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Timed Artificial Insemination in Postpartum Beef Cows Using Melengestrol Acetate, Prostaglandin F2α, and Gonadotropin-Releasing Hormone
The Professional Animal Scientist 21 (2005):17–21
Timed Artificial Insemination in Postpartum Beef Cows Using Melengestrol Acetate, Prostaglandin F2α, and Gonadotropin-Releasing Hormone N. M. POST, D. L. KREIDER1, R. W. RORIE, and T. D. LESTER Department of Animal Science, University of Arkansas, Fayetteville 72701
Abstract This study evaluated the use of melengestrol acetate (MGA), prostaglandin F2α (PGF2α) and gonadotropin-releasing hormone (GnRH) in a timed AI (TAI) protocol in postpartum beef cows. One hundred twenty-one Angus-crossbred cows in two herds were ranked by parity, days postpartum, body condition score (BCS), and BW and then randomly assigned to one of two treatments. Cows in both treatments received 1.8 kg of supplement containing 0.5 mg/d of MGA for 14 d, then received supplement without MGA for 17 d. After MGA withdrawal, cows in both groups received an injection of PGF2α. Cows receiving the MGAPGF2α (MPG) treatment were subjected to estrus detection and AI for 8 d. Cows receiving the MGA-PGF2α-GnRH (MPGG) treatment were injected with GnRH 48 h after PGF2α, followed by TAI 16 to 18 h after PGF2α. There was a difference (P<0.01) between herds in conception rate (CR) and pregnancy rate to AI (PR) and overall pregnancy rate
1
To whom correspondence should be addressed: [email protected]
(OP). The CR was not different between treatments (P≥0.24), but PR was greater in MPGG vs MPG (P≤0.08). There was no difference in OP between MPG and MPGG treatments (P≥0.25). The TAI procedure described in this study yielded results that were equal to those obtained with cows in a standard 14- to 17-d MGA-PGF2α with estrus detection and AI protocol and similar to previously described TAI procedures. (Key Words: Postpartum, Time Artificial Insemination, Melengestrol Acetate, Prostaglandin F2α, Gonadotropin-Releasing Hormone.)
Introduction An ideal synchronization program should elicit a fertile, synchronized estrus in a high percentage of treated females. Distribution of estrus is also important in evaluating the potential for timed AI (TAI) (Odde, 1990). Adoption of AI in beef herds would be increased with the development of an economical method of TAI with high fertility (Wood et al., 2001). Melengestrol acetate (MGA) is easily administered orally and is approved for use in reproductive classes of beef
and dairy cattle (Wood et al., 2001). Brown et al. (1988) and Patterson et al. (1995) obtained estrus synchronization and acceptable conception rates in heifers and cows fed MGA for 14 d, followed by injection of prostaglandin F2a (PGF2α) on d 17 after MGA withdrawal. Synchrony of estrus and conception rates were acceptable, but variability in onset of estrus makes this procedure inadequate for TAI (Beal, 1998). Wood et al. (2001) observed improved synchrony of estrus in heifers treated with gonadotropinreleasing hormone (GnRH) to synchronize follicular development at 12 d after MGA withdrawal. When cows without a functional corpus luteum (CL) are fed MGA, a persistent follicle develops and ovulates following MGA withdrawal (Sirois and Fortune, 1990; Savio et al., 1993), but fertility is reduced (Ahmed et al., 1995). Although formation of a persistent follicle is considered a detrimental event, presence of the persistent follicle would presumably synchronize follicular waves (Adams et al., 1994; Kastelic et al., 1990; Ko et al., 1991). Withdrawal of progestogen leads to estrus and ovulation or regression of the dominant follicle (Twagiramungu et al., 1995; Yelich et al.,
18
1997) and development of a new follicular wave in 3 to 4 d. Therefore, it is likely that a large proportion of animals exposed to 14 d of MGA followed by PGF2α 17 d after MGA withdrawal would possess follicles that are relatively synchronized in development and could be induced to ovulate in a synchronous manner by GnRH (Twagiramungu et al., 1995). This study was conducted to evaluate the effects of adding a GnRH injection at 48 h after PGF2α to a MGAPGF2α protocol on conception rate (CR) and pregnancy rate (PR) to TAI in postpartum beef cows.
Materials and Methods Animals and Treatments. Lactating, predominantly straightbred, Angus cows were used in this study. All animal procedures utilized in this study were approved by the University of Arkansas Institutional Animal Care and Use Committee and conformed to accepted guidelines (FASS, 1999). One hundred twenty-one cows from Herd 1 (n = 69) and Herd 2 (n = 52) were arranged by parity, days postpartum, body condition score (BCS; Richards, et al., 1986), and BW and randomly assigned to one of two treatments. Cows were classified as either primiparous or multiparous. There were 12 primiparous cows in Herd 1 and 10 primiparous cows in Herd 2. Cows had to be at least 14 d postpartum at the start of MGA feeding to be included in the study. Any cow that lost a calf and was therefore no longer lactating was not included in the study. Herds were managed separately, but pasture and management conditions were not different between herds. Within herd, treatment groups were managed as a contemporary group, grazing predominantly fescue (Festuca arundinacea) pastures before, during, and after the treatment period. Ad libitum access to water was provided, and fescue hay was provided anytime forage availability became limiting.
