Fed Melengestrol Acetate Research Articles

142 Reproductive performance of land race hair sheep ewes in different breeding seasons under a semi-continuous production

Abstract A system for semi-continuous lamb production was evaluated using Barbados Blackbelly and St. Croix hair sheep ewes under 8-mo accelerated mating. Ewes were managed in a forage-based production system using rotational grazing. Two flocks of ewes (n = 30–35), equally representing the breeds, were bred 4 mo apart. Ewes were synchronized by feeding melengestrol acetate for 10 d, and mated in two single sire groups to like breed sires in November (NOV), March (MAR), and July (JUL) in a 25-d mating period. Pregnancy was recorded by transrectal ultrasound 22 d after breeding. Ewes were supplemented with corn/soybean meal during late gestation (0.75% of BW) and during lactation (1.5% of BW). They lambed on pasture with limited supervision, and lambs were weaned at 2 mo of age. Lambs had access to the supplement provided to ewes. Ewe performance (pregnancy rate, litter size, lamb survival, and birth and weaning weight) was recorded during a 2 yr production cycle, and analyzed for the effect of mating season. Pregnancy rate was not statistically different (P > 0.1) between season, and ranged from 93.9% in NOV, to 85.9 and 86.6% in MAR and JUL. Litter size (lambs born/ewe lambing) was higher (P < 0.05) in NOV (2.09) than MAR (1.68) and JUL (1.82). Lamb survival at birth was reduced (P < 0.001) in JUL (86.5%) compared to NOV (99%) and MAR (100 %), but was not different between seasons for survival to weaning (86 to 90%). Lamb birth weights were higher (P < 0.05) in MAR (3.16 kg) than JUL (2.99 kg), with NOV (3.05 kg) intermediate, while lamb weaning weights were higher (P < 0.001) in NOV (13.6 kg) compared with MAR (11.9 kg) and JUL (11.4 kg) mating. Data suggest that dual flock, accelerated mating of landrace hair sheep ewes can produce lamb cohorts of similar size in 4 mo intervals under a low-input, forage-based production system.

Effect of delayed insemination of nonestrus beef heifers in a melengestrol acetate–prostaglandin F2α timed artificial insemination protocol

ABSTRACT A study evaluated a 19-h-delayed AI following GnRH administration in a hybrid estrus detection and timed AI protocol. Angus-based, crossbred heifers (n = 453) managed at the Kelly Ranch (Sutherland, NE) or the University of Nebraska West Central Research and Extension Center (North Platte, NE) were synchronized using the melengestrol acetate–PGF2α protocol. Heifers were fed melengestrol acetate for 14 d (d 1 through d 14). Nineteen days later, on d 33, heifers received a PGF2α injection. Heifers (n = 319) expressing estrus were assigned to the first treatment group (EST), were removed from the herd, and received AI 12 h later. Seventy-two hours following the PGF2α injection, heifers not detected in estrus were administered GnRH and randomly assigned to 1 of 2 remaining treatment groups: immediate AI (n = 70, GNRH-I) or AI 19 ± 1 h following GnRH injection (n = 64, GNRH-D). Heifers artificially inseminated that had exhibited estrus had greater (P

Timed insemination of beef heifers using the 7–11 Synch protocol1

Two experiments were conducted over 3 yr to determine pregnancy rates in beef heifers after a timed AI in response to the 7-11 Synch protocol. In Exp. 1, 179 heifers were either fed melengestrol acetate (MGA; 7-11 Synch) or given an intravaginal progesterone (P4)-releasing insert [controlled intravaginal drug releasing device (CIDR); 7-11 CIDR] for 7 d. Prostaglandin F2αwas administered on the last day of MGA feeding or at CIDR removal followed by the CO-Synch protocol (GnRH-PGF2α-GnRH) beginning 4 d after MGA withdrawal or 2 d after CIDR removal. Heifers received a timed AI with GnRH beginning 48 h after the second PGF2α. Blood samples were collected at d -10, 0 (start of MGA feeding), and 18 (second PGF2α injection). In Exp. 2, 298 beef heifers were treated with the 7-11 Synch protocol with (7-11 Synch) or without (7 Synch) the first GnRH injection. Fixed time AI and GnRH was given 54 h after PGF2α. Blood samples were collected at d -10 and 0 in yr 1 and d -10, 0, 18 and at AI in yr 2. In Exp. 1, no differences were detected between 7 and 11 Synch and 7-11 CIDR for attainment of puberty in noncyclic heifers (94 vs. 78%; P = 0.21), the proportion of heifers that had luteal tissue on d 18 (87 vs. 83%; P = 0.41), or pregnancy rates after timed AI (47 vs. 46%; P = 0.99). In Exp. 2, administration of GnRH 4 d after the last MGA (7-11 Synch) feeding tended (P = 0.07) to induce more prepubertal heifers to cycle (88 vs. 61%) and increased (P < 0.01) the proportion of heifers with luteal tissue on d 18 (88 vs. 72%) compared with heifers in the 7 Synch treatment. Pregnancy rates after the 54 h timed AI were greater (P < 0.01) in the 7-11 Synch treatment (55%) than in the 7 Synch (38%). We conclude that heifer pregnancy rates did not differ whether feeding MGA for 7 d or applying a CIDR insert for 7 d before a CO-Synch protocol. In contrast, use of GnRH at the beginning of the CO-Synch protocol improved pregnancy rates after a timed AI by inducing more prepubertal heifers to ovulate and increasing the proportion of heifers with luteal tissue at the PGF2α injection.

