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ANIMAL PRODUCTION |
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Select Sires, Inc., Plain City, OH 43064
Abstract
The efficacy of various combinations of melengestrol acetate (MGA), GnRH, and PGF2
for the synchronization of estrus in Angus-based beef cattle was compared. Hormones were administered as follows: MGA, 0.5 mg•animal-1•d-1 mixed in a grain carrier; GnRH, 100 µg i.m.; PGF2, 25 mg i.m. In Exp. 1, 2, and 3, cows were randomly assigned to treatments by parity and interval postpartum. The detection of estrus and AI were conducted from d -2 until 72 to 96 h after PGF2, at which time cows not detected to be in estrus received GnRH and fixed-time AI (TAI). Data were analyzed separately for primiparous and multiparous cows. In Exp. 1, cows (n = 799) at three locations received GnRH on d -7 and PGF2 on d 0 and either no further treatment (GnRH-PGF) or short-term MGA from d -6 through d -1 (STMGA). Among multiparous cows, conception rate at TAI was greater (P < 0.05) for STMGA (41%, 47/115) than for GnRH-PGF treated cows (26%, 24/92). Across herds and parity, synchronized AI pregnancy rate (SPR) was not affected (P > 0.10) by treatment (GnRH-PGF vs. STMGA; 54%, 210/389 vs. 57%, 228/402). In Exp. 2, cows (n = 484) at three locations received either STMGA or long-term MGA from d -32 through d -19, GnRH on d -7, and PGF2 on d 0 (LTMGA). Among primiparous cows, SPR was greater (P < 0.01) in LTMGA (65%, 55/85) than STMGA-treated cows (46%, 40/87). Treatment had no effect (P > 0.10) on SPR among multiparous cows (STMGA vs. LTMGA; 59%, 92/155 vs. 64%, 101/157). In Exp. 3, cows (n = 838) at four locations received the LTMGA treatment and either no further treatment or an additional period of MGA exposure from d -6 through d -1 (L&STMGA). Among primiparous cows, SPR tended to be influenced (P < 0.10) by the herd x treatment interaction and was greater (P < 0.01) among L&STMGA (86%, 19/22) than LTMGA-treated cows (56%, 14/25) at a single location. Among multiparous cows, SPR was lower (P < 0.05) in L&STMGA (46%, 165/358) than LTMGA-treated cows (55%, 184/336). In Exp. 4, Angus heifers (n = 155) received either STMGA or 14 d of MGA (d -32 through d -19) and PGF2 on d 0 (MGA-PGF). The detection of estrus and AI were conducted from d -2 to d 6. Interval to estrus was greater (P < 0.05) and estrous response was lower (P < 0.05) in STMGA than MGA-PGF-treated heifers. In conclusion, primiparous cows responded more favorably to longer-duration MGA treatments than did multiparous cows. All protocols achieved sufficient SPR to justify their use for improved reproductive management of postpartum beef cows.
Key Words: Beef Cattle • Synchronization of Estrus • GnRH • Melengestrol Acetate • Prostaglandin
Introduction
The efficacy of GnRH-PGF2-based (7-d interval) protocols for the synchronization of estrus is a function of induced cyclicity among anestrous cows (Thompson et al., 1999
; Stevenson et al., 2000) and of a more-precise control of follicular development and luteal regression in cyclic cows (Wolfenson et al., 1994; Twagiramungu et al., 1995). However, the ability of GnRH to induce follicle turnover is influenced by the stage of follicular development at the time of injection (Vasconcelos et al., 1999; Geary et al., 2000). The absence of follicle turnover in response to GnRH results in 8 to 10% of treated cows exhibiting estrus before PGF2 (Geary et al., 2000; DeJarnette et al., 2001b) and represents a limitation of this approach for the control of estrus in cattle. Presynchronizing cows to the early to midluteal phase of the cycle using PGF2 (Moreira et al., 2001) or a long-term progestin treatment (melengestrol acetate, MGA; Patterson et al., 2000; Wood et al., 2001) facilitates optimum response to GnRH and improves synchronized reproductive performance.
