Endocrine response of postpartum anestrous beef cows to GnRH or PMSG

Forty-one postpartum anestrous Hereford cows, maintained under range conditions, were used to determine the influence of gonadotropin releasing hormone (GnRH) or pregnant mare serum gonadotropin (PMSG) on ovarian function. Anestrous cows were identified by estrous detection with sterile bulls and concentrations of progesterone in plasma obtained weekly. At 45 +/- 2 days postpartum, cows were allotted to the following treatments: (1) control (saline), (2) 100 mug GnRH, (3) 200 mug GnRH, (4) 200 mug GnRH in carboxymethyl cellulose (CMC), (5) 500 IU PMSG, (6) 1,000 IU PMSG or (7) 2,000 IU PMSG. Cows were bled frequently the first day after treatment and then every other day until 85 days postpartum. The LH responses after 100 and 200 mug of GnRH were not significantly different and mixing 200 mug GnRH with CMC before injection did not significantly alter the LH response. During the first 20 days after treatment, neither GnRH nor 500 IU PMSG altered estradiol concentrations in plasma, but treatment of cows with 1,000 or 2,000 IU PMSG resulted in increased (P<0.01) concentrations of estradiol. The time postpartum required for concentrations of progesterone in plasma to exceed 1 ng/ml was reduced (P<0.05) by all treatments except 100 mug GnRH. These data indicate that GnRH causes LH release in anestrous range cows and that treatment with 1,000 or 2,000 IU PMSG initiates ovarian activity as evidenced by increased concentrations of estradiol in plasma.     

Pituitary responses to a challenge test of GnRH and Oestradiol Benzoate in postpartum and regularly cyclic dairy cows

Six cows at different times postpartum (days 1, 7, 14, 21, 28, 35, 42 and 49) were treated with 20 μg gonadotrophin releasing hormone (GnRH) and 1.0 mg oestradiol benzoate. There was a gradual regain of plasma luteinizing hormone (LH) response to GnRH up to day 14 postpartum. No response of LH was achieved after oestradiol benzoate treatment on day 1, and thereafter the response continued to increase until day 21, occurring between 14 and 34 h (24.6 ± 2.6, mean ± SE) after injection. There was a significant negative correlation between the time to peak concentration and day postpartum. Cows which had plasma progesterone concentrations > 0.3 ng/ml did not respond to oestradiol benzoate treatment.Cows challenged in the follicular and luteal phases of established cycles had LH responses to GnRH which were significantly (P < 0.0005) greater than in the postpartum cows, but there was no difference between the responses in the follicular and luteal phases (P > 0.1). In those cows which responded to oestradiol benzoate, the peak LH release was greater than that achieved in the responding postpartum cows (P < 0.05) and the increased LH values occurred 18–30 h (24.7 ± 2.5 h) after injection.A physiological endocrine challenge test has been established to investigate changes in pituitary responses to GnRH and oestradiol benzoate in dairy cows.     

Basal and estradiol-induced release of gonadotropins in dairy cows with naturally occurring ovarian cysts

A study was conducted to identify relationships between serum sex steroid concentrations and release of gonadotropins in dairy cows with ovarian cysts. Cows with ovarian cysts were grouped according to sex steroid profiles as being under estrogenic (n = 6) or low steroid (n = 6) influence. All cows were submitted to a sampling and treatment protocol to 1) record basal pulsatile release of gonadotropins and 2) determine whether luteinizing hormone (LH) or follicle stimulating hormone (FSH) was released after sequential administration of exogenous estradiol and gonadotropin releasing hormone (GnRH) treatments were given 30 h apart. Basal LH was higher in the estrogen-influence group (P < 0.05). There were no differences between groups in basal FSH concentrations or frequency and amplitude of pulsatile LH or FSH release. Only one of the twelve cows, an individual from the low steroid group, had a preovulatory-like surge of gonadotropins after exogenous estradiol. All cows released LH and FSH in response to GnRH treatment, with no differences between groups. These results show that 1) there is considerable variation in pulsatile release of gonadotropins in cows with ovarian cysts, even among individuals with similar sex steroid profiles, and 2) suggest that a factor in the persistence, and perhaps initiation, of the cystic condition is refractoriness to the positive feedback effect of estradiol on gonadotropin release.     