MGA and growth promotants administered to beef feedlot heifers have no effect on subsequent oocyte quality or in vitro embryo production (2012)

Introduction Beef feedlot heifers have the potential to serve as viable donors of oocytes post-slaughter for in vitro embryo production. Oocyte quality is a critical factor affecting the success of in vitro embryo production and can be influenced by factors such as age and reproductive status, ovarian follicle size, and nutritional status of the donor female. In a conventional feedlot setting, heifers are typically administered steroid-based growth promotants and fed melengestrol acetate (MGA) for suppression of estrus, which increases circulating concentrations of reproductive steroids, particularly estradiol. The effects of these management practices on oocyte quality and numbers are unknown. The purpose of this study was to compare oocytes harvested from traditionally managed beef feedlot heifers implanted with growth promotants and fed MGA with oocytes from heifers given neither MGA nor growth promotants, and to evaluate potential effects of these feedlot management practices on early embryo development.

A Comparison of Melengestrol Acetate Fed at Two Dose Levels to Feedlot Heifers

A randomized complete block design was used to compare the effects of feeding melengestrol acetate (MGA) at 0.4 (0.4M) and 0.5 (0.5M) mg/d per head on feedlot performance, estrus activity, and carcass characteristics of commercial beef heifers (n = 1,418; 10 pens/treatment). Within 48 h of arrival at the research site, heifers were assigned to treatment, processed according to feedlot protocol, and administered an abortifacient. After adaptation to a 95% concentrate diet, MGA was supplied at either 0.4 or 0.5 mg/head daily. Estrus activity was monitored twice daily and summarized as a count of heifers showing estrus within a pen over each 21-d interval throughout the study. Dry matter intake, ADG, G:F, and all other carcass measurements were not different (P > 0.10) between treatments. Overall treatment effects were different only for estrus activity (P = 0.03; 3.2 vs. 2.1% for 0.4M and 0.5M, respectively) and tended to be different for the percentage of dark-cutting carcasses (P = 0.10; 3.0 vs. 1.7% for 0.4M and 0.5M, respectively). Results of this study showed little difference between treatment groups for performance and carcass characteristics of feedlot heifers. The decrease in estrus activity and percentage of dark-cutting carcasses, however, may suggest an economic advantage of feeding a higher level of MGA to finishing heifers.

Effect of Feeding Melengestrol Acetate, Monensin, and Tylosin on Performance, Carcass Measurements, and Liver Abscesses of Feedlot Heifers

Approximately 4,000 yearling heifers (initial BW = 306 ± 1.9 kg) were randomly allotted into 4 treatment groups to determine the effect of melengestrol acetate (MGA) on feedlot performance and carcass parameters. Treatments included a diet containing monensin (Rumensin) and tylosin (Tylan; RT) fed during the entire feeding period; RT plus MGA (RTM) also fed during the entire feeding period; RT withdrawn 35 d preslaughter (RTwd); and RTM withdrawn 35 d preslaughter (RTMwd). All cattle were implanted at arrival with Revalor-IH and reimplanted with Revalor-200, and were fed a standard feedlot finishing diet (that met or exceeded 1996 NRC requirements) for approximately 150 d. Final BW, ADG, hot carcass weight (HCW), DMI, and percentage of cattle grading Choice were greater (P < 0.05) for RTM than for all other treatments. Withdrawal of MGA (RTMwd) significantly (P < 0.01) decreased ADG, HCW, 12th-rib fat thickness, calculated YG, and percentage of carcasses grading Choice, but significantly increased the percentage of dark-cutting carcasses, LM area, and YG 1, 2, or 3 carcasses compared with RTM. There were no significant differences (except for dressing percentage) between RT and RTwd. Heifers continuously fed MGA increased in ADG, HCW, and carcass quality traits of economic importance compared with other treatments. Withdrawal of MGA, Rumensin, and Tylan 35 d preslaughter adversely affected carcass weight and carcass quality traits of economic importance when compared with a feeding program including MGA, Rumensin, and Tylan.

