Timing of artificial insemination in postpartum beef cows following administration of the CO-Synch + controlled internal drug-release protocol 1

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1 Published December 5, 2014 Timing of artificial insemination in postpartum beef cows following administration of the CO-Synch + controlled internal drug-release protocol 1 D. C. Busch,* D. J. Schafer, D. J. Wilson,* D. A. Mallory,* N. R. Leitman,* J. K. Haden, M. R. Ellersieck, M. F. Smith,* and D. J. Patterson* 2 *Division of Animal Science, S132 ASRC, University of Missouri, Columbia 65211; MFA Inc., 201 Ray Young Dr., Columbia, MO 65201; and Agriculture Experiment Station, University of Missouri, Columbia ABSTRACT: This experiment was designed to compare pregnancy rates in postpartum beef cows resulting from fixed-time AI (FTAI) at 54 or 66 h after administration of the CO-Synch + controlled internal drug-release (CIDR) protocol. Cows (n = 851) at 2 locations over 2 yr (yr 1, n = 218 and 206; and yr 2, n = 199 and 228 at the 2 locations, respectively) were stratified by age, BCS, and days postpartum to 1 of 2 FTAI intervals. Cows were administered GnRH (100 g, i.m.) and were equipped with a CIDR insert (1.38 g of progesterone) on d 0. Controlled internal drug-release inserts were removed 7 d later at the time PGF 2α (25 mg, i.m.) was administered (d 7). Continuous estrus detection was performed at location 2 by using the HeatWatch Estrus Detection System; the transmitters were fitted at the time of PGF 2α and removed at the time of AI. Artificial insemination was performed at predetermined fixed times [54 h (FTAI 54; n = 424) or 66 h (FTAI 66; n = 427) after PGF 2α ] and all cows were administered GnRH (100 g, i.m.) at AI. Two blood samples were collected on d 10 or 8 and immediately before treat- ment initiation to determine the pretreatment estrous cyclicity status of cows [progesterone 0.5 ng/ml (FTAI 54, 288/424 = 68%; FTAI 66, 312/427 = 73%; P = 0.07)]. Pregnancy rates were greater (P < 0.01) among cows that exhibited estrus than among those that did not (123/163 = 76% and 150/270 = 56%, respectively). There were no treatment location interactions within year (P > 0.10) for age, days postpartum, or BCS; thus, the results were pooled for the respective treatments. Pregnancy rates were greater for FTAI 66 than FTAI 54 (P = 0.05; 286/426 = 67% vs. 257/424 = 61%, respectively). Pregnancy rates resulting from FTAI did not differ between year (P = 0.09), farm (P = 0.80), AI sire (P = 0.11), or technician (P = 0.64). There was no difference between pregnancy rates resulting from FTAI based on pretreatment cyclicity status (P = 0.30), and there was no difference between treatments in final pregnancy rates (P = 0.77). In summary, pregnancy rates resulting from FTAI following CO-Synch + CIDR at 66 h were greater than those resulting from FTAI at 54 h. Key words: artificial insemination, beef cow, controlled internal drug-release insert, estrous synchronization, pregnancy rate 2008 American Society of Animal Science. All rights reserved. J. Anim. Sci : doi: /jas INTRODUCTION Estrous synchronization and AI offer beef producers the means to introduce proven superior genetics into their herds, shorten the calving season, and increase 1 Contribution from the Missouri Agric. Exp. Stn. This project was supported by Select Sires Inc. (Plain City, OH). The authors gratefully acknowledge Pfizer Animal Health (New York, NY) for providing the Lutalyse sterile solution and Eazi-Breed CIDR cattle inserts; Merial (Athens, GA) for providing the Cystorelin; Select Sires Inc. for providing the semen; and D. S. McAtee and J. J. D. Schreffler for their dedicated support of this research at the University of Missouri Thompson Farm (Spickard, MO). calf age and uniformity. However, surveys indicate fewer than 10% of beef cows in the United States are bred by AI (National Animal Health Monitoring System, 1997), and even fewer beef operations use estrous synchronization to facilitate AI programs (National Animal Health Monitoring System, 1997). The main reason producers cited for not implementing these 2 practices was lack of available time and labor (National Animal Health Monitoring System, 1998). Therefore, to enhance the use of estrous synchronization and AI 2 Corresponding author: pattersond@missouri.edu Received February 4, Accepted March 12,

2 1520 Busch et al. by beef producers, effective estrous synchronization protocols need to minimize the number and frequency of animal handlings and minimize or eliminate the need for estrus detection. Development of methods to control estrous cycles in cattle that result in expression of a highly synchronized and fertile estrus and ovulation will more readily facilitate fixed-time AI (FTAI; Patterson et al., 2003a). Effective estrous synchronization protocols that facilitate FTAI would likely increase the adoption of AI in beef herds (Patterson et al., 2003b). Previous research demonstrated the efficacy of using the CO-Synch + controlled internal drug-release (CIDR) protocol to synchronize estrus and ovulation when FTAI was performed at 66 h after administration of PGF 2α (Schafer et al., 2007). Another study demonstrated a numerical improvement in pregnancy rates with FTAI at 66 h compared with 48 or 54 h after PGF 2α (Bremer et al., 2004). However, Dobbins et al. (2006) reported an improvement in pregnancy rates resulting from FTAI at 56 to 64 h compared with 48 or 72 h after PGF 2α. Therefore, the objective of this experiment was to compare FTAI pregnancy rates among lactating beef cows synchronized with the CO-Synch + CIDR protocol and inseminated at 54 or 66 h after PGF 2α, and to characterize the estrous response of cows in each treatment group before FTAI. MATERIALS AND METHODS The experimental procedures were approved by the University of Missouri-Columbia Animal Care and Use Committee. Animals Crossbred lactating beef cows (n = 851) at 2 locations over 2 yr (yr 1, n = 218 and 206; and yr 2, n = 199 and 228 at the 2 locations, respectively) were assigned within age group (2 to 15 yr) by calving date (days postpartum; DPP) and BCS (1 to 9 scale; 1 = emaciated and 9 = obese; Richards et al., 1986) to 1 of 2 FTAI times (Table 1). Cows synchronized with the CO-Synch + CIDR protocol received GnRH (100 g, i.m.; Cystorelin, Merial, Athens, GA) and an Eazi-Breed CIDR insert (1.38 g of progesterone; Pfizer Animal Health, New York, NY) on d 0 and PGF 2α (25 mg, i.m.; Lutalyse, Pfizer Animal Health, New York, NY) and CIDR insert removal on d 7. Cows at location 2 were fitted with HeatWatch Estrus Detection System transmitters (DDx Inc., Denver, CO) at the time of PGF 2α to characterize the estrous response (estrus was defined as cows receiving 3 mounts, each of which was 2 s in duration, within a 4-h period) before FTAI, at which time the transmitters were removed. Fixed-time AI was performed at 54 (n = 424) or 66 h (n = 427) after PGF 2α (Figure 1). Times of PGF 2α administration and AI were recorded for each cow. All cows were administered GnRH (100 g, i.m) at the time of insemination, and AI was performed by 1 of 2 experienced technicians. During yr 1 of the experiment, 1 sire was used at location 1 and 3 sires were used at location 2. During yr 2 of the experiment, the same 2 sires were used at both locations. The sire used during yr 1 at location 1 was also one of the sires used at location 2 during yr 1, and at both locations during yr 2. The AI sires were assigned to cows within each treatment by age, calving date, and BCS. Artificial insemination technicians were assigned to cows within each treatment by AI sire, cow age, calving date, and BCS. Twelve to 14 d after FTAI, cows were exposed to fertile bulls for the remainder of the 60-d breeding season. Blood Collection and RIA Blood samples were collected via jugular venipuncture into 10-mL Vacutainer tubes (Fisher Scientific, Pittsburgh, PA) at d 10 or 8 and immediately before treatment initiation (d 0) to determine pretreatment estrous cyclicity status. Blood samples were allowed to clot and were stored at 4 C for 24 h. Serum was collected by centrifugation (2,000 g for 20 min) and was stored at 20 C until hormone analyses were performed. Serum concentrations of progesterone were determined with a Coat-A-Count Kit (Diagnostic Products Corporation, Los Angeles, CA; Kirby et al., 1997), with intraand interassay CV of 2.3 and 11.4% and an assay sensitivity of 0.1 ng/ml. Cows were considered cyclic if their progesterone concentrations were 0.5 ng/ml in one or both blood samples before treatment initiation (Ciccioli et al., 2003). Pregnancy Diagnosis Pregnancy rate to AI was determined by transrectal ultrasonography (Aloka 500V equipped with a 5.0-MHz linear-array transducer; Aloka, Wallingford, CT) 56 to 78 d after FTAI. Final pregnancy rates were determined by transrectal ultrasonography or rectal palpation 60 to 110 d after the end of the 60-d breeding season. Statistical Analysis Differences in age, DPP, and BCS between treatments were analyzed by ANOVA using the statistical model with year, location, treatment, and all relevant interactions (PROC GLM; SAS Inst. Inc., Cary, NC). Pretreatment cyclicity status was analyzed by χ 2 analysis (PROC GENMOD of SAS) using the model with year, location, treatment, and all relevant interactions. Estrous response, pregnancy rate to FTAI, and final pregnancy rate at the end of the breeding season were analyzed by χ 2 analysis (PROC GENMOD of SAS) using the model with year, location, treatment, AI technician, pretreatment estrous cyclicity status, and all relevant interactions. The odds ratio was calculated by taking the inverse natural log of the regression coefficient estimate from the GENMOD procedure. Differences in interval from PGF 2α to estrus and FTAI (location 2) were

3 Timing of AI following the CO-Synch + controlled internal drug-release protocol 1521 Table 1. Number of cows at each location, age, days postpartum, BCS, and cyclicity status for cows before the initiation of treatment (means ± SE) Cows with elevated progesterone 4 Postpartum Item Location Treatment 1 n Age, yr interval, 2 d BCS 3 Proportion % Year 1 Location 1 54 h ± ± ± / h ± ± ± / Combined ± ± 1.1 a 5.3 ± 0.03 a 158/ a Year 1 Location 2 54 h ± ± ± / h ± ± ± / Combined ± ± 1.1 ab 5.9 ± 0.03 b 181/ b Year 2 Location 1 54 h ± ± ± / h ± ± ± / Combined ± ± 1.1 b 5.6 ± 0.03 c 93/ c Year 2 Location 2 54 h ± ± ± / h ± ± ± / Combined ± ± 1.0 c 5.4 ± 0.03 d 172/ a Year 1 total ± ± ± 0.02 e 339/ e Year 2 total ± ± ± 0.02 f 265/ f Location 1 total ± ± 0.8 g 5.4 ± 0.02 g 251/ g Location 2 total ± ± 0.7 h 5.6 ± 0.02 h 353/ h 54-h total ± ± ± / h total ± ± ± / a d Combined year location means within a column with different superscripts are different, P < e,f Year 1 and 2 total means within a column with different superscripts are different, P < g,h Location 1 and 2 total means within a column with different superscripts are different, P < Cows received a controlled internal drug-release (CIDR) insert (1.38 g of progesterone) and were administered GnRH (100 g, i.m.) on d 0. On d 7, the CIDR insert was removed and PGF 2α (25 mg, i.m.) was administered. At 54 or 66 h after CIDR insert removal and PGF 2α, the cows received GnRH and fixed-time AI (FTAI). 2 Number of days postpartum at the initiation of treatments. 3 BCS of cows at the time of the first blood sample before initiation of treatments (1 to 9 scale, where 1 = emaciated and 9 = obese). 4 Estrous cyclicity = the percentage of cows with elevated ( 0.5 ng/ml) concentrations of progesterone in serum before treatment. Cows were considered to be cyclic if progesterone was elevated in either or both of 2 blood samples collected either 8 or 10 d before and immediately before the initiation of treatment. determined by ANOVA using the statistical model with year, treatment, pretreatment cyclicity status, and all relevant interactions (PROC GLM of SAS). RESULTS The number of cows, mean age, DPP, BCS, and cyclicity rate of cows before the initiation of treatments are shown for each location during each year in Table 1. There were no differences between treatments at the respective locations for age (P = 0.94), DPP (P = 0.96), BCS (P = 0.77), or cyclicity status (P = 0.07) at the initiation of treatment; however, there were differences in DPP, BCS and cyclicity rate between locations and years, and a location year interaction in DPP and BCS (P < 0.05; Table 1). The interval from PGF 2α to FTAI (mean ± SD) was 54.2 ± 0.5 and 66.2 ± 0.4 for cows in the FTAI at 54- and 66-h treatments, respectively. There was no effect of year (P = 0.09), location (P = 0.80), technician (P = 0.64), sire (P = 0.11), or pretreatment cyclicity status before initiation of treatment (P = 0.30; Table 2) on pregnancy rates resulting from FTAI. However, there was a significant effect (P = 0.05) of treatment on pregnancy rates resulting from FTAI among all cows (Table 3). On the basis of the odds ratio, cows inseminated at 66 h following PGF 2α administration are 1.32 times more likely to conceive to the FTAI than cows inseminated at 54 h following PGF 2α administration. Final pregnancy rate at the end of the 60-d breeding season did not differ (P = 0.77) between treatments (Table 3). Estrous response (location 2) following PGF 2α and before FTAI was greater (P < 0.01) among cows inseminated at 66 h (50%) than among cows inseminated at 54 h (26%). However, there was no difference (P > 0.10) in estrous response (location 2) within treatment between cyclic and anestrous cows and no overall difference (P = 0.21) between cyclic and anestrous cows. Mean interval from PGF 2α to estrus was shorter (P < 0.01) for FTAI at 54 h (mean ± SE, 46.7 ± 1.1 h) compared with FTAI at 66 h (53.8 ± 1.0 h); however, these differences occurred as a result of the HeatWatch transmitters being removed at AI. Cows that exhibited estrus before FTAI had significantly greater pregnancy rates (76%; P < 0.01) than cows that did not exhibit estrus before AI (56%; Table 4). In addition, cows that exhibited estrus before FTAI at 66 h had a greater pregnancy rate (81%; P = 0.03) than cows that exhibited estrus before FTAI at 54 h (65%; Table 4). DISCUSSION Estrous synchronization for beef heifers and cows has been commercially available to producers for more than

4 1522 Busch et al. Figure 1. Treatment schedule for cows assigned to the CO-Synch + controlled internal drug-release (CIDR) protocol with fixed time AI (FTAI) at 54 or 66 h. Cows synchronized with the CO-Synch + CIDR protocol received an Eazi-Breed CIDR insert (Pfizer Animal Health, New York, NY; 1.38 g of progesterone) and were administered GnRH (Cystorelin, Merial, Athens, GA; 100 g, i.m.) on d 0. On d 7, the CIDR insert was removed and PGF 2α (PG; 25 mg, i.m. Lutalyse, Pfizer Animal Health) was administered. At 54 or 66 h after CIDR insert removal and PG, the cows received GnRH and FTAI. 30 yr; however, the beef industry has been slow to adopt this technology. Estrous synchronization protocols that include treatment with a progestin offer the potential to initiate estrous cyclicity in anestrous cows, more effectively facilitate AI, and shorten the subsequent calving period, resulting in a more uniform calf crop. In addition, recently developed estrous synchronization protocols for postpartum beef cows facilitate FTAI and eliminate the time and labor required for estrus detection without compromising pregnancy rates resulting from AI (Bader et al., 2005; Larson et al., 2006; Schafer et al., 2007). Several studies involving the CO-Synch + CIDR protocol have evaluated the effect of timing of AI on subsequent pregnancy rates resulting from FTAI, with insemination times varying from 48 to 72 h after PGF 2α (Bremer et al., 2004; Dobbins et al., 2006). However, there are no conclusive results demonstrating that AI should be performed at a specific time following the CO-Synch + CIDR protocol. Timing of insemination following the CO-Synch + CIDR protocol was based on recommendations from the pharmaceutical and AI industries of 60 ± 6 h (54 to 66 h; North Central Region Bovine Reproductive Task Force, 2006) and other reports in which the timing of AI included 48, 54, 56, 60, 64, 66, and 72 h post PGF 2α (Lamb et al., 2001; Bremer et al., 2004; Dobbins et al., 2006; Larson et al., 2006; Schafer et al., 2007). In this study, performing FTAI at 66 h after PGF 2α administration and CIDR insert removal following treatment with the CO-Synch + CIDR protocol resulted in an improved estrous response before FTAI and a greater FTAI pregnancy rate compared with insemination performed at 54 h. Improved pregnancy rates following FTAI at 66 h appear to be associated with the increased estrous response before FTAI. Larson et al. (2006) reported that the peak estrous response following the Select Synch + CIDR protocol occurred 48 to 60 h after PGF 2α administration, with a mean interval to estrus of 53.4 ± 0.8 h. Those results are similar to data reported in this study (53.8 ± 1.0 h) for cows inseminated at 66 h. On the basis of the Larson et al. (2006) data, using the general rule of inseminating cows 12 h after the onset of estrus, the peak time of insemination with the CO-Synch + CIDR protocol should occur 60 to 72 h after PGF 2α, which includes the time period (66 h) in this study in which the greatest FTAI pregnancy rate was obtained. The onset of estrus before FTAI in beef cows was shown to improve pregnancy rates compared with cows Table 2. Pregnancy rates after fixed-time AI (FTAI) based on estrous cyclicity before the initiation of treatments 1 FTAI at 54 h FTAI at 66 h Cyclic Anestrus Cyclic Anestrus Year Location Proportion % Proportion % Proportion % Proportion % / / / / / / / /10 50 Combined 102/ / / / / / / / / / / /22 59 Combined 76/ / / /72 68 Total 178/ / / / Cows received a controlled internal drug-release (CIDR) insert (1.38 g of progesterone) and were administered GnRH (100 g, i.m.) on d 0. On d 7, the CIDR insert was removed and PGF 2α (25 mg, i.m.) was administered. At 54 or 66 h after CIDR insert removal and PGF 2α, the cows received GnRH and fixed-time AI (FTAI).

5 Timing of AI following the CO-Synch + controlled internal drug-release protocol 1523 Table 3. Pregnancy rates of cows in response to fixed-time AI (FTAI) and at the end of the breeding season 1 Pregnancy rate Pregnancy rate at the end to FTAI 2 of the breeding season 3 Item Location Proportion % Proportion % Year 1 Location 1 FTAI at 54 h 66/ / FTAI at 66 h 66/ / Combined 132/ / Location 2 FTAI at 54 h 59/ / FTAI at 66 h 72/ / Combined 131/ / Combined FTAI at 54 h 125/ / FTAI at 66 h 138/ / Year 2 Location 1 FTAI at 54 h 67/ /98 94 FTAI at 66 h 71/ / Combined 138/ / Location 2 FTAI at 54 h 65/ / FTAI at 66 h 77/ / Combined 142/ / Combined FTAI at 54 h 132/ / FTAI at 66 h 148/ / FTAI at 54 h total 257/ x 385/ FTAI at 66 h total 286/ y 389/ x,y FTAI pregnancy rates within a column with different superscripts are different, P = Cows received a controlled internal drug-release (CIDR) insert (1.38 g of progesterone) and were administered GnRH (100 g, i.m.) on d 0. On d 7, the CIDR insert was removed and PGF 2α (25 mg, i.m.) was administered. At 54 or 66 h after CIDR removal and PGF 2α, the cows received GnRH and FTAI. 2 Pregnancy rate to FTAI determined by ultrasound 56 to 78 d after AI. 3 Pregnancy rate at the end of the breeding season was determined 60 to 110 d after the end of a 60-d breeding season. that did not exhibit estrus (Perry et al., 2005). Our data are similar to those of Perry et al. (2005), who reported that cows that exhibited estrus before or at AI had greater pregnancy rates (90%) than cows that had not yet exhibited estrus (29%; P < 0.01) following administration of the CO-Synch protocol. Busch et al. (2008) reported that cows that exhibited estrus following administration of the CO-Synch protocol had greater serum estradiol concentrations during the 2 d before insemination compared with cows that were induced to ovulate. Cows that exhibited estrus in the current study may have attained concentrations of estradiol necessary to effectively prepare follicular cells for luteinization, or may have induced an adequate number of uterine progesterone receptors, or both (Zelinski et al., 1980), thus providing an adequate uterine environment for pregnancy establishment and maintenance. These data indicate that greater estrous response rates before FTAI in beef cows should result in greater pregnancy rates resulting from FTAI, provided that AI is per- Table 4. Pregnancy rate based on estrous response (location 2) 1 54 h 66 h Combined Estrus Proportion % Proportion % Proportion % Yes 37/57 65 x 86/ a,y 123/ a No 87/ / b 150/ b a,b Percentages within a column with different superscripts are different, P < x,y Percentages within a row with different superscripts are different, P = Cows received a controlled internal drug-release (CIDR) insert (1.38 g of progesterone) and were administered GnRH (100 g, i.m.) on d 0. On d 7, the CIDR insert was removed and PGF 2α (25 mg, i.m.) was administered. At 54 or 66 h after CIDR insert removal and PGF 2α, the cows received GnRH and fixed-time AI (FTAI).

6 1524 Busch et al. Figure 2. Percentage of anestrous or cyclic cows in estrus during the interval between receiving PGF 2α (PG) and fixed-time AI at location 2. formed within an acceptable time period following the peak estrous period. However, to date no studies have characterized the full estrus distribution following the CO-Synch + CIDR protocol to identify the peak estrous period. It is important to note that there was no difference within treatments in pregnancy rates resulting from FTAI between cows that were classified as cyclic or anestrus before treatment initiation. These results indicate that the CO-Synch + CIDR protocol effectively induced cyclicity in anestrous cows as measured by estrus, ovulation, and pregnancy outcome. The authors acknowledge the similarity in pregnancy rates between treatments at location 1; however, a difference was observed at location 2 during both years of the study (Table 3). This may be partially explained by the slight difference that was noted in the distribution of cows exhibiting estrus among anestrous and cyclic cows at location 2 following PGF 2α (Figure 2). At location 2, anestrous cows seemed to express estrus earlier than cyclic cows following CIDR removal and PGF 2α administration, based on the small number of anestrous cows (n = 36) that expressed estrus in this study. Therefore, the similarity in FTAI pregnancy rates at location 1 may be due to the increased number of anestrous cows (40 vs. 19% anestrous; location 1 and 2, respectively) that expressed estrus earlier and the fact that more of those cows conceived to the 54-h FTAI compared with location 2. A key observation from this study lies with the fact that pregnancy rates resulting from FTAI at 66 h following administration of the CO-Synch + CIDR protocol were consistent across locations and years. These results too, as they relate to pregnancy rates following FTAI, compare favorably with the study by Schafer et al. (2007) and with the field trials in Missouri reported by Patterson et al. (2007). Interestingly, these results are consistently greater than those reported in the literature (Lamb et al., 2001; Bremer et al., 2004; Dobbins et al., 2006; Larson et al., 2006) when cows were inseminated at 48, 56, 60, 64, or 72 h following treatment administration, all of which point to the importance of timing of insemination and the associated impact on pregnancy outcome. Finally, the authors acknowledge the potential for misclassification of cows on the basis of cyclicity determined from 2 blood samples before treatment initiation and the use of progesterone values 0.5 ng/ml to confirm cyclicity. However, the potential for committing a type II error is greatly minimized, if not negated, in describing cows as anestrus when using a 0.5-ng/mL cutoff. These results indicate that greater pregnancy rates may be achieved through a single insemination following administration of the CO-Synch + CIDR protocol when FTAI is performed at 66 versus 54 h after PGF 2α. This protocol affords producers the opportunity to perform AI at a predetermined fixed time and eliminate the time and labor required to detect estrus and inseminate cows over multiple days. LITERATURE CITED Bader, J. F., F. N. Kojima, D. J. Schafer, J. E. Stegner, M. R. Ellersieck, M. F. Smith, and D. J. Patterson A comparison of progestin-based protocols to synchronize ovulation and facilitate fixed-time artificial insemination in postpartum beef cows. J. Anim. Sci. 83: Bremer, V. R., S. M. Damiana, F. A. Ireland, D. B. Faulkner, and D. J. Kesler Optimizing the interval from PGF to timed AI in the CO-Synch+CIDR and 7 11 Synch estrus synchronization protocols for postpartum beef cows. J. Anim. Sci. 82(Suppl. 2):106. Busch, D. C., J. A. Atkins, J. F. Bader, D. J. Schafer, D. J. Patterson, T. W. Geary, and M. F. Smith Effect of ovulatory follicle size and expression of estrus on progesterone secretion in beef cows. J. Anim. Sci. 86: Ciccioli, N. H., R. P. Wettemann, L. J. Spicer, C. A. Lents, F. J. White, and D. H. Keisler Influence of body condition at calving and postpartum nutrition on endocrine function and reproductive performance of primiparous beef cows. J. Anim. Sci. 81: Dobbins, C. A., D. E. Tenhouse, D. R. Eborn, K. R. Harmony, S. K. Johnson, and J. S. Stevenson Conception rates after altered timing of AI associated with the CO-Synch + CIDR protocol. J. Anim. Sci. 84(Suppl. 1):50. Kirby, C. J., M. F. Smith, D. H. Keisler, and M. C. Lucy Follicular function in lactating dairy cows treated with sustained-release bovine somatropin. J. Dairy Sci. 80: Lamb, G. C., J. S. Stevenson, D. J. Kesler, H. A. Garverick, D. R. Brown, and B. E. Salfen Inclusion of an intravaginal progesterone insert plus GnRH and prostaglandin F 2α for ovulation control in postpartum suckled beef cows. J. Anim. Sci. 79: 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 Synchronization of estrus in suckled beef cows for detected estrus and artificial insemination and timed artificial insemination using gonadotropin-releasing hormone, prostaglandin F 2α, and progesterone. J. Anim. Sci. 84: National Animal Health Monitoring System Part 1: Reference of 1997 beef cow-calf management practices. usda.gov/beefcowcalf/beef97/bf97pt1/pdf Accessed Oct. 24, 2007.

7 Timing of AI following the CO-Synch + controlled internal drug-release protocol 1525 National Animal Health Monitoring System Part IV. Changes in the U.S. Beef Cow-Calf Industry USDA-Animal and Plant Health Inspection Service, Center for Epidemiology and Animal Health, Fort Collins, CO. North Central Region Bovine Reproductive Task Force Estrus synchronization protocols for cows. beefrepro/pdfs/cowaisynchprotocols.pdf. Accessed Jan. 24, Patterson, D. J., D. C. Busch, N. R. Leitman, D. J. Wilson, D. A. Mallory, and M. F. Smith Estrus synchronization protocols for heifers. Pages in Proc. Appl. Reprod. Strategies in Beef Cattle, Billings, MT. Patterson, D.J., F.N. Kojima, and M.F. Smith. 2003a. A review of methods to synchronize estrus in replacement beef heifers and postpartum cows. J. Anim. Sci. 81 (E. Suppl. 2):E166 E177. Patterson, D. J., F. N. Kojima, and M. F. Smith. 2003b. Methods to synchronize estrous cycles of postpartum beef cows with melengestrol acetate. Prof. Anim. Sci. 19: Perry, G. A., M. F. Smith, M. C. Lucy, J. A. Green, T. E. Parks, M. D. MacNeil, A. J. Roberts, and T. W. Geary Relationship between follicle size at insemination and pregnancy success. Proc. Natl. Acad. Sci. USA 102: Richards, M. W., J. C. Spitzer, and M. B. Warner Effect of varying levels of postpartum nutrition and body condition at calving on subsequent reproductive performance in beef cattle. J. Anim. Sci. 62: Schafer, D. J., J. F. Bader, J. P. Meyer, J. K. Haden, M. R. Ellersieck, M. C. Lucy, M. F. Smith, and D. J. Patterson Comparison of progestin-based protocols to synchronize estrus and ovulation before fixed-time artificial insemination in postpartum beef cows. J. Anim. Sci. 85: Zelinski, M. B., N. A. Hirota, E. J. Keenan, and F. Stormshak Influence of exogenous estradiol-17β on endometrial progesterone and estrogen receptors during the luteal phase of the ovine estrous cycle. Biol. Reprod. 23: