Postpartum Estrous Cycles Research Articles

Follicular growth and estradiol concentrations in foaling and nonparturient mares

Summary Plasma estradiol concentration and follicular development were evaluated daily during the first postpartum estrus and the subsequent cycle of five foaling mares. For comparison, one estrous cycle was monitored in the same fashion for five nonparturient mares. The first postpartum estrous cycles were shorter but similar to non-pregnant cycles in ovarian steroid production and follicular activity. However, estradiol production from postpartum follicles was lower per mm follicular diameter than from follicles in nonpregnant cycles (p

Interrelationships of lactation and postpartum anovulation in suckled and milked cows

Abstract Reproduction is a major factor limiting efficiency of beef and dairy cattle production. Onset of lactation and reestablishment of postpartum estrous cycles are energy-competing processes, with lactation having a greater priority for dietary nutrients and body reserves through homeorhetic controls. Prolonged postpartum anestrus or anovulation limits reproductive efficiency by delaying or preventing conception. Suckling or milking frequency (> 4 × daily) delays first ovulation longer than 2 × milking. However, mere presence of a calf (own or foster) to which the cow (udder-intact or mastectomized) is bonded delays onset of postpartum ovulation. Furthermore, anovulation was prolonged when cows maintained a bond with their own calves while their milk is harvested by an unrelated suckling calf, but not when removed by 2 × milking. Although continuous presence of a nonsuckling calf prolongs anovulation only slightly, yield and composition of milk for cows milked 2 × daily are greater due to increased synthesis or ejection of milk associated with calf presence. When lactation is interrupted 13 to 18 days after calving and normal suckling, cows ovulated during the next 4 weeks while neither milked nor suckled. When these cows were reunited with their calves after 4 weeks to provide ad libitum suckling, milk secretion was reinitiated successfully. Suckling plus milking increases milk yield beyond milking alone at similar harvest frequencies. Greater milk yield in milked cows, associated with increased energy deficits and increased milking frequency, inhibits estrual expression to a greater extent than the onset of postpartum ovulation. Energy balance is the key to the timing of first postpartum ovulation in milked cows. Acute, dynamic changes in energy balance during early lactation or at weaning, rather than absolute magnitude of energy balance, provide the cow information about its metabolic status. Cows respond to positive changes in energy balance by resuming ovarian cycles during energy deficiency. Ovarian follicular development and resumption of follicular waves in suckled and milked cows do not limit early ovulation but rather the failure of dominant follicles to ovulate in the face of inadequate LH pulse frequency. Further work is needed to elucidate which metabolic signals serve as effectors of increased pulsatile LH secretion and their pivotal role in reinitiating first postpartum follicular maturation and ovulation in cattle.

An increase in serum lipids increases luteal lipid content and alters the disappearance rate of progesterone in cows.

To determine whether an increase in serum lipids alters the area occupied by lipid droplets in steroidogenic luteal cells and(or) clearance rates of progesterone from serum, pregnant beef heifers received control (n = 6) or treatment (n = 5) diets. To increase serum lipids, the treatment diet contained calcium soaps of fatty acids. Control and treatment diets were formulated to be isocaloric and isonitrogenous. Feeding of diets was initiated approximately 100 d before parturition and continued through the third postpartum estrous cycle. On d 12 or 13 of the third postpartum cycle, corpora lutea were collected by ovariectomy and a center slice was processed for electron microscopy. Eight samples from each slice were sectioned, stained, and examined at a magnification of 2,500x. Five micrographs per sample were analyzed for area occupied by small (SLC) and large (LLC) luteal cells, percentage of the area of each steroidogenic cell type occupied by lipid, and total steroidogenic area (SLC + LLC) occupied by lipid. Jugular blood was collected before and after ovariectomy, and progesterone, cholesterol, high-density lipoprotein (HDL), and low-density lipoprotein (LDL) were quantified. Cows consuming treatment diets had approximately twice (P < .05) the concentration of cholesterol, HDL, and progesterone in serum that controls had. The percentage of the area of SLC, LLC, and total area occupied by lipid was greater (P < .05) in treated than in control cows. The average time required for serum concentrations of progesterone to decrease by 50% after ovariectomy was greater (P < .05) in treated than in control cows (170 +/- 16 vs 113 +/- 15 min).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of intrauterine bacterial infusions and subsequent endometritis on prostaglandin F2 alpha metabolite concentrations in postpartum beef cows.