Use of Gonadotropin-Releasing Hormone at Fixed-Time Artificial Insemination at Eighty or Ninety-Seven Hours Post Prostaglandin F2α in Beef Cows Administered the Long-Term Melengestrol Acetate Select Synch

<h2>ABSTRACT</h2> In a 2-yr trial, the pregnancy response of 549 Angus-based cows (BCS 5.1 ± 0.56, postpartum interval 69.7 ± 19.1 d) was evaluated when estrous cycles were synchronized by feeding melengestrol acetate for 14 d with gonadotropin-releasing hormone (GnRH) given on d 26 and prostaglandin F_2α (PG) on d 33. Of all cows, those observed in estrus within 72h after PG were inseminated 9.5 ± 4.01h after estrus was first observed. The remaining cows were fixed-time inseminated (TAI) and randomly assigned to receive or not receive GnRH at TAI, and randomly assigned to be inseminated at 79.7 ± 2.26h (early) or 96.6 ± 1.94h (late) post-PG. There were no GnRH × year (<i>P</i> = 0.47) or GnRH × time of TAI (<i>P</i> = 0.96) interactions. Cows that received GnRH at TAI had a greater (<i>P</i> < 0.001) pregnancy rate than those receiving no GnRH. There was a tendency (<i>P</i> = 0.07) for a year × time of TAI interaction. In yr 1, late TAI cows had a greater (<i>P</i> = 0.01) pregnancy rate than early cows; however, in yr 2, pregnancy rate was not different (<i>P</i> > 0.10) between early and late cows. Results indicate that within the long-term melengestrol acetate Select Synch protocol, pregnancy rate of cows not observed in estrus within 72h of PG could be increased by adding a second GnRH injection at TAI, whereas the effect of delaying time of TAI until 97h post-PG on pregnancy rate was inconsistent.

Effects of Ractopamine (Optaflexx) Fed in Combination with Melengestrol Acetate on Feedlot Heifer Performance

Two commercial feedlot experiments were conducted to determine the effects of feeding melengestrol acetate (MGA) or MGA plus ractopamine (MGA+OPT) on the performance and carcass characteristics of finishing heifers. In Nebraska (Exp. 1), 1,807 heifers (337.3 ± 20.0 kg) and in Texas (Exp. 2), 1,964 heifers (331.5 ± 6.1 kg) were fed 0.4 mg of MGA daily. For heifers fed MGA+OPT, 200 mg of ractopamine was fed daily the last 29 (Exp. 2) or 36 d (Exp. 1). Live and carcass-adjusted performance data were collected. On a carcass-adjusted basis, G:F for the entire feeding period was improved (P 0.47) between treatments in Exp. 1. Carcasses from heifers fed MGA+OPT had decreased marbling scores (P = 0.01) and greater LM area (P = 0.01) than carcasses from heifers fed MGA in Exp. 2. In Exp. 1, in which G:F was improved by 8.1%, no effect on QG was observed. In Exp. 2, in which G:F was improved by 27.2%, QG decreased. Based on these results, feeding MGA+OPT increased ADG and improved G:F, with variable effects on carcass characteristics.

Utilization of soybeans or corn milling by-products in beef heifer development diets1,2

Whole raw soybeans (SB), wet corn gluten feed (WCGF), and corn dried distillers grains (DDG) are sources of protein in heifer development rations. The objectives of this study were to compare puberty status before synchronization of estrus, response to synchronization, and AI and final pregnancy rates in heifers developed on diets containing SB, WCGF, or DDG that were formulated to be similar in energy and CP. These ingredients vary substantially in fat content, which may affect reproductive performance. Rate of gain during the feeding period and post-AI performance were also compared. In a preliminary experiment, 104 crossbred heifers were fed diets containing either 1.25 kg of SB/d or 2.0 kg of WCGF/d for 110 d (DM basis), beginning at 10 mo of age. In Exp. 1, 100 crossbred heifers received either 1.25 kg of SB/d or 2.5 kg of WCGF/d from approximately 7 to 10 mo of age (91 d; 4 pens/diet), and then were fed 1.25 kg of SB/d for an additional 114 d (4 pens/diet). In Exp. 2, 1.25 kg of SB/d or 1.25 kg of DDG/d was fed to 100 crossbred heifers for 226 d, beginning at 6 mo of age (4 pens/diet). At approximately 13 mo of age, heifers were fed melengestrol acetate (0.5 mg/d) for 14 d, followed by an i.m. injection of PGF(2 alpha) (25 mg) 19 d later to synchronize estrus. Heifers (14 mo of age) received AI for 5 d after PGF(2 alpha), at which time the dietary treatments were ended. Heifers were commingled while grazing on native pasture and were exposed to bulls for approximately 60 d beginning 10 d after the last day of AI. Pregnancy to AI was determined by ultrasound 45 d after the last day of AI. Heifers fed SB in the preliminary experiment had a lower (P < 0.05) synchronization rate (81 vs. 96%) and longer interval (P = 0.05) from PGF(2 alpha) to estrus (76.6 vs. 69.2 h) compared with heifers fed WCGF. In Exp. 1, the age at which the heifers were begun on SB diets did not alter (P > 0.10) the synchronization rate (79%) or timing of estrus after PGF(2 alpha) (77.8 h). In Exp. 2, the synchronization rate (86%) and timing of estrus after PGF(2 alpha) (69.3 h) did not differ (P > 0.10) because of diet. No differences (P > 0.10) were due to diet for AI conception rates (overall mean for each experiment: 76.5, 60, and 68.5%), percentage of all heifers becoming pregnant to AI (67, 46, and 59%), or final pregnancy rates (92, 90, and 90%) in the preliminary experiment, Exp. 1, or Exp. 2, respectively. In summary, SB, DDG, and WCGF can be used as sources of protein in heifer development diets at the inclusion rates used in these studies.

