Response to GnRH on day 6 of the estrous cycle is diminished as the percentage of Bos indicus breeding increases in Angus, Brangus, and Brahman x Angus heifers

Angus (n=6), Brangus (5/8 Angus x 3/8 Brahman, n=6), and Brahman x Angus (3/8 Angus x 5/8 Brahman, n=6) heifers exhibiting estrous cycles at regular intervals were used to determine if the percentage of Bos indicus breeding influenced the secretory patterns of LH in response to a GnRH treatment on Day 6 of the estrous cycle. Heifers were pre-synchronized with a two-injection PGF(2 alpha) protocol (25 mg i.m. Day -14 and 12.5 mg i.m. Day -3 and -2 of experiment). Heifers received 100 microg GnRH i.m. on Day 6 of the subsequent estrous cycle. Blood samples were collected at -60, -30, and -1 min before GnRH and 15, 30, 60, 90, 120, 150, 180, 240, 300, 360, 420, and 480 min after GnRH to determine concentrations of serum LH. Estradiol concentrations were determined at -60, -30, and -1 min before GnRH. On Day 6 and 8, ovaries were examined by ultrasonography to determine if ovulation occurred. On Day 13, heifers received 25 mg PGF(2 alpha) i.m. and blood samples were collected daily until either the expression of estrus or Day 20 for heifers not exhibiting estrus to determine progesterone concentrations. There was no effect (P>0.10) of breed on ovulation rate to GnRH as well as size of the largest follicle, mean estradiol, and mean corpus luteum volume at GnRH. Mean LH was greater (P<0.05) for Angus (7.0+/-0.8 ng/mL) compared to Brangus (4.6+/-0.8 ng/mL) and Brahman x Angus (2.9+/-0.8 ng/mL), which were similar (P>0.10). Mean LH peak-height was similar (P>0.10) for Brangus (13.9+/-3.4 ng/mL) compared to Angus (21.9+/-3.4 ng/mL) and Brahman x Angus (8.0+/-3.4 ng/mL), but was greater (P<0.05) for Angus compared to Brahman x Angus. Interval from GnRH to LH peak was similar (P>0.10) between breeds. As the percentage of Bos indicus breeding increased the amount of LH released in response to GnRH on Day 6 of the estrous cycle decreased.     

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.     

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.     

Effect of alfaprostol, lasalocid, and once-daily suckling on postpartum interval in Brahman and Brahman crossbred cattle

Brahman cows (n = 49) and primiparous heifers (n = 11), Brahman x Hereford primiparous F1 heifers (n = 86) and Simmental x Brahman primiparous F1 heifers (n = 13) were randomly allotted by breed, age and date of calving to one of eight treatment groups: 1) control; 2) once-daily suckling; 3) lasalocid (200 mg/hd/d); 4) alfaprostol (5 mg intermuscular injections on Days 21 and 32 post partum); 5) lasalocid + once-daily suckling; 6) alfaprostol + once daily suckling; 7) alfaprostol + lasalocid; 8) alfaprostol + lasalocid + once daily suckling. All animals received 2.3 kg/hd/d of a concentrate (6 corn : 1 cottonseed meal) and lasalocid was mixed and fed in the concentrate. Body weights and condition scores were taken on Day 1 post partum and every 28 d thereafter. All animals were maintained with sterile marker bulls with Brahman and Simmental x Brahman cattle artificially inseminated at first estrus. Blood samples were collected at weekly intervals starting on Day 21 post partum until estrus and at nine to twelve days post estrus when the ovaries were palpated for corpora lutea. After the first postpartum estrus with a corpora lutea, cows were placed with fertile bulls. Mean serum progesterone concentrations were below 0.5 ng/ml prior to treatment. Calf weight gains to 90 d were not affected by age (P > 0.10) but were lower in the once-daily suckling group (P < 0.05). Treatment did not affect cow weight or condition score (P > 0.10). Cows had a shorter postpartum interval (P < 0.0001) than heifers. Once-daily suckling shortened postpartum interval (P < 0.0001) and positively influenced the cumulative frequency of return to estrus by 40 d post partum (P < 0.02). Alfaprostol did not affect postpartum interval (P > 0.10) but did increase the cumulative frequency of return to estrus by 90 d post partum (P < 0.03). Lasalocid did not affect postpartum interval or cumulative frequency of return to estrus (P > 0.10). Both once-daily suckling and alfaprostol were effective in increasing the numbers of animals inseminated by 90 d post partum. The once-daily suckling + alfaprostol treatment resulted in the shortest postpartum interval.     

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.     

Effects of immunization against GnRH, melengestrol acetate, and a trenbolene acetate/estradiol implant on growth and carcass characteristics of beef heifers

A 2 x 3 factorial experiment was conducted to determine the effects of an implant (trenbolene acetate/estradiol or no implant) and method of estrus suppression (immunization against GnRH, melengestrol acetate, or no suppression) on growth performance and carcass characteristics of heifers fed for slaughter. At the start of a 21-d feed adaption phase, crossbred beef heifers (n = 144, 390+/-2.8 kg) were given their first dose of an anti-GnRH vaccine or started on melengestrol acetate (MGA). Thereafter, heifers were fed a high-concentrate diet (78% barley grain) for 84 d (Days 0 to 83), received implants on Day 0, a second vaccination on Day 21, and were slaughtered on Days 84 or 85. Implanting increased average daily gain (1.72 vs 1.50 kg/d, P < 0.01), feed efficiency (6.02 vs 6.75 kg dry matter intake/kg gain, P < 0.01), preslaughter weight (532 vs 513 kg, P < 0.01), carcass weight (301 vs 289 kg, P < 0.01), and ribeye area (88.6 vs 85.9 cm2, P < 0.05), but had no affect (P > 0.05) on dry matter intake, grade fat thickness, marbling score, or lean yield. Compared to heifers fed MGA, those immunized against GnRH had a greater ribeye area (90.0 vs 84.6 cm2) and lean yield (63 vs 61%), and had thinner grade fat (7.5 vs 8.6 mm; P < 0.05 for each). Furthermore, immunized heifers had lower (P < 0.001) plasma progesterone concentrations than control heifers on Days 42, 63 and 83. Heifers fed MGA had less estrus mounting activity (P < 0.05) and lower plasma progesterone concentrations (P < 0.001) than the remaining heifers. Method of estrus suppression did not affect (P > 0.05) preslaughter weight, average daily gain, dry matter intake, feed efficiency, carcass weight, or marbling score. In conclusion, implanting significantly increased growth performance and preslaughter and carcass weights. Compared to heifers fed MGA, immunization against GnRH significantly increased ribeye area and lean yield, and reduced grade fat thickness     

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.     

Comparison of four synchronization protocols for fixed-time bovine embryo transfer in Bos indicus x Bos taurus recipients

The objective was to evaluate the effects of 400 IU of eCG given on Days 5 or 8 of an estrus synchronization protocol with progesterone-releasing intravaginal devices (PRID) and estradiol benzoate (EB), in recipients for fixed-time embryo transfer. A secondary objective was to determine the effects of injectable progesterone (given concurrent with EB treatment). Three-hundred-and-four crossbred Bos taurus x Bos indicus beef heifers were randomly assigned to one of four treatment groups (2 x 2 factorial design). At unknown stages of the estrous cycle (Day 0), all heifers received a progesterone-releasing intravaginal device (PRID), plus 2mg of EB i.m., with or without a concurrent treatment of 50mg of progesterone i.m. Heifers were further subdivided to receive 0.15 mg of d-cloprostenol (PGF) i.m. and 400 IU of eCG i.m. on Days 5 or 8. In all heifers, intravaginal devices were removed on Day 8 and 1mg of EB was given i.m. on Day 9 (Day 10 was arbitrarily considered the day of estrus). On Day 17, all heifers with >1 CL or a single CL with a diameter > or =18 mm (based on ultrasonographic examination), received an in vitro produced (IVP) embryo by non-surgical transfer. On Day 17, there was an effect of day of eCG administration on the number of CL (1.35 +/- 0.08 versus 1.13 +/- 0.04, for Day 5 versus Day 8, respectively; P = 0.02) and (in a subset of 154 heifers) mean (+/-S.E.M.) plasma progesterone concentrations (2.41 +/- 0.26 versus 1.74 +/- 0.19 ng/mL; P = 0.03). Although the proportion of recipients transferred/treated and pregnant/transferred did not differ among groups, the proportion of recipients pregnant/treated tended (P = 0.1) to be higher in heifers treated with eCG on Day 5 versus Day 8 (47.0% versus 40.7%, respectively). Progesterone treatment had no significant effect. In conclusion, treatment with eCG (and D-cloprostenol) on Day 5 significantly increased the number of CL and plasma progesterone concentrations and tended to increase pregnancy rates, although progesterone treatment had no significant effect.     

Effect of early luteolysis in progesterone-based timed AI protocols in Bos indicus, Bos indicus x Bos taurus, and Bos taurus heifers

The objective of this study was to evaluate the effects of treatment with an intravaginal progesterone-releasing device (CIDR) and estradiol benzoate (EB) on follicular dynamics in Bos indicus (n=23), Bos taurus (n=25), and cross-bred (n=23) heifers. To assess the influence of reduced serum progesterone concentrations during 8 days of treatment with a progesterone-releasing device on follicular dynamics, half of the heifers received PGF at CIDR insertion (Day 0; 3 x 2 factorial design). Mean (+/-S.E.M.) serum progesterone concentrations during CIDR treatment varied (P<0.05) among genetic groups: B. indicus (5.4+/-0.1 ng/mL), B. taurus (3.3+/-0.0 ng/mL), and cross-bred (4.3+/-0.1 ng/mL). Maximum diameter of the dominant follicle (DF) was smaller (P<0.01) in B. indicus heifers (9.5+/-0.5 mm) than in cross-bred (12.3+/-0.4 mm) or B. taurus heifers (11.6+/-0.5 mm). B. indicus experienced lower (P<0.01) ovulation rate (39.1%) than did B. taurus (72.7%) and cross-bred (84.0%). Heifers treated with PGF on Day 0 had lower (P<0.05) serum progesterone concentrations during progesterone treatment. The PGF treatment on Day 0 increased (P<0.01) the diameter of the DF (11.9+/-0.4 mm vs. 10.5+/-0.4 mm). Moreover, greater (P=0.02) ovulation rates (78.8 vs. 54.0%) occurred in heifers treated with PGF on Day 0. In summary, B. indicus heifers had greater serum progesterone concentrations, smaller DF diameter, and a lower ovulation rate compared to B. taurus heifers. Prostaglandin treatment on the day of CIDR insertion reduced serum progesterone during treatment, and resulted in increased maximum DF diameter and ovulation rate.     

Evaluation of two progestogen-based estrous synchronization protocols in yearling heifers of Bos indicus × Bos taurus breeding

Yearling Bos indicus × Bos taurus heifers (n = 410) from three locations, were synchronized with either the Select Synch/CIDR+timed-AI (SSC+TAI) or 7-11+timed-AI (7-11+TAI) treatments. On Day 0 of the experiment, within each location, heifers were equally distributed to treatments by reproductive tract score (RTS; Scale 1-5: 1 = immature, 5 = estrous cycling) and body condition score. The 7-11+TAI treatment consisted of melengestrol acetate (0.5 mg/head/d) from Days 0 to 7, with PGF_2α (25 mg im) on Day 7, GnRH (100 μg im) on Day 11, and PGF_2α (25 mg im) on Day 18. The SSC+TAI heifers received the same carrier supplement (without MGA) from Days 0 to 7, and on Day 11 they were given 100 μg GnRH and an intravaginal CIDR (containing 1.38 g progesterone). The CIDR were removed on Day 18, concurrent with 25 mg PGF_2α im For both treatments, estrus was visually detected for 1 h twice daily (0700 and 1600 h) for 72 h after PGF_2α, with AI done 6 to 12 h after a detected estrus. Non-responders were timed-AI and received GnRH (100 μg im) 72 to 76 h post PGF_2α. The 7-11+TAI heifers had a greater (P < 0.05) estrous response (55.2 vs 41.9%), conception rate (47.0 vs 31.3%), and synchronized pregnancy rate (33.5 vs 24.8%) compared to SSC+TAI heifers, respectively. Heifers exhibiting estrus at 60 h (61.7%) had a greater (P < 0.05) conception rate compared to heifers that exhibited estrus at ≤ 36 (35.3%), 48 (31.6%), and 72 h (36.2%), which were similar (P > 0.05) to each other. As RTS increased from ≤ 2 to ≥ 3, estrous response, conception rate, synchronized pregnancy rate, and 30 d pregnancy rate all increased (P < 0.05), irrespective of synchronization treatment. In conclusion, the 7-11+TAI treatment yielded greater synchronized pregnancy rates compared to SSC+TAI treatment in yearling Bos indicus × Bos taurus heifers.     

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.     

Ovulation, ovarian function, and reproductive performance after treatments with GnRH in postpartum suckled cows

Two experiments were conducted to determine whether treatments with gonadotropin releasing hormone (GnRH) during the early postpartum period in suckled cows would induce ovulation and initiate regular estrous cycles. In Experiment I, 0, 100 or 200mug of GnRH was given to 22 suckled Angus x Holstein cows at three and again at five weeks postpartum. Serum luteinizing hormone (LH) responses did not differ between cows given 100 or 200mug of GnRH. Treatment with GnRH tended to increase the percentage of cows exhibiting estrus by 30 and 60 days postpartum, but reproductive performance during the breeding season did not differ among groups. In Experiment II, 70 suckled Hereford cows were given either no treatment or 200mug of GnRH at 7 weeks postpartum. Cows given GnRH received either no treatment prior to GnRH or were separated from their calves for 24 hr prior to GnRH treatment. Half of the cows that were separated from their calves also received progesterone via a progesterone intravaginal device (PRID) for 12 days prior to calf removal. Treatment with GnRH alone tended to increase the percentage of anestrous cows which ovulated by 8 days after treatment. Calf removal did not increase the ovulatory response to GnRH, but PRID treatment did. More estrous periods were detected in GnRH-treated cows than in control cows during 20 days after GnRH treatment.     

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).     

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 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.     

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.     

Effect of estrus synchronization with Syncro-Mate-B((R)) on serum luteinizing hormone, progesterone and conception rate in Brahman heifers

Twenty-two estrous cyclic, 2-yr-old Brahman heifers were randomly assigned to receive either estrus synchronization with Syncro-Mate-B((R)) (SMB; 11) or no treatment (Control; 11). Blood samples were collected via tail vessel puncture at onset of estrus and daily thereafter until Day 11 after estrus. Blood samples were also collected from five SMB and five Control heifers at 0, 4, 8 and 12 h after the onset of estrus. All samples were processed to yield serum and stored at -20 degrees C until radioimmunoassay. Heifers were inseminated by one technician using semen from a single ejaculate of a Brahman bull 12 h after the onset of estrus. All SMB heifers exhibited estrus within 72 h of implant removal. All heifers had corpora lutea (CL) detected by rectal examination 8 to 12 d following estrus. Serum luteinizing hormone (LH) was not affected by treatment, time (4 - h intervals) or an interaction of treatment by time (P > 0.10). Independent analysis with h indicated that at h 12, SMB (2.2 +/- 0.06 ng/ml) had lower LH than did control heifers (8.9 +/- 2.1 ng/ml). Serum progesterone increased from Day 1 through Day 12 in all heifers, which is indicative of functional CL. Serum progesterone was affected by treatment (P < 0.0001) and time (d intervals; P < 0.10). Progesterone elevation was lower (P < 0.05) and area under the progesterone curve was lower (P < 0.03) in SMB (5.6 +/- 0.5 ng/ml, 32.0 +/- 4.5 units, respectively) when compared with control heifers (7.0 +/- 4 ng/ml, 43.7 +/- 2.4 units, respectively). Conception rate was lower (P < 0.01) in SMB heifers (2 of 11) than in control heifers (8 of 11). The lowered conception rate in SMB treated Brahman heifers may be due to altered timing of LH release following estrus, resulting in an altered time of ovulation.     

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.     

Pubertal traits and seasonal variation of the sexual activity in Brahman, Hereford and crossbred heifers

Age and weight at puberty and the pattern of ovulatory activity from puberty to 26 mo of age were studied in a total of 55 Brahman (B), Hereford (H), 5/8B-3/8H (B(5)H(3)) and 5/16B-11/16H (B(5)H(11)) heifers. The trial was conducted over two years. Heifers were kept during the whole experimental period on a paddock of native pasture at a stocking rate of 0.5 animals per hectare. Vasectomized bulls were used for estrus detection and blood was collected from the heifers twice weekly to monitor ovulatory activity through progesterone level as determinated by RIA. In addition to age and weight at puberty, the weights and dates at the time of cessation and resumption of ovulatory activity were also considered. There were significant effects (P<0.05 or P<0.01) of genetic group (purebreds vs crossbreds) for age and weight at puberty; the difference being 60 d and 18 kg, respectively, favoring the crossbred groups. After puberty attainment, an anestrous period was detected in all Brahman (118.6 d) and B(5)H(3) (113.4 d) heifers; anestrus was observed in only 14% of B(5)H(11) heifers and for a shorter (43.5 days) period. Hereford heifers did not exhibit interruption of ovulatory activity. In Brahman and B(5)H(3) heifers, the pattern of interruption and resumption of estrus and ovulatory activity correlated closely (0.89 and 0.95 for Brahman and B(5)H(3) heifers, respectively) with the photoperiod curve, suggesting the probable influence of this environmental factor on the regulation of estrous cycle activity in heifers with a higher proportion of Bos indicus genes. Thus, the genetic factor needs be taken into account when establishing programs in reproductive management.     

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.