Synchronization of estrus in beef cows and heifers with prostaglandin F(2alpha) and estradiol benzoate

An experiment was conducted to study an estrous synchronization regimen that involved the use of prostaglandin F(2alpha) (PGF(2alpha)) alone or in combination with estradiol benzoate (EB) and appointment breeding. Fifty-three registered Angus yearling heifers and 167 registered Angus cows (3 to 9 yr of age) were given two injections of PGF(2alpha) 11 d apart. Forty-eight hours after the second injection of PGF(2alpha') a random sample consisting of 117 cows and heifers was injected with EB in sesame seed oil. All females in the herd were artificially inseminated 80 h after the second injection of PGF(2alpha). Nearly equal percentages (25.1 vs 25.6%; P = 0.93) of treated (EB) and control (no EB) females conceived at the appointment breeding. Use of EB tended to reduce (P = 0.06) natural service conception rate (83.4 vs 93.1% for EB and control groups, respectively). Estrous synchronization treatment did not affect interval from Day 1 of the breeding season to calving.     

Synchronization of estrus in beef heifers with a norgestomet implant and prostaglandin F2alpha

In a 5-year study (1973-1977), 281 cycling beef heifers were treated with a 7-day norgestomet (SC21009) ear implant and an intramuscular injection of prostaglandin F(2alpha) (PGF(2alpha)) at the time of implant removal or 24 hr before implant removal. Percentages of heifers in estrus by 36, 48, 60, 72, and 120 hr after implant removal were 32.4, 52.7, 71.6, 80.1, and 93.2, respectively. Onset of estrus occurred an average of 49.8 +/- 4.7 hr after treatment. Percentages of heifers in estrus 36 hr after treatment were 5.7 and 51.7 for those with a corpus luteum and those without a corpus luteum (or determined regressing by palpation) at implant removal, respectively. When PGF(2alpha) was injected 24 hr before implant removal, 55% of the heifers were in estrus by 36 hr after implant removal compared to 30% when PGF(2alpha) was injected at the time of implant removal; however, by 60 hr after implant removal the difference was 76% vs. 71%. First-service conception rates for synchronized and nonsynchronized heifers were 62.2% and 59.6%, respectively. During 1976 and 1977 heifers were checked for estrus every 4 hr and inseminated 2, 6, 10, 14, 18, 22, 26, or 30 hr after first detected to be in standing estrus. Conception rate was not significantly affected by time of insemination but tended to be higher for heifers bred 26 and 30 hr after first being detected in standing estrus (78.9% and 70.0% vs. average 59.2%). Treatment with a 7-day norgestomet implant plus a single injection of PGF(2alpha) 24 hr before or at implant removal appears to be a practical technique for synchronizing estrus in cycling heifers without affecting conception.     

Synchronization of estrus in yearling beef heifers with melengestrol acetate and prostaglandin F(2alpha)

This study was designed to test the efficacy of melengestrol acetate (MGA) in combination with prostaglandin F(2alpha) (PGF(2alpha)) in synchronizing estrus in cyclic and noncyclic heifers. One hundred thirty-one cyclic and prepubertal crossbred heifers were randomly assigned to three treatment groups: Controls (n = 43); MGA (0.5 mg/d for 7 d) and PGF(2alpha) (25 mg i.m. on Day 7; n = 44); and PGF(2alpha) (25 mg i.m. on Day 7; n = 44). Observations for estrus were made at 6-n intervals throughout the 7-d treatment period followed by a 34-d artificial insemination breeding season. A greater percentage (P < 0.05) of MGA-PGF(2alpha) noncyclic heifers showed behavioral estrus (91%) than did Control (67%) or PGF(2alpha) heifers (61%) during the 34-d artificial insemination period. There was no difference (P > 0.05) between synchronization rates of the MGA-PGF(2alpha) heifers and PGF(2alpha) heifers 7 d after PGF(2alpha) administration. The percentage of control animals in estrus during the first 25 d of the breeding season did non differ from the synchronized rates of MGA-PGF(2alpha) and PGF(2alpha) heifers (P > 0.05). Conception rates (heifers pregnant/heifers inseminated) did not differ (P > 0.05) for cyclic or prepubertal heifers among Control, MGA-PGF(2alpha) or PGF(2alpha) heifers. Though conception rates did not differ, there was a trend toward lowered conception rates in MGA-PGF(2alpha) heifers.     

Superovulatory and endocrine responses in holstein heifers treated with either prostaglandin F(2alpha), cloprostenol or fenprostalene

Fifty Holstein heifers were each superovulated three times with FSH-P. At 60 h after the first injection of FSH-P, the animals received either prostaglandin F(2alpha), cloprostenol or fenprostalene in random order. A significant decrease in serum progesterone and a significant increase in serum estradiol-17beta were observed within 24 h of prostaglandin injection, but there were no significant differences among the three treatments. Neither were there any significant differences among the treatments with respect to the frequency of nonresponse to FSH-P treatment, nor the total number of ova/embryos collected between Days 6 and 8 of gestation.     

