J. Anim Sci.
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

J. Anim Sci. 2008. 86:2539-2548. doi:10.2527/jas.2008-1122
© 2008 American Society of Animal Science
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow All Versions of this Article:
jas.2008-1122v1
86/10/2539    most recent
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Burns, M. G.
Right arrow Articles by Stevenson, J. S.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Burns, M. G.
Right arrow Articles by Stevenson, J. S.

ANIMAL GROWTH, PHYSIOLOGY, AND REPRODUCTION

Evaluation of human chorionic gonadotropin as a replacement for gonadotropin-releasing hormone in ovulation-synchronization protocols before fixed timed artificial insemination in beef cattle1

M. G. Burns*,2, B. S. Buttrey*,3, C. A. Dobbins*, C. A. Martel*, KC Olson*, G. C. Lamb{dagger} and J. S. Stevenson*,4

* Department of Animal Sciences and Industry, Kansas State University, Manhattan 66506-0201;and {dagger} North Florida Research and Education Center, University of Florida, Marianna 32446-7906


   Abstract
Top
 Abstract
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 LITERATURE CITED
 
Two experiments were conducted during 2 yr to evaluate differences in ovulation potential and fertility in response to GnRH or hCG. In Exp. 1, 46 beef cows were given 100 µg of GnRH or 500, 1,000, 2,000, or 3,000 IU of hCG. Ovulation incidence was not different between GnRH and any of the hCG doses, indicating that ovulatory capacity of at least 500 IU of hCG was equivalent to GnRH. In Exp. 2, beef cows (n = 676) at 6 locations were assigned randomly to a 2 x 3 factorial arrangement of treatments. Main effects were: 1) pre-timed AI (TAI) treatment (GnRH or hCG) and 2) post-TAI treatment (saline, GnRH, or hCG) to initiate resynchronization of ovulation in previously inseminated cattle. Blood samples were collected (d –21 and –10) to determine progesterone concentrations and assess cyclicity. Cattle were treated with a progesterone insert on d –10 and with 100 µg of GnRH or 1,000 IU of hCG. A PGF2{alpha} injection was given at insert removal on d –3. Cows were inseminated 62 h (d 0) after insert removal. On d 26 after first TAI, cows of unknown pregnancy status were treated with saline, GnRH, or hCG to initiate a CO-Synch protocol. Pregnancy was diagnosed 33 d after first TAI to determine pregnancies per AI (P/AI). Nonpregnant cows at 6 locations in yr 1 and 1 location in yr 2 were given PGF2{alpha} and inseminated 56 h later, concurrent with a GnRH injection. Five weeks later, pregnancy diagnosis was conducted to determine pregnancy loss after first TAI and pregnancy outcome of the second TAI. Injection of pre-TAI hCG reduced (P < 0.001) P/AI compared with GnRH, with a greater reduction in cycling cows. Post-TAI treatments had no negative effect on P/AI resulting from the first TAI. Serum progesterone was greater (P = 0.06) 7 d after pre-TAI hCG than after GnRH and greater (P < 0.05) after post-TAI hCG on d 26 compared with saline 7 d after treatment in association with greater frequency of multiple corpora lutea. Compared with saline, injections of post-TAI GnRH and hCG did not increase second insemination P/AI, and inconsistent results were detected among locations. Use of hCG in lieu of GnRH is contraindicated in a CO-Synch + progesterone insert protocol. Compared with a breeding season having only 1 TAI and longer exposure to cleanup bulls, total breeding season pregnancy rate was reduced by one-third, subsequent calving distribution was altered, and 50% more AI-sired calves were obtained by applying 2 TAI during the breeding season.

Key Words: beef cattle • gonadotropin releasing hormone • human chorionic gonadotropin • ovulation synchronization • timed artificial insemination


   INTRODUCTION
Top
 Abstract
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 LITERATURE CITED
 
Timed insemination after the CO-Synch protocol (injection of GnRH 7 d before and 48 h after PGF2{alpha} with AI occurring at the time of the second GnRH injection; Geary and Whittier, 1998Go) + a progesterone-releasing, intravaginally placed controlled internal drug release insert (CIDR; Pfizer Animal Health, New York, NY) generally produces pregnancies per AI (P/AI) at or greater than 50%. Compared with synchronization protocols that require detection of estrus, the CO-Synch protocol + progesterone insert produces comparable P/AI and eliminates detection of estrus (Larson et al., 2006Go). Beef producers indicate that time, labor, and complexity are primary reasons for not employing synchronization and AI (NAHMS, 1997Go)

Because hCG was more effective than GnRH at stimulating ovulation in dairy cattle (Stevenson et al., 2006Go), a proper dose of hCG might be a substitute for GnRH in various timed AI (TAI) protocols. Resynchronization of estrus in previously inseminated cattle of unknown pregnancy status was accomplished using estrogen, progesterone inserts, or both (Stevenson et al., 2003Go). Attempts to synchronize ovulation before a fixed-TAI have not been applied in beef cattle of unknown pregnancy status.

Therefore, objectives of this study were to determine 1) a minimal effective dose of hCG needed to induce ovulation; 2) concentrations of progesterone and fertility in suckled beef cattle after substituting hCG for GnRH in a standard CO-Synch protocol + progesterone insert and in an ovulation-resynchronization protocol initiated in cows of unknown pregnancy status after first TAI; and 3) effect of hCG or GnRH on multiple corpus lutea (CL) formation, concentration of progesterone, and P/AI in cows treated with the ovulation-resynchronization protocol. The resynchronization protocol was designed to allow 2 TAI (twice during the first 35 d of the breeding season) and still have sufficient time for a 25-d natural-service period in a 60-d breeding season.