Progestin treatment between GnRH and PGF2 prevents premature estrus (DeJarnette et al., 2001c) and increases estrous response and/or conception rates among anestrous cows (Stevenson et al., 1997a; Thompson et al., 1999; Lamb et al., 2001), whereas GnRH-induced turnover of dominant follicles attenuates the development of persistent follicles and reduced fertility characteristic of exogenous progestin treatments in cyclic cows (Brink and Kiracofe, 1988; Patterson et al., 1989; Kinder et al., 1996). However, data to support the use of MGA as an exogenous progestin source between GnRH and PGF2 are limiting.
These experiments compared the reproductive performance of beef cattle synchronized to estrus using GnRH and PGF2 with or without MGA treatment between injections and/or a 14-d presynchronization period of MGA feeding.
Materials and Methods
All Experiments
The descriptive characteristics of herds and cows used in Exp. 1, 2, and 3 are presented in Table 1. Angus or Angus-based crossbred cattle were randomly assigned to treatments by parity and interval postpartum, except in Exp. 4, in which virgin heifers were randomized by weight or size. Hormones used in the various synchronization protocols were administered as follows: melengestrol acetate (Pharmacia Animal Health, Kalamazoo, MI) was mixed in a grain carrier and fed at a rate of 0.5 mg•animal-1•d-1; GnRH, 100 µg i.m. (Fertagyl, Intervet, Inc., Millsboro, DE); and PGF2, 25 mg i.m. (ProstaMate, Phoenix Scientific, Kansas City, MO). Because MGA is not readily administered in the absence of grain supplementation and the experimental objectives were to evaluate the efficacy of various MGA-based "systems" for synchronizing estrus in cattle, treatments that did not require MGA did not receive the grain carrier. Although a portion of the observed effects may be in response to elevated nutrient intake, previous studies have indicated beneficial effects of MGA on reproductive performance over and above those of grain alone (Patterson et al., 1989). Detection aids (Kamar Heatmount Detectors, Steamboat Springs, CO) were applied at the time of PGF2 injection to facilitate 
2 times daily observations for signs of estrus. The interval from PGF2 injection to detected estrus was recorded and categorized in 12-h intervals. The median value for each 12-h detection interval was assigned as a continuous variable for the analysis of the mean interval to estrus. Body condition score (BCS; 9 = obese, 1 = emaciated) was assessed either at the time of AI or at PGF2 injection, except in Herds B (spring 2001), D, and I, for which actual BCS were not recorded, but those cows had BCS 5. Cattle were isolated from herd bulls for 10 to 14 d after AI and pregnancy status was assessed by ultrasonographic diagnosis at 30 to 40 d after AI. Fetal dimensions were used to estimate approximate fetal age and date of conception, and to confirm pregnancies resulting from artificial insemination. Breeding season pregnancy rates were assessed by rectal palpation or ultrasonography at 30 to 60 d after the end of the 60-d breeding season.
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on d 0, and either no further treatment (GnRH-PGF) or a 6-d period of MGA feeding from d -6 through d -1 (short-term MGA = STMGA; Figure 1). Cows were observed for estrous behavior and bred by AI from d -3 until 72 h after PGF2, at which time cows not detected to be in estrus received GnRH and fixed-time AI (TAI). Each herd replicated the experiment with two groups of cows as follows: Herd A, spring 2001 and 2002; Herd B, spring and fall 2001; Herd C, fall 2000 and 2001. Within statistical analyses, each replicate was treated as a separate herd. The discrepancies in sample sizes between estrous response and conception data within Herd A in 2001 (Tables 2 and 3) are due to cows (n = 8) that were available at the time of treatment and detection of estrus but were not accessible (culled, died, or lost) at the time of pregnancy diagnosis. Only cows 25 d postpartum on d 0 were eligible for inclusion in this experiment.
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on d 0 (long-term MGA = LTMGA; Figure 1
). Cows were observed for estrous behavior and bred by AI from d -2 until 72 (Herds D and E) or 96 h (Herd F) after PGF2, at which time cows not detected to be in estrus received GnRH and TAI. Only cows that had calved before the initiation of LTMGA treatment were eligible for inclusion in this experiment (32 d postpartum on d 0).
Experiment 3
Cows (n = 838) in four locations received the LTMGA treatment as described in Exp. 2 and either no further treatment (LTMGA) or an additional 6-d period of MGA feeding from d -6 through d -1 (long- and short-term MGA = L&STMGA). Cows were observed for estrous behavior and bred by AI from d -2 until 72 (Herd G, H, and J) or 96 (Herd I) h after PGF2, at which time cows not detected to be in estrus received GnRH and TAI. The discrepancy in sample size between estrous and conception data within Herd H is due to cows (n = 5) that exhibited estrus 
d 0 and were not inseminated. These cows were included in the analyses of estrous response and breeding season pregnancy rate but not in analyses of synchronized conception or pregnancy rates. Only cows that had calved before initiation of treatments were eligible for inclusion in this experiment (32 d postpartum on d 0).
Experiment 4
Angus heifers (n = 155) at a single location (OH) were allocated to receive either the STMGA protocol described in Exp. 1 or a 14-d period of MGA feeding from d -32 through d -19 and PGF2 d 0 (MGA-PGF). Detection of estrus was performed from d -2 through d 6, and heifers detected in estrus were bred by AI 8 to 12 h later.
Statistics
Experiments 1, 2, and 3.
Data were analyzed using logistic regression and 
2 procedures. Dependent variables included the percentage of cows detected in estrus, conception rates at estrus and at TAI, synchronized pregnancy rates, and breeding season pregnancy rates. Data were analyzed by parity due to the confounding effects of herd and parity in one or more herds of each experiment (Table 1). Logistic regression models for multiparous cows included the effects of herd, treatment, postpartum interval (<70 vs. 70 d), all two-way interactions of main effects and BCS nested within herd. Logistic regression models for primiparous cows included the effects of herd, treatment, the herd x treatment interaction, postpartum interval (<80 vs. 80 d) nested within herd, and BCS (<5 vs. 5) nested within herd. Postpartum interval was nested within herd in primiparous models to avoid confounding these effects. Likewise, BCS was nested within herd in all models with the exception of multiparous cows in Exp. 2, in which the absence of cows <5 precluded the evaluation of BCS entirely. The BCS x treatment and the postpartum interval x treatment interactions could not be tested within any models due to the absence of cows within one or more cells of each experiment. Interval to estrus was evaluated using standard least square means and included model effects as previously described for logistic regression procedures. Models evaluating conception rates at standing estrus and TAI included the effects of sire and technician nested within herd. Within all analyses, interactions and nested effects lacking significance (P > 0.10) were removed from the model in descending order of P-value and data were reanalyzed. Mean comparisons of binomial data were performed using Fisher’s exact two-tail test and Tukey’s HSD test was used for all other mean comparisons. All statistical analyses were performed using SAS JMP Statistical Discovery Software, Version 5.0.1 (SAS Inst. Inc., Cary, NC).
Experiment 4.
Data were analyzed using logistic regression procedures. Independent variables included treatment, sire, and technician. Treatment differences in estrous response rate were evaluated at d 3 (72 h after PGF) and at 6 d. Data were also analyzed with and without the inclusion of weight quartile (<342, 342 to 362, 363 to 386, and >386 kg), which was only available on a subsample of heifers (n = 142).
Results
Experiment 1—GnRH-PGF and STMGA
Primiparous cows.
The interval to estrus was influenced (P < 0.01) by treatment and the herd x treatment interaction (Table 2). The interval to estrus was shorter (P < 0.05) for GnRH-PGF than STMGA-treated cows within Herd C, 2000, and tended (P < 0.10) to be shorter in Herd A, 2002. Across herds, the interval to estrus was shorter (P < 0.05) for GnRH-PGF than STMGA-treated cows. The percentage of cows detected in estrus was influenced by herd (P < 0.01; Table 2) and by BCS within herd (P < 0.05). The percentage of cows detected to be in estrus was greater (P < 0.05) for cows 5 BCS in Herd A, 2001 (5 vs. <5 BCS; 84%, 38/45 vs. 58%, 11/19), but were lower (P < 0.01) for cows 5 BCS in Herd A, 2002 (5 vs. <5 BCS; 36%, 10/28 vs. 95%, 20/21). Conception rates at detected estrus were not influenced (P > 0.10) by any model effects; however, conception rates at TAI were influenced by herd (P < 0.05; Table 2).
Synchronized AI pregnancy rates were influenced (P < 0.05) by herd (Table 2) and by BCS within herd. Within Herd A, 2002, synchronized AI pregnancy rates were greater (P < 0.01) among primiparous cows <5 (76%, 16/21) than 5 BCS (21%, 6/28). Pregnancy rates at the end of the breeding season differed (P < 0.05) among herds and were as follows (means that do not have common superscripts differ, P < 0.050: Herd A, 2001, 98%a, 61/62; Herd C, 2000, 97%a,b, 28/29; Herd B, fall, 94%a,b, 32/34; Herd C, 2001, 85%a,b, 23/27; Herd A, 2002, 84%b, 41/49.
Multiparous Cows.
The interval to estrus was influenced by postpartum interval (P < 0.05), herd (P < 0.01) and the herd x treatment interaction (P < 0.01; Table 3). In Herd A, 2002, the interval to estrus was shorter (P < 0.05) among GnRH-PGF than STMGA-treated cows. In contrast, GnRH-PGF treated cows tended (P < 0.10) to have a longer interval to estrus than STMGA-treated cows in Herd B, fall 2001, and Herd C, fall 2001. The interval to estrus was shorter (P < 0.05) for cows 70 d postpartum (n = 287; 48.0 ± 1.2 h) than for cows <70 d postpartum (n = 101; 52.8 ± 2.0 h). The percentage of cows detected in estrus was influenced by herd (P < 0.01), treatment (P < 0.01), the herd x treatment interaction (P < 0.05; Table 3), and the postpartum interval x treatment interaction (P < 0.05). Within Herd B, fall 2001, and Herd C, fall 2000, there was a tendency (P < 0.10) for more GnRH-PGF- than STMGA-treated cows to be detected in estrus. In contrast, a tendency (P < 0.10) for fewer GnRH-PGF- than STMGA-treated cows to be detected in estrus was observed in Herd A, 2002. Among cows 70 d postpartum, the percentage detected in estrus was similar (P > 0.10) among treatments (GnRH-PGF vs. STMGA; 68%, 137/202 vs. 67%, 150/224); however, among cows <70 d postpartum, more (P < 0.01) GnRH-PGF (69%, 60/87) than STMGA-treated cows (49%, 41/83) were detected in estrus and this trend appeared numerically similar across herds. Conception rates of cows detected in estrus were not influenced (P > 0.10) by any variables included in the model; however, conception rates of cows bred at TAI were influenced by herd (P < 0.001) and treatment (P < 0.05; Table 3). More (P < 0.05) STMGA- than GnRH-PGF-treated cows conceived at TAI.
Synchronized AI pregnancy rates were influenced (P < 0.01) by herd (Table 3) and the herd > postpartum interval interaction. In Herd C, 2000, synchronized pregnancy rates were greater (P < 0.01) among cows 70 d (73%, 37/51) than among cows <70 d postpartum (33%, 6/18). However in Herd B, spring 2001, cows 70 d postpartum had lower (P < 0.05) synchronized pregnancy rates (64%, 53/83) than cows <70 d postpartum (83%, 29/34). Breeding season pregnancy rates were influenced (P < 0.01) by herd and were as follows (means that do not have common superscripts differ, P < 0.05): Herd C, 2000, 100%a, 69/69; Herd A, 2001, 98%a, 86/88; Herd A, 2002, 93%a,b, 126/135; Herd B, spring, 90%a,b, 106/118; Herd C, 2001, 90%a,b, 85/95; Herd B, fall, 86%b, 73/85.
Experiment 2—STMGA and LTMGA
Primiparous Cows.
The interval to estrus was influenced (P < 0.01) by herd (Table 4); however, this effect was confounded by the varying length of the period for detection of estrus between the two herds (Herd D vs. Herd F; 72 vs. 96 h, respectively). The percentage of cows detected to be in estrus was influenced (P < 0.01) by herd and treatment (Table 4). More (P < 0.01) LTMGA- than STMGA-treated were detected in estrus; however, this effect was primarily evident in Herd D and was confounded by the shorter detection period (72 h) than in Herd F (96 h). Conception rates at estrus tended to be greater (P 0.10) among LTMGA- than STMGA-treated cows, whereas conception rates at TAI were not influenced (P > 0.10) by any model effects (Table 4).
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). Breeding season pregnancy rates (91%, 157/172) were not influenced (P < 0.10) by any model effects.
Multiparous Cows.
The interval to estrus was shorter (P < 0.05) among LTMGA- than STMGA-treated cows; however, this effect was primarily evident in a single location (Herd F; Table 4). The percentage of cows detected to be in estrus was influenced by postpartum interval (P < 0.05), herd (P < 0.01), and the herd x treatment interaction (P < 0.05; Table 4). The percentage of cows detected to be in estrus was greater (P < 0.05) among LTMGA-treated cows than STMGA-treated cows within Herd F. The effect of herd on the percentage of cows detected in estrus was confounded by the length of the detection period between the two herds (Herd E vs. Herd F; 72 vs. 96 h, respectively). The percentage of cows detected in estrus was greater (P < 0.01) among cows <70 (81%, 95/117) than 70 d postpartum (64%, 124/195), and within each herd, the numeric trend was consistent. Conception rates of cows detected in estrus were not influenced (P > 0.10) by any model effects, whereas conception rates at TAI were influenced (P < 0.05) by herd and the herd x treatment interaction (Table 4). Conception rates at TAI were similar (P > 0.10) among treatments within Herd F but tended to be greater (P < 0.10) among LTMGA- than STMGA-treated multiparous cows in Herd E.
Synchronized AI pregnancy rates were influenced (P < 0.05) by herd (Table 4). Breeding season pregnancy rates were greater (P < 0.01) in Herd E (99%, 100/101) than in Herd F (85%, 177/209).
Experiment 3—LTMGA and L&STMGA
Primiparous Cows.
The interval to estrus was influenced (P < 0.01) by herd and by treatment (Table 5). The interval to estrus was shorter (P < 0.01) in LTMGA- than L&STMGA-treated cows. The interval to estrus was shorter (P < 0.01) in Herd H than in Herd I and Herd J; however, Herd I was confounded by the longer period (96 h) of detection. The percentage of cows detected to be in estrus tended (P 0.10) to be greater in Herd H and Herd I than in Herd J (Table 5); however, Herd I was confounded by the longer period of detection. The conception rates of cows at detected estrus and at TAI were not influenced (P > 0.10) by any model effects (Table 5).
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). In Herd H, synchronized pregnancy rates were greater (P < 0.01) among L&STMGA- than LTMGA-treated cows but were not different in Herds I or J. The herd x treatment interaction was primarily one of magnitude and across herds, synchronized AI pregnancy rate tended (P < 0.10) to be greater among L&STMGA- than LTMGA-treated cows. Breeding season pregnancy rates were greater (P < 0.05) among L&STMGA- (99%, 66/67) than LTMGA-treated cows (89%, 65/73).
Multiparous Cows.
The interval to estrus was influenced (P < 0.01) by herd and the herd x treatment interaction (Table 5). The interval to estrus was shorter (P < 0.01) among LTMGA-treated cows in Herd H and Herd I than L&STMGA treatment. In contrast, interval to estrus was longer (P < 0.01) in LTMGA- than L&STMGA-treated cows in Herd J. The percentage of cows detected to be in estrus was influenced (P < 0.05) by herd (Table 5). More (P < 0.01) cows were detected in estrus in Herds G, H, and I than in Herd J; however, Herd I was confounded by a longer period of detection of estrus (96 h). The conception rates of cows detected to be in estrus were influenced by herd (P < 0.01) and treatment (P < 0.05; Table 5). More (P < 0.05) LTMGA- than L&STMGA-treated cows conceived to AI at detected estrus. Conception rate at TAI was only influenced by herd (P < 0.05; Table 5).
Synchronized AI pregnancy rates were influenced by herd (P < 0.05), treatment (P < 0.05; Table 5), and postpartum interval (P < 0.01). More (P < 0.05) LTMGA- than L&STMGA-treated cows conceived during the synchronized AI period. Synchronized AI pregnancy rates were greater (P < 0.01) among cows 70 d postpartum (54%, 288/535) than cows <70 d postpartum (38%, 61/159). Breeding season pregnancy rates were influenced (P < 0.05) by herd and postpartum interval. Breeding season pregnancy rates were greater (P < 0.01) among cows 70 d postpartum (94%, 503/536) than cows <70 d postpartum (87%, 138/159).
Experiment 4—Angus heifers; STMGA and MGA-PGF
The interval to estrus was shorter (P < 0.01) among MGA-PGF- than STMGA-treated heifers (Table 6). The percentage of heifers detected to be in estrus was greater (P < 0.05) among MGA-PGF- than STMGA-treated heifers (Table 6). At 72 h after PGF, more (P < 0.05) MGA-PGF- (69%, 52/75) than STMGA-treated heifers (30%, 24/80) were detected as being in estrus. Treatment had no effect (P > 0.10) on conception rates (Table 6); however, conception rates were lower (P < 0.05) among heifers in the highest weight quartile group (>386 kg; 29%, 6/21) than among heifers in the lowest three weight quartiles groups (55%, 48/88). A significant (P < 0.05) technician effect was observed (Technician A and B vs. C; 63%, 15/24 & 68%, 17/25 vs. 41%, 29/71, respectively); however, the effect was balanced across treatments and no interaction with treatment was detected.
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) but were lower (P < 0.05) in heifers in the highest weight quartile (> 386 kg; 22%, 6/27), highest in the lowest weight quartile (<342 kg; 51%, 18/35), and intermediate in the intermediate weight group (342 to 386 kg; 38%, 30/80). Breeding season pregnancy rates (96%, 148/155) were not affected (P > 0.10) by any model effects. Discussion
In Exp. 1, the herd x treatment interaction influenced the interval to estrus in both primiparous and multiparous herds. Among herds of primiparous cows, the herd x treatment interaction was primarily one of magnitude and the main effect of STMGA treatment was to extend the interval to estrus across herds (Table 2). In contrast, the herd x treatment interaction among multiparous cows differentially influenced the interval to estrus, estrous response rate, and synchronized AI pregnancy rate (Table 3). Factors responsible for these conflicting results between herds and/or parities were not readily apparent. Also not apparent is an explanation for the reduced estrous response rate among STMGA-treated multiparous cows <70 d postpartum. However, these observations emphasize the need to exercise caution when interpreting the results of single location studies.
The greater conception rates at TAI among STMGA-treated multiparous cows (Table 3) is consistent with other studies using short-term MGA treatment (DeJarnette et al., 2001c) and implies that follicular development and/or estrogenic activity may be delayed by STMGA treatment. In contrast to the 8 to 10% incidence reported in previous studies (Geary et al., 2000; DeJarnette et al., 2001a), only 2% of GnRH-PGF-treated cows displayed estrus before d 0 in Exp. 1. Therefore, a statistically significant effect of STMGA treatment (1% incidence) to attenuate the occurrence of early estrus was not apparent. In contrast to our hypothesis, the STMGA treatment failed to consistently improve synchronized AI pregnancy rate compared to GnRH-PGF treatment alone.
In Exp. 2, both estrous response rate and conception rate at detected estrus among primiparous cows tended to be improved by LTMGA compared to STMGA treatment, resulting in greater synchronized AI pregnancy rate (Table 4). However, as in Exp. 1, herd x treatment interactions among multiparous cows failed to clearly indicate one treatment as superior to the other despite significant within herd effects favoring LTMGA treatment.
The L&STMGA treatment used in Exp. 3 was designed to provide an additional opportunity for progestin-mediated induction of cyclicity and perhaps to further improve synchrony of estrus of the LTMGA treatment. Among primiparous cows, the interval to estrus was extended by L&STMGA treatment with no impact on the percentage of cows detected in estrus. Thus, the greater synchronized AI pregnancy rate observed among L&STMGA-treated primiparous cows in Herd H appeared to be mediated through improved conception rates (Table 5). This pregnancy advantage was maintained in greater breeding season pregnancy rates among L&STMGA-treated primiparous cows. These observations indicate a favorable response to L&STMGA treatment among primiparous cows. In contrast, the interval to estrus among multiparous cows was differentially influenced by the herd x treatment interaction (Table 5). Additionally, conception rates and synchronized AI pregnancy rates among L&STMGA-treated multiparous cows were lower than those of LTMGA-treated cows and indicates that MGA feeding between GnRH and PGF2 injections was counterproductive to reproductive efficiency among multiparous cows.
The negative effect of L&STMGA treatment on conception rates at estrus of multiparous cows was not anticipated. Exogenous progestin treatments immediately before estrus and in the absence of a functional corpus luteum stimulate the development of persistent follicles and negatively impact conception rates (Kinder et al., 1996). However, the 14-d period of MGA feeding should have precluded persistent follicle development by "presynchronizing" cows to the early to midluteal phase (d 8 to 10) of the estrous cycle at the initiation of the final short-term MGA treatment. Alternatively, anestrous cows that developed a "short-lived" corpus luteum (Inskeep, 1995) in response to the initial 14-d MGA feeding period would be staged for premature CL regression proximal to the final MGA feeding period of the L&STMGA treatment. Progestin stimulation initiated concurrently with CL regression could then potentially "rescue" these dominant follicles from atresia and thereby facilitate the development of persistent follicles. This speculative explanation is supported by the observation that among the anestrous cows, multiparous cows are more responsive to the induction of estrus than are primiparous cows (Stevenson et al., 1997a). Regardless, these data indicate that the reproductive performance of multiparous cows may be compromised by L&STMGA treatment and caution should be used in implementation (whole-herd vs. primiparous only) until a more complete understanding of these interactions can be achieved.
The 24-h delay between GnRH and initiation of MGA treatment (STMGA and L&STMGA) was selected due to results of previous studies that suggest concurrent administration of exogenous progestin attenuates the ability of GnRH to induce the development of a functional accessory corpus luteum (Pancarci et al., 1999; Mussard et al., 2001). Across experiments, MGA feeding between GnRH and PGF2 injections (STMGA or L&STMGA) tended to lengthen the interval to estrus among primiparous cows. However, primiparous cows tended to achieve greater pregnancy rates in response to the treatment with the longer duration of MGA exposure (Exp. 1, STMGA; Exp. 2, LTMGA; Exp. 3, L&STMGA) and are evidence of a positive effect of progestin stimulation among cows predisposed to reduced levels of cyclicity (Stevenson et al., 1997a, 1997b & 2000). In contrast, feeding MGA to multiparous cows between GnRH and PGF2 injections (Exp. 1 and 2, STMGA; Exp. 3, L&STMGA) resulted in numerous herd x treatment interactions on parameters of reproductive performance, especially estrous response rates. With a single exception (Herd A, 2002, Table 3), there was little indication of a beneficial effect of MGA feeding between GnRH and PGF2 on synchronized AI pregnancy rates of multiparous cows. Across herds and parities in Exp. 2 & 3, the LTMGA treatment achieved synchronized AI pregnancy rates similar to those reported in other laboratories (~60%; Patterson et al., 2000; Patterson et al., 2001; Perry et al., 2002).