Synchronization of estrus in cyclic beef heifers with the prostaglandin analog alfaprostol

Seventy-eight Hereford-Angus crossbred heifers were injected intramuscularly twice with 6 mg of alfaprostol^b in 6 ml of propylene glycol. On each representative day of a 20-day estrous cycle (estrus = Day 0), either three or four heifers received their first injection. The second injection was given 12 days after the first, regardless of the response to the first injection. Thirty-nine heifers were not treated. The first alfaprostol injection reduced serum progesterone to less than 1 ng/ml in all heifers injected after Day 4. A total of 79.5% ($\text{62}\text{78}$) of the heifers exhibited estrus by five days after the first injection. Average interval from injection to estrus was 63 hours. The second injection occurred on Days 6 through 16 for all but one heifer, with 75.6% ($\text{59}\text{78}$) falling on Days 8 through 11 of the estrous cycle. Estrus was detected in 93.6% ($\text{73}\text{78}$) of the heifers within five days after the second injection, with an average interval to estrus of 66 hours.Day of cycle at second injection did not affect the interval to estrus. Conception occurred in 79.4% ($\text{58}\text{73}$) of the heifers inseminated in the five days after the second injection. Occurrence of estrus and conception was no different in treated heifers after five days of the insemination period than in nontreated heifers after 21 days of the insemination period, where 94.9% ($\text{37}\text{39}$) were observed in estrus and 83.8% ($\text{31}\text{37}$) conceived. Overall percent conception for a 55-day insemination period was 89.7 ($\text{70}\text{78}$) for treated and 87.2 ($\text{34}\text{39}$) for nontreated heifers. Day of cycle at first or second injection did not affect conception after the second injection. Some signs of estrus were observed in 11 of the 16 heifers injected before Day 5.A second trial to determine if alfaprostol induced luteolysis early in the cycle was conducted. Twenty purebred Angus, Hereford, or Simmental heifers received either one or two injections of alfaprostol on either Day 1, 2, 3, or 4. Only five heifers showed any signs of estrus, and the three that were inseminated did not conceive. Subsequent cycle length indicated that luteolysis occurred in only one heifer.Data suggest that alfaprostol is an effective luteolytic agent in cyclic beef heifers after Day 4 and that two injections 12 days apart will effectively synchronize estrus in heifers when distributed throughout the cycle at the first injection without affecting conception rate.     

Synchronization of estrus after treatment with Luprostiol in beef cows and in beef and dairy heifers

Four trials were conducted to study synchronous estrous response in beef cows and in beef and dairy heifers to Luprostiol (13, thia-PG-F(2)alpha analog) in comparison with other prostaglandin products. In Trial 1, 60 virgin beef heifers were observed for estrus for 5 d and artificially inseminated. Heifers not observed in estrus within 5 d were randomly assigned to receive 15 mg Luprostiol or 25 mg Lutalyse. In Trial 2, 75 multiparous, lactating beef cows were randomly assigned to receive either 15 mg Luprostiol, 25 mg Lutalyse or 500 mcg Estrumate. All cows received a second injection of the respective treatment 11 d later. In Trial 3, 96 multiparous, lactating beef cows were randomly assigned to receive 15 mg Luprostiol or 25 mg Lutalyse. All cows received a second injection of the respective treatment 11 d later. In Trial 4, virgin dairy heifers were palpated per rectum. Seventy-seven heifers with a palpable corpus luteum (CL) were randomly assigned to receive 15 mg Luprostiol or 500 mcg Estrumate. In all trials animals were artificially inseminated 12 h following observed estrus. Estrous response during the 5-d synchronized period was 44% for Luprostiol and 42% for Lutalyse treated heifers in Trial 1. It was 52, 56 and 60%, respectively, for Luprostiol, Lutalyse and Estrumate treated cows in Trial 2; 23% for Luprostiol and 19% for Lutalyse treated cows in Trial 3; and 68% for Luprostiol and 70% for Estrumate treated heifers in Trial 4. Treatment with Luprostiol results in a similar synchronous estrous response as with the other prostaglandin products used in these studies.     

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.     

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.     

