E. A. Hiers*, C. R. Barthle*, MK. V. Dahms*, G. E. Portillo*, G. A. Bridges*, D. O. Rae, W. W. Thatcher*, and J. V. Yelich* 3

Transcription

1 Synchronization of Bos indicus Bos taurus cows for timed artificial insemination using gonadotropin-releasing hormone plus prostaglandin F 2α in combination with melengestrol acetate 1,2 E. A. Hiers*, C. R. Barthle*, MK. V. Dahms*, G. E. Portillo*, G. A. Bridges*, D. O. Rae, W. W. Thatcher*, and J. V. Yelich* 3 *Department of Animal Sciences and College of Veterinary Medicine, University of Florida, Gainesville, ABSTRACT: Nonlactating Bos indicus Bos taurus cows were used in three herds to determine the efficacy of different PGF 2α treatments in combination with GnRH and melengestrol acetate (MGA) for a timed artificial insemination protocol. The start of the experiment was designated as d 0, at which time cows were assigned a body condition score and received 100 g of GnRH. Cows were fed MGA (0.5 mg cow 1 d 1 )ond1to7.on d 7, cows received either a single injection of PGF 2α (Lutalyse sterile solution; 25 mg; n = 297), a single injection of cloprostenol sodium (Estrumate; 500 g; n = 297), or half the recommended dose of PGF 2α (12.5 mg; n = 275) on d 7 and 8. On d 10, all cows were artificially inseminated and received 100 g of GnRH. Pregnancy rates to the timed artificial insemination (39%) were not affected by treatment, herd, or treatment herd. There was an effect (P < 0.01) of artificial insemination sire on timed artificial insemination pregnancy rate for one herd, but not the other two herds. Herd influenced (P < 0.05) 30-d pregnancy rates, but there were no treatment or treatment herd effects as 72.3% of the cows became pregnant during the first 30 d of the breeding season. Results indicate that the type of PGF 2α treatment administered 7 d after GnRH did not influence timed artificial insemination pregnancy rates in nonlactating Bos indicus Bos taurus cows. Key Words: Artificial Insemination, Bos indicus, Gonadotropin-Releasing Hormone, Synchronization 2003 American Society of Animal Science. All rights reserved. J. Anim. Sci : Introduction Administration of GnRH followed 7 d later with PGF 2α to synchronize cattle for a timed-ai (TAI) isan effective synchronization system in dairy (Pursley et al., 1995; Schmitt et al., 1996a) and beef cattle (Stevenson et al., 2000; Geary et al., 2001). However, a potential problem of this protocol is that as many as 15% of the cows may exhibit estrus 1 to 2 d prior to the PGF 2α injection (Pursley et al., 1995; Schmitt et al., 1996a; Moreira et al., 2000), thus reducing the opportunity for these cows to conceive to a TAI. 1 This is Journal Series No. R of the Florida Agric. Exp. Station. 2 The authors thank Schering-Plough for the donation of the Estrumate; Intervet, Boxmeer, The Netherlands, for providing the Fertagyl; Rollins Ranch, Okeechobee, FL, and Deseret Cattle and Citrus, Deer Park, FL, for procurement and care of cattle used in the project and for graciously providing the protein supplement containing the melengestrol acetate. 3 Correspondence: phone ; fax ; yelich@animal.ufl.edu. Received July 15, Accepted November 18, Utilization of the progestogen melengestrol acetate (MGA) to suppress estrus (Zimbelman and Smith, 1966) between the GnRH and PGF 2α injections should eliminate premature expression of estrus and enhance fertility of the TAI. Administration of GnRH at the initiation of a 7-d MGA treatment increases fertility of the synchronized estrus compared with no GnRH (Martinez et al., 1998). Short-term MGA treatments have also been shown to induce estrous cycles in some anestrous cattle (Beal and Good, 1986; Patterson et al., 1989). Synchronized pregnancy rates of GnRH + PGF 2α protocols appear to be greater in Bos taurus cattle (Geary et al., 1998; Stevenson et al., 2000) compared with Bos indicus Bos taurus cattle (Lemaster et al., 2001). The reason(s) for this difference is (are) unclear, but it has been hypothesized that the corpus luteum (CL) of the Bos indicus Bos taurus female is less responsive to PGF 2α than the CL of the Bos taurus female (Lemaster et al., 2001). Whether luteolysis could be enhanced with administration of a PGF 2α analog (i.e., cloprostenol sodium) or half the recommended dose of PGF 2α (Lutalyse sterile solution, Pharmacia Animal Health, Kalamazoo, MI) administered 24 h apart is unclear. The latter has 830

2 Timed artificial insemination in Bos indicus Bos taurus cows 831 been shown to be more effective in enhancing luteolysis in Bos indicus cattle (Santos et al., 1988). The objective of this experiment was to evaluate the effectiveness of three different PGF 2α treatments in a GnRH + PGF 2α protocol combined with MGA to synchronize Bos indicus Bos taurus cows for a TAI. Materials and Methods Nonlactating Bos indicus Bos taurus cows (n = 869) were synchronized with a GnRH + PGF 2α protocol combined with short-term MGA feeding in three herds of cattle. For all herds, the genotype of cows was between ¹ ₄ and ¹ ₂ Bos indicus breeding with the remainder being of Bos taurus breeding. Herd A was located at Rollins Ranch, Okeechobee, FL, and herds B and C were located at Deseret Cattle and Citrus, Deer Park, FL. The start of the experiment was designated as d 0, at which time cows were assigned a body condition score (BCS; 1 = emaciated, 5 = moderate, 9 = extremely fat; Richards et al., 1986) and received 100 g of GnRH i.m. (Fertagyl, Intervet, Boxmeer, The Netherlands, or Factrel, Fort Dodge Animal Health, Overland Park, KS). In herd A, cows received either Fertagyl (n = 285) or Factrel (n = 148). In herds B (n = 182) and C (n = 254), all cows received Fertagyl. All cows received 0.5 mg cow 1 d 1 of MGA (MGA Premix, Pharmacia Animal Health, Kalamazoo, MI) on d 1 to 7 of the experiment. The MGA was administered in a protein cube for all herds at a rate of 0.9 kg cow 1 d 1 and cows were group fed the supplement on the ground under pasture conditions. On the last day of MGA feeding (d 7), cows were randomly assigned to one of three treatments within each herd to receive either 25 mg of PGF 2α i.m. (Lutalyse sterile solution, Pharmacia Animal Health, Kalamazoo, MI), 500 g of cloprostenol sodium i.m. (Estrumate; Schering-Plough Animal Health, Union, NJ), or 12.5 mg of PGF 2α i.m. (Lutalyse sterile solution; half-dose of PGF 2α ) on d 7 and 8. After administration of PGF 2α treatments, cows from all treatments were managed in a single pasture at all locations. In herd A, cows were TAI approximately 72 to 80 h after the last MGA feeding and received the same GnRH treatments they received on d 0 (100 g of Fertagyl or 100 g of Factrel). In herds B and C, cows were TAI approximately 72 to 80 h after the last MGA feeding and received 100 g of Fertagyl. Frozen-thawed semen from 4 sires in herd A, 10 sires in herd B, and 8 sires in herd C were used for the TAI. At the start of an insemination period within a herd, a sire was chosen at random and approximately 5 to 10 cows were consecutively inseminated to that sire. This process was repeated until all sires represented within a herd had been used once. The process was repeated starting with the first sire used until all cows were inseminated within that herd. A cumulative summary was recorded and adjustments made accordingly to ensure that all AI sires were equally distributed across treatments throughout the time period needed to complete the insemination within each herd. Cows were exposed to bulls approximately 10 d following TAI for a breeding season of approximately 60 d for all herds. Pregnancy status was determined 52 (herds B and C) and 56 d (herd A) postinsemination using a real-time B-mode ultrasound (Aloka 500V, Corometrics Medical Systems, Wallingford, CT) equipped with a 5.0-MHz transducer. Because cows were not exposed to bulls until 10 d after TAI, fetal size (Curran et al., 1986) was used to designate whether the pregnancy was from the TAI or natural service. Any fetus without the anatomical characteristics (Curran et al., 1986) of either a 60- (herds B and C) or a 56-d (herd A) fetus was classified as having become pregnant by natural service sires. In addition, any fetus with the anatomical characteristics or estimated fetal size of a (Curran et al., 1986) 30- (herds B and C) or 26-d (herd A) fetus was classified as having become pregnant in the first 30 d of the breeding season. Timed-AI pregnancy (number of animals pregnant to the TAI divided by the number treated) and 30-d pregnancy rates (number of cows pregnant during the first 30 d of the breeding season divided by the number treated) were analyzed using the GENMOD procedure of SAS (SAS Inst., Inc., Cary, NC). The independent variables tested included treatment and herd, and the interaction and effects of BCS were also tested as a covariate in each model. The effects of GnRH product on TAI pregnancy rates in herd A, Factrel (n = 148; 40.5%) vs. Fertagyl (n = 285: 42.5%), were similar so data were pooled. Because AI sires and technicians, were confounded by herd; these effects were not tested in the main model. However, within each herd, treatment, AI sire, AI technician, and their interactions were tested using the GENMOD procedure of SAS. Results There were no treatment (P = 0.38), herd (P = 0.14), or treatment herd (P = 0.51) effects on TAI pregnancy rates (Table 1). The mean BCS of cows in herd A (5.1) was greater (P < 0.001) than the mean BCS of cows in herds B and C (4.1 and 4.3, respectively). Body condition tended (P = 0.07) to influence TAI pregnancy rates, but was not significant (P = 0.20) when adjusted for herd. There was neither a treatment BCS effect (P = 0.35) nor a herd BCS effect (P = 0.38) on TAI pregnancy rates. There was an effect (P < 0.01) of AI sire on TAI pregnancy rates for herd B (Figure 1), but not for herds A or C. Within each herd, there was no treatment AI sire effect (P > 0.05) on TAI pregnancy rates. Although AI sire differences were not detected in herds A and C, TAI pregnancy rates ranged from 38 to 48% in herd A and from 0 to 53% in herd C. In herd C, only two of the 10 sires had TAI pregnancy rates 40%, whereas the remaining sires were < 40%. There were no treatment or treatment herd effects on 30-d pregnancy rates (Table 1) as 72.3% of all cows became pregnant during the first 30 d of the breeding

3 832 Hiers et al. Table 1. Pregnancy rates to a timed artificial insemination (TAI) in nonlactating Bos indicus Bos taurus cows synchronized with GnRH + PGF 2α in combination with melengestrol acetate (MGA) TAI pregnancy Pregnancy rate Item n rate, % b at 30-d, % c Treatments a PGF 2α Half-dose PGF 2α Cloprostenol Herd A d B e C f a The start of the experiment was designated as d 0. Cows were injected with GnRH (100 g) on d 0 of experiment and received MGA on d 1 to 7. On d 7, cows received either a single injection of PGF 2α (25 mg), a single injection of cloprostenol (500 g), or half the recommended dose of PGF 2α (half-dose PGF 2α ; 12.5 mg) on both d 7 and 8 of the experiment. Cows were TAI and received GnRH (100 g) on d 10 of the experiment. b Percentage of cows pregnant to TAI of total treated. c Percentage of cows pregnant during the first 30 d of the breeding season of total treated. Treatment herd (P > 0.05); herd (P < 0.01). d,e,f Means within columns that do not have a common superscript differ (P < 0.05). Figure 1. Timed artificial insemination (TAI) pregnancy rates by AI sire for herd B in nonlactating Bos indicus Bos taurus cows synchronized with GnRH + PGF 2α in combination with melengestrol acetate. Numbers in parentheses indicate the total number of cows submitted to AI for each sire. The start of the experiment was designated as d 0. Cows were injected with GnRH (100 g) on d 0 of experiment and received melengestrol acetate on d 1 to 7. On d 7, cows received either a single injection of PGF 2α (25 mg), a single injection of cloprostenol (500 g), or half the recommended dose of PGF 2α (half-dose PGF 2α ; 12.5 mg) on both d 7 and 8 of the experiment. Cows were TAI and received GnRH (100 g) on d 10 of the experiment. No treatment sire effect was detected (P > 0.10). season. However, there was a herd effect (P < 0.01) on 30-d pregnancy rates (Table 1) since more (P < 0.01) cows in herd A became pregnant in the first 30 d of the breeding