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.     

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

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

Synchronization of estrus and fertility in beef cattle with two injections of buserelin and prostaglandin

Postpartum beef cows and heifers in Group 1 received 8 mug of buserelin on Day 0 (the beginning of the experiment) and 500 mug of cloprostenol (PGF) on Day 6 (GnRH I, n = 54). In Group 2 (GnRH II, n = 54), the females were injected with buserelin on Day 0 (8 mug) and Day 3 (4 mug), and PGF on Day 6 and Day 9 for females not detected in estrus previously. Animals were bred by AI 12 hours after the onset of estrus. Blood samples were collected on Day -11 and Day 0 to assess cyclicity and on Day 3 and Days 6 to 12 to examine luteal activity. Progesterone levels did not differ between the 2 groups between Days 0 to 9. In both groups, the proportion of spontaneous estruses from Days 0 to 6 was reduced. Precision of estrus was higher (P < 0.005) in the GnRH II group than in the GnRH I group of cows that were detected in estrus between Days 6 and 9. The synchronization rate, interval to estrus, pregnancy and conception rates were similar in GnRH I and GnRH II groups. The conception rate and interval to estrus were similar in cyclic and acyclic cows. Increasing the number of buserelin injections enhanced the precision of estrus, but not the conception rate, without any detrimental effect on luteal activity and induced more estruses in postpartum acyclic beef cattle.     

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.     

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.     

Resynchronization of previously timed-inseminated beef heifers with progestins

The objective was to determine the efficacy of a previously used CIDR or melengestrol acetate (MGA; 0.5mg/head/day) for resynchronization of estrus in beef heifers not pregnant to timed-AI (TAI). In three experiments and a field trial, heifers were reinseminated 6-12 h after first detection of estrus. Pregnancy diagnosis was done from approximately 25-43 days after either TAI or reinsemination. In Experiment 1, 79 heifers received a once-used CIDR from 13 to 20 days after TAI and 80 heifers were untreated controls. For these two groups, there were 34 and 35 heifers, respectively, not pregnant to TAI; median +/- S.E. intervals from TAI to onset of estrus were 22 +/- 0.2 days versus 20 +/- 0.6 days (P < 0.001); estrus rates were 70.6% versus 85.7% (P = 0.1); conception rates were 62.5% versus 76.7% (P < 0.3); and pregnancy rates were 44.1% versus 65.7% (P = 0.07), for CIDR and untreated (control) groups, respectively. In Experiment 2, heifers (n = 651) were TAI (Day 0) and 13 days later randomly assigned to one of seven groups (n = 93 per group) to receive a once-used CIDR (three groups; Days 13-20), MGA (three groups; Days 13-19), or no treatment (control group). Groups given a CIDR or MGA also received: no further treatment (CIDR or MGA alone); 1.5mg estradiol-17beta (E-17beta) and 50 mg progesterone (P4) in 2 mL canola oil on Day 13; or E-17beta and P4 on Day 13 and 0.5 mg E-17beta on Day 21 (24 h after CIDR removal or 48 h after the last feeding of MGA). Pregnancy rate to TAI was lowest (P < 0.05) for the group given a CIDR plus E-17beta and P4 on Day 13 and E-17beta on Day 21. Variability in return to estrus was greater (P < 0.001) in the control and MGA groups than in CIDR groups. Conception and pregnancy rates in heifers given a CIDR (65.1 and 61.4%) were higher (P<0.01) than those fed MGA (49.6 and 40.4%), but not different from controls (62.2 and 54.9%, respectively). In Experiment 3, 616 heifers received a once- or twice-used CIDR for 7 days, beginning 13+/-1 days after TAI, with or without a concurrent injection of 150 mg of P4 (2 x 2 factorial design). Pregnancy rate to TAI was 47.2%. In heifers that returned to estrus, there was no significant difference between a once- or twice-used CIDR for rates of estrus (68.8%, P < 0.3), conception (65.9%, P < 0.6) and pregnancy (45.3%, P < 0.8). Injecting progesterone at CIDR insertion increased the median interval from CIDR removal to onset of estrus (P < 0.05) and reduced rates of estrus (63.8% versus 73.8%, P<0.05), conception (60.5% versus 70.6%, P = 0.1) and pregnancy (38.6% versus 52.2%, P < 0.02). In a field trial, 983 heifers received a once-used CIDR for 7 days, beginning 13 +/- 1 days after TAI. Pregnancy rate to TAI was 55.2%. The median (and mode) of the interval from CIDR removal to estrus was 2.5 days. Estrus, conception and pregnancy rates were 78.2, 70.3 and 55.0% (overall pregnancy rate to TAI and rebreeding, 78.7%). In summary, a once- or twice-used CIDR for 7 days, starting 13 +/- 1 days after TAI resulted in the majority of nonpregnant heifers detected in estrus over a 4-day interval, with acceptable conception rates; however, injecting progesterone at CIDR insertion significantly reduced both estrus and pregnancy rates, and estradiol treatment after CIDR removal was associated with a decreased pregnancy rate to TAI. Fertility was higher in heifers resynchronized with a once-used CIDR than with MGA.     