Consistent with other studies (Short et al., 1990), both BCS and postpartum interval were positively associated with various measures of reproductive performance within several herds and/or experiments. However, the effects were not consistent and negative associations of BCS or postpartum interval with estrous response or synchronized pregnancy rates were also evident (primiparous cows in Herd A, 2002; multiparous cows in Herd B, spring 2001; multiparous cows in Exp. 2). The lack of an effect of postpartum interval among primiparous cows was possibly a function of several factors in combination, including: 1) small sample sizes; 2) a longer average interval postpartum; and 3) greater variation among herds. Likewise, the effects of BCS on reproductive performance were confounded by 1) large variation across herds, 2) lack of variation within several locations, and 3) different evaluators at each location. Additionally, all herds used in the present study were considered well-managed with multiple consecutive years of experience with estrus synchronization and AI. With the exception of Herd C in 2001, suffering the effects of an impending local drought, most herds had very few cows of marginal BCS or postpartum interval. Therefore, management tended to minimize the opportunity for BCS or prolonged postpartum interval to impact reproductive performance, whereas the nature of the data obtained dictated these variables to often be confounded with herd effects.
The STMGA treatment of virgin heifers reduced the overall estrous response and delayed the interval to estrus compared to MGA-PGF treatment. These effects are similar to trends observed in STMGA-treated cows but appear to be more pronounced, as only 30% of STMGA-treated heifers were detected as being in estrus by 72 h after PGF compared to >60% response rates during this interval in STMGA-treated cows or MGA-PGF-treated heifers. This effect could be a function of herd-specific differences in the efficiency of MGA administration and/or heifer specific differences in the rate of MGA clearance. Alternatively, these effects may be due to inherent differences among heifers (vs. cows) in relation to follicular responsiveness to exogenous GnRH (Schmitt et al., 1994; Silcox et al., 1995; Pursley et al., 1997). In either case, these data do not support the use of the STMGA treatment in beef heifers.
Implications
Each of the protocols evaluated achieved synchronized artificial insemination pregnancy rates sufficient to justify their use for improved reproductive management of postpartum beef cows. Synchronized artificial insemination pregnancy rates of primiparous cows seemed to increase as a function of the duration of MGA exposure; however, protocol length and extended calving seasons may limit the number of cows eligible for treatment. Synchronized AI pregnancy rates among multiparous cows indicate that all treatments had comparable efficacy, except for the combined long- and short-term melengestrol acetate treatment, which interacted with herd to negatively impact reproductive performance. The mechanism(s) of numerous parity x treatment and herd x treatment interactions warrant further investigation and demand prudence in the interpretation of results of single location studies.
Footnotes
1 The authors thank the owners and employees at the following locations for use of their cattle and facilities to conduct these on-farm research trials: Phelp’s Creek Angus, Brookneal, VA; Silver Spring Ranch, Bellevue, ID; Brookview Farm, LLC, Winchester, KY; Woods & Clark, Mays Lick, KY; R. D. Robinson, Orleans, IN; M. Johnson, Fairfield, MT; TCPM Land and Cattle, Ledger, MT; Whiterock Ranch, Whitehall, MT; M. Sanford, Orange, VA; and Welcome In Farm, Mechanicsburg, OH. We thank the following organizations for product contributions: Intervet, Inc., Millsboro, DE, (Fertagyl: GnRH) and Phoenix Scientific, Inc., St Joseph, MO, (Prostamate: PGF2). Thanks to P. Rajala-Schultz, Ohio State University, for technical assistance and counseling regarding the statistical analysis of these experiments. 
2 Correspondence: 11740 US 42 (phone: 614-873-4683; fax: 614-873-5751; e-mail: jmdejarnette{at}selectsires.com).
Received for publication December 4, 2002. Accepted for publication October 31, 2003.
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