A Meta-Analysis Evaluation of Feeding Melengestrol Acetate to Feedlot Heifers Implanted with Estradiol, Trenbolone Acetate, or the Combination of Estradiol and Trenbolone Acetate

AbstractMixed models were used to study feeding melengestrol acetate (MGA) to feedlot heifers implanted with an estrogenic compound (EST), trenbolone acetate (TBA), or the combination of EST and TBA. One hundred one treatment means from 18 research trials were included in the analyses. Estrogenic implants increased DMI (p < 0.01) and LM area (P = 0.08). Estrogenic implant or MGA had no effect on percentage of carcasses grading low Choice or greater. Heifers implanted with TBA had greater (p < 0.05) final BW, ADG, hot carcass weight (HCW), and lower feed-togain ratio (FG) and YG. Trenbolone acetate tended to reduce the percentage YG 4 and 5 carcasses (P = 0.14) and percentage of carcasses grading low Choice or greater (P = 0.15). Interactions between MGA and EST were important (p < 0.01) for final BW, ADG, FG, and HCW, suggesting that the effect of MGA on these measurements depended upon EST implant. For heifers not implanted with EST, MGA improved (p < 0.01) FG and increased final BW, ADG, and HCW. For heifers implanted with EST, MGA had no effect on performance. Heifers fed MGA had greater (P = 0.05) DMI, increased (p < 0.01) fat depth, YG, and percentage YG 4 and 5 carcasses, and decreased (p < 0.01) LM area, regardless of implant status. In the absence of EST, MGA is effective in improving performance. However, when EST implants are used, there appears to be little performance benefit from feeding MGA.

The effects of cattle sex on carcass characteristics and longissimus muscle palatability1,2

Two experiments were conducted to determine the effects of sex on carcass traits and cooked beef steak palatability. In Exp. 1, steers (n = 99), heifers (n = 51), and intravaginally spayed heifers (n = 46) were fed a high-energy diet for 161 d. No implants were administered, and heifers were not fed melengestrol acetate to suppress estrus. In Exp. 2, 60 steers and 60 intact heifers from the same ranch source used in Exp. 1 were fed in 2 locations (sites 1 and 2). All management factors were equal across experiments except that intact heifers were fed melengestrol acetate to suppress estrus in Exp. 2. Steers in Exp. 1 were 25 kg heavier (P < 0.01) in HCW than heifers at comparable (P = 0.39) carcass fat thickness. Spayed heifers (Exp. 1) had a 5.7% smaller (P < 0.05) LM area compared with steers and intact heifers, which were similar. In Exp. 2, there was no difference (P = 0.2) in carcass weight, and heifers had greater (P < 0.01) 12th rib fat thickness compared with steers. Calculated yield grades were similar (P = 0.21) among treatments in Exp. 1 and tended (P = 0.08) to be greater for heifers compared with steers in Exp. 2. In Exp. 1, USDA quality grades and marbling scores were lower (P < 0.01) for steers compared with intact and spayed heifers, which were similar. The effects of sex on tenderness were examined at a common level of fat-thickness and marbling by covariate analysis. Steaks from steers, compared with those from nonimplanted, intact heifers, in the 2 experiments combined were: (a) superior (P < 0.05) in 2 of 9 palatability assessments when subcutaneous fat thickness (at the 12th rib) was adjusted to a common level, and (b) superior (P < 0.05) in 6 of 9 palatability assessments when marbling score was adjusted to a common level. In Exp. 1, steaks from nonimplanted steers compared with those from nonimplanted spayed heifers were: (a) superior (P < 0.05) in 0 of 8 palatability assessments when subcutaneous fat thickness (at the 12th rib) was adjusted to a common level, and (b) superior (P < 0.05) in 3 of 8 palatability assessments when marbling score was adjusted to a common level. These findings suggest that sex should be added to the list of antemortem factors contributing to variation in cooked beef steak tenderness. However, more research is needed to precisely identify those factors contributing to the lower tenderness observed for steaks from heifer carcasses.

Comparison of CIDR to MGA in a 7-11 cosynch protocol with timed insemination of beef heifers (2006)

Previous research has shown that the 7-11 Cosynch protocol using melengestrol acetate (MGA) is effective in synchronizing beef heifers. This study compared MGA and a vaginal insert containing progesterone (CIDR) in the 7-11 Cosynch protocol on beef heifers. Replacement beef heifers (n=179) from three herds were assigned to MGA or CIDR treatments. Beginning on day 1, heifers on the MGA treatment were fed to consume 0.5 mg daily of MGA for 7 days. On day 7, the last day of MGA feeding, the MGA heifers received an injection of Lutalyse (PGF2α). Heifers on the CIDR treatment received a CIDR on day 3; on day 9 the CIDR was removed, and heifers received an injection of Lutalyse. On day 11, all heifers received an injection of OvaCyst (gonadotrophinreleasing hormone; GnRH), followed by another injection of Lutalyse 7 days later (day 18). At 48 hours after the final Lutalyse injection, all heifers were time inseminated and received an injection of OvaCyst. Pregnancy status was determined 33 days after breeding by ultrasonography. No difference in pregnancy rate was observed between the CIDR (46%) and MGA (47%) treatments.

Synchronization of Estrus in Fat Tailed Sheep Using Melengestrol Acetate (MGA) in the Breeding Season

Gimenez Diaz, C.A., Emsen, E., Köycegiz, F., Emsen, B., Yaprak, M. and Kutluca, M. 2005. Synchronization of estrus in fat tailed sheep using melengestrol acetate (MGA) in the breeding season. J. Appl. Anim. Res., 28: 25–27. To assess the effectiveness of MGA in synchronizing oestrus sixty-eight Awassi and Redkaraman ewes were group-fed a diet either containing MGA (0.25 mg/ewe/d for 9d) or no MGA. At the end of the MGA feeding regime, ewes were exposed to one ram per group. The percentage of ewes exhibiting estrus within a 6 d period following withdrawal of MGA was significantly higher (P<0.05) for treated (80%) compared to the control ewes (45%). Number of lambs born per ewe lambing and fertility of ewes were not affected by treatment. It was concluded that MGA is effective to synchronize estrus in fat tailed Awassi and Redkaraman sheep in breeding season.

208 FOLLICULAR DYNAMICS, ESTRUS, AND OVULATION IN COWS SYNCHRONIZED TO OVULATE FIRST OR SECOND WAVE DOMINANT FOLLICLES USING SHORT- OR LONG-TERM MELENGESTROL ACETATE- OR EAZI-BREED CIDR INSERT-BASED PROTOCOLS

The objective of this experiment was to determine the feasibility in substituting EAZI-BREED CIDR inserts (CIDR; Pfizer Animal Health, Groton, CT, USA) for melengestrol acetate (MGA) in progestin-based protocols to synchronize estrus in beef cows. Follicular dynamics, timing of estrus, and ovulation were compared in beef cows synchronized to ovulate first or second wave dominant follicles using short- or long-term MGA- or CIDR-based protocols. The study was conducted with 48 nonsuckled, estrous cycling, crossbred beef cows assigned to one of four treatments (T1 to T4; n = 12/T) by age and body condition. Cows were synchronized to ovulate first wave (T1 and T2) or second wave (T3 and T4) dominant follicles based on assignment to treatment. Cows in T1 were fed MGA (0.5 mg h−1 d−1) for 7 days, and were injected with PGF2α (PG; 25 mg Lutalyse; Pharmacia Animal Health, Kalamazoo, MI, USA) on Day 7, GnRH (100 μg Cystorelin; Merial, Athens, GA, USA) on Day 11, and PG on Day 18. Cows in T2 had CIDR (1.38 g progesterone) inserted for 7 days, and were injected with PG on Day 7, GnRH on Day 9, and PG on Day 16. Cows in T3 were fed MGA for 14 days, and were injected with GnRH on Day 26, and PG on Day 33. Cows in T4 had CIDR inserted for 14 days, and were injected the GnRH on Day 23, and PG on Day 30. Transrectal ultrasonography was performed daily to monitor follicular dynamics from GnRH to estrus after PG; and every 4 h from 20 h after the onset of estrus until ovulation. Estrus detection was performed continuously using the HeatWatch® estrus detection system (DDx, Denver, CO, USA). Blood samples for progesterone (P4) were collected daily beginning one day prior to the initiation of treatment and continuing through ovulation following PG. Data were analyzed using the General Linear Models procedure of SAS (SAS Institute, Inc., Cary, NC, USA) and are summarized in the following table. Animals that responded to treatment and were included in the analysis were those that initiated a new follicular wave after administration of GnRH and that displayed estrus within 144 h after PG. Although estrous response was similar among treatments, there were differences in follicular dynamics, steroid secretion patterns, and timing of events that culminated in differences in timing and synchrony of estrus and ovulation among the short- and long-term groups. These differences may be important in relation to fixed-time AI programs. These data suggest that in situations that are not conducive to feeding MGA, substituting CIDR inserts into MGA-based protocols to synchronize estrus may be feasible. This work was supported by USDA-NRI grant 2000-02163; Pfizer Animal Health, New York, NY; and Merial, Athens, GA, USA.