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.     

Effect of estradiol valerate on ovarian follicle dynamics and superovulatory response in progestin-treated cattle

Three experiments evaluated the effects of estradiol valerate (EV) on ovarian follicular and CL dynamics, intervals to estrus and ovulation, and superovulatory response in cattle. Experiment 1 compared the efficacy of two norgestomet ear implants (Crestar and Syncro-Mate B; SMB) for 9 d (with PGF at implant removal), combined with either 5 mg estradiol-17beta and 100 mg progesterone (EP) or 5 mg EV and 3mg norgestomet (EN) im at the time of implant insertion on CL diameter and follicular wave dynamics. Ovaries were monitored by ultrasonography. There was no effect of norgestomet implant. Diameter of the CL decreased following EN treatment (P < 0.01). Mean (+/- S.D.) day of follicular wave emergence (FWE) was earlier (P < 0.0001) and less variable (P < 0.0001) in EP- (3.6 +/- 0.5 d) than in EN- (5.7 +/- 1.5 d) treated heifers. Intervals from implant removal to estrus (P < 0.001) and ovulation (P < 0.01) were shorter in EN- (45.7 +/- 11.7 and 74.3 +/- 12.6 h, respectively) than in EP- (56.4 +/- 14.1 and 83.3 +/- 17.0 h, respectively) treated heifers. Experiment 2 compared the efficacy of EP versus EN in synchronizing FWE for superovulation in SMB-implanted cows. At random stages of the estrous cycle, Holstein cows (n = 78) received two SMB implants (Day 0) and were randomly assigned to receive EN on Day 0 or EP on Day 1. Folltropin-V treatments were initiated on the evening of Day 5, with PGF in the morning and evening of Day 8, when SMB were removed. Cows were inseminated after the onset of estrus and embryos were recovered 7 d later. Non-lactating cows had more CL (16.7 +/- 11.3 versus 8.3 +/- 4.9) and total ova/embryos (14.7 +/- 9.5 versus 7.9 +/- 4.6) than lactating cows (P < 0.05). EP-treated cows tended (P = 0.09) to yield more transferable embryos (5.6 +/- 5.2) than EN-treated cows (4.0 +/- 3.7). Experiment 3 compared the effect of dose of EV on ovarian follicle and CL growth profiles and synchrony of estrus and ovulation in CIDR-treated beef cows (n = 43). At random stages of the estrous cycle (Day 0), cows received a CIDR and no further treatment (Control), or an injection of 1, 2, or 5 mg im of EV. On Day 7, CIDR were removed and cows received PGF. Follicular wave emergence occurred within 7 d in 7/10 Control cows and 31/32 EV-treated cows (P < 0.05). In responding cows, interval from treatment to FWE was longer (P < 0.05) in those treated with 5 mg EV (4.8 +/- 1.2 d) than in those treated with 1 mg (3.2 +/- 0.9 d) or 2 mg (3.4 +/- 0.8 d) EV, while Control cows were intermediate (3.8 +/- 2.0 d). Diameter of the dominant follicle was smaller (P < 0.05) at CIDR removal and tended (P = 0.08) to be smaller just prior to ovulation in the 5 mg EV group (8.5 +/- 2.2 and 13.2 +/- 0.6 mm, respectively) than in the Control (11.8 +/- 4.6 and 15.5 +/- 2.9 mm, respectively) or 1mg EV (11.7 +/- 2.5 and 15.1 +/- 2.2 mm, respectively) groups, with the 2mg EV group (10.7 +/- 1.5 and 14.3 +/- 1.7 mm, respectively) intermediate. Diameter of the dominant follicle at CIDR removal was less variable (P < 0.01) in the 2 and 5mg EV groups than in the Control group, and intermediate in the 1mg EV group. In summary, treatment with 5mg EV resulted in a longer and more variable interval to follicular wave emergence than treatment with 5mg estradiol-17beta, which affected preovulatory dominant follicle size following progestin removal, and may have also affected superstimulatory response in Holstein cows. Additionally, 5 mg EV appeared to induce luteolysis in heifers, reducing the interval to ovulation following norgestomet removal. Conversely, intervals to, and synchrony of, follicular wave emergence, estrus and ovulation following treatment with 1 or 2 mg EV suggested that reduced doses of EV may be more useful for the synchronization of follicular wave emergence in progestogen-treated cattle.     

