Serum luteinizing hormone levels in Brangus cows following variable suckling intensity and administration of various levels of estrogen

Fifty Brangus cows were randomly allotted to suckled (S) or nonsuckled (NS) treatment groups on day 20 postpartum. Suckled cows were nursed at 6 hr intervals for 72 hours. Nonsuckled cows were separated from their calves for the entire 72 hours. At 24 hr after initial separation from calves, S and NS cows were given an I.M. challenge of 0, 0.5, 1.0, 2.0 or 4.0 mg estradiol-17beta (E2) to induce a luteinizing hormone (LH) surge (five cows per treatment group). Blood samples were taken at the time of E2 injection and at 2 hr intervals until hr 48 post-injection. Blood serum was analyzed for LH content via radioimmunoassay. Suckled and NS cows manifesting an LH surge after receiving less than 4 mg E2 were 2 of 15 vs 9 of 15 (P<.01), or 4 mg E2 dose were 5 of 5 vs 5 of 5, respectively. Greater serum LH concentrations in NS than S cows were found with dose levels of 0, 0.5 and 1.0 mg E2 (P<.005), but there was no difference by period. Differences by treatment (P<.05) and by period (P<.005) were found at the 2 mg E2 dose. Suckled and NS cows having an LH surge at less than a 4 mg E2 challenge had no differences in LH concentration or timing parameters. Four mg E2 hastened the time of onset of the LH surge (P<.025), time till peak height of the surge (P<.025) and completion of the surge (P<.10). No differences in postpartum interval or conception rate were found between S and nonsuckled. Suckling impairs hypothalamic/pituitary response to low E2 challenge dose and elicits changes in timing parameters in response to high E2 dosage.     

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.     

Effect of bleeding stress and variable suckling intensity upon serum luteinizing hormone in Brangus heifers

In the first experiment, the effect of the stress of blood collection (via tail vessel puncture) on serum luteinizing hormone (LH) was evaluated in six nonsuckled first calf Brangus heifers. The animals were bled on days 22 and 31 postpartum at 15 minute intervals for a period of two hours. Blood was processed to yield serum and analyzed for LH via radioimmunoassay (RIA). There were no significant differences or fluctuations in serum LH levels between bleeding periods or between cows. Serum LH concentrations in nonsuckled cows were not affected by the stress of blood collection. In the second experiment, 24 first calf Brangus heifers were randomly assigned to one of four treatment groups. Treatment 1 cows were suckled once daily for approximately 30 min starting day 21 postpartum. Treatment 2 cows were suckled twice daily for approximately 30 min each time, starting 21 days postpartum. Treatment 3 cows were suckled once daily for approximately 30 min starting 30 days postpartum. Treatment 4 cows were suckled twice daily for approximately 30 min each time starting 30 days postpartum. Each cow was bled via tail vessel puncture on days one and nine following the start of each treatment. The blood sampling regime was similar to that used in Experiment 1 and consisted of four presuckling samples taken at 15 min intervals, one midsuckling sample (the calf was allowed to suckle for 15 min) and four postsuckling samples taken at 15 min intervals. Blood was collected, processed to yield serum and assayed for LH via RIA. Suckling intensity (SI) was found to have a significant effect on serum LH levels. The once daily suckled cows had higher (P<.01) mean serum LH levels than did the twice daily suckled cows (1.70 +/- .03 and 1.53 +/- .03 ng/ml, respectively). The LH concentrations decreased (P<.01) from the first to last bleeding time (BT). The mean serum LH levels for the presuckling, midsuckling and the first postsuckling samples were higher (P<.05) than the last postsuckling sample. The mean serum LH level for the first time period prior to suckling was higher (P<.05) than the last postsuckling sample. The mean serum LH level for the first time period prior to suckling was higher (P<.05) than the last two periods after suckling (1.73 +/- .08 ng/ml vs 1.51 +/- .06 and 1.41 +/- .06 ng/ml). Bleeding day (BD) and weaning day (WD) did not alter serum LH levels. The interactions found to be significant (P<.01) were SIxBD, SIxWD, BDxWD and BTxSIxBDxWD.     