Multiparous Angus and crossbred Angus cows were used to determine the effect of induced endometritis on plasma concentrations of 13,14-dihydro-15-keto-prostaglandin F2 alpha (PGFM) and progesterone (P4) and on duration of the estrous cycle of treatment. Beginning on the day of calving (d 0), blood samples were collected on alternate days. On three consecutive days, ranging from d 8 to 14 of the first postpartum estrous cycle, uterine horns were inoculated transcervically with either 3 x 10(9) colony forming units (cfu) of Actinomyces pyogenes and 1.5 x 10(9) cfu of beta-hemolytic Escherichia coli (treated; n = 9) in sterile PBS or with sterile PBS alone (control; n = 9). Samples of uterine fluid were collected by transcervical aspiration twice weekly from just before the start of each series of inoculations until the end of the experiment. Endometrial biopsies were collected transcervically between d 4 to 6 and 11 to 13 after inoculation. Based on clinical observations and results of bacterial cultures, all treated cows developed acute uterine infections. Controls did not develop uterine infections. Endometrial biopsies indicated that there were no significant diffuse or focal cellular reactions in response to the infection. The interestrous interval was greater (P less than .0003) for treated (27.7 +/- 1.0 d) than for control (20.6 +/- 1.0 d) cows, but P4 concentrations were similar between the two groups. Mean PGFM concentration and PGFM profiles were similar (P greater than .10) between treated and control cows before bacterial infusions. Bacterial infusions increased mean PGFM concentration (P less than .0001) and changed the shape of the PGFM profile (P less than .02).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of Progesterone on the Expression of Estrus at the First Postpartum Ovulation in Dairy Cattle

Fifty-two lactating Holstein cows were randomly assigned to receive either a progesterone-releasing (2g of progesterone) or a control-releasing intravaginal device (0g of progesterone). Intravaginal devices were inserted on d 10 and removed on d 15 postpartum. Daily blood samples were collected from d 10 to 90 postpartum for subsequent determination of progesterone concentrations. Observations for estrus were conducted three times daily in a dirt paddock containing a testosterone-treated cow.Serum concentrations of progesterone in the progesterone-releasing intravaginal device group were elevated on d 11, 12, 13, 14, and 15 compared with those of the control group. The days to first postpartum ovulation were similar between the treated and control groups, respectively (30.6 vs. 30.5 d). Also similar was the proportion of cows expressing estrus at first, second, and third postpartum ovulations (9/27 vs. 3/24, 14/23 vs. 15/ 21, and 14/21 vs. 9/15, respectively), length of the first postpartum estrous cycle (17.9 vs. 18.3 d), and peak serum concentrations of progesterone during the first estrous cycle (3.5 vs. 2.9ng/ml). These data indicate that administration of progesterone early postpartum did not increase the proportion of cows expressing estrus at the first ovulation.

Effects of calcium soaps of fatty acids on postpartum reproductive function in beef cows

Twelve multiparous Simmental cows (584 kg) were used to determine the influence of calcium soaps of fatty acids (CSFA) incorporated in a range supplement on postpartum reproductive characteristics. Cows were assigned randomly to receive a control [C; containing grain sorghum (GS) and soybean meal (SBM)] or CSFA-based (containing Megalac [a source of CSFA], GS, and SBM) supplement. Supplements plus prairie hay were individually fed. Diets were isonitrogenous and met the NEm requirement for heavy-milking beef cows in early lactation. Supplement feeding and daily blood collection began at parturition. Calves were removed permanently from cows at 25 +/- 2 d postpartum. Duration of first postpartum estrous cycles was determined by both visual observations and changes in concentrations of progesterone in serum. Concentrations of LH in serum (15-min intervals for 6 h) were determined 12 h before and 48 and 96 h after calf removal. Concentrations of progesterone and estradiol-17 beta in serum were determined daily. Cows receiving CSFA had higher (P = .06) mean concentrations of LH than those receiving C (1.47 vs 1.12 +/- .13 ng/ml). Concentrations of estradiol-17 beta were lower (P less than .02) and serum progesterone were higher (P less than .02) between d 6 and 8 of the induced cycle in CSFA-fed cows. Plasma cholesterol was greater (P less than .01) in cows fed CSFA although plasma triglyceride concentrations were similar between treatments.(ABSTRACT TRUNCATED AT 250 WORDS)

Ovarian endocrine activity and development of ovarian follicles during the postpartum interval in beef cows

To assess differences between multiparous and primiparous beef cows, largest follicle diameter, serum estradiol-17β (E 2) and progesterone (P 4) were monitored during the postpartum interval. Beginning 6–10 days postpartum, nine primiparous and 10 multiparous Angus cows underwent ovarian ultrasonography and jugular venipuncture every 48 h until cows either exhibited a normal estrous cycle or, owing to the ending of breeding season, first estrus. No difference was observed in average daily gain (±s.d.) for primiparous and multiparous cows (0.76±0.27 and 0.51±0.43 kg, respectively). Within 2 weeks postpartum, 5–10 mm follicles were found in all cows. The interval from appearance of the first follicle ≥ 14 mm to ovulation was longer ( P<0.01) in primiparous than in multiparous cows (42.7 and 13.5 days, respectively). No difference in mean follicle diameter was found during the postpartum interval prior to ovulation or during the first postpartum estrous cycle. Length of first postpartum estrous cycle was not different between primiparous (17.0 days) and multiparous (16.9 days) cows. Average days postpartum to first ovulation for primiparous and multiparous cows was 112 and 46.1 days, respectively ( P<0.001). Estradiol-17β remained low throughout the postpartum interval with occasional increases corresponding to increases in follicular size. Average baseline P 4 concentrations were 0.04 and 0.03 ng ml −1 for primiparous and multiparous cows, respectively. All cows had increased P 4 preceded by a drop in largest follicle diameter, prior to first postpartum estrous cycle. Only those primiparous cows with unusually long (> 115 day, n=5) postpartum intervals were likely to have an increase in largest follicle diameter after parturition. Achieving an ovulatory size follicle (≥ 14 mm) is no indication of impending estrus in primiparous cows as it may be in multiparous cows. In addition, we found no difference in mean largest follicle diameter between primiparous and multiparous cows during the postpartum interval or first postpartum estrous cycle.

Endocrine changes and ultrasonography of ovaries in suckled beef cows during resumption of postpartum estrous cycles.

Changes in follicular and luteal structures were assessed and concentrations of estradiol and progesterone were measured in 13 Hereford X Angus suckled beef cows during resumption of estrous cycles. Transrectal ultrasonography was used to monitor follicular size, ovulation, and formation and regression of the corpus luteum (CL). The interval from parturition to first postpartum ovulation (FO) was 82 +/- 4.7 d. Serum progesterone remained low before FO. One cow exhibited standing estrus, two cows showed other signs of estrus, and 10 displayed no signs of behavioral estrus preceding FO. All cows exhibited standing estrus before the second postpartum ovulation (SO). All cows had a short luteal phase after FO, with an average interval of 8.5 +/- .2 d between FO and SO. Concentrations of estradiol in serum during the 8 d preceding ovulation were similar before FO and SO. Maximal diameter of the preovulatory follicle was similar before FO and SO. However, the ovulatory follicle was larger in diameter at 2 d (P = .02) and 3 to 8 d (P less than .005) before FO than before SO. The time from detection until ovulation was less (P = .005) for the ovulatory follicle preceding SO than for the follicle associated with FO (8.5 vs 10.2 d, respectively, SE = .4). The second-largest follicle was larger (P less than .005) in diameter during the 8 d preceding the FO than before the SO. The difference in size between the ovulatory follicle and the second-largest follicle on the day before ovulation was greater (P less than .005) preceding SO than preceding FO (8.7 vs 6.6 mm, respectively, SE = .4).(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of ovarian activity in postpartum dairy cows using ultrasound imaging and progesterone profiles

Ten postpartum dairy cows were monitored for ovarian activity by using a combination of ultrasound imaging and progesterone (P 4) profiles. Blood and milk samples were collected and ovaries scanned rectally starting 2 and 14 days postpartum, respectively, and continuing until 90 days after calving. Ultrasound imaging of the ovaries for the presence of a corpus luteum revealed that four of the ten animals ovulated by 14 days and eight of the ten by 25 days postpartum. Follicular growth occurred in two waves in most of the postpartum estrous cycles. There was no difference between primiparous and pluriparous animals with regard to time to first postpartum ovulation, duration of first estrous cycle, peak plasma P 4 levels or time to uterine involution. Time to first observed estrus was non-significantly greater ( P>0.05) in primiparous than pluriparous cows. The second postpartum cycle was significantly longer ( P<0.05) with greater plasma P 4 concentrations than the first cycle. Maximum corpus luteum diameter as measured by ultrasound imaging was not correlated with the length of the first and second postpartum estrous cycles or with the maximum plasma P 4 in in the first estrous cycle, but was positively correlated with the maximum plasma P 4 in the second postpartum estrous cycle. Milk P 4 profiles were similar in pattern to plasma P 4 profiles, and correlated with the ultrasound results. The present study found ultrasound imaging useful for monitoring postpartum ovarian activity in cattle.

Ovarian Regulation of Pulsatile Luteinizing Hormone Secretion During Late Gestation in the Rat*

Abstract The object of this study was to examine the influence of both estradiol (E(2)) and progesterone (P) alone or in combination on luteinizing hormone (LH) pulse amplitude and frequency during the interval between Days 21 and 22 of gestation. This was done by analyzing pulsatile LH release in rats bled on Days 21 and 22 of gestation, and in animals ovariectomized (OVX) on Day 21, implanted with silastic capsules producing plasma levels of E(2) and/or P characteristic of the Day 21 to 22 interval, and bled on Day 22 Pulsatile LH release increased between Days 21 and 22 due to an increase in pulse frequency and a small elevation in pulse amplitude. OVX produced no further increase in pulse frequency but markedly enhanced the small change in pulse amplitude. Preventing either the decline in plasma P that normally occurs between Days 21 and 22, or just the small additional decrease in plasma P levels produced by OVX, had no suppressive effect on pulse amplitude or frequency, although Day 22 levels of P alone augmented the normal increase in pulse frequency occurring between Days 21 and 22. Restoration of physiological plasma E(2) levels had no effect on the normal increase in pulse frequency, but partially attenuated the OVX-induced increase in pulse amplitude. Replacement of physiological Day 22 levels of both E(2) and P also decreased LH pulse amplitude, although amplitude was not significantly different from that seen following E(2) replacement alone, and was still greater than the normal Day 22 value. In contrast, restoration of physiological plasma levels of E(2)+ P caused a suppression of LH pulse frequency below that normally seen on Day 22. While E(2)+ P did not completely prevent the OVX-induced increase in pulse amplitude, administration of charcoal-extracted porcine follicular fluid to rats OVX on Day 21, and in which physiological plasma levels of E(2)+ P were restored, caused a further reduction in pulse amplitude. These data demonstrate that 1) marked increases in LH pulse amplitude are prevented from occurring between Days 21 and 22 of gestation by ovarian steroids, notably E(2), and that this suppression is enhanced by a non-steroidal factor present in porcine follicular fluid, 2) neither E(2) or P alone suppresses LH pulse frequency on Day 22 of gestation; LH pulse frequency increases on Day 22 because the plasma level of one of these steroids, P, markedly declines, and 3) restoration of physiological plasma levels of both steroids in the absence of the ovary produces an unphysiological suppression of pulse frequency, i.e. results in a lower pulse frequency than normally occurs in the presence of these same plasma steroid levels in animals with their ovaries intact. One hypothesis consistent with the latter observation is that at the end of gestation in the rat the ovary may produce a factor which 'protects' the frequency of the LH pulse generator from the negative feedback action of ovarian steroids. This allows an increase in LH pulse frequency and mean blood LH levels, and thereby facilitates ovarian follicular development and the normal progress of the first postpartum estrous cycle.

Association Between Energy Balance and Luteal Function in Lactating Dairy Cows

Our objective was to determine the relationship between energy balance and secretion of progesterone in lactating dairy cows. Eight primiparous and 24 multiparous lactating Holstein cows were studied from parturition to 100 d postpartum or conception. Cows calved normally and remained healthy throughout the study. All cows were fed ad libitum a total mixed diet formulated to satisfy requirements for maintenance and lactation. Intake of feed and production of milk per cow were measured twice daily. Body weight was determined weekly. Daily energy balance was determined by subtracting energy required for maintenance and lactation from intake of energy. Concentrations of progesterone were determined in milk sampled every 3rd d. For at least 4 successive d postpartum, 81% of cows were in negative energy balance. Variation in energy balance was explained largely by intake of energy. Duration of luteal phases was not associated with energy balance. Energy balance within 9 d postpartum was correlated positively with concentration of progesterone within second and third postpartum luteal phase. Postpartum interval to nadir and magnitude of nadir of energy balance interacted to reduce progesterone within second and third postpartum estrous cycles. Thus, in lactating cows, secretion of progesterone is reduced by spontaneous caloric deficit and is modulated by timing and magnitude of maximal caloric deficit. Spontaneous caloric deficit is a potential source of infertility in lactating dairy cows.

Plasma endotoxin and concentrations of stable metabolites of prostacyclin, thromboxane A 2, and prostaglandin E 2 in postpartum dairy cows

The presence of endotoxin in plasma and patterns of stable metabolites of prostacyclin (PC), thromoxane A 2 (TXA 2) and prostaglandin E 2 (PGE 2) were determined during the first postpartum estrous cycles in sixteen dairy cows. These included 8 cows with uterine infections which exhibited shortened luteal phases (SC) and 8 cows which had normal luteal phases (NC) after the first post partum ovulations. Endotoxin was consistently detected in all SC cows during the abbreviated estrous cycles while plasma samples of NC cows were free of endotoxin. Plasma concentrations of TXA 2 metabolite was higher in SC cows (p<0.05) (1785–3452 pg/ml) compared to NC cows (723–1240 pg/ml). Similarly, plasma concentrations of PC metabolite was higher in SC cows (p<0.07) (423–1847 pg/ml) compared to NC cows (159–325 pg/ml). In contrast, plasma concentrations of PGE 2 metabolite was higher in NC cows (p<0.05) (850–2219 pg/ml) compared to SC cows (455–628 pg/ml). The results of this study suggest that postpartum uterine infections mediate the release of prostaglandins from the uteri by means of the endotoxin and endotoxin appears to stimulate selectively the production of PC and TXA 2 favoring early demise of corpora lutea formed after first postpartum ovulations in dairy cows.

Photoperiodic alteration of postpartum reproductive function in suckled cows

Three experiments were conducted using a total of 41 cows to determine if photoperiod modulates the establishment of postpartum estrous cycles and conception. Cows calving in the autumn and winter were exposed to either 18 hr light/day (18L:6D) or natural photoperiods. In Exp. 1, cows receiving 18L:6D had shorter (P<0.025) intervals from calving to estrus (61 ± 3.8 days) than cows not receiving supplemental light (154 ± 23.9 days). The same was true for primiparous cows in Exp. 2 (76 ± 5.5 days vs 153 ± 38.0 days; P<0.06) but not for the multiparous cows in Exp. 2 (56 ± 5.2 days vs 40 ± 7.4 days) or for all cows in Exp. 3 (60 ± 10.1 days vs 70 ± 13.5 days). Because conception rate was higher for cows exposed to 18L:6D for the multiparous cows in Exp. 2 and all cows in Exp. 3, interval to conception was significantly shorter for animals exposed to 18L:6D in all experiments. Generally, interval from calving to uterine involution was also reduced by exposure to 18L:6D. No effects of photoperiod were observed on body weight changes, serum levels of luteinizing hormone (LH), follicle stimulating hormone (FSH) or estradiol or on estradiol-induced release of gonadotropins. It was concluded that in certain situations day length can hasten estrus and conception in postpartum cows. The effect of photoperiod interacts with other conditions, one of which is parity. The endocrine basis for these effects are not known.

Luteal competency during the resumption of ovarian cyclicity in postpartum Brahman cows

Twenty pluriparous, spring-calving Brahman cows were used to determine luteal competency, as measured by serum progesterone concentrations, during the first and the second postpartum estrous cycles. Prior to and after calving, all cows were maintained in good body condition on Coastal bermudagrass pasture (IFN 1-00-703). The calves were allowed to suckle ad libitum, and sterile marker bulls were maintained with the cow herd as an aid in estrus detection throughout the trial. Cow weight and body condition score were recorded within 24 hours after calving and again at the first behavioral estrus observed. From day 1 through day 14 (day 0 = estrus) of both the first and the second postpartum estrous cycles, blood samples were collected from each cow, processed to yield serum and analyzed by radioimmunoassay for progesterone concentrations. There was a higher incidence of abnormal estrous cycles following the first postpartum estrus (35%) than following the second (5%) postpartum estrus (P<0.05). The abnormal first estrous cycles were characterized by either a short luteal phase (four cows) or by standing estrus behavior without luteal tissue formation (three cows). When serum progesterone concentrations were compared for all cows during the first estrous cycle with those during the second estrous cycle, there was less progesterone released during the cycle (P<0.05) and lower peak progesterone concentrations (P<0.10) during the first estrous cycle. However, if the abnormal cows were excluded from the analyses, there was no difference (P>0.10) in either progesterone concentrations through the 14 days measured or in peak progesterone concentrations between the first and the second postpartum estrous cycles. It can be concluded from this study that the higher incidence of abnormal luteal function following the first postpartum estrus may contribute to the decreased conception rates observed when cows are bred at their first postpartum estrus.

Reproductive Traits of Holsteins and Jerseys. Effects of Age, Milk Yield, and Clinical Abnormalities on Involution of Cervix and Uterus, Ovulation, Estrous Cycles, Detection of Estrus, Conception Rate, and Days Open

Two hundred and twelve Holstein and Jersey cows were in a study to determine factors that affected reproductive traits. First ovulation occurred about 3 wk postpartum, and interval to first ovulation was greater in cows that had clinical abnormalities postpartum than in normal cows. Jerseys producing more milk ovulated sooner postpartum than lower producing herdmates. Involution of cervix and uterus occurred later postpartum in cows that had clinical problems postpartum. Involution of genital tract occurred later postpartum in older cows and sooner postpartum in cows that had higher milk yields. Duration of first postpartum estrous cycle was 4 days less than for second postpartum cycle. Percentages of estrous cycles detected by standing estrus were 43 and 73% for Holsteins and Jerseys. Estrous detection rates were highest for cows that produced slightly above the mean milk yield and did not differ between cows in highest and lowest milk production quartiles. First detected estrus and days to first insemination occurred later postpartum in Holsteins as milk yield deviation from herdmates increased, regardless of sign. In Jerseys, days to first insemination and days open increased linearly as milk yield increased. Days to first insemination and conception were greater in cows with postpartum clinical problems. Conception rate at first insemination postpartum increased in proportion to concentration of progesterone in blood samples collected during 12 days before first insemination. Overall, clinical problems at parturition and postpartum lowered reproduction performance in both breeds. There was a slight antagonism between milk yield and reproduction performance (days open) in Jerseys but not in Holsteins.

Ovarian and endocrine responses and reproductive performance following GnRH treatment in early postpartum dairy cows

At calving forty-eight Holstein and Guernsey cows were assigned according to age and breed to one of six postpartum periods (1 or 2, 3 or 4, 5 or 6, 7 or 8, 12 or 13 and 18 or 19 days postpartum). Thirty-six of the cows (6 cows per postpartum period) received a single intramuscular injection of 100 μg GnRH. The other twelve cows (2 cows per postpartum period) served as controls and received a single intramuscular injection of the carrier vehicle for GnRH. Four of 36 cows administered GnRH and three of the 12 control cows ovulated by the day following treatment. Four of the cows were 12 or 13 days postpartum (1 control and 3 GnRH treated) and three were 18 or 19 days postpartum (2 controls and 1 GnRH treated). Six of the seven cows that ovulated the day following treatment had a follicle > 1.0 cm the day prior to treatment. Follicular growth was detected in the earlier postpartum periods but ovulation the following day was not detected for either control or GnRH treated cows. Following estrus or silent estrus, plasma progesterone concentrations increased to about 4 ng/ml on day 13. However, in cows ovulating the day following GnRH treatment, plasma progesterone declined from about 3 ng/ml on day 9 to approximately 1 ng/ml on day 13 postestrus. In addition, LH in plasma was higher (P < .01) ł through 13 days following estrus or silent estrus in cows ovulating the day after GnRH treatment in comparison to cows during the first or subsequent postpartum estrous cycles. In summary, in addition to days postpartum other factors including follicular development and maturity are probably involved in GnRH induced ovulation.

Concurrent changes in bovine plasma hormone levels prior to and during the first postpartum estrous cycle.

Concurrent changes in bovine plasma hormone levels prior to and during the first postpartum estrous cycle.

Serum Luteinizing Hormone, Prolactin, Glucocorticoid, and Progestin in Dairy Cows from Calving to Gestation

In a survey of 91 Holstein cows, blood serum prolactin increased from 50±4ng/ml at 2 to 4 wk before parturition to 234±14ng/ml at parturition and fell to 69±4ng/ml during the first 6 wk postpartum. Serum luteinizing hormone remained near .5ng/ml until parturition, then increased to 1.5±.3ng/ml at 5 to 6 wk postpartum. In 12 of the same cows, glucocorticoid increased three-fold to a peak at parturition and returned to prepartum values by 1 wk postpartum; progestin fell from 8.0±.6ng/ml at 1 to 2 wk prepartum to .9±.2ng/ml at parturition.During diestrus of the first postpartum estrous cycle, luteinizing hormone (23 cows) was 50% higher and progestin (9 cows) was 34% lower than during the second cycle. During the 18 days following inseminations, luteinizing hormone was 22% lower in fertile than in infertile cows. Progestin concentration was similar until day 11 in fertile and infertile cows; thereafter, progestin in nine pregnant cows increased to 12.2±2.1ng/ml at day 20, whereas in nine nonpregnant cows progestin peaked at 7.8±.6ng/ml at day 15 and declined thereafter to the next estrus. Correlations within day between luteinizing hormone and progestin were mostly positive in fertile cows, in contrast to the analogous correlations which were mostly negative in open cows.

Latent Effects of Pregnancy on Postpartum Estrous Cycle Length in Dairy Cattle

Latent Effects of Pregnancy on Postpartum Estrous Cycle Length in Dairy Cattle