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.     

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.     

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

Treatment with GnRH at the onset of standing estrus increased pregnancy percentages and circulating concentrations of progesterone in repeat breeder dairy cows. The objective of this study was to determine the effect of treatment with GnRH at AI on concentrations of progesterone and conception rates in beef cattle that exhibited estrus. Two hundred ninety-three heifers at four locations were synchronized with the Select Synch plus CIDR protocol (given GnRH and a CIDR was placed into the vagina, and 7 d later, given PGF_2α and CIDR removed; n = 253) or the 14-19 melengestrol acetate (MGA) protocol (MGA fed at 0.5 mg/head/d for 14 d, with PGF_2α 19 d after MGA withdrawal n = 40) and AI was done after detection of estrus. At Location 1, blood samples were collected on Day 2, 4, 6, 10, 15, and 18 after AI (Day 0 = AI). Two hundred and fifty postpartum cows at two locations were synchronized with the Select Synch plus CIDR protocol, and AI was performed after detection of estrus. At AI, cattle were alternately assigned to one of two treatments: (1) treatment with GnRH (100 μg) at AI (n = 127 heifers and n = 108 cows); or (2) non-treated control (n = 120 heifers and n = 119 cows). Concentrations of progesterone tended to be greater in control heifers compared to GnRH-treated heifers on Days 6 (P = 0.08), 10 (P = 0.07), and 15 (P = 0.11). Overall conception rates were 68% and 66% for GnRH treated and control, respectively, and were not different between treatments (P = 0.72). In summary, treatment with GnRH at time of AI had no influence on conception rates in cattle that had exhibited estrus.     

Effect of treatment with estradiol valerate on endocrine changes and ovarian follicle populations in dairy cows