   MATERIALS AND METHODS
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 LITERATURE CITED
 
The Kansas State University Animal Care and Use Committee approved animal housing, handling, and sample collection procedures.

Experiment 1

The purpose of Exp. 1 was to assess ovarian follicular and luteal responses to GnRH or increasing doses of hCG. Forty-six purebred Angus, Hereford, and Simmental cattle located at the Kansas State University Purebred Beef Unit, Manhattan, were lactating (fall calving; n = 19) or dry (spring calving; n = 27) during autumn 2005. Cattle were assigned randomly to 5 treatments based on ovarian characteristics and suckling status. They were injected i.m. with either 100 µg of GnRH (2 mL of OvaCyst, IVX Animal Health, St. Joseph, MO) or 500, 1,000, 2,000, or 3,000 IU of hCG (0.5, 1, 2, or 3 mL, respectively, of Chorulon, Intervet Inc., Millsboro, DE) on d 0. Ovaries were scanned by using transrectal ultrasonography (5 MHz transrectal probe, Aloka 500V, Corimetrics, Wallingford, CT) before treatment (d 0) and 7 d later to determine if ovulation had occurred. Follicles ≥ 5 mm in diameter were measured and recorded. Number of follicles on d 0 and number of new CL on d 7 were used to determine ovulation incidence. Approximately 5 mL of whole blood was collected by puncture of a coccygeal vessel on d 0 and 7 to later assess concentrations of progesterone. Blood was held overnight at 5°C, and serum was separated by centrifugation (1,200 x g). Serum was stored at –4°C until analyzed.

Experiment 2

Beef cattle at 6 locations and breeds of cattle assigned to treatments during spring 2006 included 1 location of purebred Angus, Hereford, and Simmental cows and 3 locations of Angus x Hereford crossbred cows at Kansas State University, Manhattan; 1 location of Angus x Hereford x Simmental crossbred cows at Dorrance, KS; and 1 location of purebred Angus cows at the North Central Research and Outreach Center, University of Minnesota, Grand Rapids, MN. Location of cattle assigned to treatments in 2007 included 1 location of purebred Angus, Hereford, and Simmental cows and 2 locations of Angus x Hereford crossbred cows at Kansas State University, Manhattan. Other cattle characteristics are summarized by location and year in Table 1Go


View this table:
[in this window]
[in a new window]

  		Table 1. Location characteristics (Exp. 2)
 
Year 1 (2006). Cattle were assigned randomly to a 2 x 3 factorial arrangement of treatments consisting of 6 treatments (Figure 1Go) at each location balanced for days postpartum, BCS, parity, and breed (2 locations). The pre-TAI treatment main effects included comparing injections of GnRH and hCG (d –10) to initiate ovulation at the time of intravaginal placement of a progesterone insert (CIDR EAZI-Breed; Pfizer Animal Health, New York, NY). The post-TAI treatment main effects consisted of comparing injections of saline, GnRH, and hCG (d 26) to initiate ovulation in previously inseminated cattle as part of a resynchronization protocol.


Figure 1
View larger version (14K):
[in this window]
[in a new window]

  		Figure 1. Experimental design of treatments for Exp. 2. Pre-timed AI (TAI) treatments were 100 µg of GnRH or 1,000 IU of hCG. Post-TAI treatments were saline, 100 µg of GnRH or 1,000 IU of hCG. All cows received 25 mg of PGF2{alpha} at removal of the controlled internal drug release (CIDR) progesterone insert. Nonpregnant cows were treated with PGF2{alpha} at pregnancy diagnosis followed by the second TAI at 56 h plus 100 µg of GnRH. Blood also was collected at location 3 during yr 1 at insert removal, first TAI, d 26 and 33, and before the second TAI. During yr 2, only GnRH was administered upfront before first TAI and blood samples were collected from all cows on d 26 and 33.

 
For pre-TAI treatments (before first TAI), ovulation in beef cattle was synchronized using a CO-Synch protocol + progesterone insert (Figure 1Go). A progesterone insert was placed intravaginally and 100 µg of GnRH (2 mL of OvaCyst, IVX Animal Health) or 1,000 IU of hCG (1 mL of Chorulon, Intervet Inc.) were injected i.m. on d –10. On d –3, the insert was removed and 25 mg of PGF2{alpha} (Prostamate, IVX Animal Health) were injected i.m. Initial TAI was carried out 60 to 64 h (d 0) after administration of PGF2{alpha}. A 100-µg injection of GnRH was administered before the TAI. Blood samples were collected on d –21 and –10 at all 6 locations to later determine cycling status of cows based on serum concentrations of progesterone. Blood samples were collected at each subsequent handling of cows at location 3 and processed as described in Exp. 1 for later progesterone analysis.

For post-TAI treatments (before second TAI), the first eligible estrus was bypassed at 5 locations, whereas estrus was detected and cows were reinseminated at location 4 (continuous AI) until post-TAI treatment occurred 7 d before pregnancy diagnosis (Figure 1Go). Each cow of unknown pregnancy status at all 5 locations (or those not detected in estrus at location 4) then received 100 µg of GnRH (OvaCyst; IVX Animal Health), 1,000 IU of hCG (Chorulon, Intervet Inc.), or 2 mL of saline (control) on d 26 after the first insemination to resynchronize ovulation. Seven days later on d 33 (range of d 32 to 35) after initial TAI, pregnancy diagnosis was carried out using transrectal ultrasonography. Nonpregnant cattle received 25 mg of PGF2{alpha} (Prostamate, IVX Animal Health) and a second fixed TAI 56 h later. A 100-µg injection of GnRH was administered before the second TAI.