Post et al.
The first day of MGA feeding was designated as d 1. Cows in both treatment groups received 1.8 kg of a supplement containing 0.5 mg of MGA daily for 14 d (d 1 to 14), then received supplement without MGA for 17 d (d 15 to 31). At 17 d after the last MGA feeding (d 31 of the experiment), cows in both treatment groups received an i.m. injection of 25 mg of PGF2α (Lutalyse威; Pfizer Animal Health, New York, NY). Cows receiving the MGA-PGF2α (MPG) treatment were fitted with HeatWatch威 (DDx Inc., Denver, CO) detectors 1 d prior to the PGF2α injection. Following the PGF2α injection, cows were monitored continuously for estrus for an 8-d period (d 32 to 39). Cows exhibiting standing estrus were inseminated at 12 to16 h after the start of estrus, by an experienced AI technician, using semen from the same Angus sire. Cows receiving the MGA-PGF2αGnRH (MPGG) treatment received an i.m. injection of 100 µg of GnRH (Fertagyl威; Intervet Inc., Millsboro, DE) 48 h after the PGF2α injection (d 33) and were inseminated 16 to 18 h later. Cows receiving MPGG were visually checked for estrus at least twice daily from the time of the PGF2α injection until the TAI to determine the occurrence of estrus. The BCS for animals in each treatment group were also assigned at the time of AI. At 14 d following AI, cows in both treatment groups were placed with a fertile Angus bull for the remainder of a 60-d breeding season. Cows receiving the MPG treatment that did not show estrus during the 8-d AI period were immediately placed with a bull for the remainder of the breeding season. Pregnancy to AI was confirmed by transrectal ultrasonography (Aloka 500V; Corometrics Medical Systems, Wallingford, CT) with a 5-MHz linear transducer probe at approximately 35 d after AI. Fetal size was used to determine whether pregnancy was from AI or natural service (Curran et al., 1986). Overall pregnancy rate was determined by rectal palpation at approximately 60 d after bull removal.
Blood Collection and Hormone Assays. Ten-milliliter blood samples were collected by jugular venipuncture from all cows on d −7, 1, and 31. Samples were placed on ice, allowed to clot, and then centrifuged at 4°C at 1000 × g for 20 min. Serum was harvested and stored at −20°C until assayed for progesterone by radioimmunoassay (Coat-A-Count威; Diagnostic Products Corporation, Los Angeles, CA). Use of this assay in the bovine has been previously validated by Vizcarra et al. (1997). Intra- and interassay coefficients of variation for progesterone were 6.3 and 15.4%, respectively. Cows with a serum progesterone concentration ≥1 ng/mL on either d −7 or 1 were judged to be cycling at the start of the experiment (Chenault et al., 2003). Cows with a serum progesterone concentration ≥1 ng/mL on d 31 were judged to have a functional CL at the time of the PGF2α injection (Chenault et al., 2003). Statistical Analysis. Treatment effects on dependant variables were compared using the general linear models procedure of SAS威 (SAS Inst. Inc., Cary, NC). Effects in the model included treatment, herd, parity, and the two-way interactions. Three-way interactions were not tested because of limited degrees of freedom within herd and parity. With the exception of the actual time required for AI, animals in both treatments, within each herd, were managed together on the same pastures in the same contemporary group; therefore, animal was considered the experimental unit. The FREQ procedure and the chi-square test were used to compare percent cycling, percentage of cows with a functional CL, CR to AI, PR to AI, and overall pregnancy rate (OP). The CR was defined as the percentage of cows that conceived to AI that were bred to AI, and PR was the percentage of cows that conceived to AI out of the total in the treatment group. The OP was the percentage of cows that were pregnant to either AI or natural service during the entire breeding season.