Synchronization of estrus in yearling beef heifers with the melengestrol acetate/prostaglandin F(2alpha) system: efficiency of timed insemination 72 hours after prostaglandin treatment

Two experiments were conducted to determine the conception rates of heifers time-inseminated following melengestrol acetate/prostaglandin F(2alpha) (MGA/PG) estrous synchronization treatment. In Experiment 1, timed insemination of heifers at 72 h after the PG injection, without regard for behavioral estrus, tended to improve (P < 0.15) the percentage of heifers pregnant to artificial insemination (AI) compared with that of synchronized heifers bred 12 h after they were first detected in estrus. In the timed-insemination treatment, heifers exhibiting behavioral estrus 48 to 72 h after PG tended to have an increased (P < 0.15) conception rate to AI compared with heifers exhibiting estrus within 48 h of PG administration. In Experiment 2, the number of heifers conceiving to AI following the MGA/PG estrous synchronization regimen was increased by mass insemination of all heifers not exhibiting estrus by 72 h after PG. The pregnancy rate to AI was higher in heifers with serum progesterone (P(4)) concentrations higher than 1 ng/ml compared with that of heifers with concentrations lower than 1 ng/ml. Of heifers with serum P(4) greater than 1 ng/ml, the pregnancy rate to AI tended to be higher when concentrations exceeded 2 ng/ml than when concentrations were 1 to 2 ng/ml. In cyclic heifers, timed insemination can increase the percentage of heifers pregnant after being synchronized with MGA/PG.     

Estrus induction with prostaglandin F2alpha, cloprostenol or fenprostalene during the normal estrous cycle, superovulation and after embryo collection

Holstein heifers used as embryo donors were treated with three luteolytic agents (PGF2alpha, cloprostenol, fenprostalene) during the normal estrous cycle, superovulation or after embryo collection to determine the interval from treatment to estrus. A similar return-to-estrus interval was observed for each luteolytic agent among the three groups of heifers. Nevertheless, after embryo collection, fenprostalene had a tendency to induce the longest delays (p = 0.08). This tendency is supported by a higher proportion of delayed luteolysis and more heifers showing estrus later than 11 d post treatment. Also, during normal estrous cycles, 5/10 and 0/8 fenprostalene- and cloprostenol-treated heifers, respectively, showed progesterone concentrations higher than 1 ng/mL 48 h after treatment. Regardless of the luteolytic agent used, estrus was induced earlier (P < 0.005) during superovulation than when heifers were treated between Days 9 to 16 of the normal estrous cycle or after embryo collection. However, the return-to-estrus interval was similar between heifers treated during superovulation and those treated between Days 6 to 8 of the normal estrous cycle. After embryo collection, intervals before the return to estrus increased with the number of Corpora lutea (CL) palpated except in the nonresponding group (0 to 1 CL), which returned to estrus later than the low responding group (2 to 4 CL).     

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.     

Control of estrus in dairy cows with a synthetic analogue of prostaglandin F2 alpha

Trials were carried out on 1184 dairy cows calved at least six weeks before treatment and 255 heifers to determine effectiveness of the prostaglandin analogue, cloprostenol to control estrus. In trial 1, following two injections of cloprostenol given 12 days apart, there was no difference in calving rate following AI either at 72 and 96 hr after treatment (71 163 ) or at a detected estrus (53 118 ) compared to control cows bred at estrus (54 110 ). In trial 2, treated cows were injected once after 5 to 7 days of estrous detection and AI. The calving rate following AI either at 72 and 96 hr after cloprostenol (46 100 ) or at a detected estrus (39 71 ) was similar to that in control cows bred at estrus (45 86 ). In trial 3, cows were bred at a detected estrus after the first cloprostenol injection. Twelve days after this injection, cows not bred were given a second injection and bred 72 and 96 hr later. The calving rate in treated cows bred at estrus after the first injection (66 138 ) was similar to calving rate in controls (55 95 ). However, calving rate in cows given a second injection and bred 72 and 96 hr later was significantly (P angle 0.05) lower (30 98 ). Similar results were obtained in heifers, except calving rate in trial 3 after the second cloprostenol injection was not reduced.     

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.     

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.     

Synchronization of estrus in early diestral dairy heifers with Prostaglandin F(2)alpha and estradiol benzoate

Following observation of estrus, 134 Holstein heifers were given injections of Prostaglandin F(2)alpha (PGF(2)alpha) between Days 5 and 10 of their cycle (estrus = Day 0). They were then randomly assigned to either a group receiving 400 mug of estradiol benzoate (E(2)B) 40 h or maintained as controls. Heifers observed in estrus within 120 h of PGF(2)alpha administration were inseminated (approximately 12 h after initial observation of estrus). Blood samples for progesterone determination were drawn from the coccygeal vein on Days 15 and 21 after insemination. Pregnancy was confirmed by palpation per rectum between Days 5.0 and 60 post insemination. When control and treated heifers were compared it was found that a higher percentage of heifers treated with E(2)B exhibited estrus after PGF(2)alpha, but there had been no effect on subsequent progesterone concentrations or pregnancy rates.     

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

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