Follicular growth, estrus and pregnancy after fixed-time insemination in beef cows treated with intravaginal progesterone inserts and estradiol benzoate

An experiment was performed to compare the effects of 3 short-term treatments with progesterone and estradiol benzoate (EB) on follicular growth, synchrony of estrus and pregnancy rate after fixed-time insemination in lactating postpartum beef cows. In Treatment 1 (n = 46), each cow received a progesterone-containing intravaginal insert for 7 d with injection of EB (2 mg, i.m.) at the time of device insertion. In Treatment 2 (n = 46), the insert was used for only 5 d with injection of EB (2 mg, i.m.) at the time of insertion. Cows in Treatment 3 (n = 47) received an insert for 5 d with no EB at the time of insertion. Each cow in the 3 groups received PGF2 alpha (25 mg, i.m.) at the time of insert removal, followed by EB (1 mg, i.m.) 30 h later. The cows were then inseminated 28 to 30 h after treatment with EB (58 to 60 h after insert removal). Treatment with 2 mg EB terminated the growth of the largest ovarian follicle (> 5 mm in diameter) at device insertion in 16/16 and 14/15 cows in Treatments 1 and 2, respectively. Estrus was detected within an 8-h target period (48 to 56 h after insert removal) in 93, 87 and 81% of cows in Treatments 1, 2 and 3, respectively (P > 0.05). Pregnancy rates at 39 d post insemination were 60, 50 and 51% for Treatments 1, 2 and 3, respectively (P > 0.05). The pregnancy rates did not differ between cows that were anovulatory or those that had ovulated before the initiation of treatments (54%), or among cows that were 28 to 40, 41 to 60 or > 60 days post partum at insemination (43, 59 and 54%, respectively). Treatment with progesterone inserts for 5 or 7 d, PGF2 alpha at the time of insert removal and 1 mg EB 30 h later induced the high degree of synchrony of estrus and ovulation necessary for fixed-time insemination.     

Estrus and ovulation in beef cows following use of progesterone-releasing devices, progesterone and estradiol valerate

Two hundred nonsuckling beef cows were treated with either 1) a progesterone-releasing intravaginal device (PRID) for 12 days; 2) PRID plus an IM injection of 200 mg progesterone (PRID-P); 3) PRID plus 5-mg IM injection of estradiol valerate (PRID-EV); or 4) PRID-EV-P. Cows were started on treatment on one of the first eight days of the estrous cycle. The number of cows which had P levels above 1 ng/ml one day after PRID removal was 12 to 50% lower in PRID-EV and PRID-EV-P groups than in PRID and PRID-P groups (P < 0.05). The proportion of cows showing estrus by 96 hours after PRID removal was 38, 36, 77, and 88% (P < 0.05) for the PRID, PRID-P, PRID-EV and PRID-EV-P groups, respectively. Thirty-one percent fewer cows treated with PRID on days 5 through 8 of the estrous cycle showed estrus by four days after PRID removal than those treated on days 1 through 4. In addition, 18 to 22% more cows had P levels above 1 ng/ml among cows treated with PRID or PRID-P on days 5 through 8 than among cows treated similarly on days 1 through 4. It was concluded that effective synchronization of estrus is achieved only when estrogen is used in conjunction with PRID in cows treated for twelve days during the first eight days of an estrous cycle.     

Ovarian activity and plasma concentrations of progesterone and estradiol during pregnancy in jennies

The objectives of this study were to determine ovarian activity (with ultrasound) and plasma concentrations of progesterone and estradiol during pregnancy in jennies. There was considerable ovarian activity during the second month of pregnancy. Secondary corpora lutea (total of 2 to 7 per jenny) were formed (mainly by luteinization) starting on Day 41.8 +/- 1.0 (range Days 38 to 46; ovulation = Day 0). The echogenicity of the primary and secondary corpora lutea gradually decreased during pregnancy. Plasma progesterone concentrations increased between Days 0 and 10 (0.9 and 19.9 ng/mL, respectively), gradually decreased to Day 30 (12.1 ng/mL), increased between Days 30 and 40 (plateau, at approximately 17 ng/mL), gradually declined from Days 110 to 160 (nadir of approximately 6 ng/mL), and remained nearly constant until increasing again just before parturition. Plasma estradiol concentrations increased gradually from Day 65, peaked (1.2 ng/mL) on Day 165 (> or = 1 ng/mL on Days 150 to 210), and decreased thereafter, with very low concentrations during the last 20 d before parturition. Ovarian function and hormone profiles were generally similar to those previously reported during pregnancy in mares.     

Fixed-time AI pregnancy rate following insemination with frozen-thawed or fresh-extended semen in progesterone supplemented CO-Synch protocol in beef cows

The objective of this study was to compare fixed-time AI pregnancy rate in Angus crossbred beef cows inseminated with frozen-thawed or fresh-extended semen. Two ejaculates from each of two Angus bulls were collected by artificial vagina and pooled for each bull. The pooled semen from each bull was divided into two aliquots; Aliquot 1 was extended using Caprogen^® (LIC, Hamilton, New Zealand) to a concentration of 3 × 10⁶ sperm/straw and Aliquot 2 was extended using egg-yolk-glycerol extender to a concentration of 20 × 10⁶ sperm/straw. Semen extended with Caprogen^® was maintained at ambient temperature and semen extended with egg-yolk-glycerol extender was frozen and maintained at −196 °C until insemination. In each of two breeding seasons (Fall 2007 and Spring 2008), Angus-crossbeef cows (N = 1455) at 12 locations were randomly assigned within location to semen type [Fresh (N = 736) vs. Frozen (N = 719)] and sire [1 (N = 731) vs. 2 (N = 724)]. All cows were synchronized with 100 μg of GnRH im and a progesterone Controlled Internal Drug Release insert (CIDR) on Day 0, and on Day 7, 25 mg of PGF2_α im and CIDR removal. All cows received 100 μg of GnRH im and were inseminated at a fixed-time on Day 10, 66 h after CIDR removal. Timed-AI pregnancy rates were influenced by season (P < 0.05), cows detected in estrus prior to and at AI (P < 0.001), and dam age (P < 0.01). Pregnancy rates were not affected by semen type (Fresh = 51.5% vs. Frozen = 50.4%; P = 0.66) and there were no significant interactions of semen type by estrus expression, semen type by sire, or semen type by season (P > 0.1). In conclusion, commercial beef cows inseminated with fresh-extended semen (3 × 10⁶ sperm/straw) yielded comparable pregnancy rates to conventional frozen-thawed semen in a progesterone supplemented, CO-Synch fixed-time AI synchronization protocol and may provide an alternate to frozen semen for more efficient utilization of superior genetics.     

Effect of norgestomet on peripheral levels of progesterone and estradiol-17beta in beef cows

Peripheral levels of progesterone and estradiol 17beta were quantified in 27 cycling cows following administration of a single Hydron ear implant (G. D. Searle and Co.) containing 2, 4 or 6 mg norgestomet or controls which received no implant. Implants were inserted subcutaneously in the ear on day 15 of the estrous cycle (day of estrus = day 0) and removed 9 days later. The 4 mg (seven of seven cows) and 6 mg (six of six cows) implants suppressed estrus; however, three of eight cows in the 2 mg group exhibited estrus prior to implant removal. The 6 mg implant group had a significantly longer interval from implant removal to estrus than either the 2 or 4 mg group. Failure to detect differences in the rate at which progesterone declined indicated norgestomet treatment did not affect normal corpus luteum regression. Estradiol levels rose at a similar rate approaching estrus in all treatments. There was no indication of increased endogenous estradiol levels due to norgestomet treatment.     

Overfeeding during early pregnancy reduces peripheral progesterone concentration and pregnancy rate in sheep

The 330 Merino ewes used in the study were placed with rams at a synchronized oestrus and, on Days 2-14 after mating, the ewes were placed in a feed lot and fed daily a low, medium or high ration (25%, 100% or 200% of maintenance respectively). Progesterone supplement was given to some ewes on Days 8-14 after mating by using a device containing 340 mg progesterone. Blood samples were taken from all ewes on Day 12 for measurement of plasma progesterone concentrations. On Day 14 after mating all ewes were returned to pasture. Pregnancy rate was determined by returns to oestrus and was later confirmed using ultrasound. There was a decline in the peripheral progesterone concentrations with increasing ration. The pregnancy rate in ewes fed a high ration was significantly reduced when compared with those of ewes fed a medium or low ration (48% vs 68 and 67% respectively; P less than 0.05). In ewes fed the high ration exogenous progesterone increased the pregnancy rate from 48 to 76% (P less than 0.01). Progesterone treatment did not influence pregnancy rates in ewes fed medium or low rations. The number of fetuses per ewe pregnant was not influenced by level of nutrition or progesterone treatment.     

Induction of cyclicity in postpartum anestrous beef cows using progesterone, GnRH and estradiol cypionate (ECP)

The objective of the present study was to determine whether treatment of postpartum multiparous and primiparous anestrous beef cows with an intravaginal progesterone-releasing insert (CIDR) and PGF(2alpha), with and without the addition of GnRH or estradiol cypionate (ECP) at the time of CIDR insertion, is effective in stimulating onset of estrous cycles. Postpartum lactating Angus primiparous (n=47, 2 years of age, 495+/-6 kg) and multiparous (n=76, >or=3 years of age, 553+/-9 kg) cows were assigned by calving date to four blocks spaced 21-day apart. Cows were assigned sequentially by calving date to four treatment groups: (1) PGF(2alpha) (n=30), (2) CIDR-PGF(2alpha) (n=30), (3) GnRH-CIDR-PGF(2alpha) (n=33), and (4) ECP-CIDR-PGF(2alpha) (n=27). Intravaginal CIDR inserts were in place from days -7 to 0. A single 100 microg injection of GnRH or 2 mg ECP were administered on day -7, and 25mg PGF(2alpha) was administered on day 0. Day 0 averaged 38+/-1 day postpartum. Blood samples were collected on days -19, -9, 0, 5, 9, 12, 16, 19, 23, 26, and 30 for determination of plasma progesterone concentrations. Pre-treatment luteal activity (progesterone>or=1 ng/ml) was detected in 19% of primiparous and 8% of multiparous cows. Progesterone concentrations on day 0 were greater (P<0.001) in primiparous (3.2+/-0.3 ng/ml) than multiparous (2.0+/-0.2 ng/ml) cows. Following CIDR withdrawal, progesterone concentrations from days 5 to 30 were used to categorize response profiles as either: (1) treatment-induced onset of estrous cycles, (2) continued anestrus, or (3) spontaneous ovulation and subsequent formation of a CL. Incidence of treatment-induced onset of estrous cycles, which was defined as progesterone concentrations >or=1 ng/ml in three or more consecutive samples from days 9 to 19, was influenced by treatment and parity. Percentages of cows initiating estrous cycles were greater (P<0.001) in the three CIDR-treated groups than in the PGF(2alpha) group (55 and 8%, respectively). Percentages of cows initiating estrous cycles in the CIDR-PGF(2alpha), GnRH-CIDR-PGF(2alpha), and ECP-CIDR-PGF(2alpha) groups were 55, 58, and 52%, respectively. Incidence of treatment-induced estrous cycles in the three CIDR-treated groups of cows was greater (P=0.008) in primiparous (76%) than multiparous (43%) cows. Treatment of postpartum anestrous primiparous and multiparous beef cows with CIDR-PGF(2alpha) approximately 40-day postpartum provides an approach to increase the percentage of cows that have reinitiated estrous cycles by the start of the breeding season.     

Low peripheral progesterone and late embryonic/early fetal loss in suckled beef and lactating dairy cows

Pregnancy failure during placentation in lactating dairy cows was associated with low concentrations of serum progesterone. Beef cows have greater serum progesterone and less pregnancy failure. Experiment 1 determined that reduction of serum progesterone affected late embryonic/early fetal loss in suckled beef cows. Cows (n = 40) received progesterone from two new or used controlled internal drug releasing devices, replaced every 5 d, beginning on Day 28 of gestation (mating = Day 0); CL were enucleated on Day 29. Retention of pregnancy was 77% in treated cows and 97% in 78 control cows (P < 0.05). Experiment 2 determined how pregnant, lactating dairy cows with high or low progesterone concentrations during Days 28-34 differed in luteal function or in serum progesterone during replacement therapy. Luteal tissue from such cows was assayed for progesterone and expression of mRNA for genes of endothelin and prostaglandin (PG) systems. Secretion of progesterone and prostaglandins by dispersed luteal cells was determined during incubation with LH, endothelin-1, or arachidonic acid. Neither luteal progesterone nor mRNAs for endothelin or prostaglandin systems differed. Endothelin-1 inhibited secretion of progesterone more (P < 0.05) in luteal cells from cows with low versus high serum progesterone, when incubated with arachidonic acid. Secretion of prostaglandin F₂α was increased and that of 6-keto-PGF₁α decreased by endothelin-1 in vitro. Serum progesterone during replacement was lower (P < 0.05) for cows with low than high serum progesterone at lutectomy. Thus, clearance, more than luteal production, determined peripheral progesterone in pregnant, lactating dairy cows.     

Serum and milk progesterone in Syncro-Mate-B treated postpartum beef cows

Fifteen post-partum (61.5+/-6.1 days) cyclic beef cows (4.1+/-0.3 years old) were randomly assigned to receive the Syncro-Mate-B (SMB) treatment on day 0 (estrus), day 9 or day 18 of their cycle with implant removal nine days later. An additional five post-partum (41.7+/-5.6 days) beef cows (3.2+/-0.4 years old) not detected in estrus and without corpora lutea also received the SMB treatment. Blood samples (10ml) were obtained via venipuncture on the day of implantation and one, three, five, seven and nine days after implantation and ten days after the post-treatment estrus. Milk samples (25 ml) were obtained by hand stripping at the same periods. Nineteen of twenty cows were synchronized, i.e. estrus occurred within five days of implant removal. Progesterone concentrations in both serum and milk were similar and followed similar patterns for each group. The day-9 treated cows had greater serum and milk progesterone concentrations than the other groups of cows on days 1, 3, and 5 after implantation. In conclusion, cows treated with SMB had similar serum and milk progesterone concentrations, endogenous progesterone secretion was inhibited by SMB treatment and the inhibition was influenced by prior luteal function.     

Relationship between estradiol and progesterone concentrations and cognitive performance in normally cycling female cynomolgus monkeys

Preclinical research has demonstrated that cognitive function may be influenced by estradiol (E2) and progesterone (P4) concentrations, although few cognition studies involve normally cycling females. The present study examined cognitive performance in normally cycling female cynomolgus macaques (n = 14), a species with similarities to humans in brain organization and a nearly identical menstrual cycle to women. Initial assessments compared cognitive measures to circulating concentrations of E2 and P4 (n = 12). Once a relationship was characterized between hormones and cognitive performance, the menstrual cycle was divided into four distinct phases: early follicular (EF), late follicular (LF), early luteal (EL) and late luteal (LL), verified by the onset of menses and serum concentrations of E2 and P4. Concentrations of E2 were highest during the LF phase and P4 concentrations peaked during the EL phase. All monkeys were trained on two cognitive tasks: reversal learning, involving simple discrimination (SD) and reversal (SDR), which measured associative learning and behavioral flexibility, respectively (n = 3–4 per phase) and a delayed match-to-sample (DMS) task which assessed working memory (n = 11). P4 concentrations were positively correlated with number of trials and errors during acquisition of SD performance, but not during acquisition of the SDR task or maintenance of the reversal-learning task. Across the menstrual cycle, significantly fewer errors were made in the SDR task during the LF phase, when E2 concentrations were high and P4 concentrations low. Working memory, assessed with the DMS task, was not consistently altered based on previously characterized menstrual cycle phases. These findings demonstrate a relationship between P4, E2 and cognitive performance in normally cycling cynomolgus monkeys that is task dependent. Knowledge of these interactions may lead to a better understanding of sex-specific cognitive performance.     

GnRH treatment at artificial insemination in beef cattle fails to increase plasma progesterone concentrations or pregnancy rates

Treatment with GnRH at the onset of standing estrus increased pregnancy percentages and circulating concentrations of progesterone in repeat breeder dairy cows. The objective of this study was to determine the effect of treatment with GnRH at AI on concentrations of progesterone and conception rates in beef cattle that exhibited estrus. Two hundred ninety-three heifers at four locations were synchronized with the Select Synch plus CIDR protocol (given GnRH and a CIDR was placed into the vagina, and 7 d later, given PGF_2α and CIDR removed; n = 253) or the 14-19 melengestrol acetate (MGA) protocol (MGA fed at 0.5 mg/head/d for 14 d, with PGF_2α 19 d after MGA withdrawal n = 40) and AI was done after detection of estrus. At Location 1, blood samples were collected on Day 2, 4, 6, 10, 15, and 18 after AI (Day 0 = AI). Two hundred and fifty postpartum cows at two locations were synchronized with the Select Synch plus CIDR protocol, and AI was performed after detection of estrus. At AI, cattle were alternately assigned to one of two treatments: (1) treatment with GnRH (100 μg) at AI (n = 127 heifers and n = 108 cows); or (2) non-treated control (n = 120 heifers and n = 119 cows). Concentrations of progesterone tended to be greater in control heifers compared to GnRH-treated heifers on Days 6 (P = 0.08), 10 (P = 0.07), and 15 (P = 0.11). Overall conception rates were 68% and 66% for GnRH treated and control, respectively, and were not different between treatments (P = 0.72). In summary, treatment with GnRH at time of AI had no influence on conception rates in cattle that had exhibited estrus.     

Sire effect on the pregnancy outcome in beef cows synchronized with progesterone based Ovsynch and CO-Synch protocols

The objective was to determine the sire effect on the pregnancy outcome in beef cows in which stage of estrus was synchronized with progesterone based fixed-time artificial insemination (AI) protocols. Three Angus sires with more than 300 breedings were evaluated for differences in pregnancy outcome from 1868 inseminations. Angus cross beef cows (N=1868) were synchronized with Ovsynch-CIDR or CO-Synch-CIDR protocols for fixed-time AI. Cows in both groups that showed estrus on day 9 before 1500 h were designated to Selectsynch-CIDR group and were inseminated according to AM-PM rule. Results indicated that Sire 2 had lower fixed-time AI pregnancy rate compared to Sire 3 (48.1% versus 58.7%; P=0.01). Significant sirexsynchronization program and sirexlocation interactions were observed for fixed-time AI (P<0.05). Sire 2 had a lesser fixed-time AI pregnancy in both Ovsynch-CIDR and CO-Synch-CIDR groups compared to Sire 3. In two of four locations, Sire 2 had a lesser fixed-time pregnancy rate compared to Sire 3. No sire differences were observed in AI pregnancy for cows in Selectsynch-CIDR group. In conclusion, evidence in this study suggest that there are differences in sire fertility when they were used in fixed-time AI protocols, possibly due to the sire differences in sperm capacitation process. Further studies are needed to investigate association of the sire differences in fixed-time AI protocols with sire differences in the sperm capacitation process.     

The use of milk progesterone to monitor reproductive function in beef suckler cows

Milk progesterone analysis was used to monitor reproductive function in 134 autumn calving cross-bred suckler cows. Progesterone was measured in milk samples collected three times per week from around 4 week post-calving to around day 60 of pregnancy during 1st and 2nd lactation. The mean day of onset of luteal activity (OLA) was 40.7 +/- 1.1 with the distribution skewed towards a later return. Once cyclicity had been initiated the incidence of reproductive cycle problems (6.5%) was low, though animals with such problems (n = 14) exhibited a delayed interval to first service (P < 0.05), lower conception and calving rates (P < 0.001), increased services per pregnancy (P < 0.001) and a higher (P < 0.10) barren rate (14.3% versus 4.0%) compared to animals with normal cycles (n = 201). In conclusion, using milk progesterone analysis we found a relatively low incidence of reproductive cycle problems in beefxdairy suckler cows. However, while the incidence of cycle problems was low, those animals with problems showed significantly impaired reproductive function.     

Insemination timing affects pregnancy rates in beef cows treated with CO-Synch protocol including an intravaginal progesterone insert

Our objective was to determine the optimal time to artificially inseminate lactating beef cows (Bos taurus typicus) after using the standard CO-Synch protocol that also included a progesterone-releasing, intravaginal controlled internal drug release (CIDR) insert. Cows (N = 605) at three locations were inseminated at four different times after CIDR insert removal and the prostaglandin F_2α administration of the CO-Synch + CIDR protocol: 48, 56, 64, or 72 h. Blood samples were collected 9 to 10 d before and on the day of CIDR insertion. Based on elevated (>1 ng/mL) serum progesterone concentrations, 60% of 605 cows had previously ovulated (were cycling) at the initiation of the study, with a range of 39.6% to 67.9% among locations (P < 0.05). Age of cow, body condition score, and days postpartum affected (P ≤ 0.05) cycling status before ovulation was synchronized. Averaged across treatments, pregnancy rate to artificial insemination (AI) at Day 32 was affected (P ≤ 0.05) by pretreatment cycling status and body condition. Younger cows (≤3 yr) tended to have greater AI pregnancy rates when inseminated at 56 h, whereas older cows had similar pregnancy rates when inseminated at 56 h or later (timing of AI by age interaction; P = 0.085). Pregnancy loss between Days 32 and 63 was greatest (quadratic effect; P < 0.05) when cows were inseminated at 48 and 72 h. In summary, insemination times at or after 56 h improved AI pregnancy rates when using the CO-Synch + CIDR program. Further work is warranted to examine age effects on timing of AI in the CO-Synch + CIDR program.     

Effects of Alfaprostol and uterine palpation on postpartum interval and pregnancy rate to embryo transfer in Brahman influenced beef cows

Two trials were conducted to determine the effects of Alfaprostol, a prostaglandin F2 alpha analogue, and uterine palpation on reproductive performance in postpartum beef cows. In Trial J, 143 multiparous and 132 primiparous Brahman crossbred embryo recipient cows were assigned to receive one 5 mg injection of Alfaprostol between 29 and 65 d postpartum or to serve as controls. Alfaprostol treatment between 29 and 42 d postpartum decreased (P < 0.05) postpartum interval in multiparous but not primiparous cows. Alfaprostol treatment increased pregnancy rate to embryo transfer. Trial II utilized 369 multiparous Brangus cows, 17 to 49 d postpartum, that were assigned to one of three groups: 1) Alfaprostol plus uterine palpation (APP), 2) Uterine palpation only (UP) or 3) Control. Completion of uterine involution was determined at treatment. Alfaprostol plus uterine palpation decreased postpartum interval regardless of uterine involution, while UP had a greater effect if uterine involution was not complete. Both APP and UP increased the number of cows which were detected in estrus by 80 d postpartum. Alfaprostol and uterine palpation, alone or in combination, can increase postpartum reproductive performance in Brahman crossbred and Brangus beef cows.     

Pregnancy diagnosis based on the fecal progesterone concentration in beef and dairy heifers and beef cows

The present study was undertaken to examine whether pregnancy diagnosis was possible by measuring fecal progesterone concentrations in beef and dairy heifers and beef cows. Rectal fecal samples collected on days 18–24 after insemination or days 11–17 after embryo transfer were mixed with methanol and shaken for preparation of a fecal solution. After centrifugation, the supernatant was extracted with petroleum ether followed by an enzyme immunoassay for progesterone. All pregnant animals showed fecal progesterone concentrations greater than 50 ng/g of fecal material on days 18–24 after AI or estrus. In non-pregnant animals, however, the fecal progesterone concentrations ranged widely from 5 to 180 ng/g of fecal material. In non-pregnant cattle, the percentage of cattle with <50 ng progesterone/g of fecal material compared with the total number was 37–60% on days 18–20, whereas the percentages increased more than 70% to a maximum of 78.1% on day 23. When 50 ng/g was considered as the cut-off value, the sensitivity and specificity of positive pregnancy tests were less than 70% on days 21–24, and 100% for negative pregnancy tests on days 18–24. There were significant differences in the mean fecal progesterone concentrations between pregnant and non-pregnant cattle on days 19–24. These results suggest that feces can be utilized to substitute for plasma and milk to measure progesterone for the purpose of pregnancy diagnosis in heifers and cows.     

Regulation of estradiol and progesterone receptor concentrations in cat uteri following chronic progesterone administration

The purpose of this study was to determine the early effect of progesterone (P) on the estradiol (E2) and P receptor systems in cat uteri. Ovariectomized animals were treated with E2 for 7 days. Animals were then treated for up to 48 h with E2 and P, treated with P while being E2 withdrawn, or just E2 withdrawn. P treatment resulted in a significant decrease in P cytosol receptor (PcR) and a significant increase in P nuclear receptor (PnR) at all times included in this study when compared to levels measured in the E2-treated animal. E2 cytosol receptor (EcR) and E2 nuclear receptor (EnR) levels were significantly lower after 12 h of P treatment and remained so for the duration of this study. When EcR and EnR levels were compared after 48 h of P treatment in the presence or absence of E2, or after 48 h of E2 withdrawal, the loss of EnR following P treatment appeared to be independent of any changes in EcR levels or serum E2 levels, and only dependent on the presence of P. These results clearly illustrate that the chronic administration of P decreases the uterine concentration of its own receptor, and suggests that P decreases the E2 receptor system by a selective action within the nucleus which diminishes their ability to retain EnR.     

Ovarian follicular and corpus luteum changes, progesterone concentrations, estrus and ovulation following estradiol benzoate/progesterone based treatment protocol in cross-bred cows

The objectives of the present study were to investigate the effects of the stage of the estrous cycle at the start of an estradiol benzoate (EB) and progesterone (P) based treatment protocol on new follicular wave emergence, subsequent estrus and ovulation. The experiment was conducted using a crossover design with each cow (five cross-bred cows) being assigned to one of three groups at 3-month intervals within a 1-year period. Estrous cycle stage in individual cows was initially synchronized with prostaglandin F(2)alpha. After detection of estrus, each cow was injected intramuscularly (i.m.) with 2 mg EB and 200 mg P (EB/P) on day 5, 12 or 17 of the estrous cycle (estrus=day 0), followed by 1 mg EB i.m. 12 days after the EB/P treatment. Ovarian ultrasonographic examinations showed that the emergence of a new follicular wave occurred after EB/P treatment in all groups and the mean interval from EB/P treatment to wave emergence did not differ among the groups (3.2-3.8 days). All cows in each group exhibited behavioral estrus and ovulated the newly formed dominant follicle. However, cows in the day-17 group exhibited estrus 1-3 days before the second EB injection. The concentrations of progesterone showed faster reduction, during the treatment period, in the day-12 and -17 groups compared to the day-5 group. These results indicate that the EB/P treatment induces an emergence of a new follicular wave, irrespective of the estrous cycle stage at the start of treatment, but the effect of EB/P protocol on estrous/ovulation synchronization is influenced by the stage of the estrous cycle.     

Plasma gonadotrophin responses in beef cows two progesterone plus prostaglandin treatment

Progesterone Releasing Intravaginal Devices (Prids) were inserted into six post-partum beef cows for nine days and 0.5 mg cloprostenol was injected i m on day eight. Blood samples were taken via jugular venous catheters at frequent intervals for seven days after Prid removal and assayed for LH, FSH and progesterone. The induced pre-ovulatory type LH and FSH surges occurred between 35 and 123h after Prid withdrawal in five of the cows. In four cows which underwent surges during the time of most intensive sampling, LH levels were significantly higher during the 30h period prior to the LH surge than during the 30h period after the surge. FSH values were low for the 30h period preceding and the 14h period following the time of maximum FSH/LH concentrations. 16 - 30h after the FSH and LH surges, FSH values were again significantly raised compared with the period immediately after the surge. Despite the success of this Prid/PG regime in inducing ovulation, the variability in time between progestagen withdrawal and the LH surge and ovulation is such that the use of fixed time artificial insemination may give poor results.     

Effect of monensin, energy level, and cow body condition on pregnancy rate in nonlactating beef cows

One hundred eighty-two nonlactating cycling Brahman crossbred and Angus cows were used to study the effects of monensin, cow body condition and supplemental energy level on pregnancy rate. The cows were classified as being in either moderate or poor body condition at the start of the experiment and were fed either 9 or 18 Mcal of supplemental metabolizable energy/head/day with or without 125 mg of monensin in the ration. Feeding commenced 21 days prior to and throughout a 45-day breeding season. In the monensin treated group ruminal propionic acid levels (molar %) were increased (P<.01) while acetic and butyric acid levels were lowered (P<.05) regardless of supplemental energy intake. Pregnancy rate, pregnancy rate at first service, and the percentage of conceptions per estrus were increased (P<.05) when cows were fed 18 Mcal of supplemental energy per head per day. Neither cow body condition nor addition of monensin to the diet had an effect on the preceding reproductive endpoints. Cows in poor body condition and receiving 18 Mcal of supplemental energy per head per day had increased (P<.05) reproductive performance as evidenced by an increase in the pregnancy rate, pregnancy rate at first service, and percentage of conceptions per estrus when compared to the other groups.