A comparison of progestin-based protocols to synchronize ovulation and facilitate fixed-time artificial insemination in postpartum beef cows1

The experimental objective was to compare pregnancy rates after fixed-time AI in postpartum suckled beef cows following administration of two progestin-based protocols to synchronize ovulation. Cows (n = 424) at three locations (n = 208, 122, and 92 per location) were stratified by age, BCS, and days postpartum (DPP) and assigned randomly to one of the two treatment protocols. The MGA Select-treated cows (MGA Select; n = 213) were fed melengestrol acetate (MGA, 0.5 mg x cow(-1) x d(-1)) for 14 d and carrier for 8 d, and then GnRH (100 microg i.m. Cystorelin; d 26) was injected 12 d after MGA withdrawal, and PG (25 mg i.m. Lutalyse) was administered 7 d after GnRH. Cows assigned to the 7-11 Synch protocol (7-11 Synch; n = 209) were fed carrier for 15 d and MGA for 7 d, and then injected with PG on d 22 (d 7 of MGA), GnRH on d 26, and PG again on d 33. Artificial insemination was performed at fixed times for cows in both treatments at 60 or 72 h after d 33 PG for 7-11 Synch and MGA Select groups, respectively. All cows were injected with GnRH (100 microg of i.m. Cystorelin) at AI. There was no treatment x location interaction for age (P = 0.90), BCS (P = 0.64), or DPP (P = 0.93), and the results were therefore pooled for the respective treatments (age [7-11 Synch, 5.5 +/- 0.2; MGA Select, 5.5 +/- 0.2], BCS [7-11 Synch, 5.7 +/- 0.1; MGA Select, 5.6 +/- 0.1], and DPP [7-11 Synch, 41.1 +/- 1.1; MGA Select, 42.1 +/- 1.1]). Blood samples were collected 8 and 1 d before MGA or carrier to determine pretreatment estrous cyclicity (progesterone >or=1 ng/mL; 7-11 Synch, 59/209 [28%]; MGA Select, 54/213 [25%]; P = 0.50) and again on d 33 PG to evaluate treatment response as a percentage of cows with progesterone concentrations in serum >or=1ng/mL (7-11 Synch, 184/209 [88%]; MGA Select, 177/213 [83%]; P = 0.15). Pregnancy rates resulting from fixed-time AI did not differ (P = 0.25) between treatments (7-11 Synch, 128/209 [61%]; MGA Select, 142/213 [67%]), nor did pregnancy rates (P = 0.77) at the end of the breeding season (7-11 Synch, 198/208 [95%]; MGA Select, 204/213 [96%]). These data indicate that pregnancy rates were comparable after fixed-time AI, following administration of the 7-11 Synch and MGA Select protocols. Both protocols provide opportunities for beef producers to use AI and eliminate the need to detect estrus.

Methods to reduce or eliminate detection of estrus in a melengestrol acetate-PGF2alpha protocol for synchronization of estrus in beef heifers.