season than both herds B and C. Furthermore, more (P < 0.05) cows in herd B became pregnant during the first 30 d of the breeding season than in herd C. Discussion The synchronization protocol used in the present experiment is similar to the CoSynch protocol with two exceptions. First, MGA was administered on d 1 to 7 in the present experiment, which is not part of the CoSynch protocol. Secondly, cows were TAI and injected with GnRH 72 to 80 h after PGF 2α compared with 48 to 54 h after PGF 2α in the CoSynch protocol. In the present study, cows were not administered MGA on the day of the first GnRH injection because initiating MGA concomitantly with GnRH has been shown to reduce the effectiveness of GnRH to induce ovulation in cows with dominant follicles (Pancarci et al., 1999). Furthermore, because some cattle may not have a functional CL at PGF 2α in the GnRH + PGF 2α protocol (Twagiramungu et al., 1995; Schmitt et al., 1996b; Moreira et al., 2000), MGA was administered on the day of PGF 2α to prevent premature expression of estrus and to possibly tighten the synchrony of estrus and follicle development after PGF 2α. It was speculated that waiting until 72 h after the PGF 2α to begin TAI would allow the dominant follicles adequate time to develop and respond to the second GnRH. There was no difference in TAI pregnancy rates between PGF 2α treatments in the present study. Therefore, these data suggest that either a half-dose PGF 2α or cloprostenol treatment 7 d after a GnRH injection does not increase TAI pregnancy rates compared to a single dose of PGF 2α in nonlactating Bos indicus Bos taurus cows. This agrees with a recent report by Fernandes et al. (2001), who observed similar TAI pregnancy rates in Nelore cows using either a single dose of PGF 2α or a half dose PGF 2α in an OvSynch protocol. The TAI pregnancy rates for the PGF 2α treated cows in the present study (36%) are considerably less than reports using the CoSynch protocol in cycling and noncycling Bos taurus cows (Twagiramungu et al., 1992; Stevenson et al., 2000; Geary et al., 2001), but 5% greater compared to a report in cycling and noncycling lactating Bos indicus Bos taurus cows (Lemaster et al., 2001). Furthermore, the TAI pregnancy rates for the half-dose PGF 2α and the cloprostenol sodium treatments are only slightly greater than those reported in cycling and noncycling Bos indicus Bos taurus cows synchronized with the CoSynch protocol (Lemaster et al., 2001). It is unclear why TAI pregnancy rates are consistently lower in cattle of Bos indicus Bos taurus breeding compared to cattle of Bos taurus breeding when GnRH + PGF 2α protocols are used. The ability of the PGF 2α treatments to induce luteolysis as measured by blood progesterone concentrations

4 Timed artificial insemination in Bos indicus Bos taurus cows 833 was not determined in the present study. However, a recent experiment by Hiers et al. (2001) in cycling Bos indicus Bos taurus cattle using the same three GnRH + PGF 2α treatments as the present study observed no significant difference in luteolysis as measured by blood progesterone concentrations between PGF 2α treatments. These data, as well as data from the present study, suggest that a single injection of PGF 2α used 7 d after GnRH results in similar luteolysis and TAI pregnancy rates as the half-dose PGF 2α and cloprostenol sodium treatments in nonlactating Bos indicus Bos taurus cattle. Another possibility for the decreased TAI pregnancy rates in the present study compared with similar studies in Bos taurus cattle could be related to dominant follicle development at the time of the second GnRH injection. The ability of a dominant follicle to ovulate to GnRH depends on its stage of development (Silcox et al., 1993). In a recent experiment by Thundathil et al. (1999), cows were synchronized with GnRH + cloprostenol sodium and MGA, in a manner similar to the present study and only 35% of cows were observed in estrus within 96 h after the cloprostenol sodium injection. These data suggest that the TAI, performed 72 h after PGF 2α in the present study, may have been initiated too early relative to the developmental capacity of the ovulatory follicle, resulting in decreased pregnancy rates in the present study compared with other studies (Stevenson et al., 2000; Geary et al., 2001). In the previously cited studies, cows were synchronized with the CoSynch protocol with no MGA treatment between the first GnRH and PGF 2α injections. Therefore, one could speculate that when MGA is administered on the same day as the PGF 2α, it may be necessary to extend the time interval between the PGF 2α and second GnRH in TAI to achieve maximal pregnancy rates. Conversely, development of the ovulatory follicle could be hastened by not feeding MGA on the day of PGF 2α and TAI performed either 48 or 72 h after PGF 2α. Additional research is needed to determine the proper duration of MGA treatment and the interval from PGF 2α to TAI to maximize pregnancy rates. Although there was only a significant AI sire effect on TAI pregnancy rates for herd B, there was considerable variation in TAI pregnancy rates for AI sires across all herds. Furthermore, there was not a significant treatment AI sire interaction on TAI pregnancy rates within each herd, indicating that TAI pregnancy rates were similar between AI sires across treatments. Of the 22 sires used in the present study, only eight sires had TAI pregnancy rates 40%. These data suggest that the fertility, as measured by the TAI pregnancy rates, of frozen-thawed semen of certain AI sires used in the present study may have been compromised by the synchronization and TAI protocols used. Pursley et al. (1995) reported that cows ovulate between 24 and 32 h after the second GnRH injection of the OvSynch protocol. It has also been estimated that the maximal viability of sperm in the female reproductive tract is 24 to 30 h (Walker et al., 1996). Therefore, in TAI programs where GnRH is administered at insemination, it is probable that an asynchrony between ovulation and arrival of viable sperm capable of fertilization results in decreased pregnancy rates for some sires. Administering GnRH 48 h after PGF 2α and inseminating cows approximately 12 to 16 h later (i.e., OvSynch protocol) could minimize this asynchrony. However, this requires that cattle be handled through working facilities an additional time. It is not inferred that all TAI protocols yield unacceptable pregnancy rates because acceptable pregnancy rates with some AI sires using TAI protocols were observed in the present study and a recent report by Yelich (2002) using multiple AI sires. It appears that conventional synchronization and ovulation control protocols and differences in fertility between different AI sires are just some of the factors contributing to unacceptable TAI pregnancy rates. Factors directly associated with the sperm cell could also influence TAI pregnancy rates. These include the presence or absence of specific proteins on sperm cells (Bellin et al., 1994), the effects of processing the semen during the freezing and/ or thawing process (Ennen et al., 1976), and the rate at which the acrosome reaction takes place inside the reproductive tract (Macmillan and Watson, 1975). As previously indicated, processing semen during collection and freezing can influence fertility of semen (Ennen et al., 1976). Therefore, it is plausible that semen from different collection and processing sites (i.e., custom collection and collection at established bull studs) can differ in its fertility and this may have attributed to differences observed in TAI pregnancy rates in the present experiment. Additional research must be conducted to develop a test that can identify AI sires that can be used specifically in TAI protocols and yield acceptable and consistent pregnancy rates. There was no difference in 30-d pregnancy rates among PGF 2α treatments. The 30-d pregnancy rates observed in the present study are comparable with other reports using CoSynch (Lemaster et al., 2001), Syncromate-B (Odde, 1990), and controlled internal drug-releasing device (Beal, 1983) protocols. However, there was a herd effect on 30-d pregnancy rates. Herd A had a greater BCS at the first GnRH injection than both herds B and C, which may have resulted in greater 30-d pregnancy rates for herd A. Because BCS is used as an indirect indicator of the cycling status of cattle (Yelich et al., 1995; Stevenson et al., 2000; Moreira et al., 2001), some of the cows in herds B and C may have been anestrus at the initiation of treatment, which was reflected in their decreased 30-d pregnancy rates compared with herd A cows. Cycling cows have an increased response to GnRH + PGF 2α protocols compared to noncycling cows in Bos taurus (Stevenson et al., 2000; Moreira et al., 2001) and Bos indicus Bos taurus (Lemaster et al., 2001) cattle. Although administration of GnRH (Twagiramungu et al., 1995; Geary et al., 1998; Thompson et al., 1999) or MGA (Beal and Good, 1986) can

5 834 Hiers et al. induce cyclicity in some anestrous females, it is difficult to determine if the GnRH and MGA treatments induced cyclicity in any of the cattle in the present study since cycling status was not determined. A BCS of approximately a 5.0 (scale 1 to 9) at breeding appears to be necessary to get cows pregnant early in the breeding season and for cows to maintain a yearly calving interval (Rae et al., 1993; Kunkle et al., 1994) in cattle of Bos indicus Bos taurus breeding. However, when BCS was included in the statistical model, it was not a significant source of variation in 30-d pregnancy rates. Therefore, there are limitations of the data in determining what effects BCS had on herd fertility and 30-d pregnancy rates. There are also a multitude of other factors that could have contributed to differences in 30-d pregnancy rates among herds. These factors include differences in cow fertility among herds, differences in fertility among the different natural service sires used between herds, and a difference in total available nutrients for cows during the period after TAI between herds. For the latter factor, a decrease in total nutrient intake could have resulted in an increased number of anestrous cows during the breeding season in herd A, which had the lowest 30-d pregnancy rates of the three herds. In summary, TAI pregnancy rates in the present study were similar among a half-dose PGF 2α, a single injection PGF 2α, and an injection of cloprostenol sodium when used in GnRH + PGF 2α protocols in nonlactating cows of Bos indicus Bos taurus breeding. There was considerable variation in TAI pregnancy rates among AI sires within two herds, suggesting that AI sires differ considerably in the ability of their semen to result in a pregnancy to a TAI breeding. Further research must be conducted to answer questions regarding fertility of semen used in TAI programs. Implications These data suggest that two injections of prostaglandin F 2α are not necessary and that it does not seem to matter whether a single injection of prostaglandin F 2α or cloprostenol sodium is used 7 d after treatment with gonadotropin-releasing hormone in combination with melengestrol acetate when synchronizing nonlactating cows of Bos indicus Bos taurus breeding. Furthermore, when multiple artificial insemination sires are used in a timed artificial insemination protocol, producers should expect timed artificial insemination pregnancy rates to be compromised for some sires because of differences in bull fertility. Literature Cited Beal, W. E A note on synchronization of estrus in post partum cows with prostglandin F 2α and a progesterone-releasing device. Anim. Reprod. Sci. 37: Beal, W. E., and G. A. 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Niswender Pregnancy rates of postpartum beef cows that were synchronized using Synchro-Mate-B or the Ovsynch protocol. J. Anim. Sci. 76: Geary, T. W., J. C. Whittier, D. M. Hallford, and M. D. MacNeil Calf removal improves conception rates to the Ovsynch and CO-synch protocols. J. Anim. Sci. 79:1 4. Hiers E. A., C. R. Barthle, J. K. Fullenwider, G. E. Portillo, M. K. V. Dahms, J. M. Kramer, and J. V. Yelich Stage of cycle effects on response to different GnRH + prostaglandin F 2α (PG) treatments in Bos indicus Bos taurus cows. J. Anim. Sci. 79(Suppl. 1): (Abstr.) Kunkle W. E., R. S. Sand, and D. O. Rae Effect of body condition score on productivity in beef cattle. Pages in Factors Affecting Calf Crop. M. J. Fields and R. S. Sand, ed. CRC Press, Boca Raton. Lemaster, J. W., J. V. Yelich, J. R. Kempfer, J. K. Fullenwider, C. L. Barnett, M. D. Fanning, and J. F. Selph Effectiveness of GnRH plus prostaglandin F 2α for estrus synchronization in cattle of Bos indicus breeding. J. Anim. Sci. 79: Macmillan, K. L., and J. D. Watson Fertility differences between groups of sires relative to the stage of estrus at the time of insemination. Anim. Prod. 21: Martinez, M. F., J. P. Kastelic, G. P. Adams, E. Janzen, W. Olson, and R. J. Mapletoft Alternative methods of synchronizing estrus and ovulation for fixed-time insemination in cattle. Theriogenology 49:350. (Abstr.) Moreira, F., R. L. de la Sota, T. Diaz, and W. W. Thatcher Effect of day of the estrous cycle at the initiation of a timed artificial insemination protocol on reproductive responses in dairy heifers. J. Anim. Sci. 78: Moreira, F., C. Orlandi, C. A. Risco, R. Mattos, F. Lopes, and W. W. Thatcher Effects of presynchronization and bovine somatotropin on pregnancy rates to a timed artificial insemination protocol in lactating dairy cows. J. Dairy Sci. 84: Odde, K. G A review of synchronization of estrus in postpartum cattle. J. Anim. Sci. 68: Pancarci, S. M., J. Vonk, F. Moreira, and W. W. 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6 Timed artificial insemination in Bos indicus Bos taurus cows 835 estrous synchronization in beef cattle. Pages in Proc. 11th Int. Cong. for Anim. Reprod. and AI. Schmitt, E. J., T. Diaz, M. Drost, and W. W. Thatcher. 1996a. Use of gonadotropin-releasing hormone agonist or human chorionic gonadotropin for timed insemination in cattle. J. Anim. Sci. 74: Schmitt, E. J., M. Drost, T. Diaz, C. Roomes, and W. W. Thatcher. 1996b. Effect of a gonadotropin-releasing hormone agonist on follicle recruitment and pregnancy rate in cattle. J. Anim. Sci. 74: Silcox, R. W., K. L. Powell, and T. E. Kiser Ability of dominant follicles (DF) to respond to exogenous GnRH administration is dependent on their stage of development. J. Anim. Sci. 71(Suppl. 1):219. (Abstr.) Stevenson, J. S., K. E. Thompson, W. L. Forbes, G. C. Lamb, D. M. Grieger, and L. R. Corah Synchronization of estrus and(or) ovulation in beef cows after combinations of GnRH, norgestomet, and prostaglandin F 2α with or without timed insemination. J. Anim. Sci. 78: Thompson, K. E., J. S. Stevenson, G. C. Lamb, D. M. Grieger, and C. A. Loest Follicular, hormonal, and pregnancy responses of early postpartum suckled beef cows to GnRH, norgestomet, and prostaglandin F 2α J. Anim. Sci. 77: Thundathil, J., J. P. Kastelic, W. O. Olson, R. B. Cook, and R. J. Mapletoft Melengestrol acetate, estradiol-17β and GnRH for synchronization of estrus and ovulation in beef cows. Canadian J. Anim. Sci. 79: Twagiramungu, H., L. A. Guilbault, and J. J. Dufour Synchronization of ovarian follicular waves with a gonadotropin-releasing hormone agonist to increase the precision of estrus in cattle: A review. J. Anim. Sci. 73: Twagiramungu, H., L. A. Guilbault, J. G. Proulx, P. Villeneuve, and J. J. Dufour Influence of an agonist of gonadotropin-releasing hormone (buserelin) on estrus synchronization and fertility in beef cows. J. Anim. Sci. 70: Walker, W. L., R. L. Nebel, and M. L. McGilliard Time of ovulation relative to mounting activity in dairy cattle. J. Dairy Sci. 79: Yelich, J. V A vaginal insert (CIDR) to synchronize estrus and timed-ai. Pages in Factors Affecting Calf Crop- Biotechnology of Reproduction. M. J. Fields, R. S. Sand, and J. V. Yelich, ed. CRC Press, Boca Raton, FL. Yelich, J. V., M. D. Holland, D. N. Schutz, and K. G. Odde Synchronization of estrus in suckled postpartum beef cows with melengestrol acetate, 48-hour calf removal and PGF 2α Theriogenology. 43: Zimbelman, R. G., and L. W. Smith Control of ovulation in cattle with melengestrol acetate. I. Effect of dosage and route of administration. J. Reprod. Fertil. 11: