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ANIMAL GROWTH, PHYSIOLOGY, AND REPRODUCTION |
Veterinary Medicine Teaching and Research Center, University of California–Davis, Tulare 93274
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionLiterature Cited |
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treatments of a timed AI protocol using estradiol cypionate (ECP) to synchronize ovulation on display of estrus, ovulation rate, pregnancy rate, and late embryonic loss in lactating cows. Holstein cows, 227 from Site 1 and 458 from Site 2, were presynchronized with two injections of PGF2 on study d 0 and 14, and subjected to a timed AI protocol (100 µg of GnRH on study d 28, 25 mg of PGF2 on study d 35, 1 mg of ECP on study d 36, and timed AI on study d 38) with or without a CIDR insert. Blood was collected on study d 14 and 28 for progesterone measurements to determine cyclicity. Ovaries were scanned on d 35, 37, and 42, and pregnancy diagnosed on d 65 and 79, which corresponded to 27 and 41 d after AI. Cows receiving a CIDR had similar rates of detected estrus (77.2 vs. 73.8%), ovulation (85.6 vs. 86.6%), and pregnancy at 27 (35.8 vs. 38.8%) and 41 d (29.3 vs. 32.3%) after AI, and late embryonic loss between 27 and 41 d after AI (18.3 vs. 16.8%) compared with control cows. The CIDR eliminated cows in estrus before the last PGF2 injection and decreased (P < 0.001) the proportion of cows bearing a corpus luteum (CL) at the last PGF2 injection because of less ovulation in response to the GnRH and greater spontaneous CL regression. Cyclic cows had greater (P = 0.03) pregnancy rates than anovulatory cows at 41 d after AI (33.8 vs. 20.4%) because of decreased (P = 0.06) late embryonic loss (16.0 vs. 30.3%). The ovulatory follicle was larger (P < 0.001) in cows in estrus, and a greater proportion of cows with follicles 
15 mm displayed estrus (P < 0.001) and ovulated (P = 0.05) compared with cows with follicles <15 mm. Pregnancy rates were greater (P < 0.001) for cows displaying estrus, which were related to the greater (P < 0.001) ovulation rate and decreased (P = 0.08) late embryonic loss for cows in estrus at AI. Cows that were cyclic and responded to the presynchronization protocol (high progesterone at GnRH and CL at PGF2) had the highest pregnancy rates. Incorporation of a CIDR insert into a presynchronized timed AI protocol using ECP to induce estrus and ovulation did not improve pregnancy rates in lactating dairy cows. Improvements in pregnancy rates in cows treated with ECP to induce ovulation in a timed AI protocol are expected when more cows display estrus, thereby increasing ovulation rate.
Key Words: Dairy Cows • Estradiol Cypionate • Pregnancy • Progesterone • Timed Artificial Insemination
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionLiterature Cited |
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). Furthermore, incorporation of the CIDR into a timed AI protocol improved pregnancy rates in anovulatory dairy (Pursley et al., 2001; El-Zarkouny et al., 2004) and beef cows (Lamb et al., 2001).
Anovulation by 65 d in milk (DIM) affects more than 20% of the lactating dairy cows in the United States (Moreira et al., 2001; Cerri et al., 2004; Santos et al., 2004a), with some herds having more than 40% of the primiparous cows not cycling by 65 DIM (Cerri et al., 2004), leading to reduced conception rates (Rhodes et al., 2003; Cerri et al., 2004; Santos et al., 2004a) and increased embryonic losses (Santos et al., 2004b). Treatment of anovulatory cows with CIDR for 7 d improved detection of estrus, ovulation rate, and pregnancy rates (Rhodes et al., 2003), and incorporation of a CIDR into a timed AI protocol using GnRH to induce ovulation improved pregnancy rates (Pursley et al., 2001). Dairy cows subjected to a timed AI protocol using estradiol cypionate (ECP) to induce ovulation had greater conception and pregnancy rates when observed in estrus at AI, which was associated with cyclicity (Pancarci et al., 2002; Cerri et al., 2004).
We hypothesized that incorporating a CIDR to a timed AI protocol using ECP to synchronize ovulation would benefit cows by improving detection of estrus and ovulation, resulting in greater pregnancy rates. Therefore, the objectives of the current study were to determine the effects of a CIDR insert incorporated into a presynchronized timed AI protocol on detection of estrus, ovulation rate, pregnancy rates, and late embryonic loss in high-producing dairy cows.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionLiterature Cited |
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). Primiparous and multiparous cows were housed in the same barn, but in separate pens throughout the study. During the study period, cows were inseminated during periods of thermoneutral temperature (December 2002 to April 2003) or heat stress (May to August 2003). The mean (±SD) daily average and daily maximum temperatures for the thermoneutral and heat stress periods were 11.8 ± 3.3°C and 17.2 ± 4.4°C, and 25.3 ± 4.3°C and 33.8 ± 5.3°C, respectively. Daily average temperatures ranged from 2.8 to 20.0°C and from 12.8 to 32.2°C for the thermoneutral and heat stress periods, respectively.
At both sites, cows were milked twice daily and yield was measured for individual cows once monthly during official California Dairy Herd Improvement Association milk test performed by the Dairy Herd Improvement Association laboratories in Hanford (Site 1) and Fresno (Site 2). Milk yields during the first 3 mo postpartum were used to determine effects on reproductive responses. The body condition (Ferguson et al., 1994) of all cows was scored at study d 0 and 37, which corresponded with the day of study enrollment and the day before timed AI, respectively.
Treatments and Artificial Insemination
On study d 0 and 14, all cows were presynchronized (Moreira et al., 2001) with two i.m. injections of 25 mg of PGF2 (Lutalyse, 5 mg/mL of dinoprost tromethamine, Pfizer Animal Health, New York, NY), and 14 d later (study d 28) cows were enrolled in a timed AI protocol (Pancarci et al., 2002), which consisted of a 100-µg injection of GnRH i.m. (Cystorelin, 50 µg/mL of gonadorelin diacetate tetrahydrate, Merial Ltd., Iselin, NJ) on study d 28, followed by an i.m. injection of 25 mg of PGF2 on study d 35, and an i.m. injection of 1.0 mg of ECP (2.0 mg/mL of estradiol cypionate, Pfizer Animal Health) on study d 36. Treatments were control with no CIDR (n = 343) or CIDR (Eazi-Breed, Pfizer Animal Health, n = 342), which was inserted concomitant with the GnRH injection and removed with the injection of PGF2 of the timed AI protocol (Figure 1).
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) using paint-sticks (All-weather Paintstik; LA-CO Industries, Chicago, IL). Based on detection of estrus, cows were segregated into three groups: estrus and inseminated before the day of scheduled timed AI; inseminated at the day of timed AI and in estrus; and inseminated at the day of timed AI and not in estrus. At Site 1, two technicians inseminated cows, whereas at Site 2, a single technician inseminated all cows. Semen from different sires was randomly assigned within site to the two treatment groups to maintain an equal distribution of sires across both treatments.
Blood Sampling, Progesterone Analysis, and Cyclic Status
Blood samples were collected from all cows for determination of plasma progesterone concentrations on study d 14 (second PGF2 injection) and 28 (GnRH injection). Progesterone concentrations were used to determine whether cows were cycling, when concentration was 1.0 ng/mL in one of the two samples, or anovulatory, when both samples were <1.0 ng/mL, before the beginning of the timed AI protocols. This sampling schedule was used to facilitate the conduct of the study because they coincided with other study activities; however, the 14-d interval between samples could potentially overestimate the number of anovular cows.
Approximately 7 mL of blood was collected by puncture of the median coccygeal vein or artery using Vacutainer tubes (Becton, Dickinson and Co., Franklin Lakes, NJ) with sodium EDTA. The samples were immediately placed in ice, and later centrifuged at 2,000 x g for 15 min to separate plasma. Plasma samples were frozen at –25°C until later analysis. Progesterone was analyzed by ELISA according to Munro and Stabenfeldt (1984) with modifications. Blood was collected from a young male calf, and plasma was separated. A charcoal stripping procedure (Sharpe and Cooper, 1984) was used to remove any progesterone and steroids from plasma that could cross-react during the assay. Charcoal-stripped plasma enriched with known concentrations of progesterone was used in each 96-well plate to determine the efficiency of progesterone extraction. An intraassay CV was determined for each 96-well plate, and a plasma sample with low (2.1 ng/mL) and high (3.8 ng/mL) progesterone concentrations was used in each plate to determine the inter-assay CV. The sensitivity of the assay was 0.05 ng/mL, the intraassay CV was 9.4%, and interassay CV for samples with low and high progesterone were 12.7 and 7.2%, respectively.
Ovarian Ultrasonography and Progesterone Classes
Ultrasonographic examination of the ovaries was performed using a 7.5-MHz linear transducer (Sonovet 2000, Universal Medical System, Bedford Hills, NY) on study d 35 (PGF2 injection of the timed AI protocol), 37, and 42. A map of each ovary was drawn to include the location and size of follicles greater than 5 mm and corpus luteum (CL). The growing dominant follicle was identified and the ovulatory follicle determined. Ovulation after the synchronization protocols was determined by the disappearance of the dominant follicle and presence of a CL 7 d after the last injection of PGF2 (4 d after timed AI). In cows that experienced delayed ovulation, the CL would have been only 2 d old, making it more difficult to accurately determine its presence. Thus, it is possible that, in a small proportion of cows, some CL were not diagnosed accurately.
Cows were categorized according to progesterone concentration at the GnRH injection (high [H] 1.0 ng/ mL; low [L] <1.0 ng/mL) and presence of CL at the last PGF2 injection (CL present = H; or no CL present = L). This classification system allowed ovulatory response to the first GnRH to be evaluated for those cows with progesterone <1 ng/mL at the GnRH (LH and LL), as well as spontaneous CL regression for those cows with progesterone 1 ng/mL at the GnRH (HH and HL), and resulted in four possible progesterone class permutations: HH, HL, LH, and LL. Of the 685 cows in the study, progesterone classes were assigned to 626 cows because 59 cows had either a missing blood sample at the GnRH or an ultrasound examination of the ovary at the PGF2 injection before AI.
Pregnancy Diagnosis and Reproductive Outcomes
All cows were examined for pregnancy by ultrasonography between 27 and 28 d after the first postpartum AI, which corresponded to study d 65 and 66. The detection of an embryonic vesicle with a viable embryo (presence of heartbeat) was used as indicator of pregnancy. Cows diagnosed as pregnant were palpated per rectum for detection of an embryonic vesicle to confirm pregnancy 14 d later, on study d 79 to 80 (41 to 42 d after AI). Because the majority of the cows was examined for pregnancy on study d 65 and 79 (83.5%), corresponding to 27 and 41 d after AI, we used these as the days to indicate when pregnancy diagnosis was performed and embryonic loss occurred throughout the manuscript.
Pregnancy rates were defined as the proportion of pregnant cows at 27 and 41 d after AI in each treatment group. Late embryonic loss between 27 and 41 d of gestation was characterized in cows diagnosed with a viable pregnancy (presence of heartbeat) on gestation d 27, but nonpregnant on gestation d 41. Cows diagnosed as nonpregnant at first AI that had not been reinseminated were resynchronized for reinsemination using GnRH followed 7 d later by PGF2, and pregnancy at second AI was diagnosed by palpation per rectum 35 to 48 d after insemination.
Statistical Analyses
The experimental design was a randomized complete block design (Kuehl, 1994). Weekly, within each site, a cohort of 20 to 40 cows were blocked according to parity (first, second, and third), BCS at 27 ± 3 d postpartum, and milk yield during the first month postpartum, and within each block randomly assigned to one of the two treatments.
The sample size was calculated (Minitab Inc., State College, PA) for = 0.05 and ß = 0.20 to detect differences in pregnancy rates at first AI of 8% units between treatments when pregnancy ranges from 30 to 45%. This was based on improvements in pregnancy rates when a CIDR insert was incorporated into a timed AI protocol in dairy cows (Pursley et al., 2001).
Dichotomous outcomes, such as cyclicity, ovulation rate, estrous response, pregnancy rates, and late embryonic loss were analyzed by logistic regression using the Logistic procedure of SAS (SAS Inst., Inc., Cary, NC). A backward stepwise regression model was used (Allison, 1999), and the full model included the effects of treatment, site, parity, BCS at the moment of AI, BCS change between study enrollment and AI, average milk yield during the first 3 mo postpartum, season (heat stress vs. thermoneutral), and the interactions between treatment and the respective explanatory variables. For analyses of ovulation rate, 24 cows were excluded because of missing examination data, and the statistical model also included the effects of cyclicity status (anovulatory vs. cycling), follicle diameter 48 h after the final PGF2 injection, and estrous response (estrus and AI before the day of timed AI, estrus and AI at the day of timed AI, and no estrus at timed AI). For analyses of pregnancy rates and late embryonic loss, the model also included the effects of cyclicity status, follicle diameter 48 h after the final PGF2 injection, estrous response, and ovulation. Variables were sequentially removed from the model by the Wald statistic criterion if P > 0.20. Additional statistical analyses were performed with 625 cows to evaluate the effect of progesterone class (HH, HL, LH, and LL) on fertility responses according to the statistical model described previously. The Fisher’s exact test in SAS was used to evaluate dichotomous outcomes when the number of cows experiencing the outcome was less than five.
Milk yield, lactation number, BCS, follicle growth, and size of the ovulatory follicle at the final injection of PGF2 and 48 h later were all analyzed by ANOVA (Littell et al., 2002) using the GLM procedure of SAS, with a statistical model that included the effects of treatment, parity, site, season, and interactions between treatment and parity, treatment and site, treatment and season, and the random experimental error. For analysis of the size of the ovulatory follicle, the effects of cyclicity and the interaction between treatment and cyclicity were also included in the statistical model.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionLiterature Cited |
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). Cyclicity was affected by site (P < 0.001), and cows in Site 1 were more likely to be cycling than those in Site 2 (adjusted odds ratio [AOR] = 2.35; 95% confidence interval [CI] = 1.40 to 3.94). Likewise, BCS at AI influenced cyclicity (P = 0.01), and cows with BCS 
2.75 were less likely to be cycling than those with a BCS 3.0 (AOR = 0.56; 95% CI = 0.36 to 0.89).
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injection tended to be smaller (P = 0.09) for cows receiving a CIDR than for control cows (16.0 vs. 16.5 mm). Also, primiparous cows had smaller follicles (P = 0.04) than multiparous (16.0 vs. 16.6 mm) cows, but no treatment x parity interaction was observed (Table 1
). However, at 48 h after the last PGF2 injection, dominant follicle diameter was similar for CIDR and control cows and averaged 19.1 mm, and the proportion of cows with the dominant follicle growing during the 48 h after the last PGF2 injection (81.0 vs. 82.3%), as well as the magnitude of growth in the 48-h period (2.76 vs. 2.91 mm), did not differ between CIDR and control cows.
In cows with progesterone <1 ng/mL at GnRH injection, ovulation to GnRH was less (P = 0.005) in CIDR-treated vs. control cows (67.7 vs. 97.1%). Furthermore, spontaneous CL regression during the timed AI protocol was greater (P = 0.03) for the CIDR-treated vs. control cows (9.2 vs. 4.0%). Consequently, the proportion of cows bearing a CL at the PGF2 injection of the timed AI protocol was less (P < 0.001) for CIDR than control cows (86.3 vs. 93.9%). Presence of a CL at the PGF2 injection of the timed AI protocol resulted in greater pregnancy rates at 27 (40.0 vs. 15.8%; P < 0.001) and 41 d (32.7 vs. 13.9; P = 0.006) after AI.
Incorporation of a CIDR insert into the synchronization protocol eliminated cows in estrus before the final PGF2 injection (0 vs. 2.04% [(7/343]; P < 0.01; Table 2), and tended to decrease (P = 0.10) ovulation in the first 48 h after the last PGF2 injection (0.3 [1/334] vs. 1.55% [5/327]). Detection of estrus after the last PGF2 injection was similar between the CIDR and control groups (77.1 vs. 73.8%; P = 0.11), but multiparous cows were less likely to be found in estrus than primiparous (AOR = 0.68; 95% CI = 0.46 to 1.00; P = 0.06).
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15 mm 48 h after the last PGF2 injection were observed to be in estrus (78.4 vs. 57.1%; P < 0.001), as well as to have ovulated (87.7 vs. 73.7%; P = 0.05) compared with cows with follicle <15 mm. In fact, cows with follicle 15 mm were 3.7 times more likely to have been in estrus (95% CI = 2.24 to 6.05), and two times more likely to have ovulated (95% CI = 1.00 to 3.76) than cows with smaller follicles. Nevertheless, follicle size did not affect pregnancy rates at 27 and 41 d after AI, or late embryonic loss.
Ovulation after ECP injection was similar for cows in the CIDR and control groups (85.4 vs. 86.4%), but a tendency (P = 0.10) for a treatment x parity interaction was observed due to a decreased ovulation rate for the primiparous cows receiving CIDR (Table 2).
Pregnancy rates at 27 and 41 d after AI and embryonic loss between 27 and 41 d after AI were not affected by treatment (Table 2). Cows inseminated during heat stress were less likely (P = 0.05) to be pregnant at 27 d after AI (34.3 vs. 39.3%; AOR = 0.69; 95% CI = 0.48 to 1.00), and tended (P = 0.09) to have a decreased pregnancy rate at 41 d after AI (27.2 vs. 33.1%; AOR = 0.72; 95% CI = 0.49 to 1.05) compared with cows inseminated during the thermoneutral period, but late embryonic loss was similar (20.7 vs. 15.7%).
A greater (P = 0.04) proportion of cyclic cows were detected in estrus (76.5 vs. 69.0%), but ovulation rate was similar when compared to anovulatory cows (86.4 vs. 81.1%; Table 3). Although more cyclic than anovulatory cows were bearing a CL at the PGF2 injection of the synchronization protocol (92.2 vs. 78.4%; P < 0.001), estrous response was not associated with presence of CL at the last PGF2 injection (CL, 75.1% vs. no CL, 70.3%). Pregnancy rate at d 27 tended to be greater (P = 0.09) for cyclic compared to anovulatory cows (40.3 vs. 29.2%) and was greater (P = 0.03) at d 41 (33.8 vs. 20.4%) after AI, because of decreased (P = 0.06) late embryonic loss between 27 and 41 d of gestation (16.0 vs. 30.3%). In fact, cyclic cows were only half as likely to experience late embryonic loss compared with anovular cows (AOR = 0.41; 95% CI = 0.16 to 1.06); however, no interaction between treatment and cyclic status was observed for pregnancy rates and embryonic loss.
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). Also, the ovulatory follicle 48 h after the last PGF2 injection was larger in cows in estrus before the day of timed AI than that of cows in estrus at timed AI, and the latter was larger than that of cows not in estrus at timed AI (20.4 vs. 18.9 vs. 18.0 mm; P < 0.001). Pregnancy rates on d 27 (43.6 vs. 17.0%; P < 0.001) and 41 (36.6 vs. 12.0%; P < 0.001) were greater for cows detected in estrus compared with cows not in estrus, and the improvements in pregnancy rates in cows in estrus were observed regardless of whether estrus was observed 24 or 48 h after the ECP treatment (Table 4). Late embryonic loss tended to be less (P = 0.08) for cows detected in estrus compared with cows not in estrus (16.1 vs. 29.7%). No interaction between treatment and display of estrus was observed for ovulation rate, pregnancy rates at 27 and 41 d after AI, and late embryonic loss.
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of the timed AI protocol was evaluated, those cows classified as HH had the highest pregnancy rates at 27 and 41 d after AI, followed by LH, HL, and the lowest for LL (Figure 2). Similarly, cows that ovulated and had a visible CL 4 d after the timed AI had greater pregnancy rates at 27 (42.3 vs. 7.8%; P < 0.0001) and 41 d (35.4 vs. 3.3%; P < 0.0001) after AI, and tended to experience decreased (P = 0.07) late embryonic loss (16.5 vs. 57.1%).
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionLiterature Cited |
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; Cerri et al., 2004; Santos et al., 2004a), with some herds having more than 40% of the primiparous cows not cycling by 65 DIM (Cerri et al., 2004), leading to reduced conception rates (Rhodes et al., 2003; Cerri et al., 2004; Santos et al., 2004a) and increased embryonic losses (Santos et al., 2004b). Treatment of anovulatory cows with progesterone before first postpartum ovulation minimized the occurrence of short luteal cycles and improved conception rates (Inskeep, 2002).
The loss of 2.6% of the CIDR inserted is similar to the 2.7% reported by Chenault et al. (2003). As expected, the CIDR insert eliminated estrous behavior before its removal and tended to decrease ovulation in the first 48 h after the last PGF2 treatment because of the negative feedback of progesterone on LH secretion, which prevents estrus and the LH surge.
The smaller diameter of the dominant follicle for the CIDR group at the last PGF2 injection was observed mainly because of a decrease in follicle size in primiparous cows. This effect may be attributed to an inhibitory effect of progesterone on LH pulse frequency when CIDR is administered (Burke et al., 1996), which might have been more pronounced in primiparous cows due to the lower milk yield and consequent reduced clearance of progesterone (Sangsritavong et al., 2002). This could explain the effect of parity on follicle diameter at the final injection of PGF2 and 48 h later; however, when the CIDR was removed, follicle growth and diameter before ovulation were similar for both treatments, indicating that follicle growth was resumed similarly between groups when the CIDR was no longer inserted.
Of the 476 cows with high progesterone levels at the GnRH, only 6.7% had premature spontaneous CL regression during the timed AI protocol. This low proportion was expected because cows had their estrous cycles presynchronized with two injections of PGF2 (Moreira et al., 2001); however, more CIDR-treated than control cows experienced premature spontaneous CL regression. It is not clear why CIDR-treated cows had increased spontaneous CL regression during the timed AI protocol, but it is possible that the presynchronization with PGF2 was not as effective in cows in the CIDR compared to those in the control group.
Although ovulatory response to GnRH was not evaluated in all cows, those with progesterone <1 ng/mL at the GnRH injection and treated with a CIDR insert had a decreased incidence of CL at the final injection of PGF2 compared with control cows with progesterone <1 ng/mL at the GnRH. The decreased ovulation to GnRH in low-progesterone cows when treated with CIDR might be related to the negative feedback of progesterone on LH secretion. When ovariectomized cows received a CIDR insert, LH concentrations and pulse frequency were decreased in the first 8 h of insert administration (Burke et al., 1996). However, ovulation to GnRH, based on changes in progesterone from the day of GnRH treatment and 7 d later, did not differ in dairy (El-Zarkouny et al., 2004) or beef cows (Lamb et al., 2001) when treated with a CIDR. The decreased ovulation rate to the GnRH, and the greater premature luteolysis resulted in a reduced proportion of cows in the CIDR-treated group with a CL at the PGF2 treatment of the timed AI protocol. The presence of CL at PGF2 treatment of timed AI protocols has been shown to positively affect pregnancy rates (Moreira et al., 2001; Cerri et al., 2004), which was also observed in the current study.
Incorporation of a CIDR insert into a timed AI protocol using ECP to induce ovulation did not influence reproductive variables in lactating dairy cows. Control and CIDR-treated cows had similar detection of estrus, pregnancy rates, and late embryonic losses. When the CIDR was incorporated into the Ovsynch (d 0 GnRH, d 7 PGF2, d 9 GnRH, and timed AI 12 to 20 h after GnRH) protocol, pregnancy rates were improved on d 28 of gestation for anovulatory cows (Pursley et al., 2001) and on d 57 for all cows (El-Zarkouny et al., 2003). However, when cows were presynchronized with PGF2, incorporation of a CIDR insert into the Ovsynch protocol did not benefit pregnancy rates and embryo survival (El-Zarkouny et al., 2003). It is possible that the benefits from incorporation of a CIDR insert into Ovsynch timed AI protocol were related to preventing cows from prematurely coming into estrus and ovulating, thereby improving synchrony of ovulation and conception at timed AI. Another explanation for lack of response to the CIDR in the current study was the fact that cows were inseminated the day before scheduled timed AI if observed in estrus. When incorporation of a CIDR insert demonstrated benefits in fertility, cows were inseminated at fixed-time, 12 to 20 h after the final GnRH of the Ovsynch, which requires optimal synchronization after the GnRH-induced ovulation because of lack of estrus detection.
Surprisingly, the CIDR insert did not improve pregnancy rates or embryo survival in anovulatory cows. Previously, incorporation of a CIDR insert into a timed AI protocol improved pregnancy rates in one of the two experiments (El-Zarkouny et al., 2004). The authors attributed the improved pregnancy rates in experiment one to the greater proportion of anovulatory cows than in experiment two; however, similar to the current study, anovulatory cows in experiment two did not benefit from the CIDR insert (El-Zarkouny et al., 2004). The inconsistency in reproductive performance after progesterone supplementation is quite intriguing. Pursley et al. (2001) observed improved pregnancy rates on d 28 after timed AI when anovulatory cows were subjected to the Ovsynch protocol with a CIDR, but response was site dependent. In only two of the six sites in the study were the CIDR beneficial to pregnancy rates. Nevertheless, the benefits of the CIDR to pregnancy in anovulatory cows were no longer observed at 56 d of gestation. In beef cows, incorporation of a CIDR to the Co-Synch (d 0 GnRH, d 7 PGF2, d 9 GnRH, and timed AI) timed AI protocol improved pregnancy rates in anovulatory cows and in cyclic cows with premature CL regression (Lamb et al., 2001). The lack of positive effects to the CIDR in the current study might be attributed to the decreased proportion of CIDR-treated cows with CL at the final injection of PGF2 as a result of the decreased ovulation to the first GnRH and increased spontaneous CL regression. Furthermore, presynchronization of the estrous cycle likely minimized the occurrence of cows with premature CL regression in the timed AI protocol, which would mask potential benefits from the CIDR.
Detection of estrus was greater for cyclic cows, but they had similar ovulation rates compared with anovulatory cows. Despite similar ovulation rate, cyclicity had a major effect on pregnancy rates and late embryonic loss, which has normally been observed (Rhodes et al., 2003; Santos et al., 2004b). Because ovulation rate affected pregnancy rates and embryonic loss, but cyclic and anovulatory cows had similar ovulation rates regardless of treatment, it is likely that the negative effects of anovulation before the timed AI protocol on pregnancy were related to factors other than synchronization of ovulation. Anovulatory cows receiving ECP to induce ovulation had conception rates similar to those of cyclic cows inseminated upon detection of a synchronized estrus, but they were both less than cyclic cows treated with ECP to induce ovulation (Cerri et al., 2004). Reduction in fertility in anovulatory cows was related to subnormal exposure to progesterone before AI resulting in subsequent short luteal cycles and decreased conception rates (Garverick et al., 1992; Inskeep, 2002).
Late embryonic loss was greater for anovulatory compared to cyclic cows. Factors influencing late embryonic loss are complex and not well understood (Inskeep, 2002; Inskeep, 2004; Santos et al., 2004b). Embryo death occurred after spontaneous luteal regression associated with increased secretion of pulsatile oxytocin and PGF2 on d 30 to 35 of pregnancy (Schallenberger et al., 1989), or without prior loss of luteal function (Kastelic et al., 1991). Shahan-Albalacy et al. (2001) observed a delayed effect of low progesterone in the previous estrous cycle on the subsequent cycle PGF2 release after an oxytocin challenge. Therefore, it is possible that the same mechanism responsible for short luteal cycle associated with suboptimal exposure to progesterone in the preceding estrous cycle could be involved in late embryonic loss for anovulatory cows. A tendency for increased late embryonic loss in anovulatory cows compared with cyclic cows was observed by others (El-Zarkouny et al., 2004). In fact, anovulatory cows immediately before synchronization of estrus or ovulation were twice more likely to experience late embryonic loss than cyclic cows (Santos et al., 2004b).
Cows that displayed estrus after the ECP had greater pregnancy rates than cows not in estrus at timed AI (Cerri et al., 2004), and these results were similar to those of the current study. Furthermore, cows in the current study in estrus at AI experienced improved embryo survival compared with cows not in estrus when inseminated. Display of estrus increased ovulation rate, which could have mediated the improved pregnancy rates. In fact, cows with no detectable CL 4 d after the day of timed AI were less likely to conceive and, when nonpregnant, more likely to experience short (17 d) interinsemination interval.
The size of the ovulatory follicle influenced display of estrus, which was similar to results observed by Cerri et al. (2004). Estradiol production by the preovulatory follicle influences display of estrus and has been correlated with the number of progesterone receptors present in the uterine endometrium (Zollers et al., 1993). Progesterone stimulates embryo development, and it directly inhibits luteolysis by blocking oxytocin receptors and decreasing sensitivity to oxytocin (Grazzini et al., 1998). Furthermore, Kiebofz-Loos et al. (2003) demonstrated that estradiol was required before progesterone treatment was capable of inhibiting the release of uterine PGF2 induced by oxytocin in ovariectomized cows. Low concentrations of estradiol in the pro-estrus increased uterine secretion of PGF2 after an oxytocin challenge (Mann and Lamming, 2000). Although estradiol concentrations were not measured, cows with follicles <15 mm had decreased detection of estrus and ovulation rate, indicating that size of follicle, and probably endogenous estradiol production (Inskeep, 2002), contributed to estrous behavior and ovulation. Therefore, it is plausible that cows in estrus had greater secretion of follicular estradiol, which induced progesterone receptors and inhibited oxytocin receptor activity, thereby preventing the luteolytic cascade to be activated prematurely. This sequence of events might have favored embryonic survival in cows in estrus.
In addition to the possible uterine effects of follicular estradiol improving pregnancy rates for cows in estrus at AI, it is plausible that estrus and associated greater estradiol concentration during proestrus might have also enhanced sperm transport and fertilization rates. Sperm transport in the reproductive tract of rats was improved during proestrus compared with other stages of the estrous cycle (Orihuela et al., 1999). The same authors also demonstrated that exogenous estradiol-17ß facilitated sperm migration into the oviduct and progesterone antagonized this effect (Orihuela et al., 1999). Therefore, cows in estrus ovulating larger follicles might have had greater concentrations of estradiol, which could have influenced uterine motility and sperm transport through the reproductive tract.
Incorporation of an intravaginal progesterone insert into a presynchronized timed insemination protocol using ECP to induce estrus and ovulation did not improve detection of estrus, ovulation rate, pregnancy rates, and late embryonic loss in high-producing lactating dairy cows. Cows detected in estrus had improved pregnancy rates and decreased embryonic losses due to greater ovulation rate, which was influenced by ovulatory follicle size. Anovulatory cows subjected to a presynchronized timed AI protocol using ECP had similar ovulation rates, but decreased detection of estrus and pregnancy rates, and greater late embryonic loss than cyclic cows. Therefore, incorporation of a progesterone insert to a presynchronized timed AI protocol using ECP did not improve reproductive performance of lactating dairy cows regardless of cyclic status. Improvements in pregnancy rates are expected when display of estrus is increased in the first 48 h after treatment with ECP.
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3 Current address: Dept. de Zootecnia, Universidade Federal de Pelotas, Pelotas 96001-970, Brazil.
2 Correspondence: 18830 Road 112 (phone: 559-688-1731; fax: 559-686-4231; e-mail: jsantos{at}vmtrc.ucdavis.edu).
Received for publication June 27, 2004. Accepted for publication September 20, 2004.
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