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in combination with melengestrol acetate1,2
* Department of Animal Sciences and
and
College of Veterinary Medicine, University of Florida, Gainesville, 32611
3 Correspondence:
phone 352-392-7560; fax 352-392-7652; E-mail:
yelich{at}animal.ufl.edu.
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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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 µg of GnRH. Cows were fed MGA (0.5•mg•cow-1•d-1) on d 1 to 7. On d 7, cows received either a single injection of PGF2 (Lutalyse sterile solution; 25 mg; n = 297), a single injection of cloprostenol sodium (Estrumate; 500 µg; n = 297), or half the recommended dose of PGF2 (12.5 mg; n = 275) on d 7 and 8. On d 10, all cows were artificially inseminated and received 100 µg of GnRH. Pregnancy rates to the timed artificial insemination (39%) were not affected by treatment, herd, or treatment x 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 x 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 PGF2 treatment administered 7 d after GnRH did not influence timed artificial insemination pregnancy rates in nonlactating Bos indicus x Bos taurus cows.
Key Words: Artificial Insemination • Bos indicus • Gonadotropin-Releasing Hormone • Synchronization
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Introduction |
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to synchronize cattle for a timed-AI (TAI) is an 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 PGF2 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.
Utilization of the progestogen melengestrol acetate (MGA) to suppress estrus (Zimbelman and Smith, 1966) between the GnRH and PGF2 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 + PGF2 protocols appear to be greater in Bos taurus cattle (Geary et al., 1998; Stevenson et al., 2000) compared with Bos indicus x 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 x Bos taurus female is less responsive to PGF2 than the CL of the Bos taurus female (Lemaster et al., 2001). Whether luteolysis could be enhanced with administration of a PGF2 analog (i.e., cloprostenol sodium) or half the recommended dose of PGF2 (Lutalyse sterile solution, Pharmacia Animal Health, Kalamazoo, MI) administered 24 h apart is unclear. The latter has 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 PGF2 treatments in a GnRH + PGF2 protocol combined with MGA to synchronize Bos indicus x Bos taurus cows for a TAI.
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Materials and Methods |
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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 µg 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 PGF2 i.m. (Lutalyse sterile solution, Pharmacia Animal Health, Kalamazoo, MI), 500 µg of cloprostenol sodium i.m. (Estrumate; Schering-Plough Animal Health, Union, NJ), or 12.5 mg of PGF2 i.m. (Lutalyse sterile solution; half-dose of PGF2) on d 7 and 8. After administration of PGF2 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 µg of Fertagyl or 100 µg of Factrel). In herds B and C, cows were TAI approximately 72 to 80 h after the last MGA feeding and received 100 µg 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.
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Results |
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). 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 x BCS effect (P = 0.35) nor a herd x BCS effect (P = 0.38) on TAI pregnancy rates.
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), but not for herds A or C. Within each herd, there was no treatment x 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%.
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) as 72.3% of all cows became pregnant during the first 30 d of the breeding 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 |
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compared with 48 to 54 h after PGF2 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 PGF2 in the GnRH + PGF2 protocol (Twagiramungu et al., 1995; Schmitt et al., 1996b; Moreira et al., 2000), MGA was administered on the day of PGF2 to prevent premature expression of estrus and to possibly tighten the synchrony of estrus and follicle development after PGF2. It was speculated that waiting until 72 h after the PGF2 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 PGF2 treatments in the present study. Therefore, these data suggest that either a half-dose PGF2 or cloprostenol treatment 7 d after a GnRH injection does not increase TAI pregnancy rates compared to a single dose of PGF2 in nonlactating Bos indicus x 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 PGF2 or a half dose PGF2 in an OvSynch protocol.
The TAI pregnancy rates for the PGF2 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 x Bos taurus cows (Lemaster et al., 2001). Furthermore, the TAI pregnancy rates for the half-dose PGF2 and the cloprostenol sodium treatments are only slightly greater than those reported in cycling and noncycling Bos indicus x 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 x Bos taurus breeding compared to cattle of Bos taurus breeding when GnRH + PGF2 protocols are used.
The ability of the PGF2 treatments to induce luteolysis as measured by blood progesterone concentrations was not determined in the present study. However, a recent experiment by Hiers et al. (2001) in cycling Bos indicus x Bos taurus cattle using the same three GnRH + PGF2 treatments as the present study observed no significant difference in luteolysis as measured by blood progesterone concentrations between PGF2 treatments. These data, as well as data from the present study, suggest that a single injection of PGF2 used 7 d after GnRH results in similar luteolysis and TAI pregnancy rates as the half-dose PGF2 and cloprostenol sodium treatments in nonlactating Bos indicus x 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 PGF2 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 PGF2 injections. Therefore, one could speculate that when MGA is administered on the same day as the PGF2, it may be necessary to extend the time interval between the PGF2 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 PGF2 and TAI performed either 48 or 72 h after PGF2. Additional research is needed to determine the proper duration of MGA treatment and the interval from PGF2 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 x 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 PGF2 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 PGF2 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 + PGF2 protocols compared to noncycling cows in Bos taurus (Stevenson et al., 2000; Moreira et al., 2001) and Bos indicus x 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 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 x 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 PGF2, a single injection PGF2, and an injection of cloprostenol sodium when used in GnRH + PGF2 protocols in nonlactating cows of Bos indicus x 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.
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Implications |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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are not necessary and that it does not seem to matter whether a single injection of prostaglandin F2 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 x 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.
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Footnotes |
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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.
Received for publication July 15, 2002. Accepted for publication November 18, 2002.
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Literature Cited |
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and a progesterone-releasing device. Anim. Reprod. Sci. 37:305–308.
J. Anim. Sci. 63:343–347.
and estradiol benzoate (EB). Theriogenology 55:1521–1532.[Medline]
(PG) treatments in Bos indicus x Bos taurus cows. J. Anim. Sci. 79(Suppl. 1):462–463. (Abstr.)
for estrus synchronization in cattle of Bos indicus breeding. J. Anim. Sci. 79:309–316. and GnRH. Theriogenology 44:915–923.
for estrous synchronization in beef cattle. Pages 459–461 in Proc. 11th Int. Cong. for Anim. Reprod. and AI.
with or without timed insemination. J. Anim. Sci. 78:1747–1758.J. Anim. Sci. 77:1823–1832.Theriogenology 43:401–410.[Medline]
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