A linear-array ultrasound instrument was used to monitor the dynamics of follicular cyst formation following estradiol valerate (EV) administration in postpartum dairy cattle. Twelve cyclic cows were given two intramuscular (i.m.) injections of prostaglandin and F(2alpha) (PGF(2alpha)) 12 d apart to synchronize estrus. On Day 16 (Day 0 = day of estrus) six cows received 10 mg of EV in 1 ml sesame oil; the remaining six cows were treated with 1 ml sesame oil. The ovaries of all cows were scanned rectally each morning from Day 9 until 14 or 30 d post treatment. Plasma concentrations of luteinizing hormone (LH) and progesterone (P(4)) were also determined as objective indices of treatment effects. Day 0 to 16 ultrasound pictures of the ovaries of both control and treated cows were characterized by the presence of a corpus luteum (CL; 19 to 38 mm), several small follicles (<5 mm) and a medium-sized follicle (6 to 28 mm). Following treatment in control cows, the CL regressed gradually, and a preovulatory follicle was identifiable by Day 17 to 18, it increased in size and reached a maximum of 28 to 30 mm by Day 20 after ovulation and was identifiable throughout the rest of the cycle. Administration of 0 mg of EV resulted in a rapid reduction in the size of the CL. Growth of a large follicle was observed in all treated animals around Days 16 to 20, but having reached a maximum diameter of 12 to 24 mm it regressed without resulting in ovulation. Subsequent ultrasound pictures of EV-treated cows were characterized by the absence of a new CL and the presence of medium-sized persistent follicles. Estradiol valerate treatment induced early luteolysis (43 +/- 05 h post EV vs 101 +/- 22 h) and an LH surge (41 +/- 11 h vs 125 +/- 17 h).     

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.     

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.     

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.     

Effects of oestradiol and some of its esters on gonadotrophin release and ovarian follicular dynamics in CIDR-treated beef cattle

Three experiments were conducted to: (1) compare the effect of three oestradiol formulations on gonadotrophin release in ovariectomised cows; (2) compare the effects of either oestradiol-17beta (E-17beta) or oestradiol benzoate (EB), given at two doses, on the synchrony of ovarian follicular wave emergence in CIDR-treated beef cattle; and (3) determine the timing of ovulation of the dominant follicle of a synchronised follicular wave following administration of E-17beta or EB 24h after progesterone withdrawal. In Experiment 1, ovariectomised cows (n = 16) received a once-used CIDR on Day 0 (beginning of the experiment) and were allocated randomly to receive 5mg of E-17beta, EB or oestradiol valerate (EV) plus 100mg progesterone i.m. The CIDR inserts were removed on Day 7. There were effects of time, and a treatment-by-time interaction (P < 0.0001) for plasma concentrations of both oestradiol and FSH. Plasma oestradiol concentrations peaked 12h after treatment, with highest (P < 0.01) peak concentrations in cows given E-17beta; estradiol concentrations subsequently returned to baseline by 36 h in E-17beta-treated cows and by 96 h in EB- and EV-treated cows. Plasma FSH concentrations decreased by 12h after oestradiol treatment in all groups (P < 0.0001), reached a nadir at 24h, and increased by 60 h in all groups; plasma FSH reached higher (P < 0.02) concentrations in E-17beta-treated than in EB- or EV-treated cows. In Experiment 2, non-lactating Hereford cows (n = 29) received a new CIDR on Day 0 (beginning of the experiment), and were assigned randomly to receive 1 or 5mg of E-17beta or EB i.m. on Day 1. On Day 8, CIDR were removed and PGF was given. Transrectal ultrasonography was done once daily from 2 days before CIDR insertion to 2 days after CIDR removal, and then twice-daily to ovulation. Although there was no difference among groups in the interval from oestradiol treatment to follicular wave emergence (4.2 +/- 0.3 days; P = 0.5), 5mg of E-17beta resulted in the least variable interval to wave emergence (P < 0.005), compared with the other treatment groups which were not different (P = 0.1). For the interval from CIDR removal to ovulation, there were no differences among groups for either means (P = 0.5) or variances (P = 0.1). In Experiment 3, beef heifers (n = 32) received a once-used CIDR on Day 0 (beginning of the experiment) plus 100mg progesterone i.m. and were assigned randomly to receive 5mg E-17beta or 1mg EB i.m. On Day 7, CIDR were removed and all heifers received PGF. On Day 8 (24h after CIDR removal), each group was subdivided randomly to receive 1mg of either E-17beta or EB i.m. There was no effect of oestradiol formulation on interval from treatment to follicular wave emergence (4.1 +/- 0.2 days; P = 0.7) or on the median interval (76.6h; P = 0.7) or range (72-120 h; P = 0.08) from CIDR removal to ovulation. In summary, oestradiol treatments suppressed FSH in ovariectomised cows, with the duration of suppression dependent on the oestradiol formulation. Both E-17beta and EB effectively synchronised ovarian follicular wave emergence and ovulation in CIDR-treated cattle, and the interval from CIDR removal to ovulation did not differ in heifers given either E-17beta or EB 24h after CIDR removal.     

