The effects of varying the interval from follicular wave emergence to progestin withdrawal on follicular dynamics and the synchrony of estrus in beef cattle

The objective of this experiment was to examine the effects of varying the interval from follicular wave emergence to progestin (controlled internal drug-releasing insert, CIDR) withdrawal on follicular dynamics and the synchrony of estrus. A secondary objective was to assess the effects of causing the dominant follicle (DF) to develop in the presence or absence of a corpus luteum (CL) on follicular dynamics and the synchrony of estrus and ovulation. The experiment was designed as a 2 x 2 x 2 factorial arrangement of treatments with injection of GnRH or estradiol-17 beta and progesterone (E2 + P4) at treatment initiation, duration of CIDR treatment, and injection of PG (prostaglandin F2 alpha) or saline at the time of CIDR insertion as main effects. Estrous cycles (n = 49) in Angus cows were synchronized, and treatments commenced on d 6 to 8 of the estrous cycle. Cows were randomly assigned to receive a CIDR containing 1.9 g of P4 for 7 or 9 d. Approximately half the cows from each CIDR group received either GnRH (100 micrograms) or E2 + P4 (1 mg of E2 + 100 mg of P4) at CIDR insertion. Cows in GnRH or E2 + P4 groups were divided into those that received PG (37.5 mg) or saline at CIDR insertion. All cows received PG (25 mg) 1 d before CIDR removal. Daily ovarian events were monitored via ultrasound. The intervals from GnRH or E2 + P4 treatment to follicular wave emergence were 1.4 and 3.3 d, respectively (P < 0.05). The interval from follicular wave emergence to CIDR removal was longer (P < 0.05) for cows treated with GnRH (6.6 d) than those treated with E2 + P4 (4.7 d) and longer (P < 0.05) for those fitted with a CIDR for 9 d (6.5 d) than those with a CIDR in place for 7 d (4.8 d). Cows treated with PG or GnRH at CIDR insertion had a larger (P < 0.05) DF at CIDR removal than those treated with saline or E2 + P4. Treatment with a CIDR for 9 d also resulted in a larger (P < 0.07) DF at CIDR removal compared with cows fitted with a CIDR for 7 d. The interval from CIDR removal to estrus was shorter (P < 0.05) in cows treated with PG than those treated with saline. The synchrony of estrus and ovulation was not affected by any of the treatments (P > 0.05). Altering the interval from follicular wave emergence to progestin removal or creating different luteal environments in which the DF developed caused differences in the size of the DF at CIDR removal and the timing of the onset of estrus, but it did not affect the synchrony of estrus or ovulation.     

A comparison of progestin-based protocols to synchronize estrus in postpartum beef cows

Two progestin-based protocols for estrus synchronization in postpartum beef cows were compared following treatment administration on the basis of estrous response, interval to and synchrony of estrus, and pregnancy. Cows were assigned to one of the two treatment protocols by age, body condition score (BCS), and days postpartum (DPP). The MGA Select-treated cows (MGA Select; n = 109) were fed melengestrol acetate (MGA; 0.5mg x cow-1 x d(-1)) for 14 d, fed carrier for 8 d, GnRH (100 microg of Cystorelin) was injected i.m. 12 d after MGA withdrawal, and PG (25 mg of Lutalyse) was administered i.m. 7 d after GnRH. Cows assigned to the 7-11 Synch protocol (7-11 Synch; n = 111) were fed carrier for 15 d, fed MGA for 7 d, injected with PG on d 22 (d 7 of MGA), injected with GnRH on d 26, and injected with PG on d 33. Mean BCS (4.8 +/- 0.1, MGA Select; 4.7 +/- 0.1, 7-11 Synch) and DPP (40 +/- 1, MGA Select; 40 +/- 1, 7-11 Synch) did not differ between treatments. Blood samples were collected 8 d and 1 d before feeding of MGA or carrier to determine the pretreatment estrous cyclicity (progesterone > or = 1 ng/mL; 10/109 [9%], MGA Select; 12/111 [11%], 7-11 Synch), and again at PG on d 33 to evaluate treatment response (81/109 [74%], MGA Select; 84/111 (76%), 7-11 Synch). Serum concentrations of progesterone at PG on d 33 differed (P < 0.01) between treatments (3.3 +/- 0.3 ng/mL [MGA Select] vs. 1.7 +/- 0.1 ng/mL [7-11 Synch]). HeatWatch was used for 6 d after PG on d 33 to detect estrus, and AI was performed 12 h after the onset of estrus. Estrous response did not differ between treatments (100/109 [92%], MGA Select; 101/111 [91%], 7-11 Synch). Mean interval to estrus (65 +/- 2.7 h, MGA Select; 52 +/- 1.8 h, 7-11 Synch) and synchrony of estrus differed (P < 0.01) between treatments. Synchronized conception and pregnancy rates (61/100 [61%], 61/109 [56%], MGA Select; 71/101 [70%], 71/111 [64%], 7-11 Synch), and final pregnancy rates (94/109 [86%], MGA Select; 99/110 [90%], 7-11 Synch) did not differ between treatments. In summary, estrous response and fertility did not differ among cows assigned to the MGA Select or 7-11 Synch protocols. Synchrony of estrus, defined as the variance in the interval to estrus from PG, however, was improved following treatment with the 7-11 Synch protocol.     

Dynamics of ovarian follicular development in cattle following hysterectomy and during early pregnancy

Three experiments were conducted to evaluate ovarian follicular dynamics and functional activity during pregnancy in cattle. In 11 pregnant Charolais cows of Experiment I, size of largest follicle, number of follicles and accumulated follicle size were reduced by day 27 of pregnancy on the ovary bearing the corpus luteum (CL) but not on the non-CL bearing ovary. In experiment II, local attenuation of ovarian follicular development on the CL bearing ovary of seven pregnant heifers was evident compared to the contralateral ovary without the CL. However, in four hysterectomized heifers, follicular development was sustained on both the CL- and non-CL bearing ovaries when CL maintenance was achieved without presence of the uterus or conceptus. In Experiment III, steroidogenic characteristics of the largest and second largest follicles at 17 d postestrus were evaluated for seven pregnant and six cyclic cattle. Follicle by physiological status interactions were detected for both aromatase activity of the follicle and follicular fluid concentrations of estradiol and progesterone. In cyclic cows, the largest follicle had appreciably more aromatase activity than did the second largest follicle; whereas, aromatase activity of the largest follicle from pregnant cows was less than that of cyclic cows. However, in pregnant cows the second largest follicle became the estrogen-active follicle, and this follicle occurred with a higher frequency on the ovary contralateral to the CL-bearing ovary. These changes in aromatase activity were reflected by parallel changes in estrogen concentrations of follicular fluid. The higher progesterone concentration in follicular fluid of the largest follicle in pregnant cows provided further confirmation of their atretic status. In conclusion, during early pregnancy the conceptus and/or uterus ipsilateral to the conceptus appear to secrete compounds which alter local follicular steroidogenic activity and attenuate subsequent follicular growth between 17 to 34 d of pregnancy on the CL-bearing ovary. This local mechanism acting within the ovary may contribute to the antiluteolytic effects of early pregnancy in cattle.     

Comparison of protocols to synchronize estrus and ovulation in estrous-cycling and prepubertal beef heifers

The objective of the experiment was to compare follicular dynamics, ovulatory response to GnRH, and synchrony of estrus and ovulation among estrous-cycling and prepubertal beef heifers synchronized with a controlled internal drug-release (CIDR)- based or GnRH-PGF(2alpha) (PG) protocol. Estrous-cycling beef heifers were randomly assigned to 1 of 4 treatments (C1, C2, C3, C4), and prepubertal beef heifers were randomly assigned to 1 of 2 treatments (P1, P2) by age and BW. Blood samples were taken 10 and 1 d before treatment to confirm estrous cyclicity status (progesterone > or =0.5 ng/mL estrous cycling). The CIDR Select (C1, n = 12; P1, n = 14)-treated heifers received a CIDR insert (1.38 g of progesterone) from d 0 to 14, GnRH (100 microg, i.m.) on d 23, and PG (25 mg, i.m.) on d 30. Select Synch + CIDR (C2, n = 12; P2, n = 11)-treated heifers received a CIDR insert and GnRH on d 23 and PG at CIDR removal on d 30. The CIDR-PG (C3, n = 12)-treated heifers received a CIDR insert on d 23 and PG at CIDR removal on d 30. Select Synch (C4, n = 12)-treated heifers received GnRH on d 23 and PG on d 30. HeatWatch transmitters were fitted at CIDR removal (C1, C2, C3, P1, and P2) or at GnRH administration (C4) for estrus detection. Ultrasound was used to determine the response to GnRH and the timing of ovulation after estrus. Among the estrous-cycling heifers, ovulatory response to GnRH and estrous response did not differ (P > 0.05). Among the prepubertal heifers, more (P = 0.02) P1 heifers responded to GnRH than P2 heifers, but estrous response did not differ (P > 0.05). Among the estrous-cycling heifers, variance for interval to estrus after PG was reduced (P < 0.05) for C1 compared with each of the other treatments, and C3 [corrected] was reduced (P < 0.05) compared with C2 [corrected] Variance for interval to ovulation after PG was reduced (P < 0.05) for C1 compared with each of the other treatments. Among the prepubertal heifers, there was no difference (P > 0.05) in variance for interval to estrus or ovulation. Results from C1 and P1 (T1) and C2 and P2 (T2) were combined to compare T1 and T2 among mixed groups of estrous-cycling and prepubertal heifers. Response to GnRH was greater (P < 0.01; 81% T1 and 39% T2), and variances for interval to estrus and ovulation for T1 were reduced (P < 0.01) compared with T2. In summary, CIDR Select improved (P < 0.01) the synchrony of estrus and ovulation compared with Select Synch + CIDR.     

Experimental use of prostaglandin F2 alpha to synchronize estrus in cattle]

Experiments were conducted for the use of prostaglandin to synchronise oestrus in dairy cows. One single dose of 5 mg F2-Alpha prostaglandin was administered via the intrauterine route. The reference values of the control groups were matched by first insemination following induced oestrus among the best of the test animals which were inseminated twice. These values reached 72 and 96 hours following application accounted for 46.7 and 50.0 per cent. The yellow body phase, a prerequisite for oestrus production by prostaglandins, had been induced in the test animals by cycle blocking, using gestagen. Possible use of the method in a bio-engineering context for large-scale oestrus synchronisation in cows is discussed.     

Distribution of time to first postpartum estrus in beef cattle

The function of a distribution that describes postpartum interval (PPI) under any experimental treatment is useful for simulation modeling, understanding the effects of stimuli on the endocrine system, and estimating the average PPI in experiments terminated before all animals have expressed estrus. This study was undertaken to compare the fit of three statistical distributions, the Weibull, the log-normal, and the linear hazard rate (LHR), to the empirical distribution of PPI for five treatment regimens: no bull exposure postpartum, bull exposure from 53 d postpartum, bull exposure from 3 d postpartum, and bull exposure from an average of 63 d postpartum for 2-yr-old cows and for mature cows. The Weibull and the log-normal distributions deviated considerably from the empirical distribution. The LHR distribution with parameters changing over three different regions gave an excellent fit. The resulting hazard rate (instantaneous probability of a cow expressing her first estrus at time t postpartum) revealed a low probability of expressing estrus within 27 d postpartum (43 d for 2-yr-olds). For cows not exposed to bulls, the hazard rate increased slowly with time. For cows exposed to bulls after 3 d postpartum, the hazard rate increased rapidly between d 27 and d 50. For cows exposed to bulls after 53 d postpartum, the hazard rate increased instantaneously approximately 12 d after initial exposure to bulls. This increase was also seen when cows were exposed to bulls beginning at a constant date (at an average of 63 d postpartum). Because of lack of fit, the Weibull and the log-normal distributions should not be used in survival analysis of PPI.(ABSTRACT TRUNCATED AT 250 WORDS)     

Comparison of long-term progestin-based protocols to synchronize estrus before fixed-time artificial insemination in beef heifers

Two experiments were conducted to compare pregnancy rates resulting from fixed-time AI (FTAI) after administration of 1 of 2 long-term controlled internal drug release (CIDR)-based protocols. Heifers were assigned to treatment by age, BW, and pubertal status. The CIDR Select-treated heifers (Exp. 1, n = 37; Exp. 2, n = 192) received a CIDR (1.38 g of progesterone) from d 0 to 14, followed by 100 μg of GnRH, intramuscularly (i.m.) 9 d after CIDR removal (d 23) and PGF(2α) (25 mg, i.m.) 7 d after GnRH treatment (d 30). Heifers assigned to the Show-Me-Synch protocol (Exp. 1, n = 40; Exp. 2, n = 200) received a CIDR from d 0 to 14, followed by PGF(2α) 16 d later (d 30). Artificial insemination was performed at 72 or 66 h after PGF(2α) treatment for the CIDR Select- and Show-Me-Synch-treated heifers, respectively, and each heifer was given GnRH (100 μg, i.m.) at the time of AI. In Exp. 1, ovaries of each heifer were examined by transrectal ultrasonography on d 23 and 30 to characterize follicular dynamics. Follicles ≥5 mm and the presence of corpora lutea were recorded. On d 25, ovaries of each heifer were examined to characterize the status of dominant follicles recorded on d 23. Heifers were fitted with HeatWatch (DDx Inc., Denver, CO) estrus-detection transmitters at PGF(2α) to characterize estrus distribution up to FTAI. The diameter of dominant follicles on d 23 at PGF(2α) and on d 30, and the estrous response after PGF(2α) treatment up to the point of FTAI did not differ between CIDR Select- and Show-Me-Synch-treated heifers. Concentrations of progesterone in serum at PGF(2α) were greater (P = 0.07) in Show-Me-Synch- than CIDR Select-treated heifers (6.0 vs. 4.8 ng/mL, respectively). Pregnancy rates of heifers resulting from FTAI did not differ (P = 0.33) between CIDR Select- and Show-Me-Synch-treated heifers (CIDR Select, 59%; Show-Me-Synch, 70%). In Exp. 2, FTAI pregnancy rates tended (P = 0.07) to be greater in Show-Me-Synch-treated (62%) than in CIDR Select-treated (51%) heifers. Pregnancy rates at the end of the breeding season did not differ (P = 0.72; CIDR Select, 85%; Show-Me-Synch, 83%) between treatments. In summary, pregnancy rates resulting from FTAI were comparable for heifers assigned to each of the 2 long-term progestin-based protocols. The reduced treatment cost and animal handling associated with administration of the Show-Me-Synch protocol offer distinct advantages over the CIDR Select protocol despite similarities in pregnancy rates resulting from FTAI.     

Comparison of progestin-based protocols to synchronize estrus and ovulation before fixed-time artificial insemination in postpartum beef cows

This experiment was designed to compare pregnancy rates in postpartum beef cows resulting from fixed-time AI (FTAI) after treatment with 1 of 2 protocols to synchronize estrus and ovulation. Cross-bred, suckled beef cows (n = 650) at 4 locations (n = 210; n = 158; n = 88; and n = 194) were assigned within a location to 1 of 2 protocols within age group by days postpartum and BCS. Cows assigned to the melengestrol acetate (MGA) Select treatment (MGA Select; n = 327) were fed MGA (0.5 mg x head(-1) x d(-1)) for 14 d, GnRH (100 microg of Cystorelin i.m.) was injected on d 26, and prostaglandin F2alpha (PG; 25 mg of Lutalyse i.m.) was injected on d 33. Cows assigned to the CO-Synch + controlled internal drug release (CIDR) protocol (CO-Synch + CIDR; n = 323) were fed a carrier for 14 d, were injected with GnRH and equipped with an EAZI-BREED CIDR insert (1.38 g of progesterone, Pfizer Animal Health, New York, NY) 12 d after carrier removal, and PG (25 mg of Lutalyse i.m.) was injected and the CIDR were removed on d 33. Fixed-time AI was performed at 72 or 66 h after PG for the MGA Select or CO-Synch + CIDR groups, respectively. All cows were injected with GnRH (100 microg of Cystorelin i.m.) at the time of insemination. Blood samples were collected 8 and 1 d before the beginning of MGA or carrier to determine estrous cyclicity status of the cows (estrous cycling vs. anestrus) before treatment [progesterone > or = 0.5 ng/mL (MGA Select, 185/327, 57%; CO-Synch + CIDR, 177/323, 55%); P = 0.65]. There was no difference (P = 0.20) in pregnancy rate to FTAI between treatments (MGA Select, 201/327, 61%; CO-Synch + CIDR, 214/323, 66%). There was also no difference (P = 0.25) between treatments in final pregnancy rate at the end of the breeding period (MGA Select, 305/327, 93%; CO-Synch + CIDR, 308/323, 95%). These data indicate that pregnancy rates to FTAI were comparable after administration of the MGA Select or CO-Synch + CIDR protocols. Both protocols provide opportunities for beef producers to utilize AI and potentially eliminate the need to detect estrus.     

Relevance of ovarian follicular development to the seasonal impairment of fertility in weaned sows

A field study was conducted to estimate seasonal differences in follicular development in weaned sows and to evaluate the implication of these differences on seasonal infertility. A total of 110 sows were selected at weaning during winter–spring (WS, n = 58) and summer–autumn (SA, n = 52). Ovaries were scanned once daily from weaning to the onset of oestrus and twice daily from then until ovulation. Six sows during WS were removed from study for not showing growing follicles at weaning. Oestrus was evaluated twice daily from day 1 after weaning to day 14 post-weaning. One of 52 (1.9%) sows in WS and 9/52 (17.3%) in SA showed no signs of oestrus within 14 days of weaning (P < 0.05). The diameters of the follicles at weaning, at the onset of oestrus and just before ovulation were smaller (P < 0.01) in SA sows than in WS sows. There were fewer follicles in SA sows than in WS sows just before ovulation (P < 0.05). Fifty of 51 (98.0%) sows in WS and 31/43 (72.1%) sows in SA experienced a weaning-to-oestrus interval (WOI) of 3–6 days (P < 0.05). Fifty-one of 52 (98.1%) sows in WS and 43/52 (82.7%) sows in SA were inseminated; the percentage of pregnant sows that failed to farrow was lower in WS (1/51, 2.0%) than in SA (5/43, 11.6%; P < 0.05). The percentage of farrowed sows was greater in WS (46/51, 90.2%) than in SA (32/43, 74.4%; P < 0.05). Sows in WS had on average 1.5 more piglets than sows in SA (P < 0.05). Sows with a WOI of 3–6 days had lower rates of pregnancy losses (P < 0.05) and higher farrowing percentages (P < 0.01) than those with a WOI > 6 days, irrespective of season.     

Resynchronization of estrus in cattle of unknown pregnancy status using estrogen,progesterone, or both

Our objective was to develop treatments applied to cattle of unknown pregnancy status that would resynchronize the repeat estrus of nonpregnant females. In Exp. 1, previously inseminated dairy and beef heifers were assigned randomly to each of three treatments 13 d after AI: 1) no treatment (controls; n = 44); 2) 0.5 mg of estradiol cypionate (ECP) i.m. on d 13 and 20 at the time of insertion and removal of a used intravaginal progesterone (P4)-releasing insert (CIDR; P4 + ECP; n = 44); and 3) same as P4 + ECP without injections of ECP (P4; n = 42). The P4 + ECP (>90%) and P4 (>75%) protocols effectively synchronized repeat periods of estrus to 2 d and did not harm established pregnancies. In Exp. 2, treatments similar to those in Exp. 1 were applied to previously inseminated beef heifers (n = 439). Feeding 0.5 mg of melengestrol acetate (MGA) from d 13 to 19 after AI replaced the CIDR as a source of progestin. Of those heifers not pregnant (n = 65) after the initial AI, more than 86% were reinseminated, but conception was decreased (P < 0.05) by 28 to 39% compared with controls. In Exp. 3, previously inseminated lactating beef cows at four locations were assigned within herd to each of three treatments: 1) no treatment (control; n = 307); 2) same as in Exp. 1, but with P4 + 1 mg of estradiol benzoate on d 13 and 20 (P4 + EB; n = 153); and 3) same as in Exp. 1, P4 + ECP (n = 149). Treatments with P4 plus estrogen did not decrease conception rates in pregnant cows at any location, but increased (P < 0.05) the percentage of nonpregnant cows returning to estrus between 19 and 23 d after timed AI from 29% in controls to 86% in P4 + EB and 65% in P4 + ECP cows. Conception rates at the return estrus were not decreased when treatments occurred between d 13 and 20. In Exp. 4, lactating beef cows were assigned as in Exp. 3 to each of three treatments: 1) no treatment (controls; n = 51); 2) P4 + ECP (n = 47), as in Exp. 1; and 3) a single injection of ECP on d 13 (n = 48). Previously established pregnancies were not harmed (P = 0.70), and return rates of nonpregnant cows did not differ (P = 0.78) among treatments. In summary, in both heifers and lactating beef cows, the P4-based resynchronization treatments increased synchronized return rates when estrus detection rates were low, had no negative effects on established pregnancies, and decreased or tended to decrease conception rates at the resynchronized estrus.     

Postpartum nutrient intake and body condition: effect on pituitary function and onset of estrus in beef cattle

From calving through first estrus, 30 Brangus females were assigned equally to one of three diets to study the effect of postpartum nutrient intake and body condition on the ability of the pituitary to release luteinizing hormone (LH) and on the postpartum interval to estrus (PPI). The postpartum diets were calculated to achieve a 1) low [90% of the National Research Council (NRC) recommendations], 2) maintenance (100% of the NRC recommendations), or 3) high (110% of the NRC recommendations) level of nutrient intake. The females were group-fed within a treatment and calves were allowed to suckle ad libitum. Cow weight, body condition score and calf weight were recorded 24 h aftercalving, d 20 postpartum and at first behavioral estrus. On d 21 postpartum, blood samples were collected via jugular cannulae at 15-min intervals for 4 h, followed by a 100-micrograms im injection of gonadotropin-releasing hormone (GnRH) and continued sampling at 15-min intervals for an additional 6 h to determine serum LH. Although there was a significant decrease in PPI with increasing levels of nutrient intake (low = 57.5 +/- 8.8 d; maintenance = 40.3 +/- 6.6 d; high = 34.7 +/- 5.1 d), there were no differences in any of the observed LH characteristics (or variables) due to treatment. There were, however, marked differences in both the PPI and LH characteristics when data were analyzed on the basis of ability to maintain body condition from calving through 20 d postpartum, regardless of calculated dietary treatments. Cows that maintained body condition (MBC) had a shorter PPI [MBC, 31.7 +/- 2.8 vs lost (LBC) 60.0 +/- 7.5 d; P less than .01], higher basal levels of endogenous LH (MBC, .83 +/- .09 vs LBC, .61 +/- .04 ng/ml; P less than .025), higher GnRH-induced peak LH concentration (MBC, 58.99 +/- 11.15 vs LBC, 38.86 +/- 8.37 ng/ml. P less than .10), higher LH levels throughout the GnRH-induced LH surge (P less than .001), and greater release curve areas for the endogenous (MBC, 124.6 +/- 13.3 vs LBC, 91.7 +/- 5.6 units; P less than .025), GnRH-induced (MBC, 4370.8 +/- 699.5 vs LBC, 3039.7 +/- 683.3 units; P less than .10) and total (MBC, 4510.7 +/- 706.7 vs LBC, 3141.9 +/- 684.7 units; P less than .10) LH release.(ABSTRACT TRUNCATED AT 400 WORDS)     

Synchronization of estrus in beef cattle with norgestomet and estradiol valerate

Fifty-six cows received a norgestomet implant and an injection of norgestomet and estradiol valerate; half (n = 28) received 500 IU equine chorionic gonadotrophin (eCG) at implant removal, 9 d later. A third group (n = 25) received 2 doses of cloprostenol (500 micrograms) 11 d apart. Estrous rate was higher (P < 0.05) for cows given norgestomet and estradiol plus 500 IU eCG (75.0%) than for those receiving cloprostenol (44.0%); for those receiving norgestomet and estradiol alone, it was intermediate (67.8%). Pregnancy rates to artificial insemination (after estrus or timed) were higher (P < 0.05) for cows given norgestomet and estradiol than for those given cloprostenol (23 of 28, 82.1% vs 13 of 25, 52.0%), and intermediate (67.8%) for those given norgestomet and estradiol plus eCG. In a second experiment, for heifers treated with norgestomet and estradiol plus eCG (n = 15) or with 2 doses of cloprostenol (n = 16), estrous rates were 66.7% vs 56.2% (P > 0.5), ovulation rates were 100.0% vs 81.2% (P = 0.08), intervals from implant removal or cloprostenol treatment to estrus were 48.0 +/- 4.4 hours vs 61.3 +/- 7.0 hours (P = 0.12) and to ovulation were 70.4 +/- 4.4 hours vs 93.2 +/- 7.5 hours (P < 0.01), respectively; pregnancy rates were 41.7 and 35.7%, respectively (P > 0.5). Norgestomet and estradiol were as good as (heifers) or superior to (cows) a 2-dose cloprostenol regimen. In cows given norgestomet and estradiol, injecting eCG at implant removal did not significantly improve estrous or pregnancy rates.     

Effects of estradiol on gonadotrophin release, estrus and ovulation in CIDR-treated beef cattle

The effects of estradiol-17beta (E-17beta) or estradiol benzoate (EB) on gonadotrophin release, estrus and ovulation in beef cattle were evaluated in two experiments. In experiment 1, 16 ovariectomized cows received a previously used CIDR insert from days 0 to 7 and 1mg of EB on day 8; they also received 5mg of E-17beta on days 0 or 1, or 5mg of E-17beta+100mg of progesterone on day 0. There was only an effect of time (P<0.0001) on plasma concentrations of progesterone, estradiol, FSH, and LH. Following treatment with E-17beta, plasma FSH concentrations were suppressed for approximately 36 h, whereas plasma LH concentrations were reduced (P<0.05) for 6 h, but surged within 24 h. Injecting 1mg of EB 24 h after CIDR removal decreased (P<0.02) plasma LH concentrations for 6h, followed by an LH surge at 18 h. In experiment 2, ovary-intact heifers (n=40) received a used CIDR and 5mg of E-17beta+100mg of progesterone on day 0. On day 7, CIDR were removed, PGF given, and heifers received nothing (control) or 1mg of EB 12, 24, or 36 h later. In these groups, plasma LH peaked (mean+/-SEM) 78.0+/-23.0, 37.8+/-8.5, 44.4+/-10.3, and 51.0+/-5.1 h after CIDR removal (means, P<0.001; variances, P<0.001) and intervals from CIDR removal to ovulation were 102.0+/-6.7, 63.6+/-3.6, 81.6+/-3.5, and 78.0+/-4.1h (P<0.05). The interval from CIDR removal to ovulation was shorter and less variable in EB-treated groups; the interval from EB to ovulation was shortest (P<0.05) in the 12-h group. In summary, E-17beta or EB decreased both FSH and LH, but LH increased after 6h (despite elevated progesterone concentrations). Following CIDR removal, 1mg of EB effectively synchronized LH release, and ovulation (in intact cattle), but the interval from CIDR removal to EB treatment affected the time of ovulation.     

Dynamics of ovarian follicular development in cattle during the estrous cycle, early pregnancy and in response to PMSG

Ovarian follicular dynamics of cattle were examined during the estrous cycle, early pregnancy and in response to PMSG. Number and size of follicles were monitored by ultrasonographic examinations. During the estrous cycle, distinct periods of follicular dominance (measured by the increase in difference in size between the largest and second largest follicle) occurred in both the luteal (Days 6-8) and proestrus (18-22) phases of the estrous cycle (two follicular waves). Associated with the well timed development of the first dominant follicle was a change in distribution of follicle numbers in small (less than 5 mm; increased on Days 2-4), medium (6-8 mm; increased on Days 3-5) and large (greater than or equal to 9 mm; increased on Days 6-9) follicular size classes. Follicular development was greater on the ovary bearing the CL for the period that the CL was present. The dominant follicle formed during the first follicular wave was capable of ovulating (6 of 8 heifers) following an injection of a synthetic analogue of prostaglandin F-2 alpha on Day 9 of the estrous cycle. During early pregnancy (Days 6-34), follicular development (size of largest follicle, number of follicles and total accumulated size of all follicles) on the ovary bearing the CL was suppressed between Days 24 and 34 of pregnancy. This was a local effect in that follicular development was sustained on the contralateral ovary. Therefore, the CL or conceptus may be regulating follicular development in a manner to help prevent luteolysis. Associated with the injection of PMSG was an initial increase in the number of small follicles followed by their recruitment into medium and large size classes leading to ovulation. Number of follicles greater than 5 mm on the Day of estrus was related (r = .97) to the number of subsequent embryos and oocytes collected. Ultrasonography is a valuable technique to monitor ovarian follicular dynamics in cattle, and can thereby be used to infer changes in physiological and endocrine states.     

Effects of body condition and supplementation on ovarian function,growth factors and response to estrus synchronization in postpartum beef cows in Tamaulipas,Mexico

ABSTRACT

Eighteen beef cows with their calves were assigned to 3 groups considering body conditions scores at calving (BCSC): T1 (high BCSC, maintained throughout the study); T2 (low BCSC, increased during the study); and T3 (low BCSC, maintained throughout). An estrus synchronization protocol was implemented, with fixed time artificial insemination on day 70. Follicular or luteal growth, and pregnancy rate were similar between groups. Of all cows, five showed multiple ovulations (2–6); increasing number of ovulations reduced preovulatory follicle diameter and individual luteal volume (P?<?0.05), but total luteal volume was greater in cows with 2 or 3 ovulations on day 14 after insemination (P?<?0.05). Estradiol and progesterone levels were associated to follicular and luteal growth, respectively. No difference was found in IGF1 or GDF9 levels between groups or associated to fertility. No association was found between BCS, IGF1 or GDF9 levels and the reproductive process.     

Insulin-like growth factor I in follicular development and function in postpartum beef cows

The objective of this study was to determine whether nutrition affects follicular growth and(or) steroid and insulin-like growth factor I (IGF-I) concentrations in follicular fluid. Beginning 6 d after calving, Hereford-cross cows (n = 28) were fed either 14 (ad libitum) or 7 (restricted) kg.animal1.d-1 of chopped alfalfa-brome hay. Half the cows in each treatment were ovariectomized on d 20 (OVX-20) and the remaining half on d 35 (OVX-35) postpartum. Cow weight and condition score were recorded weekly, and blood was collected thrice weekly for determination of insulin, IGF-I, glucose, and free fatty acid (FFA) concentrations. At ovariectomy, follicular fluid from each follicle greater than or equal to 4 mm in diameter was aspirated for determination of IGF-I, progesterone (P4), and estradiol-17 beta (E2) concentrations. Restricted cows lost more weight after calving than did ad libitum cows (P less than .0001), although all cows lost similar amounts of body condition (time postpartum, P = .008). Concentrations of FFA were elevated (P less than .0001) in restricted cows from wk 2 through 5 after calving but did not change with time in ad libitum cows. Plasma concentrations of glucose were lower in restricted than in ad libitum cows (59.6 +/- .4 vs 61.8 +/- .4 mg/dl; P = .05), but insulin and IGF-I were similar (P greater than .10) between dietary treatments.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effect of pretreatment with different doses of GnRH to synchronize follicular wave on superstimulation of follicular growth in dairy cattle

The objective of this study was to evaluate the efficiency of gonadotropin releasing hormone (GnRH) and GnRH doses in synchronizing follicular wave emergence as a pretreatment for superovulation in cattle. Fourteen Holstein-Friesian cows 6 days from estrus were randomly assigned to receive 100 microg (n=4), 50 microg (n=5), or 25 microg (n=5) of GnRH. Superovulation was induced with injections of porcine FSH (pFSH) twice daily, decreasing the dose (total 42 AU) over 5 days beginning 2.5 days after receiving GnRH. On the 7th and 8th injections of pFSH, 750 microg of PGF(2alpha) was also given. With the exception of one cow that was given 50 microg of GnRH, ovulation was induced in all cows from the three groups and the new follicular wave emergence was observed. The total number of follicles for the 25 microg GnRH group was less than that observed for the 100 microg GnRH group (P<0.05), although there were no differences between the 100 microg, 50 microg and 25 microg GnRH groups with respect to the number of preovulatory follicles (>or=10 mm) and CL. The numbers of normal embryos were greater for the 25 microg GnRH group than the 100 or 50 microg GnRH groups (P<0.01); however, the numbers of ova/embryos did not differ significantly between the three groups. These results suggest that 25 microg of GnRH was sufficient to induce ovulation and follicular wave emergence. On day 6 of the estrous cycle, a reduction of the dose of GnRH to synchronize follicular wave emergence as a pretreatment for superstimulation promotes transferable embryos.     

热带牛同期发情-定时输精和胚胎移植效果分析

研究了同期发情-定时输精和胚胎移植以及同期发情方法的效果和影响因素.结果表明,婆罗门牛和BMY牛同期发情率和妊娠率无显著差异( P＞0.05);同期发情后,表观发情牛和表观未发情牛的妊娠率没有显著差异(P＞0.05).结果提示,对没有观察到发情症候的母牛实施定时输精是必要的,可以有效提高情期受胎率;体况(4.5)较肥的繁殖母牛可以获得良好受胎率,情期受胎率高达78.9%,在实际工作中,主要防止体况差影响繁殖率,而不是防止牛体过肥影响繁殖率;同期发情②法比①法能极显著提高胚胎移植受体牛可用率,移植妊娠率差异不显著(P＞0.05);埋栓时存在优势卵泡或发情,对胚胎移植受体牛可用率没有显著影响.     

Ovarian follicular growth, function and turnover in cattle: a review

Studies in cattle assessing changes in number and size of antral follicles, concentrations of estradiol, androgens and progesterone in serum and follicular fluid, and numbers of gonadotropin receptors per follicle during repetitive estrous cycles and postpartum anestrus are reviewed. The rate of growth of small follicles (1 to 3 mm) into larger follicles increases as the estrous cycle progresses from d 1 to 18 (d 0 = estrus). Size of the largest antral follicle present on the ovary also increases with advancement of the estrous cycle. Most large follicles (greater than 10 mm) persist on the ovarian surface for 5 d or more between d 3 and 13 of the bovine estrous cycle. After d 13, most of these large follicles are replaced more frequently by new growing follicles (turnover) with an increased probability for recruitment of the ovulatory follicle after d 18. More research is needed to determine the time required for growth of bovine follicles from small to large antral size and evoke recruitment of the ovulatory follicle. Factors that regulate selection of the ovulatory follicle are unknown but may involve increased frequency of LH pulses in blood, altered blood flow and(or) changes in intrafollicular steroids and proteins. Quantitative evaluation of ovarian follicles indicated occurrence of consistent short-term changes in fluid estradiol and numbers of luteinizing hormone receptors in cells of large follicles only during the pre-ovulatory period. Presumably, low concentrations of follicular estradiol found during most of the estrous cycle are not due to a lack of aromatizable precursor or follicle-stimulating hormone receptors. Follicular fluid concentrations of progesterone increase only near the time of ovulation. Little is known about changes in follicular growth, turnover and function during postpartum anestrus in cattle. However, preliminary data suggest that the steroidogenic capacity of large follicles changes markedly during the postpartum period.