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Timed AI with Melengestrol Acetate in Beef Cows
TABLE 1. Least squares means and standard errors for days postpartum at start of treatment, initial BW, initial body condition score (BCSa), BCS at breeding, and percentage of anestrous cows. Treatment MPGb
MPGGc
P
59 42.4 ± 1.9 474.1 ± 12.1 5.2 ± 0.1 5.1 ± 0.1 36 (21/59)
62 40.3 ± 1.8 489.5 ± 12.0 5.3 ± 0.1 5.1 ± 0.1 48 (30/62)
— 0.60 0.19 0.39 0.80 0.15
Item Cows (n) Postpartum (d) BW (kg) BCS at start of MGA BCS at breeding Anestrous cows, % (no.) a
Nine-point scale, where 1 = emaciated to 9 = obese (Richards et al., 1986). MPG = MGA (melengestrol acetate) − prostaglandin F2α + estrus detection and AI. c MPGG = MGA − prostaglandin F2α − gonadotropin-releasing hormone + timed AI. b
Results and Discussion Initial Variables. Average BCS at breeding did not differ (P=0.11) in Herd 1 vs Herd 2. Other initial variables measured also did not vary between herds (P>0.41). Effects of parity (primiparous vs multiparous) were significant (P<0.01) for days postpartum at the start of the study, but not for other variables. Averages of days postpartum at the start of treatment were 35 ± 1 for multiparous cows vs 48 ± 3 for primaparous cows. The difference in days postpartum is due to the fact that heifers in both of these herds are managed to calve earlier than cows. There were no significant interactions between treatment and parity or between treatment and herd for any variables examined in this study (P>0.41). Summaries of initial measures for MPG and MPGG treatments are given in Table 1. By design, initial days postpartum, initial BW, initial BCS, BCS at breeding, and percentage of cows anestrous at the start of the experiment did not differ between treatments (P≥0.15). Reproductive Performance. Conception rate, PR, and OP differed (P<0.01) between herds (48% vs 81%, 38% vs 65%, and 79% vs 96% for Herds 1 and 2, respectively). There were no significant (P>0.37) interac-
tions between treatment and herd; therefore, data are presented by treatment for the combined herds. There was no main effect of parity on CR, PR, or OR (P>0.14), and there was no parity by treatment interaction for any measured reproductive response (P>0.27); therefore, data for primiparous and multiparous cows were combined in the analysis. Conclusions regarding parity should be considered cautiously, as there was a total of only 22 primiparous cows in the study. Summaries of CR, PR, and OP by treatment are given in Table 2. The CR was not different (P=0.24) between MPG and MPGG treatments (69% vs 56%, respectively). Pregnancy rate to AI was greater in MPGG vs MPG treatments (P=0.08; 56% vs
41%). The OP to both TAI and natural service did not differ between treatments (P=0.33). There was no interaction between cycling status and herd (P>0.59) or cycling status and treatment (P=0.82) for any of the reproductive variables evaluated. There was also no difference in CR (P=0.48), PR (P=0.55), or OP (P=0.74) between cycling and non-cycling cows (means not shown). The precise control of the estrous cycle needed for the successful use of a TAI procedure in cows requires the synchronization of both follicular waves and CL regression (Fogwell et al., 1986). Kojima et al. (1995) and Kinder et al. (1996) demonstrated that feeding of MGA for a period beyond the normal lifespan of a CL does not suppress the pulsatile release of luteinizing hormone and leads to the persistence of the dominant follicle. In this study, we presumed that presence of a persistent dominant follicle would suppress the development of subordinate follicles and prevent the emergence of the next follicular wave as described by Adams et al. (1994), Kastelic et al. (1990), and Ko et al. (1991) and that the withdrawal of MGA (progestogen) would lead to estrus and ovulation of the dominant follicle (Roche et al., 1999), with the emergence of a new follicular wave in 3 to 4 d (Twagiramungu et al., 1995). Based on these assumptions, we reasoned that follicles in a large number of cows would be, in effect, synchronized and would be responsive to an
TABLE 2. Conception rate to AI (CR), pregnancy rate to AI (PR), and overall pregnancy rate (OP) by treatment. Treatment Item CR, % (no.) PR, % (no.) OP, % (no.)
MPGa
MPGGb
P
69 (24/35) 41 (24/59) 90 (53/59)
56 (35/62) 56 (35/62) 84 (52/62)
0.24 0.08 0.33
MPG = MGA (melengestrol acetate) − prostaglandin F2α + estrus detection and AI. MPGG = MGA − prostaglandin F2α − gonadotropin-releasing hormone + timed AI.
a
b
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injection of GnRH to induce ovulation. Although follicular waves and follicular development were not specifically characterized in this study, the CR and PR observed suggested that a large number of cows in the MPGG treatment group did have follicles that were responsive to GnRH. The CR and PR were equal to or greater than those observed in cows treated with MGA and PGF2α and bred 12 to 16 h after the onset of estrus. Additional studies will be required to characterize the response of follicular waves to the treatment regime used in this study. The CR observed in the present study was similar to that observed by Perry et al. (2002a) in a study in which cows on a 14-d MGA treatment received a GnRH injection on d 12 after MGA withdrawal and a second GnRH injection at 72 h after PGF2α at the time of AI. Results were also similar to those observed by Martinez et al. (2002) using 7 d of MGA + Co-Synch or a controlled intravaginal drug releasing device + Co-Synch protocol and were similar to or greater than observed with other TAI protocols (Schmitt et al., 1996; Perry et al., 2002b; Stevenson et al., 2003). No difference was observed in CR of anestrous MPG and MPGG cows in this study. In contrast, Patterson et al. (2002) demonstrated an increase in estrus response in postpartum beef cows in which anestrus rates were high. In that study, cows received GnRH injections at 7 d prior to PGF2α rather than at 48 h after PGF2α. Patterson et al. (1995) reported a high incidence of twinning in cows fed MGA for 14 d followed by the injection of PGF2α. However, no increase in twinning was observed in the current study. Further, Perry et al. (2002a) observed no effect of MGA on the number of follicles recruited or growth rate of dominant follicles in either cycling on noncycling postpartum beef cows. In summary, an injection of GnRH at 48 h after the PGF2α injection in a standard 14- to 17-d MGA-PGF2α estrus synchronization protocol can be
Post et al.
used to achieve pregnancy rates to TAI that are comparable with other currently described methods. The use of MGA for the synchronization of estrus with TAI may offer advantages for some producers. It is easily administered in feed and does not require animals to be handled or restrained and is economical to use at approximately $0.02/d per head (Wood et al., 2001).
Implications The use of TAI for breeding postpartum beef cows can eliminate the time, labor, and difficulties of estrus detection associated with breeding at estrus and could potentially increase the use of AI in cow herds. This protocol (14 d of MGA, followed by PGF2α 17 d after MGA, GnRH 48 h after PGF2α with insemination 16 to 18 h later) offers a relatively simple and inexpensive alteration to an existing protocol and gave acceptable results relative to other TAI procedures. The expanded use of AI would be beneficial to many beef producers. The exact TAI protocol best suited to individual producers is influenced by a number management factors specific to individual operations.
Acknowledgments This research was supported by the University of Arkansas, Agricultural Experiment Station. The authors gratefully acknowledge Pfizer Inc. (New York, NY) for the contribution of Lutalyse威 sterile solution for this research.
Literature Cited Adams, G. P., L. F. Nasser, G. A. Bo, A. Garcia, M. R. Del Campo, and R. J. Mapletoft. 1994. Superstimulatory response of ovarian follicles of wave 1 versus wave 2 in heifers. Theriogenology. 42:1103.
Ahmad, N., F. N. Schrick, R. L. Butcher, and E. K. Inskeep. 1995. Effect of persistent follicles on early embryonic losses in beef cows. Biol. Reprod. 52:1129. Beal, W. E. 1998. Current estrus synchronization and artificial insemination programs for cattle. J. Anim. Sci. 76(Suppl. 3):30. Brown, L. N., K. G. Odde, M. E. King, D. G. LaFever, and C. J. Neubauer. 1988. Comparison of melengestrol acetate-prostaglandin F2α to Syncro-mate B for estrous synchronization in beef heifers. Theriogenology. 30:1. Chenault, J. R., J. F. Boucher, and H. D. Hafs. 2003. Symposium paper: Synchronization of estrus in beef cows and beef and dairy heifers with intravaginal progesterone inserts and prostaglandin F2α with or without gonadotropin-releasing hormone. Prof. Anim. Sci. 19:116. Curran S., R. A. Pierson, and O. J. Ginther. 1986. Ultrasonographic appearance of the bovine conceptus from days 20 through 60. J. AVMA 189(10):1295. Fogwell, R. L., B. M. Kanyima, A. Villa-Gody, W. J. Enright, and J. J. Ireland. 1986. Enhanced precision of estrus and luteinizing hormone after progesterone and prostaglandin in heifers. J. Dairy Sci. 62:2179. Kastelic, J. P., J. C. H. Ko, and O. J. Ginther. 1990. Suppression of dominant and subordinate ovarian follicles by a proteinaceous fraction of follicular fluid in heifers. Theriogenology. 34:499. Kinder, J. E., F. N. Kojima, E. G. M. Bergfeld, M. E. Wehrman, and K. E. Fike. 1996. Progestin and estrogen regulation of pulsatile LH release and development of persistent ovarian follicles in cattle. J. Anim. Sci. 74:1424. Ko, J. C. H., J. P. Kastelic, M. R. Del Campo, and O. J. Ginther. 1991. Effects of a dominant follicle on ovarian follicular dynamics during the estrous cycle in heifers. J. Reprod. Fertil. 91:511. Kojima, F. N., J. R. Chenault, M. E. Wehrman, E. G. Bergfeld, A. S. Cupp, L. A. Werth, V. Mariscal, T. Sanchez, R. J. Kittock, and J. E. Kinder. 1995. Melengestrol acetate at greater doses than typically used for estrous synchrony in bovine females does not mimic endogenous progesterone in regulation of secretion of luteinizing hormone and 17β-estradiol. Biol. Reprod. 52:455. Martinez, M. F., J. P. Kastelic, G. P. Adams, and R. J. Mapletoft. 2002. The use of a progesterone-releasing device (CIDR-B) or melengestrol acetate with GnRH, LH, or estradiol benzoate for fixed-time AI in beef heifers. J. Anim. Sci. 80:1746. Odde, K. J. 1990. A review of synchronization of estrus in postpartum cattle. J. Anim. Sci. 68:817. Patterson, D. J., J. B. Hall, N. W. Bradley, K. K. Schillo, B. L. Woods, and J. M. Kearnan. 1995. Improved synchrony, conception rate, and fecundity in postpartum suckled beef cows fed melengestrol acetate prior to prostaglandin F2α. J. Anim. Sci. 73:954.