Plasma progesterone profiles and fertility status of anestrus Zebu cattle treated with norgestomet-estradiol-eCG regimen

Zebu cattle are notorious for poor fertility characterized by late maturity and long intercalving intervals attributed to a variety of factors, including genetic, nutritional and climatic. The aim of the present investigation, therefore, was to induce fertile estrus in acyclic pubertal heifers and postpartum anestrous Zebu cows by hormonal intervention. Pubertal Hariana and Sahiwal anestrous heifers (n=51) and postpartum cows (n=55) were either assigned a placebo (controls, N=6 for each breed and parity) or treated with 10-d norgestomet (3 mg) subcutaneous ear implants, with an initial injection of 3 mg, im norgestomet + 5 mg estradiol valerate, followed by 500 IU eCG at implant withdrawal (NOR-treated groups). Jugular venous plasma samples were obtained from a total of 28 animals (controls : 4 heifers and 4 cows; NOR-treated : 12 heifers and 8 cows) on Days 0 (implant insertion), 3, 7, 9 and Day 10 (implant withdrawal), every 12 h on Days 11 and 12, and then once daily on Days 17, 24 and 31. All the samples were assayed for progesterone. Almost all (97%) heifers and 81% cows were induced to estrus, the majority (92% heifers and 79% cows) within 120 h of implant removal. Synchrony of the induced estrus was better in cows, but interval to estrus and estrus duration were significantly longer in heifers (P<0.05). Post-treatment fertility, based on Day 28 nonretum rate, first service, and overall conception rates, was better in heifers (78.9, 60.5 and 73.7%, respectively) than cows (77.1, 48.6 and 62.9%, respectively), but the differences were significant only for the overall pregnancy rate (71.8% for heifers and 51.2% for cows; P<0.05). Low pre-treatment plasma progesterone values (<0.5ng/mL) were consistent with ovarian inactivity, confirming the true anestrus status of experimental animals. Controls failed to exhibit estrus and maintained low progesterone concentrations throughout the study. In treated animals, high progesterone values from Day 17 onwards suggested ovulatory estrus. These early luteal phase progesterone concentrations in nonpregnant (P=0.06) and nonpregnant, nonretum (P<0.05) animals were low in comparison with those of pregnant animals. Good fertility resulting from breeding according to estrus, inspite of variable intervals to estrus and estrus duration, advocates its advantage over fixed-time insemination in norgestomet-treated anestrous Zebu cattle.     

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.     

The use of estradiol and/or GnRH in a two-dose PGF protocol for breeding management of beef heifers

The objective was to determine reproductive performance following AI in beef heifers given estradiol to synchronize ovarian follicular wave emergence and estradiol or GnRH to synchronize ovulation in a two-dose PGF-based protocol. In Experiment 1, 561 cycling (confirmed by ultrasonography), Angus heifers received 500 microg cloprostenol, i.m. (PGF) twice, 14 days apart (days 0 and 14) and were equally allocated to four groups in a 2 x 2 factorial design. On Day 7, heifers received either 2 mg estradiol benzoate (EB) and 50 mg progesterone (P), i.m. in oil (EBP group) or no treatment (NT group). Half the heifers in each group received 1mg EB, i.m. in oil on Day 15 (24h after the second PGF treatment) with TAI 28 h later (52 h after PGF), and the other half received 100 microg GnRH, i.m. on Day 17 (72 h after PGF) concurrent with TAI. All heifers were observed for estrus twice daily from days 13 to 17; those detected in estrus more than 16 h before scheduled TAI were inseminated 4-16 h later and considered nonpregnant to TAI. Overall pregnancy rate (approximately 35 days after AI) was higher in heifers that received EBP than those that did not (61.6% versus 48.2%, respectively; P < 0.002); but was lower in heifers that received EB after PGF than those that received GnRH (50.0% versus 59.8%; P < 0.02). Although estrus was detected prior to TAI in 77 of 279 heifers (27.6%) treated with EBP (presumably due to induced luteolysis), they were inseminated and 53.2% became pregnant. Overall pregnancy rates were 51.4, 68.3, 45.0, and 55.0% in the NT/GnRH, EBP/GnRH, NT/EB, and EBP/EB groups, respectively (P < 0.05). In Experiment 2, 401 cycling, Angus heifers were used. The design was identical to Experiment 1, except that 1.5mg estradiol-17beta (E-17beta) plus 50mg progesterone (E-17betaP) and 1mg E-17beta were used in lieu of EBP and EB, respectively. All heifers receiving E-17beta 24h after the second injection of PGF (NT/E-17beta and E-17betaP/E-17beta) were TAI 28 h later without estrus detection, i.e. 52 h after PGF. Heifers in the other two groups received 100 microg GnRH, i.m. 72 h after PGF and were concurrently TAI; heifers in these two groups that were detected in estrus prior to this time were inseminated 4-12h later and considered nonpregnant to TAI. Estrus rate during the first 72 h after the second PGF treatment was higher (P < 0.05) in the E-17betaP/GnRH group (45.0%; n = 100) than in the NT/GnRH group (16.0%; n = 100), but conception rate following estrus detection and AI was not different (mean, 57.4%; P = 0.50). Overall pregnancy rate was not significantly different among groups (mean, 46.9%; P = 0.32). In summary, the use of EB or E-17beta to synchronize follicular wave emergence and estradiol or GnRH to synchronize ovulation in a two-dose, PGF-based protocol resulted in acceptable fertility to TAI. However, when 2mg EB was used to synchronize follicular wave emergence, early estrus occurred in approximately 28% of heifers, necessitating additional estrus detection. A combination of estrus detection and timed-AI in a two-dose PGF protocol resulted in highly acceptable pregnancy rates.     

Effect of a CIDR insert and flunixin meglumine, administered at the time of embryo transfer, on pregnancy rate and resynchronization of estrus in beef cattle

The objectives of this study were to evaluate the effects of flunixin meglumine (FM), an inhibitor of PGF(2alpha) synthesis, and insertion of an intravaginal progesterone-releasing device (CIDR), on pregnancy rates in beef cattle embryo transfer (ET) recipients, and to examine the effect of a CIDR after embryo transfer on the synchrony of the return to estrus in non-pregnant recipients. Cows (n=622) and heifers (n=90) at three locations were assigned randomly to one of four groups in a 2x2 factorial arrangement of treatments with FM administration (500 mg i.m.) 2-12 min prior to ET, and insertion of a CIDR (1.38 g progesterone) immediately following ET as main effects. Fresh or frozen embryos (Stage=4 or 5; Grade=1 or 2) were transferred on Days 6-9 of the estrous cycle and CIDR devices were removed 13 days after ET. Recipients at Location 2 only were observed for signs of return to estrus. Recipients that returned to estrus at Location 2 were either bred by AI or received an embryo 7 days after estrus. Following the initial ET, there was an FMxlocation interaction on pregnancy rate (P<0.01; Location 1, 89% versus 57%; Location 2, 69% versus 64%; Location 3, 64% versus 67% for FM versus no FM, respectively). Pregnancy rates of embryo recipients were not affected by CIDR administration (P>0.05; 65% with CIDR, 70% without CIDR), however, the timing of the return to estrus was more synchronous (P<0.01) for recipients given a CIDR. Pregnancy rate of recipients bred following a return to estrus did not differ between cows receiving or not receiving a CIDR for resynchronization (P>0.13). Effects of FM on pregnancy rate were location dependent and CIDR insertion at ET improved synchrony of the return to estrus.