Three experiments were conducted to evaluate methods to decrease or eliminate the detection of estrus inherent to a melengestrol acetate (MGA)-PGF2alpha (PGF) protocol for synchronization of estrus in heifers. In each experiment, all heifers received 0.5 mg of MGA x animal(-1) x d(-1) for 14 d (d -32 to -19) and PGF (25 mg, i.m.; d 0, 0 h) 19 d after the last feeding of MGA (MGA-PGF protocol). In Exp. 1, heifers (n = 709) were assigned to each of the following protocols: 1) the MGA-PGF protocol with AI 6 to 12 h after detection of estrus (estrus AI; MGA-PGF); 2) MGA-PGF plus 100 microg, i.m. of GnRH on d -7 (1x GnRH) and estrus AI; or 3) MGA-PGF, GnRH on d -7, and GnRH (100 microg, i.m.) at 48 h after PGF, coincident with insemination (2x GnRH-TB48). In Exp. 2, heifers (n = 559) received the MGA-PGF protocol and were inseminated by either estrus AI or fixed-time AI (TAI) at 60 h, coincident with an injection of GnRH (GnRH-TB60). In Exp. 3, all heifers (n = 460) received the MGA-PGF protocol and were inseminated by estrus AI when detected up to 73 h. Heifers not observed in estrus by 73 h received TAI between 76 and 80 h. Half the heifers inseminated by TAI received no further treatment (TB80), and the remaining half was injected with GnRH at insemination (GnRH-TB80). Variance associated with the interval to estrus and the proportion in estrus from d 0 to 5 was similar for 1x GnRH and MGA-PGF treatments in Exp. 1. Pregnancy rate (d 0 to 5) did not differ for the MGA-PGF and 1x GnRH treatments (62.5 and 60.4%, respectively), and both were greater (P < 0.05) than TAI pregnancy rate in the 2x GnRH-TB48 treatment (42.3%). In Exp. 2, the peak estrous response occurred 60 h after PGF. Pregnancy rate during the synchrony period was greater (P < 0.05) for the MGA-PGF (255/401; 63.6%) than the GnRH-TB60 (74/158; 46.6%) treatment. In Exp. 3, 75.7% of heifers (348/460) were detected in estrus by 73 h and were inseminated, with a conception rate of 74.4%. Pregnancy rates after TAI did not differ between TB80 and GnRH-TB80 (14/56 = 25% and 19/ 56 = 33.9%, respectively). Total pregnancy rate was 63.5% for heifers inseminated after detected estrus and by TAI. Collectively, these data indicate that the exclusive use of TAI for heifers treated with the MGA-PGF protocol resulted in lower pregnancy rates than when AI was performed after detection of estrus. However, estrus AI for 3 d and TAI at the end of d 3 could result in pregnancy rates similar to those achieved after a 5-d period of detecting estrus.

Evaluation of Three Estrous Synchronization Protocols in Beef Heifers

Objectives of this study were to evaluate synchronization, conception, and pregnancy rates of heifers synchronized with melengestrol acetate (MGA)-prostaglandin F2α(PGF2α,), Select Synch, or Select Synch preceded by MGA (MGA-Select Synch). Heifers in the MGA-PGF2αgroup (n = 209; BW = 378kg) received MGA (0.5 mg/ d per heifer) for 14 d and PGF2α(25 mg) 19 d later. Select Synch heifers (n = 213; BW = 374kg) received gonadotropin-releasing hormone (GnRH; 100 μg) followed by PGF2α(25 mg) 7 d later. The MGA-Select Synch heifers (n = 210; BW = 373kg) were fed MGA (0.5 mg/d per heifer) for 7 d, GnRH (100 μg) the day following the last MGA feeding, and PGF2α(25 mg) 7 d after GnRH. More (P<0.01) heifers were in estrus 1 to 4 d before PGF2a administration in both the Select Synch (20%) and MGA-Select Synch (24%) groups than in the MGA-PGF2α(4%) group. Pregnancy rates for heifers in estrus early (d 1 to 4 before PGF2α) were greater (P<0.05) for both Select Synch (55%) and MGA-Select Synch (63%) compared with MGA-PGF2αheifers (18%). Synchronization rate (detected after PGF2α) was greater (P<0.01) for MGA-PGF2αheifers (86%) compared with Select Synch (66%) and MGA-Select Synch (68%) heifers; however, conception rate did not differ (P=0.13) and averaged 72, 63, and 62% for MGA-PGF2α, Select Synch, and MGA-Select Synch heifers, respectively. Select Synch (52%), MGA-Select Synch (58%), and MGA-PGF2αprotocols (61%) provided similar (P=0.18) overall AI pregnancy rates; however, more heifers were in estrus before PGF2αadministration in protocols using GnRH.

Effects of growth-promoting agents and season on yearling feedlot heifer performance.

Angus x crossbred heifers (270 per trial) were used in an experiment conducted over one 105-d summer and one 104-d winter feeding period. Treatments were identical for each trial and included: 1) control, 2) estrogenic implant (E), 3) trenbolone acetate implant (TBA), 4) E + TBA (ET), 5) melengestrol acetate (MGA) in the feed, and 6) ET + MGA (ETM). Each treatment was replicated in five pens, with nine heifers per pen in each season. Initial weights (mean = 384 kg, SE = 57) were the same for each season. There were no treatment x season interactions for final BW, ADG, G:F, water intake, or carcass characteristics. Heifers receiving a growth-promoting agent were 11.6 kg (SE = 4.08) heavier and gained 0.108 kg/d (SE = 0.04) more (P < 0.05) than control heifers. Heifers receiving ET gained 0.09 kg/d (SE = 0.032) more (P = 0.05) than heifers not receiving ET. Heifers receiving ET (with and without MGA) had greater G:F (P < 0.05) than control, E, and TBA heifers. Carcass weights of ET-treated heifers were greater (P < 0.05) than carcass weights for unimplanted heifers, those fed MGA only, and heifers receiving either E or TBA implants. Marbling scores were increased (P < 0.05) by feeding MGA to ET-treated heifers. Water intake was greater (P < 0.01) in the summer (31 L/d) than in the winter (18 L/ d), with no difference among implant treatments. Heifers fed in the winter had heavier carcasses, less 12th-rib fat, greater marbling scores, larger LM area, and a greater incidence of liver abscesses than heifers finished in the summer (P < 0.01). A treatment x season interaction (P = 0.07) was evident for DMI during the 35-d coldest and hottest portions of the year. Heifers fed MGA and implanted with ET tended (P = 0.07) to have greater DMI in the summer but lesser DMI in the winter. In general, differences among growth-promotant programs were relatively similar over the entire summer and in winter.

