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).     

Ovulation, ovarian function, and reproductive performance after treatments with GnRH in postpartum suckled cows

Two experiments were conducted to determine whether treatments with gonadotropin releasing hormone (GnRH) during the early postpartum period in suckled cows would induce ovulation and initiate regular estrous cycles. In Experiment I, 0, 100 or 200mug of GnRH was given to 22 suckled Angus x Holstein cows at three and again at five weeks postpartum. Serum luteinizing hormone (LH) responses did not differ between cows given 100 or 200mug of GnRH. Treatment with GnRH tended to increase the percentage of cows exhibiting estrus by 30 and 60 days postpartum, but reproductive performance during the breeding season did not differ among groups. In Experiment II, 70 suckled Hereford cows were given either no treatment or 200mug of GnRH at 7 weeks postpartum. Cows given GnRH received either no treatment prior to GnRH or were separated from their calves for 24 hr prior to GnRH treatment. Half of the cows that were separated from their calves also received progesterone via a progesterone intravaginal device (PRID) for 12 days prior to calf removal. Treatment with GnRH alone tended to increase the percentage of anestrous cows which ovulated by 8 days after treatment. Calf removal did not increase the ovulatory response to GnRH, but PRID treatment did. More estrous periods were detected in GnRH-treated cows than in control cows during 20 days after GnRH treatment.     

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.     

The effect of method of GnRH administration and short-term calf removal on ovarian function and reproductive performance in postpartum suckled beef cows administered PGF(2)alpha for estrous synchronization

The first experiment was a 2 x 2 factorial experiment with calf removal (none or short-term) and method of GnRH administration (intramuscularly in saline or subcutaneously in gelatin capsules) as main effects. The durations of the GnRH-induced LH surges were similar among groups but the LH surges were delayed in the cows that received GnRH subcutaneously in gelatin capsules. Calf removal enhanced the GnRH-induced LH release for cows administered GnRH subcutaneously in a gelatin capsule but not for cows administered GnRH intramuscularly in saline. In the second experiment, 191 postpartum suckled beef cows were administered two injections of prostaglandin F(2)alpha(PGF(2)alpha) 11 days apart. After the second PGF(2)alpha injection, the cows were assigned to a 2 x 2 factorial experiment as in Experiment 1 plus one control group. Short-term calf removal (47 h) began 28 h after the second PGF(2)alpha injection. GnRH was administered 30 h after the time of calf removal. The number of cows that ovulated following the time of the GnRH treatment, the number that had abnormal luteal phases and the first-service pregnancy rates among treatment groups within the anestrous and cyclic cows classifications were not significantly different. However, several effects were detected and are reported.     

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.     

Effect of repeated injections of GnRH on reproductive parameters in postpartum anestrous dairy cows

To induce cyclicity in dairy cattle with prolonged postpartum anestrous, repeated dosages of gonadotrophin releasing hormone (GnRH) were administered. Twenty-one (21) Holstein dairy cows and heifers calving between October 1, 1989, and January 1, 1990, at the Louisiana State University Dairy were used in the study. The animals were defined as anestrous if their plasma progesterone remained < 1.0 ng/ml until 32 to 36 days post partum. They were randomly assigned to one of two treatment groups. Group 1 (n=6) received two injections 1 hour apart of a GnRH analogue (50 mug) (i.m.). The treatment was repeated twice weekly at 3- to 4-day intervals. Group 2 controls (n=6) received saline (1 ml, i.m.) on the same schedule as Group 1. A maximum of 12 to 13 treatments were given. Cattle that had plasma progesterone >1.0 ng/ml by 32 to 36 days post partum were identified as Group 3, or cyclic contemporaries (n=9). Postpartum anestrous in the herd was 46.2% (18 39 ). Cows in Group 1 had significantly fewer days to first plasma progesterone > 1.0 ng/ml than those in Group 2 (P < 0.05), but more days than Group 3. Cows in Group 1 also had significantly fewer treatments to induce plasma progesterone > 1.0 ng/ml than those in Group 2 (P < 0.05). There were no significant differences among treatment groups in the number of days from calving to first observed estrus or the number of days open (P > 0.05).     

