3 mg, s.c. implant and 3 mg, i.m.) and estradiol valerate (5 mg, i.m.) treatment on Day -10, NOR implant removal and 400 IU, i.m. PMSG on Day 0, and AI after observed estrus through to Day 5. Treatment 3 (14-d NOR+PGF protocol; n = 168) constituted a NOR implant (3 mg, sc) on Day -14, NOR implant removal on Day 0, PGF on Day 16, and AI after observed estrus through to Day 21. All heifers were examined for return to estrus at the next cycle and inseminated after observed estrus. The heifers were then exposed to bulls for at least 21 d. During the period of estrus observation (5 d) after treatment, those heifers treated with the PGF protocol had a lower (P<0.01) rate of estrual response (58%) than heifers treated with the 10-d NOR (87%) or 14-d NOR+PGF (88%) protocol. Heifers treated with the 10-d NOR protocol displayed estrus earlier and had a closer synchrony of estrus than heifers treated with either the PGF or the 14-d NOR+PGF protocol. Heifers treated with the 14-d NOR+PGF protocol had higher (P<0.05) conception and calving rates (51 and 46%) to AI at the induced estrus than heifers treated with the PGF (45 and 27%) or the 10-d NOR (38 and 33%) protocol. Calving rate to 2 rounds of AI was greater (P<0.05) for heifers treated with the 14-d NOR-PGF (50%) protocol than heifers treated with the 10-d NOR (38%) but not the PGF (43%) protocol. Breeding season calving rates were similar among the 3 protocols. The results show that the 14-d NOR+PGF estrus synchronization protocol induced a high incidence of estrus with comparatively high fertility in Brahman 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.     

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.     

Growth and ovarian function of weanling and yearling beef heifers grazing endophyte-infected tall fescue pastures

Growth and ovarian function of crossbred beef heifers grazing low and high endophyte-infected tall fescue pastures were studied for 2 successive years. In April of each year, 20 weanling and 20 yearling heifers were included in the study. All heifers were weighed at 28-d intervals for 112 d. Blood samples were collected from each heifer on Day 0 and +7 of each of five 28-d periods and analyzed for progesterone concentration. Heifers with progesterone concentrations >/= 1.5 ng/ml on either or both Day 0 and +7 were classified as having normal cyclic ovaries. High endophyte-infected fescue pastures adversely altered the ovarian activity (P < 0.05) of weanling heifers in both years. In each trial, average weight gains were lower (P < 0.05) in yearling and weanling heifers grazing the high endophyte-infected pastures than in heifers grazing low endophyte-infected pastures. In 1992, heifers were synchronized with PGF(2alpha) administered on Days 101 and 112. Blood samples were collected on 0, 4, 8 and 12 d after the second PGF(2alpha) injection for progesterone analysis. Heifers grazing high and low endophyte-infected pastures were pastured separately with 4 bulls each and were given heatmount detectors. At 96 h, less estrus activity was observed (P < 0.10; power=0.63) in weanling heifers grazing the high vs. low endophyte pastures although pregnancy rates were similar for all groups. Progesterone concentrations suggested that weanling heifers on the high endophyte pastures had a higher incidence of luteal dysfunction after PGF(2alpha) synchronization. In summary, high endophyte-infected pastures decreased growth in both weanling and yearling heifers, ovarian activity and luteal function were adversely altered in weanling heifers with subsequent decreased estrus response to estrus synchronization.     

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.     

Response of prepubertal Bos taurus and Bos indicus x Bos taurus heifers to melengestrol acetate with or without gonadotropin-releasing hormone

The effectiveness of treatments to induce estrus in prepubertal beef heifers was evaluated. Angus x Hereford (n = 148) and Brahman x Hereford (n = 148) heifers were sorted after weaning by body weight into light and heavy weight blocks. Heifers were assigned to diets, calculated to reach a target weight of 55% or 65% of their projected mature weight by the start of breeding. Cyclicity was determined after a 160-d observation period and from concentrations of progesterone in serum determined 10 d before and on the day that treatments began to induce puberty. The remaining nonpubertal heifers, with concentrations of progesterone in serum of less than 1 ng/ml (0 or 10 d before treatment), were assigned randomly within breed and nutrition group to either a melengestrol acetate + saline (MGA+S) or MGA + gonadotropin-releasing hormone (MGA+GnRH) treatment. Prepubertal Angus x Hereford heifers (n = 11) and Brahman x Hereford heifers (n = 49) were fed 0.5 mg MGA for 7 d. Forty-eight hours after MGA, heifers were injected with 500 ug s.c. GnRH or 5 ml of saline. Blood samples were collected from all prepubertal heifers every 3 d after GnRH or saline for 30 d. There was no difference between treatments in the proportion of heifers that exhibited estrus by Day 7 after treatment. However, a larger (P<0.05) proportion of MGA+S-treated heifers exhibited estrus within 14 d after treatment than MGA+GnRH-treated heifers (87 vs 63%). Among heifers that exhibited estrus during that time period, the proportion with increased progesterone was higher (P<0.10) for the MGA+GnRH group than for the MGA+S group (71 vs 41%, Day 7; 79 vs 54%, Day 14). There was no difference in conception rate at first service between treatment groups. Thirty-seven and 53%, respectively, of the MGA+S and MGA+GnRH-treated heifers had short estrous cycles after treatment, and 44 and 50%, respectively, of those short cycles were repeated. Pregnancy rates at the end of 45 d were numerically higher for MGA+S heifers than for MGA+GnRH treated counterparts (63 vs 53%).     

Estrus synchronization in cattle using estradiol, melengestrol acetate and PGF

In Experiment 1, all cattle were fed MGA (0.5 mg/head/d) for 7 d (designated Days 0 to 6) and given PGF on Day 6. One-half were administered estradiol valerate (EV; 5 mg, im) on Day 0. At Location 1, a higher proportion (P < 0.005) of EV-treated heifers were detected in estrus and bred by AI between Days 7 and 13 than control heifers not receiving EV (27 of 33 versus 15 of 32), but the number of pregnancies (12 vs 10) was not significantly different. Eighty-three of 104 EV-treated and 89 of 106 control cows were inseminated, resulting in 50 and 45 pregnancies, respectively (not significant). At Location 2, cattle were similarly treated and exposed to bulls on Days 7 to 49. Fall pregnancy rate was higher (P < 0.015) for EV-treated than control heifers (44 of 48 vs 33 of 46), but was not significantly different for cows (22 of 26 vs 19 of 23). In Experiment 2, estradiol 17beta (E17beta; 5 mg, im) and progesterone (100 mg, im) were administered on Day 0 (instead of EV). In a third group (designated the PGF group), cattle were bred on Days 0 to 6, and PGF was administered on Day 6 to those not yet bred. For 213 cows, the percentage pregnant to a synchronized estrus was greater in the PGF group (72%) than in either the control group treated with MGA (49%; P = 0.005) or the group receiving MGA and E17beta (54%; P < 0.025). Fall pregnancy rates were 91, 89, and 96% for the 213 cows (not significant) and 89, 93, and 98% for 131 heifers (not significant) in the PGF, MGA and E17beta groups, respectively. In cattle without a functional CL, the average diameter of the largest follicle at Day 6 was 1 to 2 mm smaller in the E17beta + MGA group than in the MGA group (difference significant only in cows at Location 1). Combined for both locations, the synchronized pregnancy rate in heifers without a functional CL on Day 6 was higher (P < 0.05) in the E17beta + MGA group than in the MGA group (11 of 21, 52% versus 4 of 20, 20%). Estrogen treatment caused regression of ovarian follicles with emergence of a new follicular wave. Including estrogen in an estrus synchronization program utilizing MGA and PGF significantly increased fall pregnancy rate in heifers (at 1 location) and the synchronized pregnancy rate of heifers without a functional CL at the time of PGF treatment (combined for both locations).     

Attenuation of premature estrous behavior in postpartum beef cows synchronized to estrus using GnRH and PGF2alpha

The efficacy of GnRH and PGF2alpha (7-day injection interval) for estrus synchronization is diminished by estrous expression before PGF2alpha (premature estrus; PE). Effects of modifications to GnRH-PGF2alpha protocols on the incidence of PE and other indicators of reproductive performance were evaluated. In Experiment 1, Angus-based crossbred cows (n=51) received 25 mg of PGF2alpha i.m. on Day 0. Animals were randomly assigned by parity and interval postpartum to receive GnRH 100 microg i.m. on either Day -7 or Day -6. Estrous detection and AI were conducted from Day -3 to Day 5. Treatment had no effect on the incidence of PE, estrous response, conception rate per AI or synchronized pregnancy rate (6- vs. 7-day interval; 8 vs. 15%; 92 vs. 93%; 77 vs. 76%; 71 vs. 70%, respectively). In Experiment 2, Angus cows (n=150) received GnRH 100 microg i.m. on Day -7 and 25 mg PGF2alpha i.m. on Day 0. Animals were randomly assigned by parity, interval postpartum, and body condition score to receive either no further treatment (Control) or 0.5 mg melengestrol acetate/hd/d from Day -7 to Day -1 (MGA). Estrous detection and AI were conducted from Day -2 to Day 7. Fewer (P < 0.05) MGA-treated cows were detected in PE (0%) compared to controls (7%). Treatment had no effect on estrous response or synchronized pregnancy rates (Control vs. MGA; 78 vs. 84%; 52 vs. 60%, respectively). Conception rate per AI of cows > or = 60 days postpartum were not affected by treatment (Control vs. MGA; 79 vs. 73%) however, control cows < 60 days postpartum tended (P < 0.10) to have lower conception rates per AI (39%) than did their MGA-treated counterparts (69%). In summary, 6- and 7-day GnRH-PGF2alpha injection intervals resulted in similar synchronized reproductive performance. Inclusion of MGA feeding between GnRH and PGF2alpha injections eliminated the occurrence of premature estrus and improved conception rate per AI of late-calving cows.     