Follicular dynamics and steroid profiles in cows during and after treatment with progestin-based protocols for synchronization of estrus.

Two progestin-based protocols for the synchronization of estrus in beef cows were compared. Cyclic, nonlactating, crossbred, beef cows were assigned by age and body condition score to one of two treatments. Cows assigned to the MGA Select protocol were fed melengestrol acetate (MGA; 0.5 mg x cow(-1) x (-1)) for 14 d, GnRH was administered (100 microg i.m. of Cystorelin) 12 d after MGA withdrawal, and PGF2alpha (25 mg of i.m. Lutalyse) was administered 7 d after GnRH. Cows assigned to the 7-11 Synch protocol were fed MGA for 7 d and were injected with PG on d 7 of MGA, GnRH on d 11, and PG on d 18. Transrectal ultrasonography was performed daily to monitor follicular dynamics from the beginning of MGA feeding through ovulation after the synchronized estrus. All cows exhibited estrus in response to PG. Mean interval to estrus was shorter (P < 0.01) for 7-11 Synch-treated cows (56 +/- 1.5 h) than for cows assigned to the MGA Select protocol (73 +/- 4.7 h). Mean interval from estrus to ovulation did not differ between treatments (P > 0.10). Variances for interval to estrus differed (P < 0.01) between treatments. Mean follicular diameter at GnRH injection, PG injection, and estrus did not differ (P > 0.10) between treatments. Relative to MGA Select, serum estradiol-17beta concentrations were higher (P < 0.01) for 7-11 Synch 2 d and 1 d before, on the day of GnRH injection, in addition to 4 d after GnRH, and 24 h after PG. Mean progesterone concentrations were greater (P < 0.01) for MGA Select cows from 4 d before to 7 d after GnRH. Forty-four percent of the variation in interval to estrus between treatments was explained by differences in estradiol-17beta concentrations 24 h after PG. This study suggests that follicular competence is likely related to steroidogenic capacity of the follicle and the endocrine environment under which growth and subsequent ovulation of the dominant follicle occurs.

Follicular dynamics and steroid profiles in cows during and after treatment with progestin-based protocols for synchronization of estrus12

Two progestin-based protocols for the synchronization of estrus in beef cows were compared. Cyclic, nonlactating, crossbred, beef cows were assigned by age and body condition score to one of two treatments. Cows assigned to the MGA Select protocol were fed melengestrol acetate (MGA; 0.5mg•cow−1•−1) for 14 d, GnRH was administered (100 μg i.m. of Cystorelin) 12 d after MGA withdrawal, and PGF2α (25 mg of i.m. Lutalyse) was administered 7 d after GnRH. Cows assigned to the 7-11 Synch protocol were fed MGA for 7 d and were injected with PG on d 7 of MGA, GnRH on d 11, and PG on d 18. Transrectal ultrasonography was performed daily to monitor follicular dynamics from the beginning of MGA feeding through ovulation after the synchronized estrus. All cows exhibited estrus in response to PG. Mean interval to estrus was shorter (P < 0.01) for 7-11 Synch-treated cows (56 ± 1.5 h) than for cows assigned to the MGA Select protocol (73 ± 4.7 h). Mean interval from estrus to ovulation did not differ between treatments (P > 0.10). Variances for interval to estrus differed (P < 0.01) between treatments. Mean follicular diameter at GnRH injection, PG injection, and estrus did not differ (P > 0.10) between treatments. Relative to MGA Select, serum estradiol-17β concentrations were higher (P < 0.01) for 7-11 Synch 2 d and 1 d before, on the day of GnRH injection, in addition to 4 d after GnRH, and 24 h after PG. Mean progesterone concentrations were greater (P < 0.01) for MGA Select cows from 4 d before to 7 d after GnRH. Forty-four percent of the variation in interval to estrus between treatments was explained by differences in estradiol-17β concentrations 24 h after PG. This study suggests that follicular competence is likely related to steroidogenic capacity of the follicle and the endocrine environment under which growth and subsequent ovulation of the dominant follicle occurs.