The effect of days postpartum, indomethacin and oxytocin on prostaglandin metabolite concentrations in postpartum suckled beef cows

In Experiment 1, blood samples were collected on days 1, 4, 7, 10, 13, 16, 19, 22, and 25 postpartum from the jugular veins of 10 suckled beef cows to determine 13, 14-dihydro-15-keto prostaglandin F(2)alpha (PGFM) concentrations during the early postpartum period. PGFM concentrations on days 1 and 4 were 207.8 +/- 33.9 and 283.6 +/- 45.6 pg/ml and then declined linearly (r = -0.71; P < 0.05) to 44.1 +/- 5.7 and 44.0 +/- 5.3 pg/ml on days 22 and 25 postpartum. Two groups of postpartum (25.3 +/- 0.5 and 37.7 +/- 1.1 days) suckled beef cows (10 cows/group) were used in the second experiment. Five cows of each group received intrauterine infusions of indomethacin for 5.5 days while the other five cows of each group served as controls. All cows had calves removed at the time of the last indomethacin infusion and were subcutaneously administered oxytocin six hours later. During the infusion period, PGFM concentrations decreased (P < 0.01) across time for both groups of indomethacin-treated cows. Concentrations of PGFM increased (P < 0.05) after oxytocin treatment for both groups of control and indomethacin-treated cows, but concentrations were higher for the control cows than for the indomethacin-treated cows.     

Effects of infusion of GnRH pulses and level of body condition on ovarian function in postpartum beef cows

An experiment was conducted to test the hypothesis that postpartum anoestrus in beef cows is prolonged in cows in low body condition (BC) because they have a reduced LH pulse frequency compared with cows in high BC. Thirty-six multiparous Blue-Grey (White Shorthorn × Galloway) cows were fed so that they calved in either low (L) (BC score 2.07; SE 0.05; n = 24) or high (H) (BC score 2.81; SE 0.08; n = 12) body condition. They were then fed to maintain BC after calving. Twelve L cows were infused (i.v.) with 2 μg GnRH in 2 ml saline every 2 h from 5 to 7 weeks postpartum (LG) while the remaining L cows and all H cows were infused with saline only (LS and HS). Ovulations, as indicated by the presence of a morphologically normal corpus luteum, were recorded in one, one and ten of the cows of the LS, HS and LG groups, respectively. Mean LH concentrations and pulse frequencies were not affected by either GnRH treatment or BC but mean LH pulse amplitudes were lower (P < 0.05) in LG and LS cows than in HS cows at Week 5 and in LG cows at Week 6. At Week 7 postpartum, the numbers of small (3–7.9 mm diameter) and large (≥ 8 mm diameter) ovarian follicles, mean granulosa cell numbers per follicle and mean concentrations of LH receptors (pg per mg thecal and granulosa tissue) were not affected by GnRH treatment or BC. Granulosa cells from oestrogen active follicles of HS and LG cows secreted more oestradiol in vitro (P < 0.01) than cells from LS cows. However, there were no significant differences with treatment in the intrafollicular concentrations of oestradiol, testosterone or insulin-like growth factor-1. It was concluded that, since infusion of GnRH pulses enhanced both follicular steroidogenesis and the incidence of ovulation in low BC cows, the frequency of GnRH pulses is one determinant of follicle development in the postpartum cow. While H cows also exhibited a degree of enhancement of oestradiol synthesis by the granulosa cells of oestrogenic follicles, compared with L cows there was no difference in the LH pulse frequency or in the incidence of ovulation. It is concluded that there may be a threshold level of oestrogen synthesis by granulosa cells below which the final stages of follicle maturation and ovulation cannot be initiated, or that a high rate of oestradiol synthesis by this tissue is not the only factor mediating the effects of body condition on follicle development in the postpartum cow.     

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.     

The effect of short term calf removal in combination with GnRH treatment and the effect of limited nursing on reproductive performance of postpartum suckled beef cows administered PGF2alpha for ovulation control

Over a two year period, postpartum suckled Hereford and Angus Cows (n=213) were administered two injections of PGF(2)alpha (25 mg/injection) and divided into three groups. No additional treatments were administered to cows in Group I and calves were allowed to nurse their dams ad libitum. In Group II, calves were removed for 48 hours beginning on the third day following the initial PGF(2)alpha injection. These cows were given a subcutaneous injection of 250 mug GnRH dissolved in 2% carboxymethylcellulose midway through the 48 hour period. In Group III, calves were allowed to nurse their dams for only one hour per day for the first 7 days after the initial PGF(2)alpha injection. In year 1, PGF(2)alpha was administered 14 days apart whereas in year 2, PGF(2)alpha was administered 11 days apart. Cows were artificially inseminated at 72 and 96 hours after the second injection of PGF(2)alpha. In year 1, the numbers of cows that conceived to the timed inseminations were similar (P > .10) for the three groups. In year 2, a higher percentage of cows in groups II (P < .10) and III (P < .05) conceived to the timed inseminations than in group I. Other reproductive performance parameters were similar (P > .10) between groups for both years 1 and 2. In summary, limited nursing and short term calf removal in conjunction with GnRH treatment may improve the pregnancy rate in cows administered PGF(2)alpha for ovulation control.     

Induction of ovulation in postpartum suckled beef cows: a review

Prolonged postpartum acyclicity in suckled beef cows reduces the calf crop, and causes economic loss to beef cattle producers. Once anterior pituitary LH stores have been replenished between Days 15 and 30 post partum in suckled beef cows, methods to initiate cyclicity include non-hormonal methods such as weaning of calves (either complete, temporary or partial), or exposure to bulls, and hormonal methods such as administration of GnRH (either single injection, intermittent injections, or continuous infusion), gonadotropins (eCG, FSH, hCG), and steroids (estrogens, anti-estrogens, and progestogens). Weaning is costly, reduces growth rate of weaned calves, and short cycles are common after weaning-induced ovulation. Exposure of cows to bulls is not practical and its effect is not predictable. Repeated injections of GnRH, or a single injection of hCG are not always effective; ovulation is always followed by a short cycle, and usually a return to acyclicity. Estrogens and anti-estrogens do not consistently shorten postpartum anestrus. Exogenous progestogens include intravaginal devices, such as controlled-internal drug release (CIDR) or progesterone-releasing intravaginal device (PRID), norgestomet implants, and the feed-additive melengestrol acetate (MGA). Administration of exogenous progestogens is more practical than, and offers more advantages over, other treatments to shorten postpartum acyclicity in suckled beef cows. Mimicking the short cycle after Week 3 post partum, by maintaining circulating progesterone at subluteal concentrations or circulating progestin at intermediate concentrations, extends the life-span and allows terminal maturation of the postpartum dominant follicle as in cyclic cows, by initiating endogenous GnRH and LH pulses. This is followed by an LH surge, ovulation and normal cycles. The benefit from using exogenous progestogens after Week 3 post partum in suckled beef cows is that ovulation is induced, cyclicity is initiated, the resulting CL has a normal life-span and function, and there is no need to change management, such as weaning of calves. We present a model for the induction of ovulation and initiation of cyclicity using exogenous progestogens after Week 3 post partum in suckled beef cows.     

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.     

The effect of treatment with a GnRH analogue on postpartum reproductive performance in Friesian cows

Three trials were conducted to examine the effect of GnRH injection on the reproductive performance of Friesian cows. In the first trial 100 μg gonadotrophin releasing hormone synthetic analogue (GnRH) was given at the time of A.I. to 32 cows while a control group received a saline placebo injection. Conception rate to first insemination was significantly higher in treated than control cows (81.3 v. 54.8%, P < 0.05).In the second trial, two groups of 19 cows each received either 100 μg GnRH or saline at 15 days postpartum. A total of 60 cows was used in the third trial in which GnRH was given at either 50 or 100 μg dose level on either days 7 or 15 postpartum while control cows were untreated. In both trials GnRH treatment reduced the intervals from calving to complete uterine involution, first ovulation and first detected oestrus. The effect was most pronounced when 100 μg GnRH was given on day 7 postpartum. Number of days open and number of services per conception were appreciably reduced when 100 μg GnRH was given at either 7 or 15 days postpartum with maximal effect when given 15 days after calving.     

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.     

GnRH induced LH release in suckled beef cows. 1. The effects of days postpartum and estradiol-17beta concentrations on the release of LH following administration of GnRH

Twenty suckled CharloixxHereford beef cows (5 cows/group) were assigned at random to receive 100 microg GnRH (IM) at either 2 to 3, 7 to 8, 15 to 16, or 31 to 32 days postpartum, Groups 1 through 4, respectively. Blood samples for hormone determinations were collected at time 0 (pre-GnRH), every half hr for 3 hr, and at 4.0 hr and 6.0 hr post-GnRH. Mean plasma LH, estradiol-17beta, or progesterone concentrations were not different among groups prior to GnRH. Plasma LH increased (P<.05) following GnRH in Groups 2, 3 and 4, but not in Group 1. Peak GnRH induced LH release was greater (P<.05) in Groups 3 and 4 than in Groups 1 or 2. Correlation coefficients between days postpartum and peak LH release (r=.72), and estradiol-17beta concentrations and time of LH peak (r=-.42) were significant (P<.05). These data indicate that LH release in response to GnRH, in suckled beef cows is not fully restored until 15 to 16 days postpartum.     

Effects of cloprostenol sodium and clenbuterol HCl on reproductive performance in postpartum anestrous cows

Two experiments were conducted to study effects of cloprostenol sodium (cloprostenol) and clenbuterol HCl (clenbuterol) during postpartum anestrus on subsequent reproductive performance in cows. In Experiment I, 96 cows received either 0.5 mg cloprostenol (PGF, n = 25), 364 mg clenbuterol (CLEN, n = 24), 0.5 mg cloprostenol and 364 mg clenbuterol (CLEN+PGF, n = 21) or no treatment (Control, n = 26) on Day 20 post partum. Treatments failed to influence postpartum interval, pregnancy rate or the incidence of short estrous cycles preceding the first normal estrous cycle. In Experiment II, anestrous cows were administered cloprostenol (0.5 mg) on either Day 20 (PGF20, n = 27) or Day 35 post partum (PGF35, n = 25), or served as untreated controls (Control, n = 26). Neither postpartum interval nor pregnancy rate were affected by cloprostenol treatment. In conclusion, treatment of postpartum cows with PGF did not alter the resumption of normal estrous cycles following parturition.     

Effect of passive immunization against testosterone on reproductive hormone secretion and ovarian function in dairy cows and pubertal beef heifers

Multiparous dairy cows were divided in 3 groups from Day 5 up to Day 56 post partum: high energy level (Group H, n=10), low energy level (Group L, n=10) and low energy level plus anti-testosterone bovine immunoglobulins (Group LI, n= 10). Undernutrition decreased body weight, body condition score, milk yield and energy balance in Groups L and LI compared to Group H (P<0.05), but had no effect on secretory pattern of LH. Passive immunization against testosterone increased LH secretion in Group LI (P<0.05). Follicular score and the presence of follicles >/= 10mm on the ovary were not affected by underfeeding but were higher in Group LI than in Group L after immunization (P<0.01). The duration of the first luteal phase was shorter in Group H than in Groups L and LI and maximum progesterone levels reached were higher in Group LI than in Group H (P<0.01). Reproductive performance was not depressed by underfeeding and immunization. In the pubertal beef heifers maintained in anestrus by undernutrition had very low LH secretion. After passive immunization against testosterone, the increase of LH pulses number became almost significant (P=0.07). Following injection of exogenous LH, the number of follicles >/= 9mm was higher in immunized (Group I, n=8) than in control heifers (Group C, n=7). Group I developed a dominant follicle sooner and of greater size than Group C. Passive immunization against testosterone increased LH secretion and follicular development.     

Effect of dietary lipid supplementation on progesterone concentration and reproductive performance in suckled beef cows

Using whole cottonseed as a lipid source, silage-based diets that were isocaloric and isonitrogenous yet varied in lipid level were fed to multiparous cows. In Experiment 1, 48 cows (n = 12 per treatment) were allotted to 1 of 4 treatments where diets were formulated to supply 3.9, 4.3, 5.3 and 6.3% of total lipid. In Experiment 2, 66 cows (n = 22 per treatment) were allotted to 1 of 3 treatments where diets were formulated to supply 3.1, 5.5 and 8.3% of total lipid. Length of the first ovarian cycle, length of the first normal estrous cycle, postpartum intervals to onset of ovarian luteal activity and to first estrus were not affected by diet (P>0.10) in either experiment. Mean progesterone (P(4)) concentrations for first normal estrous cycles were not different (P>0.10) in either experiment. Anestrous periods were divided into 3 phases for analyses: Phase I) parturition to onset of ovarian luteal activity, Phase II) first ovarian luteal activity and Phase III) first normal estrous cycle. No differences were observed in P(4) concentrations during any phase of the postpartum period. In conclusion, isocaloric and isonitrogenous diets with increasing levels of lipid had no effect on reproductive performance in suckled beef in these experiments.     

Synchronization of estrus in suckled postpartum beef cows with melengestrol acetate and PGF2alpha

A total of 402 two suckled postpartum beef cows at 2 locations (CSU and ECRC) were utilized in 3 trials to evaluate the effectiveness of a combination melengestrol acetate (MGA)-PGF2alpha estrus synchronization system in spring-calving cows. The cows were allocated by days post partum, body condition score (1 = emaciated; 9 = obese), sire breed, and dam age at the beginning of treatment to 1 of 2 treatments within location and trial: MGA-PGF2alpha (0.5 mg MGA/head/d for 14 d with 25 mg of PGF2alpha injected 17 d after MGA withdrawal), and unsynchronized controls. All cows were observed for estrus at 12-h intervals for at least 5 d post injection. Cows observed in standing estrus were inseminated 12 to 18 h later. There was a location effect on response to treatment that was attributed to differences in body condition score between locations so data were analyzed within a location. Body condition score at the CSU location was 5.7 compared with 4.0 at the ECRC location. The CSU MGA-PGF2alpha treated cows had higher (P < 0.05) 5-day estrus and 5-d pregnancy rates (78.6 and 61.0%, respectively) than the CSU controls (11.1 and 6.9%, respectively). Similar results for 5-d estrus and pregnancy rates but of decreased magnitude were also observed for the ECRC MGA-PGF2alpha treated (31.6 and 21.4%, respectively) cows compared with that of the ECRC controls (11.9 and 8.5%, respectively). The CSU MGA-PGF2alpha treated cows had higher (P < 0.05) 25- and 60-d pregnancy rates (82.5 and 94.8%) than the CSU controls (65.3 and 87.5%). The 25- and 60-d pregnancy rates were similar between the ECRC MGA-PGF2alpha treated cows and ECRC control cows. The MGA-PGF2alpha estrus synchronization system appears to contribute to pregnancy early in the breeding season in postpartum beef cows, although its effectiveness is limited by cow body condition.