The effect of progestin and GnRH treatments on ovarian function and reproductive hormone secretions of anestrous postpartum suckled beef cows

Eighteen anestrous crossbred suckled beef cows were assigned to one of three treatment groups. Treatments were as follows: Group 1 cows (n = 3) were untreated and served as controls, Groups 2 cows (n = 6) were intramuscularly administered 250 mug GnRH, and Group 3 cows (n = 9) were subcutaneously administered a progestin ear implant for eight days prior to the administration of 250 mug GnRH. The GnRH was given to cows in Group 3 24 h after the time of progestin implant removal. Cows were 21 to 31 days postpartum at the time of GnRH treatment. The percent of cows that ovulated after the time of GnRH treatment was 0%, 83% and 100% for Groups 1, 2 and 3, respectively. For the cows that ovulated, more (P < 0.05) cows in Group 2 (80%) had abnormal luteal phases than in Group 3 (33%). The GnRH-induced LH release and peak LH concentrations were greater (P < 0.01) in the cows in Group 3 (214.3 +/- 37.1 ng/ml) than in the cows in Group 2 (142.7 +/- 19.0 ng/ml). The LH concentrations of the control cows remained very low throughout the sampling period. Although prostaglandin metabolite (PGFM) concentrations were not significantly (P > 0.10) different among groups, mean concentrations were higher and more variable for cows in Groups 1 (39.2 +/- 5.2 pg/ml) and 2 (39.4 + 6.1 pg/ml) than for cows in Group 3 (25.1 + 1.4 pg/ml).     

An alteration in the hypothalamic action of estradiol due to lack of progesterone exposure can cause follicular cysts in cattle

Many mammals, including cattle, can develop ovarian follicular cysts, but the physiological mechanisms leading to this condition remain undefined. We hypothesized that follicular cysts can develop because estradiol will induce a GnRH/LH surge on one occasion but progesterone exposure is required before another GnRH/LH surge can be induced by estradiol. In experiment 1, 14 cows were synchronized with an intravaginal progesterone insert (IPI) for 7 days, and prostaglandin F(2alpha) was given on the day of IPI removal. Estradiol benzoate (EB; 5 mg i.m.) was given 3 days before IPI removal to induce atresia of follicles. Cows were given a second EB treatment 1 day after IPI removal to induce a GnRH/LH surge in the absence of an ovulatory follicle. All cows had an LH surge following the second EB treatment, and 10 of 14 cows developed a large-follicle anovulatory condition (LFAC) that resembled follicular cysts. These LFAC cows were given a third EB treatment 15 days later, and none of the cows had an LH surge or ovulation. Cows were then either not treated (control, n = 5) or treated for 7 days with an IPI (n = 5) starting 7 days after the third EB injection. Cows were treated for a fourth time with 5 mg of EB 12 h after IPI removal. All IPI-treated, but no control, cows had an LH surge and ovulated in response to the estradiol challenge. In experiment 2, cows were induced to LFAC as in experiment 1 and were then randomly assigned to one of four treatments 1) IPI + EB, 2) IPI + GnRH (100 microg), 3) control + EB, and 4) control + GnRH. Control and IPI-treated cows had a similar LH surge and ovulation when treated with GnRH. In contrast, only IPI-treated cows had an LH surge following EB treatment. Thus, an initial GnRH/LH surge can be induced with high estradiol, but estradiol induction of a subsequent GnRH/LH surge requires exposure to progesterone. This effect is mediated by the hypothalamus, as evidenced by similar LH release in response to exogenous GnRH. This may represent the physiological condition that underlies ovarian follicular cysts.     