Timed AI with Melengestrol Acetate in Beef Cows
Patterson, D. J., J. E. Stegner, F. N. Kojima, and M. F. Smith. 2002. MGA Select improves estrus response in postpartum beef cows in situations accompanied with high rates of anestrus. Proc. Western Section Am. Soc. Anim. Sci. 53:418. Perry G. A., F. N. Kojima, B. E. Salfen, J. F. Bader, D. J. Patterson, and M. F. Smith. 2002a. Effect of an orally active progestin on follicular dynamics in cycling and anestrous postpartum beef cows. J. Anim Sci, 80:1932. Perry, G. A., M. F. Smith, and D. J. Patterson. 2002b. Evaluation of a fixed-time artificial insemination protocol for postpartum suckled beef cows. J. Anim. Sci. 80:3060. Richards, M. W., J. C. Spitzer, and M. B. Warner. 1986. Effect of varying levels of postpartum nutrition and body condition at calving on subsequent reproductive performance in beef cattle. J. Anim. Sci. 62:300.
1993. Effects of induction of low plasma progesterone concentrations with a progesteronereleasing intravaginal device on follicular turnover and fertility in cattle. J. Reprod. Fertil. 98:77. Schmitt, E´. J. -P., T. Diaz, M. Drost, and W. W. Thatcher. 1996. Use of a gonadotropin-releasing hormone agonist or human chorionic gonadotropin for timed insemination in cattle. J. Anim. Sci. 74:1084. Sirois, J., and J. E. Fortune. 1990. Lengthening the bovine estrous cycle with low levels of exogenous progesterone: a model for studying ovarian follicular dominance. Endocrinology. 127:916.
Roche, J. F., E. J. Austin, M. Ryan, M. O’Rourke, M. Mihm, and M. G. Diskin. 1999. Regulation of follicle waves to maximize fertility in cattle. J. Reprod. Fertil. Suppl. 54:61.
Stevenson, J. S., G. C. Lamb, S. K. Johnson, M. A. Medina-Britos, D. M Grieger, K. R. Harmoney, J. A. Cartmill, S. K. El-Zarkauny, C. R. Dahlen, and T. J. Marple. 2003. Supplemental norgestomet, progesterone, or melengestrol acetate increases pregnancy rates in suckled beef cows after timed inseminations. J. Anim. Sci. 81:571.
Savio, J. D., W. W. Thatcher, G. R. Morris, K. Entwistle, M. Drost, and M. R. Mattiacci.
Twagiramungu, H., L. A. Guilbault, and J. J. Dufour. 1995. Synchronization of ovarian fol-
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licular waves with a gonadotropin-releasing hormone agonist to increase the precision of estrus in cattle: A review. J. Anim. Sci. 73:3141. Vizcarra J. A., R. P. Wettemann, T. D. Braden, A. M. Turzillo and T. M. Nett. 1997. Effect of gonadotropin-releasing hormone (GnRH) pulse frequency on serum and pituitary concentrations of luteinizing hormone and follicle-stimulating hormone, GnRH receptors, and messenger ribonucleic acid for gonadotropin subunits in cows. Endocrinology 138(2):594. Wood, S. L., M. C. Lucy, M. F. Smith, and D. J. Patterson. 2001. Improved synchrony of estrus and ovulation with the addition of GnRH to a melengestrol acetate-prostaglandin F2α synchronization treatment in beef heifers. J. Anim. Sci. 79:2210. Yelich, J. V., R. D. Geisert, R. A. M. Schmitt, G. L. Morgan, and J. P. McCann. 1997. Persistence of the dominant follicle during melengestrol acetate administration and its regression by exogenous estrogen treatment in beef cattle. J. Anim. Sci. 75:745.
Timed Artificial Insemination in Postpartum Beef Cows Using Melengestrol Acetate, Prostaglandin F2α, and Gonadotropin-Releasing Hormone N. M. POST, D. L. KREIDER1, R. W. RORIE, and T. D. LESTER Department of Animal Science, University of Arkansas, Fayetteville 72701
Abstract This study evaluated the use of melengestrol acetate (MGA), prostaglandin F2α (PGF2α) and gonadotropin-releasing hormone (GnRH) in a timed AI (TAI) protocol in postpartum beef cows. One hundred twenty-one Angus-crossbred cows in two herds were ranked by parity, days postpartum, body condition score (BCS), and BW and then randomly assigned to one of two treatments. Cows in both treatments received 1.8 kg of supplement containing 0.5 mg/d of MGA for 14 d, then received supplement without MGA for 17 d. After MGA withdrawal, cows in both groups received an injection of PGF2α. Cows receiving the MGAPGF2α (MPG) treatment were subjected to estrus detection and AI for 8 d. Cows receiving the MGA-PGF2α-GnRH (MPGG) treatment were injected with GnRH 48 h after PGF2α, followed by TAI 16 to 18 h after PGF2α. There was a difference (P<0.01) between herds in conception rate (CR) and pregnancy rate to AI (PR) and overall pregnancy rate
1
To whom correspondence should be addressed: [email protected]
(OP). The CR was not different between treatments (P≥0.24), but PR was greater in MPGG vs MPG (P≤0.08). There was no difference in OP between MPG and MPGG treatments (P≥0.25). The TAI procedure described in this study yielded results that were equal to those obtained with cows in a standard 14- to 17-d MGA-PGF2α with estrus detection and AI protocol and similar to previously described TAI procedures. (Key Words: Postpartum, Time Artificial Insemination, Melengestrol Acetate, Prostaglandin F2α, Gonadotropin-Releasing Hormone.)
Introduction An ideal synchronization program should elicit a fertile, synchronized estrus in a high percentage of treated females. Distribution of estrus is also important in evaluating the potential for timed AI (TAI) (Odde, 1990). Adoption of AI in beef herds would be increased with the development of an economical method of TAI with high fertility (Wood et al., 2001). Melengestrol acetate (MGA) is easily administered orally and is approved for use in reproductive classes of beef
and dairy cattle (Wood et al., 2001). Brown et al. (1988) and Patterson et al. (1995) obtained estrus synchronization and acceptable conception rates in heifers and cows fed MGA for 14 d, followed by injection of prostaglandin F2a (PGF2α) on d 17 after MGA withdrawal. Synchrony of estrus and conception rates were acceptable, but variability in onset of estrus makes this procedure inadequate for TAI (Beal, 1998). Wood et al. (2001) observed improved synchrony of estrus in heifers treated with gonadotropinreleasing hormone (GnRH) to synchronize follicular development at 12 d after MGA withdrawal. When cows without a functional corpus luteum (CL) are fed MGA, a persistent follicle develops and ovulates following MGA withdrawal (Sirois and Fortune, 1990; Savio et al., 1993), but fertility is reduced (Ahmed et al., 1995). Although formation of a persistent follicle is considered a detrimental event, presence of the persistent follicle would presumably synchronize follicular waves (Adams et al., 1994; Kastelic et al., 1990; Ko et al., 1991). Withdrawal of progestogen leads to estrus and ovulation or regression of the dominant follicle (Twagiramungu et al., 1995; Yelich et al.,
18
1997) and development of a new follicular wave in 3 to 4 d. Therefore, it is likely that a large proportion of animals exposed to 14 d of MGA followed by PGF2α 17 d after MGA withdrawal would possess follicles that are relatively synchronized in development and could be induced to ovulate in a synchronous manner by GnRH (Twagiramungu et al., 1995). This study was conducted to evaluate the effects of adding a GnRH injection at 48 h after PGF2α to a MGAPGF2α protocol on conception rate (CR) and pregnancy rate (PR) to TAI in postpartum beef cows.
Materials and Methods Animals and Treatments. Lactating, predominantly straightbred, Angus cows were used in this study. All animal procedures utilized in this study were approved by the University of Arkansas Institutional Animal Care and Use Committee and conformed to accepted guidelines (FASS, 1999). One hundred twenty-one cows from Herd 1 (n = 69) and Herd 2 (n = 52) were arranged by parity, days postpartum, body condition score (BCS; Richards, et al., 1986), and BW and randomly assigned to one of two treatments. Cows were classified as either primiparous or multiparous. There were 12 primiparous cows in Herd 1 and 10 primiparous cows in Herd 2. Cows had to be at least 14 d postpartum at the start of MGA feeding to be included in the study. Any cow that lost a calf and was therefore no longer lactating was not included in the study. Herds were managed separately, but pasture and management conditions were not different between herds. Within herd, treatment groups were managed as a contemporary group, grazing predominantly fescue (Festuca arundinacea) pastures before, during, and after the treatment period. Ad libitum access to water was provided, and fescue hay was provided anytime forage availability became limiting.
Post et al.
The first day of MGA feeding was designated as d 1. Cows in both treatment groups received 1.8 kg of a supplement containing 0.5 mg of MGA daily for 14 d (d 1 to 14), then received supplement without MGA for 17 d (d 15 to 31). At 17 d after the last MGA feeding (d 31 of the experiment), cows in both treatment groups received an i.m. injection of 25 mg of PGF2α (Lutalyse威; Pfizer Animal Health, New York, NY). Cows receiving the MGA-PGF2α (MPG) treatment were fitted with HeatWatch威 (DDx Inc., Denver, CO) detectors 1 d prior to the PGF2α injection. Following the PGF2α injection, cows were monitored continuously for estrus for an 8-d period (d 32 to 39). Cows exhibiting standing estrus were inseminated at 12 to16 h after the start of estrus, by an experienced AI technician, using semen from the same Angus sire. Cows receiving the MGA-PGF2αGnRH (MPGG) treatment received an i.m. injection of 100 µg of GnRH (Fertagyl威; Intervet Inc., Millsboro, DE) 48 h after the PGF2α injection (d 33) and were inseminated 16 to 18 h later. Cows receiving MPGG were visually checked for estrus at least twice daily from the time of the PGF2α injection until the TAI to determine the occurrence of estrus. The BCS for animals in each treatment group were also assigned at the time of AI. At 14 d following AI, cows in both treatment groups were placed with a fertile Angus bull for the remainder of a 60-d breeding season. Cows receiving the MPG treatment that did not show estrus during the 8-d AI period were immediately placed with a bull for the remainder of the breeding season. Pregnancy to AI was confirmed by transrectal ultrasonography (Aloka 500V; Corometrics Medical Systems, Wallingford, CT) with a 5-MHz linear transducer probe at approximately 35 d after AI. Fetal size was used to determine whether pregnancy was from AI or natural service (Curran et al., 1986). Overall pregnancy rate was determined by rectal palpation at approximately 60 d after bull removal.