Single appointment insemination for heifers after prostaglandin or progestin synchronization of estrus

Pregnancy rates to a single appointment insemination were compared in seven groups of beef or dairy heifers following estrus synchronization with the prostaglandin F(2a) product, cloprostenol, versus the progestín product, Syncro-Mate-B. For cloprostenol synchronization, two injections of cloprostenol were given 11 d apart, with insemination occurring at 61+/-1 h after the second injection. The Syncro-Mate-B treatment consisted of a norgestomet/estradiol injection and a norgestomet implant on Day 0, followed by insemination at 49+/-1 h after implant removal on Day 9. Treatments were coordinated so all heifers in each group were housed together at the time of and for 48 h before a common insemination time so that the treatment received by individual heifers was not known. Overall pregnancy rates did not differ between treatment groups: 42% for 111 heifers treated with cloprostenol and 38% for 108 heifers treated with Syncro-Mate-B. Four blood samples per heifer taken during each replicate showed that two groups (n = 67) had many noncyclic heifers, while five groups (n = 152) had very few. Differences in pregnancy rates between these categories existed for both estrus synchronization methods, 18 versus 53% for cloprostenol and 21 versus 45% for Syncro-Mate-B. Thus the two methods were equally effective for cyclic heifers and equally ineffective for noncyclic heifers.     

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.     

Effect of prostaglandin F2alpha dosage and stage of estrous cycle on the estrous response and corpus luteum function in beef heifers

Ninety-five normal cyclic crossbred beef heifers were used to determine if the proportions of heifers showing estrus, intervals to estrus and corpus luteum (CL) function were influenced by PGF(2alpha) dosage and (or) the stage of luteal phase when PGF(2alpha) was administered. Heifers were assigned randomly to treatments in a 4 x 3 factorial arrangement. Treatments were 5, 10, 25 or 30 mg PGF(2alpha) injected either in early (5 to 9 d), mid (10 to 14 d) or late (15 to 19 d) stages of the luteal phase. Jugular samples were taken at 0 h and at 8 h-intervals for 48 h and again at 60 h after PGF(2alpha) treatment for progesterone assay. Heifers were observed for estrus continuously for 120 h PGF(2alpha) treatment. The proportion of heifers showing estrus was dependent upon (P<0.05) both dosage of PGF(2alpha) and stage of luteal phase. Heifers given 5 mg of PGF(2alpha) showed estrus only if treated during the late stage, while those given 10 mg of PGF(2alpha) showed a progressive increase of heifers in estrus as stage of luteal phase advanced. The proportion of heifers showing estrus after 25 and 30 mg of PGF(2alpha) increased from 56% for the early stage to 100% for the mid and late stages. Interval to estrus in heifers showing estrus within 120 h after PGF(2alpha) treatment did not differ (P>0.05) among dosages but tended (P=0.10) to be longer in heifers treated during the mid luteal stage (67 h) than in heifers treated in the two other stages (56 h). A greater proportion of heifers (P<0.05) showed estrus by 60 h after PGF(2alpha) when treated during the early and late luteal stages (75.5%) than for heifers treated during the mid luteal stage (30.4%). Patterns of progesterone concentrations were influenced (P=0.08) by the three way interaction of dosage, stage and time. In heifers that showed estrus, rate of decline in progesterone tended (P=0.07) to be slower during the mid luteal stage than during the early and late stages. Progesterone did not drop below 1 ng/ml until 32 h in heifers treated during the mid luteal stage; whereas progesterone dropped below 1 ng/ml by 24 h in heifers treated during the early and late stages. These data may be useful in designing more efficient systems for using PGF(2alpha) or its analogues in estrus synchronization of beef cattle.