Effect of suckling on the hypothalamic-pituitary axis in postpartum beef cows, independent of ovarian secretions

Thirty-two postpartum (PP) cows were used to investigate the effect of suckling on secretion of luteinizing hormone (LH). Calves remained with their dams (suckled; S), or they were removed within 24 h of birth (nonsuckled; NS). To evaluate the relationship between suckling and negative feedback regulation of LH, cows were ovariectomized on Day 5 PP, then injected intravenously with estradiol-17 beta (E) or vehicle (V) on Day 10 PP. To investigate the influence of suckling on the gonadotropin-releasing hormone (GnRH)-induced release of LH, cows were injected with 80 micrograms of GnRH on a single day varying from 18 to 85 days PP. Suckling inhibited the postcastration rise in LH, as LH concentrations increased at a faster rate in NS compared with S cows [0.031 +/- 0.02 ng/(ml X day) LH: P less than 0.05]; this was not influenced by basal amounts of E since amounts did not differ between S and NS cows at ovariectomy (5.37 +/- 0.36 vs. 5.34 +/- 0.48 pg/ml E; P greater than 0.05). Serum concentrations of LH were negatively related to total follicular E only in S cows (r = -0.71; P less than 0.01). Estradiol-17 beta caused a decrease not only in the level but also the variability in LH concentrations in both S and NS cows: LH in S cows was less variable after E than in NS cows (P less than 0.001), but the magnitude of LH suppression was not influenced by suckling (P greater than 0.25). The regression of LH response on days PP was essentially the same over time for both S (P greater than 0.25) and NS (P greater than 0.25) cows, indicating that LH response to a GnRH injection was not influenced by suckling or days PP. Suckled cows had a tendency to release more LH relative to their baseline in response to GnRH as time PP increased (P less than 0.10), but NS cows did not. These results indicate that even though ovarian secretions inhibit LH release from the pituitary, other inhibitory influences may have a major effect in S cows. Concentrations of LH were lower in S cows than NS cows on Day 10 PP, following removal of the ovaries on Day 5, suggesting that suckling had a direct effect on the hypothalamic-pituitary axis.     

Effect of monensin and suckling on the GnRH induced luteinizing hormone surge and the effect of monensin on the postpartum interval in Brangus cows

Twenty-seven fall calving Brangus cows were randomly allotted to one of four treatment groups: nonsuckled monensin (NSM), suckled monensin (SM), nonsuckled control (NSC), and suckled control (SC). Cows were group fed 1.82 kg/hd/day concentrate and Coastal bermuda grass hay $\text{ad}$$\text{libitum}$. Monensin cows received 200 mg monensin/hd/day in the concentrate. At 0800 hr on day 21 postcalving, the calves were separated from the cows. Suckled monensin and SC cows were allowed to suckle their calves for 30 min at 6-hr intervals. Nonsuckled monensin and NSC cows were not suckled. Calves were given free access to the cows after 1400 hr on day 22 postpartum. At 0800 hr on day 22 postpartum, a blood sample was collected. A 100 μg GnRH challenge was administered IM at 0801 hr. Blood samples were collected at 15-min intervals for 6 hr postinjection. Changes in body weight and body condition from day 21 postpartum to the day of first estrus were not different (P>0.10) by dietary treatment. Monensin cows consumed 10.7% less hay than did the control cows. Serum luteinizing hormone (LH) following GnRH was greater (P<0.005) in suckled than nonsuckled cows. Control cows released more (P<0.005) LH in response to GnRH than did the monensin cows. The postpartum interval (to first estrus) for the monensin cows (92.4±14.7 days) was shorter (P<0.025) than the controls (138.5±9.5 days). A greater proportion (P<0.005) of the monensin cows (8 of 14) exhibited estrus by 90 days postpartum compared to the control cows (0 of 13). Monensin and suckling appear to exert independent and agonistic influences on pituitary function in the postpartum beef cow.     

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.     