A second pregnancy diagnosis occurred 35 d (range of 33 to 36 d) after the second TAI. All cows previously diagnosed pregnant were reconfirmed pregnant at this time (d 68; range of 66 to 70 d) to allow estimation of pregnancy losses. A positive diagnosis was indicated when a viable (heart beat or movement) embryo or fetus was detected.

At 5 of 6 locations, cleanup bulls were turned out with cows 7 d after the second TAI and removed from pastures at the second pregnancy diagnosis (d 68 d after the first TAI). At location 4, cows were continuously observed for estrus and inseminated accordingly until d 68 after the first TAI. During yr 1, cows at 5 locations received 2 TAI and were exposed to bulls for 25 to 29 d; at location 4, cows received 2 TAI plus continuous detection of estrus and AI.

Year 2 (2007). At 3 locations, cows were inseminated as described in Exp. 2 with a standard CO-Synch protocol (using GnRH on d –10) + progesterone insert. At locations 1 and 3, cleanup bulls were turned out with cows 11 d after TAI. At location 2, cows were continuously observed for estrus and inseminated as in yr 1. Cows of unknown pregnancy status at all 3 locations (including cows at location 2 not detected in estrus by d 26) were treated randomly, as described in yr 1, with GnRH, hCG, or saline on d 26 after TAI. Only cows at the continuous AI location (location 2) received a second TAI as described for yr 1. As described previously, blood samples were collected before treatment injections (d 26) and 7 d later before the first pregnancy diagnosis on d 33 (range of d 33 to 34). At location 1, number of CL in 75 cows was counted visually by transrectal ultrasonography at the first pregnancy diagnosis to assess ovulatory response to hCG and GnRH.

Based on a second pregnancy diagnosis conducted at d 68 (range of d 68 to 70) after TAI, pregnancy was reconfirmed in previously pregnant cows to estimate pregnancy loss. Pregnancy status was staged to determine if cows conceived to matings at their first or second eligible estrus after TAI. Cows pregnant on d 68 were approximately 47 or 26 d pregnant to bulls at their first or second estrus after TAI, respectively. Bulls were removed from pastures or continuous AI was discontinued at the second pregnancy diagnosis (d 68 after TAI). During yr 2, cows at locations 1 and 3 received 1 TAI and were exposed to bulls for 57 to 58 d; at location 2, cows received 2 TAI plus continuous detection of estrus and AI as in yr 1.

RIA of Progesterone

Sera harvested from blood samples during the 2 experiments were assayed for progesterone by RIA (Skaggs et al., 1986Go). All samples from each cow were analyzed in the same assay. Inter- and intra-assay CV for 25 assays were 13.9 and 11.4%, respectively, for a serum pool that averaged 3.99 ± 0.06 ng/mL. When concentrations of progesterone were ≥1 ng/mL on either or both of the first sampling days (d –21 and –10), cows were considered to have initiated estrous cycles.

Statistical Analyses

All continuous variables (serum concentrations of progesterone) were analyzed using ANOVA (procedure GLM; SAS Inst. Inc., Cary, NC). All binomial data were analyzed using logistic regression (procedure GEN-MOD; SAS Inst. Inc.) or chi-square. When F-tests were significant (P < 0.05), differences among least-squares means were detected by the method of least-significant difference. In some cases, means were separated using orthogonal contrasts.

The model used for Exp. 1 included effects for treatment (GnRH vs. 4 doses of hCG), lactation status (dry vs. suckled), and breed (Angus, Simmental, and Hereford). In Exp. 2, the first model used to assess differences in serum progesterone at progesterone insert removal and at first TAI included effects for treatment (GnRH vs. hCG), parity, (1, 2, or 3+), cycling status (0 vs. 1), and treatment x cycling status. Days postpartum and BCS were used as covariates. A second model was used to assess potential differences in serum progesterone at d 26 after first TAI. The model included effects for treatment, cycling status, pregnancy status, treatment x cycling status, and treatment x pregnancy status. A third model assessed serum progesterone at d 33 and 35 after first TAI. It included effects for post-AI treatments (GnRH, hCG, and saline), cycling status, pregnancy status, treatment x cycling status, and treatment x pregnancy status.

Number of CL detected in yr 2 was analyzed by logistic regression using effects for treatment (saline, GnRH, or hCG) and pregnancy stage at d 33. Days postpartum and BCS were used as covariates. Pregnancy stage included cows that conceived at first TAI (d 26 pregnant), conceived to the first mating (d 5 pregnant), and failed to conceive to the first mating (d 5 nonpregnant). Concentrations of progesterone were analyzed by procedure GLM in SAS using effects for treatment, parity, and treatment x parity; days postpartum and BCS were used as covariates.

Conception rates in the d 5 nonpregnant cows represented conception at the second mating (cycle following treatment on d 26 post-first TAI). Conception rate in the d 5 pregnant cows represented conception for cows treated on d 5 postmating (before d 26 post-TAI treatments). Pregnancies per AI and pregnancy loss after first TAI were analyzed by logistic regression with a model consisting of pre-AI treatment (i.e., GnRH vs. hCG), post-TAI treatments (saline, GnRH, or hCG), cycling status, all 2-way interactions, sire nested within location, technician nested within location, parity (1, 2, and 3+), and treatment x parity; days postpartum and BCS were used as covariates.