Blood Collection and Hormone Assays. Ten-milliliter blood samples were collected by jugular venipuncture from all cows on d −7, 1, and 31. Samples were placed on ice, allowed to clot, and then centrifuged at 4°C at 1000 × g for 20 min. Serum was harvested and stored at −20°C until assayed for progesterone by radioimmunoassay (Coat-A-Count威; Diagnostic Products Corporation, Los Angeles, CA). Use of this assay in the bovine has been previously validated by Vizcarra et al. (1997). Intra- and interassay coefficients of variation for progesterone were 6.3 and 15.4%, respectively. Cows with a serum progesterone concentration ≥1 ng/mL on either d −7 or 1 were judged to be cycling at the start of the experiment (Chenault et al., 2003). Cows with a serum progesterone concentration ≥1 ng/mL on d 31 were judged to have a functional CL at the time of the PGF2α injection (Chenault et al., 2003). Statistical Analysis. Treatment effects on dependant variables were compared using the general linear models procedure of SAS威 (SAS Inst. Inc., Cary, NC). Effects in the model included treatment, herd, parity, and the two-way interactions. Three-way interactions were not tested because of limited degrees of freedom within herd and parity. With the exception of the actual time required for AI, animals in both treatments, within each herd, were managed together on the same pastures in the same contemporary group; therefore, animal was considered the experimental unit. The FREQ procedure and the chi-square test were used to compare percent cycling, percentage of cows with a functional CL, CR to AI, PR to AI, and overall pregnancy rate (OP). The CR was defined as the percentage of cows that conceived to AI that were bred to AI, and PR was the percentage of cows that conceived to AI out of the total in the treatment group. The OP was the percentage of cows that were pregnant to either AI or natural service during the entire breeding season.
19
Timed AI with Melengestrol Acetate in Beef Cows
TABLE 1. Least squares means and standard errors for days postpartum at start of treatment, initial BW, initial body condition score (BCSa), BCS at breeding, and percentage of anestrous cows. Treatment MPGb
MPGGc
P
59 42.4 ± 1.9 474.1 ± 12.1 5.2 ± 0.1 5.1 ± 0.1 36 (21/59)
62 40.3 ± 1.8 489.5 ± 12.0 5.3 ± 0.1 5.1 ± 0.1 48 (30/62)
— 0.60 0.19 0.39 0.80 0.15
Item Cows (n) Postpartum (d) BW (kg) BCS at start of MGA BCS at breeding Anestrous cows, % (no.) a
Nine-point scale, where 1 = emaciated to 9 = obese (Richards et al., 1986). MPG = MGA (melengestrol acetate) − prostaglandin F2α + estrus detection and AI. c MPGG = MGA − prostaglandin F2α − gonadotropin-releasing hormone + timed AI. b
Results and Discussion Initial Variables. Average BCS at breeding did not differ (P=0.11) in Herd 1 vs Herd 2. Other initial variables measured also did not vary between herds (P>0.41). Effects of parity (primiparous vs multiparous) were significant (P<0.01) for days postpartum at the start of the study, but not for other variables. Averages of days postpartum at the start of treatment were 35 ± 1 for multiparous cows vs 48 ± 3 for primaparous cows. The difference in days postpartum is due to the fact that heifers in both of these herds are managed to calve earlier than cows. There were no significant interactions between treatment and parity or between treatment and herd for any variables examined in this study (P>0.41). Summaries of initial measures for MPG and MPGG treatments are given in Table 1. By design, initial days postpartum, initial BW, initial BCS, BCS at breeding, and percentage of cows anestrous at the start of the experiment did not differ between treatments (P≥0.15). Reproductive Performance. Conception rate, PR, and OP differed (P<0.01) between herds (48% vs 81%, 38% vs 65%, and 79% vs 96% for Herds 1 and 2, respectively). There were no significant (P>0.37) interac-
tions between treatment and herd; therefore, data are presented by treatment for the combined herds. There was no main effect of parity on CR, PR, or OR (P>0.14), and there was no parity by treatment interaction for any measured reproductive response (P>0.27); therefore, data for primiparous and multiparous cows were combined in the analysis. Conclusions regarding parity should be considered cautiously, as there was a total of only 22 primiparous cows in the study. Summaries of CR, PR, and OP by treatment are given in Table 2. The CR was not different (P=0.24) between MPG and MPGG treatments (69% vs 56%, respectively). Pregnancy rate to AI was greater in MPGG vs MPG treatments (P=0.08; 56% vs
41%). The OP to both TAI and natural service did not differ between treatments (P=0.33). There was no interaction between cycling status and herd (P>0.59) or cycling status and treatment (P=0.82) for any of the reproductive variables evaluated. There was also no difference in CR (P=0.48), PR (P=0.55), or OP (P=0.74) between cycling and non-cycling cows (means not shown). The precise control of the estrous cycle needed for the successful use of a TAI procedure in cows requires the synchronization of both follicular waves and CL regression (Fogwell et al., 1986). Kojima et al. (1995) and Kinder et al. (1996) demonstrated that feeding of MGA for a period beyond the normal lifespan of a CL does not suppress the pulsatile release of luteinizing hormone and leads to the persistence of the dominant follicle. In this study, we presumed that presence of a persistent dominant follicle would suppress the development of subordinate follicles and prevent the emergence of the next follicular wave as described by Adams et al. (1994), Kastelic et al. (1990), and Ko et al. (1991) and that the withdrawal of MGA (progestogen) would lead to estrus and ovulation of the dominant follicle (Roche et al., 1999), with the emergence of a new follicular wave in 3 to 4 d (Twagiramungu et al., 1995). Based on these assumptions, we reasoned that follicles in a large number of cows would be, in effect, synchronized and would be responsive to an
TABLE 2. Conception rate to AI (CR), pregnancy rate to AI (PR), and overall pregnancy rate (OP) by treatment. Treatment Item CR, % (no.) PR, % (no.) OP, % (no.)
MPGa
MPGGb
P
69 (24/35) 41 (24/59) 90 (53/59)
56 (35/62) 56 (35/62) 84 (52/62)
0.24 0.08 0.33
MPG = MGA (melengestrol acetate) − prostaglandin F2α + estrus detection and AI. MPGG = MGA − prostaglandin F2α − gonadotropin-releasing hormone + timed AI.
a
b
20
injection of GnRH to induce ovulation. Although follicular waves and follicular development were not specifically characterized in this study, the CR and PR observed suggested that a large number of cows in the MPGG treatment group did have follicles that were responsive to GnRH. The CR and PR were equal to or greater than those observed in cows treated with MGA and PGF2α and bred 12 to 16 h after the onset of estrus. Additional studies will be required to characterize the response of follicular waves to the treatment regime used in this study. The CR observed in the present study was similar to that observed by Perry et al. (2002a) in a study in which cows on a 14-d MGA treatment received a GnRH injection on d 12 after MGA withdrawal and a second GnRH injection at 72 h after PGF2α at the time of AI. Results were also similar to those observed by Martinez et al. (2002) using 7 d of MGA + Co-Synch or a controlled intravaginal drug releasing device + Co-Synch protocol and were similar to or greater than observed with other TAI protocols (Schmitt et al., 1996; Perry et al., 2002b; Stevenson et al., 2003). No difference was observed in CR of anestrous MPG and MPGG cows in this study. In contrast, Patterson et al. (2002) demonstrated an increase in estrus response in postpartum beef cows in which anestrus rates were high. In that study, cows received GnRH injections at 7 d prior to PGF2α rather than at 48 h after PGF2α. Patterson et al. (1995) reported a high incidence of twinning in cows fed MGA for 14 d followed by the injection of PGF2α. However, no increase in twinning was observed in the current study. Further, Perry et al. (2002a) observed no effect of MGA on the number of follicles recruited or growth rate of dominant follicles in either cycling on noncycling postpartum beef cows. In summary, an injection of GnRH at 48 h after the PGF2α injection in a standard 14- to 17-d MGA-PGF2α estrus synchronization protocol can be
Post et al.
used to achieve pregnancy rates to TAI that are comparable with other currently described methods. The use of MGA for the synchronization of estrus with TAI may offer advantages for some producers. It is easily administered in feed and does not require animals to be handled or restrained and is economical to use at approximately $0.02/d per head (Wood et al., 2001).
Implications The use of TAI for breeding postpartum beef cows can eliminate the time, labor, and difficulties of estrus detection associated with breeding at estrus and could potentially increase the use of AI in cow herds. This protocol (14 d of MGA, followed by PGF2α 17 d after MGA, GnRH 48 h after PGF2α with insemination 16 to 18 h later) offers a relatively simple and inexpensive alteration to an existing protocol and gave acceptable results relative to other TAI procedures. The expanded use of AI would be beneficial to many beef producers. The exact TAI protocol best suited to individual producers is influenced by a number management factors specific to individual operations.
Acknowledgments This research was supported by the University of Arkansas, Agricultural Experiment Station. The authors gratefully acknowledge Pfizer Inc. (New York, NY) for the contribution of Lutalyse威 sterile solution for this research.
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Timed AI with Melengestrol Acetate in Beef Cows
Patterson, D. J., J. E. Stegner, F. N. Kojima, and M. F. Smith. 2002. MGA Select improves estrus response in postpartum beef cows in situations accompanied with high rates of anestrus. Proc. Western Section Am. Soc. Anim. Sci. 53:418. Perry G. A., F. N. Kojima, B. E. Salfen, J. F. Bader, D. J. Patterson, and M. F. Smith. 2002a. Effect of an orally active progestin on follicular dynamics in cycling and anestrous postpartum beef cows. J. Anim Sci, 80:1932. Perry, G. A., M. F. Smith, and D. J. Patterson. 2002b. Evaluation of a fixed-time artificial insemination protocol for postpartum suckled beef cows. J. Anim. Sci. 80:3060. Richards, M. W., J. C. Spitzer, and M. B. Warner. 1986. Effect of varying levels of postpartum nutrition and body condition at calving on subsequent reproductive performance in beef cattle. J. Anim. Sci. 62:300.
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