Reproductive studies of Brahman cattle IV. Luteinizing hormone levels in ovariectomized Brahman, Brahman × Hereford and Hereford cows following a 20 mg dose of Estradiol-17β

Luteinizing Hormone (LH) levels were quantitated by radioimmunoassay (RIA) in six mature, long-term ovariectomized cows each of Brahman (B), Brahman × Hereford (B×H) and Hereford (H) breeding following an in-tramuscular injection of 20 mg of Estradiol-17_β (E) suspended in corn oil. After E administration all cows were bled via coccygeal venipuncture every two hours from 0–8 hours post-injection, every hour from 9–24 hours post-injection, concluding with bleedings every two hours from 26–36 hours post-injection. An LH surge was observed in 5/6 B cows, 6/6 B×H cows and 6/6 H cows. Basal LH levels (mean of first eight data points of each breed type) did not differ (P>.10) between B (3.5 ng/ml), B×H (2.4 ng/ml) and H (2.4 ng/ml). Elapsed time from E injection to peak LH value varied significantly (P<.05) between B, B×H and H, respectively (27.8 hrs, 23.8 hrs, 22.2 hrs). Peak LH values also varied between breed (B, 20.2 ng/ml; B×H, 36.0 ng/ml; H, 113.2 ng/ml: P<.005). The area under the LH curve differed significantly between B, B×H and H (P<.05), however, the duration of the LH surge was not different between breeds; B (13.2 hrs), B×H (16.2 hrs) and H (15.3 hrs). Overall significant period effects (P<.05), breed effects (P<.10) and period × breed interactions (P<.05) were found. In summary, B are less reactive to a 20 mg dose of E than are B×H or H using the following criteria: time to peak LH value, peak LH value and area under the LH curve. These data strongly indicate inherent differences between breeds regarding estrogen feedback mechanisms at the hypophysial-hypothalamic axis.     

Effects of suckling and of a long interval after ovariectomy on hypothalamo-hypophyseal responsiveness to naloxone, morphine and GnRH in beef cows

The response of serum luteinizing hormone (LH) to morphine, naloxone and gonadotropin-releasing hormone (GnRH) in ovariectomized, suckled (n=4) and nonsuckled (n=3) cows was investigated. Six months after ovariectomy and calf removal, the cows were challenged with 1mg, i.v. naloxone/kg body weight and 1 mg i.v. morphine/kg body weight in a crossover design; blood was collected at 15-minute intervals for 7 hours over a 3-day period. To evaluate LH secretion and pituitary responsiveness, 5 mug of GnRH were administered at Hour 6 on Day 1. On Days 2 and 3, naloxone or morphine was administered at Hour 3, followed by GnRH (5 mug/animal) at Hour 6. Mean preinjection LH concentrations (3.6 +/- 0.2 and 4.7 +/- 0.2 ng/ml), LH pulse frequency (0.6 +/- 0.1 and 0.8 +/- 0.1 pulses/hour) and LH pulse amplitude (2.9 +/- 0.5 and 2.9 +/- 0.6 ng/ml) were similar for suckled and nonsuckled cows, respectively. Morphine decreased (P < 0.01) mean serum LH concentrations (pretreatment 4.2 +/- 0.2 vs post-treatment 2.2 +/- 0.2 ng/ml) in both suckled and nonsuckled cows; however, mean serum LH concentrations remained unchanged after naloxone. Nonsuckled cows had a greater (P < 0.001) LH response to GnRH than did suckled cows (area of response curve: 1004 +/- 92 vs 434 +/- 75 arbitrary units). We suggest that opioid receptors are functionally linked to the GnRH secretory system in suckled and nonsuckled cows that had been ovariectomized for a long period of time. However, gonadotropin secretion appears not to be regulated by opioid mechanisms, and suckling inhibits pituitary responsiveness to GnRH in this model.     

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.     

Pituitary luteinizing hormone-releasing hormone receptors in ovariectomized cows after challenge with ovarian steroids

Acute changes of bovine pituitary luteinizing hormone-releasing hormone (LHRH) receptors in response to steroid challenges have not been documented. To investigate these changes 96 ovariectomized (OVX) cows were randomly allotted to one of the following treatments: 1) 1 mg estriol (E3); 2) 1 mg 17 beta-estradiol (E2); or 3) 25 mg progesterone (P) twice daily for 7 days before 1 mg E2 and continuing to the end of the experiment. Serum was collected at hourly intervals from 4 animals in each group for 28 h following estrogen treatment. Four animals from each treatment were killed at 4-h intervals from 0 to 28 h after estrogen injection to recover pituitaries and hypothalami. Treatment with E3 or E2 decreased serum luteinizing hormone (LH) within 3 h and was followed by surges of LH that were temporally and quantitatively similar (P greater than 0.05). Progesterone did not block the decline in serum LH, but did prevent (P less than 0.05) the E2-induced surge of LH. Serum follicle-stimulating hormone (FSH) was unaffected (P less than 0.05) by treatment. Pituitary concentrations of LH and FSH were maximal (P less than 0.001) at 16 h for E3 and 20 h for E2, whereas P prevented (P greater than 0.05) the pituitary gonadotropin increase. Concentrations of LHRH in the hypothalamus were similar (P greater than 0.05) among treatments. Pituitary concentrations of receptors for LHRH were maximal (P less than 0.005) 12 h after estrogen injection (approximately 8 h before the LH surge), even in the presence of P. This study demonstrated that in the OVX cow: 1) E2 and E3 increased the concentration of receptors for LHRH and this increase occurred before the surge of LH; and 2) P did not block the E2-induced increase in pituitary receptors for LHRH but did prevent the surge of LH.     

Effect of suckling on cortisol, progesterone and luteinizing hormone in postpartum beef cows

Five primiparous, 3-year-old Hereford cows suckled ad libitum , were cannulated via the jugular vein and stanchioned for 2-day sampling periods, every 14 days starting 14 days after the mean calving date. On the second day of each period, calves were removed to a pen away from the cows, for 9 hours. Blood was sampled 5 min before calves were returned to their dams, as soon as possible after initiation of suckling (IOS), and at 15-min intervals for 45 min, thereafter. Cortisol, progesterone and luteinizing hormone (LH) concentrations in the serum were quantitated by radioimmunoassay. Mean serum cortisol concentrations were 7.3 +/- .7, 9.4 +/- .7, 12.1 +/- .9, 7.5 +/- .5 and 5.7 +/- .4 ng/ml (mean +/- S.E.) at -5, 0, 15, 30 and 45 min after IOS, respectively, for all cows across all periods. Cortisol concentrations, during and after suckling, tended (P<.06) to differ among sampling periods, during the postpartum interval. Serum progesterone concentrations were .28 +/- .02, .28 +/- .02, .32 +/- .05 and .24 +/- .03 ng/ml at 0, 15, 30 and 45 min after IOS, respectively, for all cows across all period, indicating that suckling had no effect on serum progesterone, and were similar at all sampling periods during the postpartum interval. Serum LH concentrations were .81 +/- .07, .77 +/- .06, .71 +/- .04, and .72 +/- .04 ng/ml at 0, 15, 30 and 45 min after IOS, respectively. During the postpartum interval, serum LH concentrations were greater (P<.01) at 71 and 85 days postpartum than at any other time.     

Reproductive studies of Brahman cattle V. The effect of various dose levels of estradiol-17β upon serum luteinizing hormone in ovariectomized Brahman cows

Twelve mature chronically-ovariectomized Brahman cows were randomly assigned to receive three of six different dosages of estradiol-17_b (E2) at three different time periods such that at the termination of the trial six animals received each E2 dosage. The E2 was suspended in 0, 1, 2.5, 5, 10 and 20 milligrams. A two week period was maintained between injections. The cows were bled via coccygeal vessel puncture immediately before E2 injection, every 2 hr from 0 to 18 hr, every hr from 18 to 42 hr and every 2 hr from 42 to 48 hr postinjection. Blood was processed to yield serum and stored at ?20 Celsius. Serum luteinizing hormone (LH) was quantitated by validated radioimmunoassay. An LH surge was defined as a sustained elevation of LH at least two standard deviations above the level of LH prior to the rise and was observed in $\text{0}\text{6}$, $\text{3}\text{6}$, $\text{5}\text{6}$, $\text{5}\text{6}$, $\text{5}\text{6}$, and $\text{6}\text{6}$ cows administered 0, 1, 2.5, 5, 10, and 20 mg of E2, respectively. All animals injected with E2 responded with a significant initial decrease (independent of E2 dosage) in LH that persisted from 2 through 12 hr post E2 injection. No significant decrease in LH levels was recorded in animals injected with the corn oil vehicle. The time to the LH surge differed (P<.05) between 1 mg E2 (10 hr) vs 20 mg E2 (19.5 hr), 1 mg E2 vs 10 mg E2 (16.2 hr), and 2.5 mg E2 (12.4 hr) vs 20 mg estradiol-17_β. Luteinizing hormone concentrations at the onset of the surge did not differ (P>.10) between E2 dosages. The elapsed time from E2 injection to the peak LH value differed (P<.05) between 1 mg E2 (20.3 hr) vs 20 mg E2 (26.8 hr), and 2.5 mg E2 (21.2 hr) vs 20 mg estradiol-17_β. The peak LH value, the area under the LH curve and the duration of the LH surge did not differ (P>.10) with E2 dosage. The time to the end of the LH surge differed (P<.05) between 1 mg E2 (25.3 hr) vs 2.5 mg E2 (31.6 hr), 1 mg E2 vs 5 mg (34.4 hr), 1 mg E2 vs 10 mg E2 (34.8 hr), 1 mg E2 vs 20 mg E2 (37.3 hr), and 2.5 mg E2 vs 20 mg estradiol-17_β. Luteinizing hormone values at the termination of the surge did not differ (P>.10) between dosages nor did the LH values at the termination of the surge differ (P>.10) from LH concentrations observed at the onset of the LH surge.     

Effect of days postpartum and exogenous GnRH on reproductive hormone and ovarian changes in postpartum suckled beef cows

Two experiments were conducted to determine the effect of days postpartum and exogenous gonadotropin releasing hormone (GnRH) on reproductive hormone and ovarian changes in postpartum suckled beef cows. In experiment 1, eight suckled cows were bled at .5 hour intervals for 4 hours on days 7, 14, 21 and 28 postpartum. Although mean concentrations of plasma luteinizing hormone (LH) were positively correlated with days postpartum, mean concentrations did not differ. The mean maximum change and the variance of plasma LH were low on days 7, 14, 21 and 28 postpartum. Although the number of cows with an ovarian follicle and follicular size increased with days postpartum, mean concentrations of estradiol-17beta did not change. The interval from parturition to the first detected ovarian follicle and the first postpartum estrus was 17.5 +/- 2.6 days and 36.0 +/- 2.2 days, respectively. An elevation in plasma progesterone was detected about one week prior to the first postpartum estrus in 6 of the eight cows in the absence of corpora lutea. In experiment 2, gonadotropin releasing hormone (GnRH) induced ovulation in 4 of the 8 cows treated on day 27, 28 or 29 postpartum whereas none of the 8 saline treated cows ovulated to treatment. The interval from parturition to first estrus and conception were similar for both groups (P >.10).     

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.     

Pituitary and ovarian function in postpartum beef cows. I. Effect of suckling on serum and follicular fluid hormones and follicular gonadotropin receptors

The effect of suckling on serum and follicular fluid hormones and on follicular gonadotropin receptors was studied. Sixteen anestrous postpartum cows were assigned to 1 of 2 groups: suckled (S) or weaned (W). All calves were allowed to suckle ad libitum from parturition to 21 days postpartum when calves from W cows were weaned. All cows were ovariectomized on Day 25 postpartum. W cows had more (P less than 0.01) pulses of LH during the 96-h period from weaning until ovariectomy than S cows (6.3 vs. 1.3 pulses). Serum concentrations of prolactin (Prl), estrone (E1), estradiol-17 beta (E2) and progesterone (P) were not different (P greater than 0.10) between groups. Furthermore, there were n differences (P greater than 0.10) in follicular in contents of luteinizing hormone (LH), E1, E2 and P between the treatment groups. However, follicular fluid content of Prl was greater (P less than 0.05) in the W cows than in the S cows (123 vs. 65.1 ng/cow). The number of follicular LH receptors was greater (P less than 0.05) in the W cows than in the S cows (71.1 vs. 48.3 fmoles/mg protein) although the number of follicular follicle-stimulating hormone (FSH) receptors was not different (P greater than 0.10) between W cows and S cows (1531 vs. 1862 fmoles/mg protein). There were no correlation between serum hormone concentrations and follicular fluid hormone content; however, the numbers of follicular LH receptors and follicular fluid Prl content were highly correlated in the W cows (r = 0.85; P less than 0.05). It is concluded that removal of the suckling stimulus increases pulsatile LH release and the accumulation of Prl in the follicular fluid. These factors, either together or separately, may at least in part be responsible for the increase in follicular LH receptor concentrations that were observed in the W cows.     

The effect of estradiol-17beta and estrone administration on GnRH-induced LH release during the early postpartum in dairy cattle

The effect of high plasma concentrations of estradiol-17beta or estrone, similar to those observed in late gestation, on the gonadotropin releasing hormone (GnRH)-induced luteinizing hormone (LH) release was studied in early postpartum dairy cows. Twenty dairy cows in late gestation were assigned to four groups of five cows each. Treatment groups were 1) no exogenous estrogens, 2) 20 mg estradiol-17beta (E(2)beta) daily, 3) 30 mg estrone (E(1)) daily and 4) 20 mg E(2)beta and 30 mg E(1) daily. Steroids were dissolved in ethanol (vehicle). Injections of the vehicle or steroids were given in two daily subcutaneous injections for seven consecutive days starting immediately following parturition. All cows (Groups 1-4) were given 100 mug GnRH intramuscularly on days 2, 10, 18 and 26 postpartum. Blood for plasma determination of E(2)beta, E(1), progesterone (P) and LH was collected daily from parturition to completion of vehicle or steroid injection and on alternate days thereafter. In addition, blood was collected on GnRH treatment days prior to GnRH and at 30-min intervals thereafter for four hours. Concentrations of hormones were determined by validated radioimmunoassays (RIA's). Effects of treatment (T), days postpartum (D) and the interaction between T and D (T x D) on the amount of LH released (area under the curve) in response to GnRH were significant (P < 0.01). More LH was released over all days combined in Group 1 compared to the other groups. LH release to GnRH increased as time postpartum increased in Groups 1 and 3, but at a ratelower for Group 3 than Group 1 (P < 0.05). In contrast, LH release to GnRH was greater (P < 0.05) on day 2 postpartum for Groups 2 and 4 compared to Groups 1 and 3, but less on days 10 and 18 postpartum. Average LH release was less (P < 0.05) on day 10 for Groups 2 and 4 than for day 2 postpartum. By day 26 postpartum, however, LH release in Groups 2 and 4 was greater than in Group 3. In summary, E(2)beta appeared to stimulate LH release early postpartum with a subsequent inhibition of LH release after prolonged E(2)beta administration, and E(1) administration did not stimulate LH release early postpartum.     

Mechanical masking of neurosensory pathways at the calf-teat interface: endocrine, reproductive and lactational features of the suckled anestrous cow

A mammary somatosensory mask was employed in suckled anestrous beef cows to attenuate signals that were hypothesized to play a direct regulatory role in postpartum anestrus. Cows (n = 20) were randomly assigned to one of three treatment groups on Days 15 to 20 postcalving. The three treatments were: 1) masked (n = 7); 2) suckled (negative control, n = 6); and 3) weaned (positive control, n = 7). Four layers of surgical glove latex were used to cover the teats and ventro-lateral prominence of the udder of masked cows with a nonhardening, nontoxic adhesive (Day 0). Masks were designed to prevent direct contact between the skin of the teat/udder and the mouth of the calf and to allow normal suckling and milk removal. Masks were left in place for 7 d, with calves in the weaned group removed to a remote location for 7 d. Calves in the suckled group were allowed ad libitum suckling. Calves in the masked group tended (P < 0.1) to suckle longer than calves in the suckled control group (11.3 +/- 1.3 vs. 7.8 +/- 1.3 min/suckle) posttreatment; however, suckling frequency and calf weight gains did not differ due to treatment. Weaned cows exhibited a four-fold increase (P < 0.01) in the frequency of luteinizing hormone (LH) pulses on Day 2 relative to suckled and masked cows. The percentage of animals ovulating within 12 d after treatment differed (P < 0.05) and was 100, 50 and 0% for weaned, suckled and masked cows, respectively. Presence of the latex mask allowed essentially normal suckling and lactation, but failed to attenuate (and may have potentiated) the negative effects of suckling on secretory patterns of LH, ovulation and estrus.     

Effect of suckling on basal and GnRH-induced LH release in post-partum dairy buffaloes

Suckling, a common practice in smallholder dairy-farming systems in the developing world, delays the onset of post-partum ovarian activity in dairy buffalo. The present study was designed to assess the effect of suckling on pituitary function in lactating buffaloes 25-35 days post-partum. Six suckled and nine non-suckled buffaloes were challenged intravenously with a bolus injection of GnRH (20microg buserelin acetate; Receptal). Heparinized venous blood samples were collected at 15min intervals for 2h before and up to 4h after GnRH for luteinizing hormone (LH) estimation. Pretreatment basal LH concentrations were similar in the suckled (0.6+/-0.2ng/ml) and the non-suckled (0.5+/-0.1ng/ml) buffaloes. All but one suckled buffaloes released a LH surge, starting 15-60min post-GnRH treatment, which lasted for 180-225min. While one suckled buffalo did not respond to GnRH, the LH response in the remaining suckled buffaloes was significantly less than in the non-suckled buffaloes in terms of peak LH concentrations (14.3+/-2.7ng/ml versus 26.2+/-4.3ng/ml) and area under the LH curve (1575.6+/-197.4mm(2) versus 2108.9+/-323.9mm(2)). The LH response was least in suckled buffaloes challenged with GnRH while in the luteal phase of an oestrus cycle and with plasma progesterone concentration >1ng/ml. In conclusion, suckling suppressed pituitary responsiveness to exogenous GnRH challenge in post-partum buffaloes.     

Acute effect of suckling on gonadotropin, prolactin and glucocorticoid concentrations in serum of intact and ovariectomized beef cows

Suckling may prolong the anovulatory period postpartum by 1) a neural-mediated inhibition of luteinizing hormone-releasing hormone (LHRH)-induced gonadotropin secretion, or 2) an inhibitory effect of hormones released by suckling on gonadotropin secretion and/or action at the ovary. In the present investigation we considered whether a suckling event caused 1) acute inhibition of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion, and 2) release of glucocorticoids and/or prolactin (PRL). Six Hereford cows remained intact and six were ovariectomized (ovx) on day 7 postpartum. Calves remained with their dams continuously. Cows were bled at 10-min intervals during 6 consecutive hr on days 14, 28 and 42 postpartum. Both LH and FSH were released episodically by day 14 in intact and ovx cows, but suckling did not acutely affect LH and FSH secretion. A PRL release accompanied suckling 67, 96 and 95% of the time. However, among all instances where PRL was released on days 14, 28 and 42 postpartum, 67, 29 and 37% occurred independent of a suckling event. Glucocorticoids were not released by suckling in intact cows but were released in ovx cows. We conclude that suckling does not acutely affect LH or FSH concentrations in serum of cows postpartum, that PRL concentrations usually increase in serum coincident with suckling but can be released at other times, and suckling-induced glucocorticoid release depends upon the presence of the ovary.     

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.