Effect of endogenous and exogenous progesterone on the oestradiol-induced LH surge in dairy cows

Four cows released an LH surge after 1.0 mg oestradiol benzoate administered i.m. during the post-partum anoestrous period with continuing low plasma progesterone. A similar response occurred in the early follicular phase when plasma progesterone concentration at the time of injection was less than 0.5 ng/ml. Cows treated with a progesterone-releasing intravaginal device (PRID) for 8 days were injected with cloprostenol on the 5th day to remove any endogenous source of progesterone. Oestradiol was injected on the 7th day when the plasma progesterone concentration from the PRID was between 0.7 and 1.5 ng/ml. No LH surge occurred. Similarly, oestradiol benzoate injected in the luteal phase of 3 cows (0.9-2.1 ng progesterone/ml plasma) did not provoke an LH surge. An oestradiol challenge given to 3 cows 6 days after ovariectomy induced a normal LH surge in each cow. However, when oestradiol treatment was repeated on the 7th day of PRID treatment, none released LH. It is concluded that ovaries are not necessary for progesterone to inhibit the release of LH, and cows with plasma progesterone concentrations greater than 0.5 ng/ml, whether endogenous or exogenous, did not release LH in response to oestradiol.     

Influence of GnRH infusion on endocrine parameters and duration of postpartum anestrus in beef cows

The effect of an intravenous infusion of gonadotrophin releasing hormone (GnRH) on the duration of postpartum anestrus in suckled beef cows was studied. Twenty-eight, mature, suckled beef cows were assigned in equal numbers to one of four treatment groups which were based on infusion with saline or GnRH (15ug/hour for 12 hours) and stage postpartum (pp) (20 or 35 days). Serum LH and progesterone were determined by radioimmunoassay for the period which began 5 days pre-infusion and ended at 55 days postpartum (ie: 35 or 20 days post-infusion). Serum LH remained below 5ng/ml during infusion in all control cows. Peak serum LH values, times of LH peaks, and duration of LH responses (means +/- SE) during infusion were 49 +/- 12 ng/ml, 162 +/- 42 minutes and 7.8 +/- 1.3 hours for the 20 day group and 44 +/- ng/ml, 144 +/- 6 minutes, and 8.2 +/- 1.1 hours for the 35 day group respectively. Serum progesterone levels indicated that the proportion of cows showing the onset of estrous cycles within 10 days of infusion was greater in the 20 day pp GnRH group (4/7) than the 20 day pp saline group (0/7) (p < .05) but was not significantly different between the 35 day pp GnRH (4/7) and 35 day pp saline (2/6) groups. The incidence of estrus was not affected by GnRH treatment and was 37% in all cows prior to 55 days pp. It was concluded that infusions of GnRH for 12 hours at a rate of 15 ug/hour could induce estrous cycles in suckled beef cows treated at 20 days postpartum.     

Length of progesterone exposure needed to resolve large follicle anovular condition in dairy cows

Hypothalamic unresponsiveness to an estradiol surge appears to be an underlying cause of large follicle anovular condition (follicular cysts), but progesterone exposure for 7 days resolves this condition. In this study, dairy cows with induced (Experiment 1) or naturally occurring (Experiment 2) follicular cysts were treated for different times with progesterone. In Experiment 1, 16 of 26 cows (62%) were induced into anovulation by causing a GnRH/LH surge when no ovulatory follicle was on the ovary. Anovular cows (n = 16) were assigned to one of four treatment groups ( 0, 1, 3, or 7 days of progesterone treatment) using an intravaginal, progesterone-releasing implant (CIDR). All anovular cows had low circulating progesterone concentrations before controlled internal drug releasing (CIDR) and greater concentrations that reached steady state (1.3 +/- 0.1 ng/mL progesterone) by 3 h after CIDR insertion. Circulating progesterone decreased to basal concentrations by 4 h after CIDR removal. Cows were treated with 5mg estradiol benzoate (EB) 12 h after CIDR removal. None (n = 4) of the control cows (0 day) had an LH surge after EB. All of the 3 days (5/5) and 7 days (4/4) CIDR-treated cows had an LH surge following EB, but only one of the 1 day (1/3) CIDR-treated cows. Magnitude of the LH peak was similar in the 3 and 7 days cows. All cows treated for 7 days ovulated (4/4), whereas, ovulation occurred in only 3/5, 1/3, and 0/4 of the cows treated for 3, 1, and 0 day, respectively. The two cows in the 3 days group that did not ovulate had a normal LH surge, but these two cows had a smaller maximal follicle size than cows that ovulated. In Experiment 2, naturally anovular lactating dairy cows (24 of 248) were identified using weekly ultrasonography. All anovular cows grew follicles to >12 mm, with 54% (13 of 24) having follicles larger than ovular size (15-24 mm) and 33% (8 of 24) having follicles that would be considered cystic (>25 mm). Anovular cows were randomly assigned to CIDR treatment for 0, 1, or 3 days. All (7/7) of 3 days, 33% (3/9) of 1 day, and 25% (2/8) of control (0 day) cows ovulated by 1 week after CIDR removal. Thus, 3 days but not 1 day of progesterone exposure appears to be sufficient to reinitiate estradiol responsiveness of the hypothalamus.     

LH surge in Nelore cows (Bos indicus), after induced estrus or after ovarian superestimulation

Considering that there is limited information about the preovulatory LH surge in Zebu cattle (Bos indicus), the purpose of the present work was to assess the LH surge in Nelore cows during the estrous cycle and after ovarian superestimulation of ovarian follicular development with FSH. This information is particularly important to improve superovulatory protocols associated with fixed-time artificial insemination. Nelore cows (n=12) had their estrus synchronized with an intravaginal device containing progesterone (CIDR-B) associated with estradiol benzoate administration (EB, 2.5 mg, i.m., Day 0). Eight days later all animals were treated with PGF2alpha (Day 8) in the morning (8:00 h) and at night, when CIDR devices were removed (20:00 h). Starting 38h after the first PGF2alpha injection, blood sampling and ovarian ultrasonography took place every 4h, during 37 consecutive hours. Frequent handling may have resulted in a stress-induced suppression of LH secretion resulting in only 3 of 12 cows having ovulations at 46.7+/-4.9 and 72.3+/-3.8 h, respectively, after removal of CIDR-B. Thirty days later, the same animals received the described hormonal treatment associated with FSH (Folltropin), total dose=200 mg) administered twice a day, during 4 consecutive days, starting on Day 5. Thirty-six hours after the first injection of PGF2alpha, to minimize stress, only seven blood samples were collected at 4h interval each, and ultrasonography was performed every 12 h until ovulation. In 11 of 12 cows (92%) the LH surge and ovulation were observed 34.6+/-1.6 and 59.5+/-1.9 h, respectively, after removal of progesterone source. The maximum values for LH in those animals were 19.0+/-2.6 ng/ml (mean+/-S.E.M.). It is concluded that, in Nelore cows submitted to a ovarian superstimulation protocol, the LH surge occurs approximately 35 h after removal of intravaginal device containing progesterone, and approximately 12h before the LH surge observed after an induced estrus without ovarian superstimulation.     

Effect of suckling and ovariectomy on the control of luteinizing hormone secretion during the postpartum period in beef cows

Twenty-two mature pluriparous beef cows were randomly assigned to one of six treatments in a 2 X 3 factorial experiment in order to study the role of suckling and ovarian factors on control of the tonic and episodic release of luteinizing hormone (LH). Twelve cows remained intact (INT) and 10 were ovariectomized (OVX) within 4 days following the day of parturition (Day 0). The suckling intensities were nonsuckled (0), suckled once daily for 30 min (1) and suckled ad libitum by two calves (2). Blood samples were collected at 15-min intervals for 6 h weekly, from Days 6 to 76 postpartum. The postpartum intervals to initiation of ovarian luteal function were 31 +/- 3, 41 +/- 4 and 67 +/- 1 days (means +/- SEM) for INT cows with 0, 1 and 2 suckling intensities, respectively. Mean LH concentrations and frequency of LH pulses increased as time of ovulation approached in INT cows. In OVX animals, both mean LH concentrations and frequency of LH pulses increased as time postovariectomy progressed. No differences were detected in mean LH concentrations or frequency of LH pulses between the two suckled OVX groups. Mean LH in the OVX-0 cows was greater on Days 13, 20 and 27 postpartum when compared to the respective days in suckled OVX cows. Frequency of LH pulses tended to be lower (P less than 0.10) in both suckled OVX groups when compared with OVX-0 cows from Day 6 to Day 55 postpartum. It is postulated that suckling and ovarian factors act together during the postpartum period to suppress LH levels and frequency of LH pulses in beef cows.     

Effects of cloprostenol, human chorionic gonadotropin and estradiol benzoate treatment on estrus synchronization in dairy cows

Two consecutive experiments were conducted. In Experiment 1, 24 Friesian lactating cows were randomly assigned to two groups. Cows in Group I received intramuscularly (i.m.) 500 mcg of cloprostenol, 1250 IU of human chorionic gonadotropin (hCG) and 5 mg of estradiol benzoate 12 h after cloprostenol treatment. Cows in Group II received 750 IU i.m. of hCG and 3 mg of estradiol benzoate 12 h after cloprostenol treatment. Treatment was given on Day 16 after estrus in both groups. All animals showed estrus within 24 to 48 h after cloprostenol treatment. The average interval from cloprostenol injection to the onset of estrus was not influenced by treatments. Four cows in Group I failed to ovulate and became cystic. In Experiment 2, 71 Friesian lactating cows were randomly assigned to two groups. Cows in Group I received 500 mcg i.m. of cloprostenol after corpus luteum detection by palpation per rectum. Cows in Group II received 500 mcg of cloprostenol plus 750 IU of hCG and 3 mg of estradiol benzoate 12 h after. When estrus ready for service was confirmed by rectal examination, cows were inseminated. The percentage of cows ready for service tended to be lower (P < 0.06) between cows in Group I (88%) and those in Group II (100%). The average interval from cloprostenol treatment to service was longest (P < 0.001) in Group I (78.7 h +/- 14.9, X +/- SD) vs Group II (48 h +/- 2.9). The degree of readiness for service synchrony was lowest (P < 0.001) in Group I (59.3%) vs Group II (94.2%). The pregnancy rates of cows synchronized or treated were not altered by hCG-estradiol benzoate treatment (P > 0.25). These results suggest that in dairy cows treated with cloprostenol following palpation per rectum of a corpus luteum and then with 750 IU of hCG and 3 mg of estradiol benzoate 12 h later, a single fixed-time insemination at 48 h after cloprostenol treatment should be performed.     

Reproductive hormones associated with the ovarian cyst response to GnRH

Eighteen cows with ovarian cysts were administered 100 mug of GnRH and bled prior to treatment, at half hour intervals for 4 hours posttreatment and on days 1, 5 and 9 posttreatment. Blood plasma was analyzed for estradiol-17beta, progesterone and LH by radioimmunoassay. Response to treatment was recorded as positive if ovulation was detected within 30 days posttreatment. Fourteen cows (78%) initiated ovarian cycles by 30 days posttreatment. Mean pretreatment concentrations of estradiol-17beta, progesterone and LH and the GnRH induced LH release were not different for positive or no response cows. However, all seven cows that had pretreatment progesterone concentrations greater than 1.0 ng/ml had a positive response to treatment. Eight of the remaining eleven cows had a progesterone response (mean progesterone concentrations on days 5 and 9 posttreatment) greater than 1.0 ng/ml; seven had a positive response to treatment. In summary, most cows with ovarian cysts administered GnRH will initiate ovarian cycles within 30 days if: 1) pretreatment progesterone concentrations are greater than 1.0 ng/ml or 2) if progesterone response is greater than 1.0 ng/ml.     

Hypothalamic proopiomelanocortin mRNA levels in suckled or nonsuckled beef cows: A preliminary study

This study was conducted to determine proopiomelanocortin (POMC) mRNA levels in the preoptic and hypothalamic brain regions of postpartum anestrous cows. An additional objective was to determine if calf suckling influences POMC mRNA concentration in these regions. Twenty cows were randomly assigned to suckled and nonsuckled treatment groups and slaughtered between 30 and 36 days postpartum. Serum luteinizing hormone (LH) concentrations were determined from blood collected every 15 minutes for 8 hours, starting 20 hours prior to slaughter. POMC mRNA levels in brain tissues were determined by dot blots. Serum LH concentrations between nonsuckled and suckled cows were 1.3 +/- 0.2 and 0.9 +/- 0.1 ng.ml(-1) (mean +/- SEM; P = 0.19), respectively. The POMC gene is expressed in the hypothalamus of postpartum anestrus cows with POMC mRNA levels higher (P<0.05) in the hypothalamus than in the preoptic region. Hypothalamic POMC mRNA levels tended (P = 0.12) to be lower in nonsuckled (14.9 +/- 3.8 ADU) than in suckled cows (23.5 +/- 3.6 ADU). Covariate analysis indicated (P = 0.10) that as mean serum LH concentrations increased, hypothalamic POMC mRNA levels decreased.     

Effect of calf removal on serum luteinizing hormone and cortisol concentrations in postpartum beef cows

Serum luteinizing hormone (LH) and cortisol concentrations were measured in ten fall calving, Angus cows averaging 38 +/- 8 days postpartum. Calves from five cows were weaned at the beginning of the study. Blood samples were collected at 20 min. intervals for 48 h after weaning and for 8 h on day 4 and day 6 postweaning. Mean serum LH concentrations increased (P<0.01) in weaned cows (W) from 0.55 +/- 0.01 ng/ml at time of calf removal to 1.3 +/- 0.04 ng/ml 48 h afterwards. Comparable LH concentrations for suckled cows (S) were 0.65 +/- 0.08 ng/ml and 0.62 +/- 0.03 ng/ml respectively. Average serum LH concentrations at 48 h after weaning were greater (P<0.01) for W cows than S cows and a treatment by time interaction occurred (P<0.01) with serum LH concentrations increasing (P<0.01) from time of calf removal to 48 h after calf removal in W cows. Frequency of LH peaks increased (P<0.01) in W cows and by 48 h after weaning was greater (P<0.01) in W cows than in S cows. Magnitude of LH peaks did not differ between the two groups. Serum cortisol concentrations were not different between W and S cows except for a transient elevation (P<0.01) in W cows from 7.6 +/- 0.9 ng/ml to 11.9 +/- 1.0 ng/ml 9 to 12 h after calf removal. Since serum LH concentrations were increased in W cows but not in S cows at 48 h and serum cortisol concentrations increased transiently in W cows we suggest that circulating cortisol levels may not be a physiological inhibitor of LH secretion in the suckled postpartum beef cow.     

The acute effect of bull presence on plasma profiles of luteinizing hormone in postpartum, anoestrous dairy cows

The objective of this study was to investigate whether bull exposure affects LH profiles in postpartum, anoestrous dairy cows. Eight cows between 10 and 17 days after parturition were used. On Day 1, blood samples were taken at 10 min intervals for 8 h. On Day 2, blood sampling continued at 10 min intervals and after 2 h a bull was introduced behind a fence, and blood sampling continued for another 8 h. Time of resumption of luteal activity was between 25 and more than 80 days after parturition for these animals and was not related (P>0.1) with frequency of LH pulses, amplitude of pulses and basal LH concentration on either Day 1 or Day 2. In 6 of the 8 cows, average and basal LH concentration were greater (P<0.001) during the 8 h of bull presence (0.56 +/- 0.33 and 0.39 +/- 0.26 ng/ml, respectively) compared to the 8 h without a bull (0.50 +/- 0.30 and 0.35 +/- 0.24 ng/ml, respectively). Pulse amplitude did not differ (P=0.85) between Day 2 (0.45 +/- 0.24 ng/ml) or Day 1 (0.45 +/- 0.14 ng/ml). LH pulse frequency was greater (P<0.1) on Day 2 (5.3 pulses/8h) compared to the Day 1 (4.6 pulses/8h). In conclusion, fenceline bull exposure early postpartum seems to have an acute effect on LH-release in anoestrous dairy cows. Whether sustained bull exposure can hasten first ovulation after calving through an effect on LH release in dairy cows is an interesting area of research.     

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.     

Profiles of LH, FSH and progesterone in postpartum dairy cows and their relationship to the commencement of cyclic functions

In 25, 3-to-13 year old, dairy cows (Braunvieh and Hoehenfleckyieh) FSH, LH and progesterone plasma profiles were determined by RIA. Blood was sampled at 6-hour intervals from parturition to 40–78 days postpartum, and the results correlated with the commencement of cyclic functions. For FSH, generally basic values were recorded, without characteristic features associating any values with the onset of cyclic ovarian activities or the occurrence of the first heat. LH profiles varied greatly between individuals with regard to the onset of elevations, regularity of patterns and peak values. The first preovulatory LH peak was recorded 17.3±9.8 days (range 4–46) postpartum. The first heat occurred on day 28.4±16 (range 6–55) postpartum, indicating that 13/23 cows ovulated without behavioral estrus, as reproductive cycles were re-established. Peak LH values increased with progressive cycles (1st peak 5.7±4.8 ng/ml; 2nd peak 11.8±8.7 ng/ml; 3rd peak 13.5±9.9 ng/ml plasma). Progesterone values also showed great variations in the profile of their first postpartum elevation. In 13/25 cows the first cycle was shortened (13.1±2.9 days), prolonged in 3 animals (34±4 days) and normal in 7 cows (20.4±1.9 days). Heat, preovulatory LH peak and progesterone profile were normal in all animals on subsequent cycles. Two animals did not start cycling.     

GnRH induced LH release in suckled beef cows. II. The effects of exogenous corticoids and estradiol benzoate on luteinizing hormone release by GnRH

In Experiment 1, 24 suckled beef cows were assigned to 4 treatment groups (6 cows/group). Group I cows calved spontaneously. Parturition was induced in Groups 2, 3 and 4 with 20 mg dexamethasone (DEX) 8 to 12 days prior to expected calving date. Additionally, cows in Groups 3 and 4 received 8 mg triamcinalone acetonide (TA) 6 days prior to DEX treatment. Animals in Group 4 also received 10 mg estradiol benzoate (EB) with TA, and on alternate days until DEX, when 20 mg EB was given. Gonadotropin releasing hormone (GnRH, 100 mug) was given intramuscular (IM) to all cows on days 2 or 3 postpartum. Plasma LH increased (P< .05) following GnRH treatment in Groups 2, 3 and 4, but not in Group 1. LH release (area under the curve) following GnRH was greater (P< .05) for cows in Group 4 compared to cows in Groups 1, 2 or 3, and differences in LH release between Groups 1, 2 or 3 were not significant. In Experiment II, 36 mature Hereford cows were assigned to a 2 x 3 factorial experiment (6 cows/group). Groups 1 and 2, 3 and 5, and 4 and 6 received 0, 100, or 200 mug GnRH (IM) at 78 hr postpartum, respectively. In addition, cows in Groups 2, 5 and 6 received 5 mg EB at 36 hr postpartum. Plasma LH concentrations were not different (P <.05) among groups from 36 to 78 hr postpartum. A surge of LH in response to EB treatment was not detected at 54 to 62 hr (18 to 26 hr post EB), indicating a lack of response by the positive feedback mechanism at this early time postpartum. Mean plasma LH concentrations were elevated 78 to 82 hr postpartum for Groups 3 through 6. Treatment with EB at 36 hr caused a significantly greater (P< .05) response to GnRH with 200 mug of GnRH releasing more LH than 100 mug of GnRH.     

Effect of variable suckling intensity and estrogen administration upon serum luteinizing hormone in Brahman cows

Ten mature Brahman cows were randomly allotted within calving intervals to either a suckled (S) or nonsuckled (NS) treatment group. All cows received a 20 mg intramuscular injection of estradiol-17beta (E2), suspended in 2 ml of corn oil, to determine the effect of suckling on the estrogen induced LH surge. Starting on day 21 postpartum the S cows were suckled at six hour intervals for 24 hours, at which time they were challenged with a 20 mg E2 injection. The suckling regimen was continued for 48 hours postinjection. The NS cows were separated from their calves on day 21 postpartum and received no suckling stimulus for 72 hours. At 24 hours after calf separation, the NS cows were challenged with a 20 mg E2 injection. Blood samples were removed at two hour intervals beginning 10 hours post E2 injection until 36 hours postinjection, at which time blood samples were removed at four hour intervals until 48 hours postinjection. Blood samples were processed to yield serum and assayed for luteinizing hormone (LH) via radioimmunoassay. The injection of a 20 mg dose of E2 induced an LH surge in all cows. The NS cows were found to exhibit a longer (P<.05) duration of the estrogen induced LH surge than the S cows, 15.6 +/- .98 and 12.4 +/- .75 hours, respectively. The timing parameters (time to start of LH surge, time to peak LH value and time to end of surge) and LH concentration parameters (LH concentration at start of LH surge, peak value of LH surge and LH concentration at end of LH surge) were not different between suckling regimens. No blockage of the LH response to estrogen challenge was found on day 22 postpartum. Suckling did depress the duration of the LH surge indicating some blockage due to suckling stimuli.     

The effect of eCG or estradiol at or after norgestomet removal on follicular dynamics, estrus and ovulation in early post-partum beef cows nursing calves

In post-partum anestrous beef cows suckling calves, neither the choice of hormonal regime to ensure the presence of a healthy dominant follicle at the end of a progestagen treatment nor the optimum hormone to induce estrus and ovulation is clear. Twenty-eight beef cows, in good body condition, 25-30 days post-partum, were assigned to one of four treatments: (i) 3mg norgestomet (N) implant with 5mg estradiol valerate (EDV) and 3mg N injection at the time of insertion (Crestar) for 5 days followed by 600 IU eCG at the time of implant removal; (ii) Crestar for 5 days as in (i) followed by 0.75 mg estradiol benzoate (EDB) 24h later; (iii) Crestar for 9 days followed by 600 IU eCG at the time of implant removal; and (iv) Crestar for 9 days followed by 0.75 mg EDB 24h later. Ovarian scanning was preformed from 4 days before implant insertion until ovulation and 4 days postovulation to detect the CL. Daily blood samples were collected from day 20 post-partum until second ovulation for FSH and E(2) assay. Data were analyzed using analysis of variance. There was no effect of the stage of follicle wave at the time of implant insertion on interval to new follicle wave emergence (range 1-7 days; mean 4.7 days). FSH concentrations were decreased to 5.9+/-2.0 and 7.7+/-1.1 ng/ml for pre- and post-selection cows 1 day after start of treatment; thereafter, they increased on Day 2 to 7.9+/-2.0 and 11.0+/-1.1 ng/ml and on Day 3 to 10.3+/-2.7 and 11.4+/-1.7 ng/ml for pre- and post-selection cows, respectively, despite high-estradiol concentrations at that time. There was no effect of treatment on the interval from implant removal to ovulation (3.2-4.0 days) or on the number of cows detected in estrus (26 of 27 cows). The size of the ovulatory follicle in cows given 0.75 mg EDB 24h post implant removal was decreased in animals at the pre-selection stage (12.2+/-0.1mm) of the follicle wave compared with those at the post-selection stage (15.3+/-0.9 mm) at implant removal. Cows given 600 IU eCG at the pre-selection phase of follicular growth had multiple ovulations (4.0+/-1.1). Cows given EDV at the start of a 5-day implant period had higher estradiol concentrations before and on the day of implant removal than those given EDV at the start of a 9-day implant period. The injection of 0.75 mg EDB 1 day after implant removal tended to increase concentrations of estradiol one day later. In conclusion, 5mg EDV and 3mg N at insertion of a 3mg N implant resulted in variable new follicle wave emergence 1-7 days later in post-partum beef cows nursing calves (22 of 27); both eCG and EDB were equally effective at inducing estrus after implant removal in cows in good BCS, but eCG resulted in a significant increase in ovulation rate in cows treated before dominant follicle selection.