Effects of post-TAI treatments on second TAI P/AI were assessed by including the fixed effects of treatments (GnRH, hCG, and saline), parity, cycling status, 2-way interactions of treatment with parity and cycling status, location, sire nested within location, AI technician nested within location, and days postpartum and BCS as covariates. The first model included only those locations where TAI was carried out. A second model included 2 locations during both years where cows received both TAI and were continuously detected in estrus and inseminated.

The model to test the effect of various AI management systems on total pregnancy rate (pregnancy status on the last day of the breeding season) included parity, AI management group (n = 3), interaction of parity by AI management group, and days postpartum and BCS as covariates. Group 1 included cows that received a single TAI at the beginning of the 68-d breeding season, after which cleanup bulls were inserted into pastures 11 d post-first TAI. Group 2 included double TAI cows, those that received a second TAI at d 35 after the beginning of the breeding season. The first eligible estrus post-first TAI was bypassed and ovulation was resynchronized beginning 26 d after first TAI. Cleanup bulls were inserted into pastures 7 to 10 d after TAI and removed 68 to 70 d after first TAI. Group 3 was similar to the double TAI group except cows were observed continuously for estrus and reinseminated until d 26 when ovulation resynchronization treatments were imposed. Breeding season continued until 69 to 70 d after first TAI.

Calving distribution of cows in locations where 2 TAI plus a 25-d period of exposure to cleanup bulls were conducted was compared with that of cows at 1 location (location 4 in yr 1 and location 2 in yr 2) where continuous AI was carried out in addition to the 2 TAI. Proportions of cows calving during the subsequent calving season (six 10-d periods) were compared using chi-square to determine relative differences in calving distribution of cows receiving the 2 TAI without any detected estrus.


   RESULTS AND DISCUSSION
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
LITERATURE CITED
 
Experiment 1

Previously, hCG doses in excess of 3,000 IU administered to cattle ensured ovulation or enhanced CL production of progesterone (Howard et al., 1990Go; Schmitt et al., 1996Go; Diaz et al., 1998Go). Experiment 1 was designed to determine the ovulation potential of 4 different doses of hCG compared with GnRH. Ovulation incidence did not differ between cows treated with GnRH and those treated with any dose of hCG (Table 2Go). No difference in ovulation incidence was detected between dry vs. suckled cows (52 vs. 78%, respectively). Only 3 of the 46 cows were not cycling; all 3 were lactating. Cows with serum progesterone ≥1 ng/mL before treatment tended (P < 0.10) to ovulate less often than those having progesterone <1 ng/mL (52 vs. 80%, respectively). The number of follicles ≥5 mm in diameter 7 d after treatment in cows that formed accessory CL was less (P < 0.001; 0.8 ± 0.1 vs. 1.7 ± 0.1; least squares means ± SE) than in cows that did not ovulate. Concentrations of progesterone were similar in cows having induced accessory CL and cows that did not ovulate (last 2 lines in Table 2Go).


View this table:
[in this window]
[in a new window]

  		Table 2. Incidence of new corpus luteum (CL) formation (ovulation), diameter of ovarian follicles, and progesterone before and 7 d after treatment injections of GnRH or hCG in beef cows (Exp. 1)
 
Treatment with hCG on d 5 or 7 of the estrous cycle caused ovulation of a dominant follicle (Rajamahendran and Sianangama, 1992Go; Schmitt et al., 1996Go). Cattle in Exp. 1 were treated at random stages of the cycle. Stage of estrous cycle affected ovulation incidence in response to GnRH (Vasconcelos et al., 1999Go) or hCG (Price and Webb, 1989Go) and, more importantly, availability of a dominant follicle to ovulate. Although 500 IU did not differ from larger doses of hCG based on outcomes of Exp. 1, the decision to use 1,000 IU rather than 500 IU of hCG in Exp. 2 was made because of potential injection difficulties of administering 0.5 mL (500 IU) vs. 1 mL (1,000 IU).

Experiment 2

Fertility. Injection of GnRH resulted in greater (P < 0.001) P/AI than hCG (53.4 vs. 38.9%; Table 3Go). Compared with GnRH, hCG decreased P/AI of cycling cows, whereas in noncycling cows, P/AI did not differ between treatments (treatment x cycling status; P = 0.052; Figure 2Go). Pregnancy loss between d 33 and 68 after first TAI was small and did not differ between GnRH and hCG treatments in cows that conceived after the first TAI (7.2 and 4.6%, respectively; Table 3Go). During both years, post-AI treatment injections had little effect on first-TAI P/AI or pregnancy loss between d 33 and 68 post-TAI (Figure 3Go).


View this table:
[in this window]
[in a new window]

  		Table 3. First timed AI (TAI) pregnancies per AI (P/AI), and pregnancy loss in Exp. 2 (yr 1)
 

Figure 2
View larger version (26K):
[in this window]
[in a new window]

  		Figure 2. First timed AI (TAI) pregnancies per AI (P/AI) of cycling and noncycling cows after initial TAI in which the CO-Synch + progesterone insert protocol was initiated by treatment of GnRH or hCG (Exp. 2). a,bA treatment x cycling status interaction was detected (P < 0.05).

 

Figure 3
View larger version (26K):
[in this window]
[in a new window]

  		Figure 3. First timed-AI (TAI) pregnancies per AI (P/AI) and pregnancy loss between d 33 and 68 after TAI in response to post-TAI treatment with saline, GnRH, or hCG on d 26 after first TAI (Exp. 2). No interaction (P = 0.54) was detected between pre-TAI and post-TAI treatments.

 
The TAI P/AI for cows treated with saline, GnRH, or hCG to initiate the resynchronization treatments did not differ among treatments (Table 4Go). A treatment by location interaction was detected (P < 0.05). At 2 locations, saline produced numerically more P/AI than GnRH or hCG treatment. At 4 locations, saline produced numerically less P/AI than GnRH or hCG. Only in cows exposed to continuous insemination after detected estrus post-TAI at location 2 in 2007 and cows at location 2 in 2006 did more P/AI occur in the GnRH and hCG treatments (Table 4Go). Given this variation in responses among locations, no conclusion is possible. If saline, however, is not different from GnRH or hCG, then reinitiating TAI after a nonpregnant diagnosis on d 33 post-AI reduces costs of hormone and labor to facilitate a resynchronization protocol.


View this table:
[in this window]
[in a new window]

  		Table 4. Pregnancies per AI after the second timed AI (TAI) in beef cows in which the first eligible estrus was bypassed and treatments were initiated at d 26 after first timed AI before second TAI or cows not detected in estrus by d 26 were treated (Exp. 2)
 
Progesterone. Seven days after pre-TAI hCG and GnRH administration (i.e., at the time of insert removal and PGF2{alpha} injection at location 3), serum progesterone tended (P = 0.067) to be greater in cows that received hCG (4.4 ± 0.5 ng/mL) than GnRH (3.2 ± 0.4 ng/mL). Further, 61 cows at location 3 that were cycling before treatment injections had greater (P < 0.05) serum progesterone at insert removal compared with 54 anestrous cows (5.1 ± 0.5 vs. 2.6 ± 0.5 ng/mL). These results are consistent with the understanding that hCG can induce ovulation of first- or second-wave dominant follicles (Price and Webb, 1989Go), induce accessory CL, and increase progesterone production of the original CL (Rajamahendran and Sianangama, 1992Go; Schmitt et al., 1996Go; Santos et al., 2001Go). Increased production of progesterone by the existing CL could be a response to the extended half-life of hCG, which exhibits prolonged LH-like stimulation of the CL (Niswender et al., 2000Go).

Serum progesterone was <1 ng/mL at location 3 for both hCG and GnRH treatments at TAI (0.2 ± 0.1 vs. 0.3 ± 0.1 ng/mL, respectively). Serum progesterone did not differ between treatments 26 d after TAI (GnRH: 4.4 ± 0.3 vs. hCG: 4.2 ± 0.3 ng/mL) but was greater (P < 0.001) in 68 pregnant cows than in 45 nonpregnant cows (6.5 ± 0.3 vs. 2.1 ± 0.3 ng/mL). Cattle that did not conceive to the first TAI should have had a new CL subsequent to their return to estrus at 20 to 22 d after TAI. Pregnant cows would have maintained their original CL beyond the duration of a normal estrous cycle because of pregnancy recognition (Niswender et al., 2000Go) and, therefore, had greater serum concentrations of progesterone than nonpregnant cows.

Injection of hCG (7 d before pregnancy diagnosis at location 3 in yr 1 and locations 1, 2, and 3 in yr 2) increased (P < 0.01) serum progesterone 7 d postinjection (6.9 ± 0.3 ng/mL; n = 166) compared with injections of saline (5.9 ± 0.3 ng/mL; n = 161) or GnRH (5.8 ± 0.3 ng/mL; n = 168). Further, cows cycling (n = 331) before TAI maintained greater (P < 0.001) concentrations of progesterone than noncycling cows (n = 156) 33 d after TAI (6.7 ± 0.2 vs. 5.5 ± 0.3 ng/mL). This difference did not result from fewer anestrous cows becoming pregnant (Figure 2Go), although concentrations of progesterone were greater in 269 pregnant cows than in 218 nonpregnant cows (7.2 ± 0.2 vs. 5.0 ± 0.2 ng/mL).

Serum progesterone at the second TAI did not differ among post-TAI treatments; however, a tendency for a post-TAI treatment x cycling status interaction was detected (P = 0.074). Concentrations of progesterone for noncycling and cycling saline-treated cows were 0.5 ± 0.2 and 0.2 ± 0.3 ng/mL, respectively, whereas those for GnRH- (0.2 ± 0.3 and 0.7 ± 0.3 ng/mL) and hCG-treated cows (0.2 ± 0.3 and 1.1 ± 0.3 ng/mL) were greater in cycling cows than noncycling cows. It is not known if the slightly elevated concentration of progesterone at the second TAI among cycling cows negatively affected the second TAI P/AI. These samples were collected at location 3 during yr 1, in which TAI P/AI were 33.3% in 18 saline controls, 38.5% in 13 GnRH-treated cows, and 0% in 15 cows treated with hCG.

Induced Luteal Structures

Increased concentration of progesterone in hCG-treated cattle at location 1 (yr 2) was associated with a greater (P < 0.05) proportion of multiple CL (Table 5Go). Incidence of multiple CL assessed on d 26 after TAI did not differ among nonpregnant cows (cycle d 5) and pregnant cows (d 5 or 26 of pregnancy; Table 5Go). Compared with saline, serum progesterone was greater (P < 0.05) after hCG but not after GnRH. Serum progesterone was greater (P < 0.05) among pregnant cows and greater (P < 0.05) on d 5 in pregnant cows.


View this table:
[in this window]
[in a new window]

  		Table 5. Proportion of cows having 2 corpora lutea (CL) and serum progesterone 7 d after treatment based on pregnancy status at treatment (Exp. 2)
 
Conception rates for cattle treated on d 5 postmating at locations 1 and 3 during yr 2 were numerically greater in 43 hCG- (65.1%) and 56 GnRH-treated (64.3%) cows than in 49 saline controls (59.2%). Similarly, nonpregnant cows treated on d 5 of the estrous cycle before conception followed a similar pattern: 15 hCG (73.7%); 20 GnRH (65%), and 20 saline controls (50%). These results were interpreted to indicate that increased serum progesterone in early pregnancy can improve early embryo survival, as demonstrated in lactating dairy cattle (Stevenson et al., 2006Go). Moreover, greater serum progesterone in dairy cattle during the luteal phase before AI had positive effects on subsequent conception rates (Fonseca et al., 1983Go; Folman et al., 1990Go).

AI Management Systems. Total pregnancy rate and number of AI calves produced by the end of the breeding season for 3 AI management systems are shown in Table 6Go. Imposing a second TAI and bypassing the first eligible estrus after the first TAI reduced (P < 0.05) total pregnancy rate by nearly one-third but increased (P < 0.05) the projected number of AI calves by nearly 50%. Using 2 TAI and continuous detection of estrus and AI at locations 4 (yr 1) and 2 (yr 2) resulted in intermediate P/AI but the most (P < 0.05) AI calves.


View this table:
[in this window]
[in a new window]

  		Table 6. Total pregnancy rate at the end of the breeding season (Exp. 2)
 
Calving distribution differed between location 4, where AI was continuous + double TAI, and 4 locations where 2 TAI were combined with approximately 25 to 29 d of bull exposure (Figure 4Go). Bypassing the first eligible estrus of the breeding season after an initial TAI resulted in redistribution of calvings after first 20 d of the calving season.


Figure 4
View larger version (27K):
[in this window]
[in a new window]

  		Figure 4. Calving distribution (Exp. 2) of cows for locations where timed AI (TAI) was conducted at first and second services and cleanup bulls were inserted 10 d after the second TAI (TAI only + bulls; n = 255) compared with locations where TAI occurred at first and second services plus detection of estrus and AI occurred during the entire breeding season (TAI + estrus-AI; n = 165). **Denotes differences (P ≤ 0.01) between groups.

 
General Discussion

The CO-Synch protocol + progesterone insert applies a GnRH injection 7 d before PGF2{alpha} (and insert removal). This ensures that a CL is present in a large percentage of noncycling cows or a CL is present in cycling cows before luteolysis is induced by PGF2{alpha} (Thompson et al., 1999Go). We hypothesized that hCG would stimulate more follicles to ovulate than GnRH and, therefore, increase synchronization and P/AI by cows treated with the CO-Synch protocol + progesterone insert. Based on increased serum progesterone at insert removal (Figure 4Go), more follicles may have ovulated in the hCG vs. GnRH treatment.

In Exp. 1, ovulation incidence in response to hCG was equal to that of GnRH. In Exp. 2, P/AI of GnRH-treated cows exceeded that of hCG in cycling cows. In previous studies, hCG administered before or after insemination was effective at inducing ovulation of follicles (Price and Webb, 1989Go; Diaz et al., 1998Go; Santos et al., 2001Go), lengthening the estrous cycle (Howard et al., 1990Go), increasing size and function of a CL (Santos et al., 2001Go; Stevenson et al., 2006Go), and improving fertility (Santos et al., 2001Go; Stevenson et al., 2006Go). Pregnancies per AI in a CO-Synch protocol, with or without calf removal, were greater when GnRH was used at the beginning of the CO-Synch protocol and at TAI compared with injecting hCG at both times (Geary et al., 2001Go). Our experiment supports these findings; hCG had a negative effect in cycling cows when used to initiate the CO-Synch + progesterone insert protocol. The mechanism of action that would cause hCG to reduce P/AI in cows is unknown. Both studies provide evidence that hCG is not a good substitute for GnRH in the CO-Synch or CO-Synch + insert protocols.

In the dairy industry, it is common to resynchronize ovulation in cows of unknown pregnancy status by injecting GnRH 7 d before pregnancy status is determined (Chebel et al., 2003Go; Fricke et al., 2003Go). Beef producers have not taken advantage of potential resynchronization protocols similar to those applied in the dairy industry. We hypothesized that successful resynchronization of ovulation in previously inseminated cattle of unknown pregnancy may not require that cattle be treated initially with GnRH or hCG, depending on when nonpregnancy status is determined. Pregnancy diagnosis and PGF2{alpha} injection were carried out at a known stage of the estrous cycle in cows previously inseminated but diagnosed not pregnant. Nonpregnant cattle that were responsive to the initial synchronization should have had a functional CL at the time of PGF2{alpha} injection on d 33. These cows should then have recycled from the first TAI by d 20 to 22 (Pierson and Ginther, 1987Go) and should have been on d 12 or 13 of the estrous cycle when PGF2{alpha} was administered. In actuality, normal returns to estrus after TAI were observed for cows in which estrus detection occurred after the first TAI at 2 locations (yr 1: 22.0 ± 0.6 d; range of 17 to 30; n = 29 and yr 2: 21.9 ± 0.5 d; range of 19 to 33; n = 37). Based on that distribution of estrus, cows treated on d 26 and given PGF2{alpha} on d 33 were on d 11 of the estrous cycle. Further, the majority of nonpregnant cows treated with GnRH or hCG on d 26 (d 5 of cycle) should have ovulated (Price and Webb, 1989Go) and produced a new dominant follicle before PGF2{alpha} was administered 7 d later. Those given saline should have had a more mature first-wave dominant follicle (Vasconcelos et al., 1999Go) 7 d later at pregnancy diagnosis.

Timing of AI, therefore, could have favored cattle that received saline because these cows should have had a mature, first-wave, dominant follicle when PGF2{alpha} was injected at pregnancy diagnosis. In contrast, most of the cattle should have ovulated in response to hCG or GnRH and likely had less mature dominant follicles at PGF2{alpha} injection. As a result, timing of saline-treated cows at 56 h after PGF2{alpha} injection might have been better suited to time of ovulation than for the remaining treated cows. Injections of GnRH or hCG as part of the CO-Synch resynchronization protocol before the second TAI had positive effects on P/AI at only 2 locations. Small average differences among treatments of saline, GnRH, or hCG on d 26 might indicate that no treatment is needed to resynchronize ovulation in cattle before a second TAI. Our results, however, cannot explain the dichotomous response among locations for the effect of post-TAI treatments on P/AI.

In our experiments, cattle that were not pregnant after the first insemination (initial TAI) were treated with GnRH, hCG, or saline to initiate a second TAI 7 d before the first pregnancy diagnosis. Within 35 d of the beginning of the breeding season, cattle had 2 chances to conceive to AI without detection of estrus. Normally, calving distribution after a breeding season in which synchronization of estrus or ovulation is applied has a large proportion of calves born at the beginning of the calving season (Larson et al., 2006Go). Then, 21 d later, another large portion of the calves are born in response to natural services from cleanup bulls or a second AI. In our protocol (Exp. 2), however, another 14 d are sacrificed to incorporate a second TAI, thereby lengthening the interval between these 2 calving-period peaks from 21 to 35 d but giving cows 2 chances to conceive to a TAI (Figure 4Go). This system may appeal to producers whose facilities are inadequate to detect estrus or to those that find detection less appealing because of time and labor costs. The trade-off, however, for 50% more AI-sired calves in the double TAI system is that, potentially, fewer cows become pregnant during a 68-d breeding season (Table 6Go). To resolve this situation, P/AI must be greater at both TAI periods to increase total P/AI without lengthening the breeding season.

In summary, treatment of cows with 500 IU of hCG ovulated follicles as effectively as GnRH treatment. Pregnancies per AI, however, were less in cycling cows treated initially with hCG than GnRH in a CO-Synch + progesterone insert protocol. Therefore, hCG might not be a suitable replacement for GnRH in a CO-Synch + progesterone insert protocol. Pregnancies per AI after resynchronization of ovulation with GnRH and hCG were similar to saline but the response was not consistent among locations. A double TAI system may reduce total pregnancy rate and alter subsequent calving distribution. Future experiments should be conducted to evaluate how to increase P/AI at the resynchronized estrus for cows not already pregnant after the first TAI of the breeding season.


   Footnotes
 
1 Contribution number 08-346-J from the Kansas Agricultural Experiment Station, Manhattan. The authors thank Gary Ritter, Wayne Adolph, and student workers at the Kansas State University (KSU) Commercial Cow-Calf Unit; Ryan Breiner and student workers at the KSU Purebred Beef Unit; Kevin Thielen, Thielen Ranch, Dorrance, KS; and Bethany Lovaas, Scott Bird, and Jamie Larson at the North Central Research and Outreach Center in Grand Rapids, MN, for providing cattle and assistance in carrying out this project. We acknowledge Select Sires Inc., Plain City, OH for partial funding; IVX Animal Health, St. Joseph, MO; Intervet Inc., Millsboro, DE; and Pfizer Animal Health, New York, NY, for donation of products [GnRH, PGF2{alpha}, hCG, progesterone (controlled internal drug release) inserts], respectively. Back

2 Present address: Clemson Extension, Clemson, SC 29634. Back

3 Present address: Hereford Veterinary Clinic, Hereford, TX 79045. Back

4 Corresponding author: jss{at}k-state.edu

Received for publication April 23, 2008. Accepted for publication June 3, 2008.


   LITERATURE CITED
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 LITERATURE CITED
 


Chebel, R. C., J. E. Santos, R. L. Cerri, K. N. Galvão, S. O. Juchem, and W. W. Thatcher. 2003. Effect of resynchronization with GnRH on day 21 after artificial insemination on pregnancy rate and pregnancy loss in lactating dairy cows. Theriogenology 60:1389–1399.[CrossRef][Medline]

Diaz, T., E. J.-P. Schmitt, R. L. de la Sota, M.-J. Thatcher, and W. W. Thatcher. 1998. Human chorionic gonadotropin-induced alterations in ovarian follicular dynamics during the estrous cycle of heifers. J. Anim. Sci. 76:1929–1936.[Abstract/Free Full Text]

Folman, Y., M. Kaim, Z. Herz, and M. Rosenberg. 1990. Comparison of methods for the synchronization of estrous cycles in dairy cows. 2. Effects of progesterone and parity on conception. J. Dairy Sci. 73:2817–2825.[Abstract]

Fonseca, F. A., J. H. Britt, B. T. McDaniel, J. C. Wilk, and A. H. Rakes. 1983. Reproductive traits of Holsteins and Jerseys. Effects of age, milk yield, and clinical abnormalities on involution of cervix and uterus, ovulation, estrous cycles, detection of estrus, conception rate, and days open. J. Dairy Sci. 66:1128–1147.[Abstract/Free Full Text]

Fricke, P. M., D. Z. Caraviello, K. A. Weigel, and M. L. Welle. 2003. Fertility of dairy cows after resynchronization of ovulation at three intervals following first timed insemination. J. Dairy Sci. 86:3941–3950.[Abstract/Free Full Text]

Geary, T. W., R. R. Salverson, and J. C. Whittier. 2001. Synchronization of ovulation using GnRH or hCG with the CO-Synch protocol in suckled beef cows. J. Anim. Sci. 79:2536–2541.[Abstract/Free Full Text]

Geary, T. W., and J. C. Whittier. 1998. Effects of a timed insemination following synchronization of ovulation using the Ovsynch or COSynch protocol in beef cows. Prof. Anim. Sci. 14:217–220.[Abstract/Free Full Text]

Howard, H. J., R. G. Scott, and J. H. Britt. 1990. Associations among progesterone, estradiol-17 beta, oxytocin and prostaglandin in cattle treated with hCG during diestrus to extend corpus luteum function. Prostaglandins 40:51–70.[CrossRef][Medline]

Larson, J. E., G. C. Lamb, J. S. Stevenson, S. K. Johnson, M. L. Day, T. W. Geary, D. J. Kesler, J. M. DeJarnette, F. N. Schrick, A. DiCostanzo, and J. D. Arseneau. 2006. Synchronization of estrus in suckled beef cows for detected estrus and artificial insemination and timed artificial insemination using gonadotropin-releasing hormone, prostaglandin F2{alpha}, and progesterone. J. Anim. Sci. 84:332–342.[Abstract/Free Full Text]

NAHMS. 1997. NAHMS Beef ’97 Part III: Reference of 1997 Beef Cow-Calf Production Management and Disease Control. http://nahms.aphis.usda.gov/beefcowcalf/beef97/bf97pt3.pdf Accessed Feb. 20, 2007.

Niswender, G. D., J. L. Juengel, P. J. Silva, M. K. Rollyson, and E. W. McIntosh. 2000. Mechanisms controlling the function and life span of the corpus luteum. Physiol. Rev. 80:1–29.[Abstract/Free Full Text]

Pierson, R. A., and O. J. Ginther. 1987. Follicular populations during the estrous cycle in heifers. I. Influence of day. Anim. Reprod. Sci. 14:165–176.[Medline]

Price, C. A., and R. Webb. 1989. Ovarian response to hCG treatment during the oestrous cycle in heifers. J. Reprod. Fertil. 86:303–308.[Abstract/Free Full Text]

Rajamahendran, R., and P. C. Sianangama. 1992. Effect of human chorionic gonadotropin on dominant follicles in cows: Formation of accessory corpora lutea, progesterone production and pregnancy rates. J. Reprod. Fertil. 95:577–584.[Abstract/Free Full Text]

Santos, J. E. P., W. W. Thatcher, L. Pool, and M. W. Overton. 2001. Effect of human chorionic gonadotropin on luteal function and reproductive performance of high-producing lactating Holstein dairy cows. J. Anim. Sci. 79:2881–2894.[Abstract/Free Full Text]

Schmitt, E. J. P., T. Diaz, C. M. Barros, R. L. de la Sota, M. Drost, E. W. Fredriksson, C. R. Staples, R. Thorner, and W. W. Thatcher. 1996. Differential response of the luteal phase and fertility in cattle following ovulation of the first-wave follicle with human chorionic gonadotropin or an agonist of gonadotropin-releasing hormone. J. Anim. Sci. 74:1074–1083.[Abstract]

Skaggs, C. L., B. V. Able, and J. S. Stevenson. 1986. Pulsatile or continuous infusion of luteinizing hormone-releasing hormone and hormonal concentrations in prepubertal beef heifers. J. Anim. Sci. 62:1034–1048.[Abstract/Free Full Text]

Stevenson, J. S., S. K. Johnson, M. A. Medina-Britos, A. M. Richardson, and G. C. Lamb. 2003. Resynchronization of estrus in cattle of unknown pregnancy status using estrogen, progesterone, or both. J. Anim. Sci. 81:1682–1692.

Stevenson, J. S., M. A. Portaluppi, D. E. Tenhouse, A. Lloyd, D. R. Eborn, S. Kacuba, and J. M. DeJarnette. 2006. Interventions after artificial insemination: Conception rates, pregnancy survival, and ovarian responses to gonadotropin-releasing hormone, human chorionic gonadotropin, and progesterone. J. Dairy Sci. 89:6378–6383.

Thompson, K. E., J. S. Stevenson, G. C. Lamb, D. M. Grieger, and C. A. Löest. 1999. Follicular, hormonal, and pregnancy responses of early postpartum suckled beef cows to GnRH, norgestomet, and prostaglandin F2{alpha}. J. Anim. Sci. 77:1823–1832.[Abstract/Free Full Text]

Vasconcelos, J. L. M., R. W. Silcox, G. J. M. Rosa, J. R. Pursley, and M. C. Wiltbank. 1999. Synchronization rate, size of the ovulatory follicle, and pregnancy rate after synchronization of ovulation beginning on different days of the estrous cycle in lactating dairy cattle. Theriogenology 52:1067–1078.[CrossRef][Medline]



This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow All Versions of this Article:
jas.2008-1122v1
86/10/2539    most recent
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Burns, M. G.
Right arrow Articles by Stevenson, J. S.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Burns, M. G.
Right arrow Articles by Stevenson, J. S.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS