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.     

Follicular dynamics and concentrations of steroids and gonadotropins in lactating cows and nulliparous heifers

Differences in follicular development and circulating hormone concentrations, between lactating cows and nulliparous heifers, that may relate to differences in fertility between the groups, were examined. Multiparous, cyclic, lactating Holstein cows (n=19) and cyclic heifers (n=20) were examined in the winter, during one estrous cycle. The examinations included ultrasound monitoring and daily blood sampling. Distributions of two-wave and three-wave cycles were similar in the two groups: 79 and 21% in cows, 70 and 30% in heifers, respectively. Cycle lengths were shorter by 2.6 days in heifers than in cows, and in two-wave than in three-wave cycles. The ovulatory follicle was smaller in heifers than in cows (13.0+/-0.3 mm versus 16.5+/-0.05 mm). The greater numbers of large follicles in cows than in heifers corresponded well to the higher concentrations of FSH in cows. The duration of dominance of the ovulatory follicle tended to be longer in cows than in heifers. Estradiol concentrations around estrus and the preovulatory LH surge were higher in heifers than in cows (20 versus 9 ng/ml). Progesterone concentrations were higher in heifers than in cows from Day 3 to Day 16 of the cycle. Circulating progesterone did not differ between two-wave and three-wave cycles. The results revealed differences in ovarian follicular dynamics, and in plasma concentrations of steroids and gonadotropins; these may account for the differences in fertility between nulliparous heifers and multiparous lactating cows.     

Behavioral and physiological changes in early-weaned multiparous and primiparous beef cows

Early weaning is used to advance postpartum ovulation in beef cows, but triggers a stress response, affecting cows' and calves' welfare. Our aim was to compare the response to early weaning in multiparous and primiparous cows. Seventy-one days after birth, calves from 14 primiparous and 11 multiparous cows were weaned. Since birth, cow's body weight (BW), body condition (BC), milk yield and calves' BW were recorded. Basophil/neutrophil relationship, total protein and albumin concentrations were measured before and after weaning. The distance between each cow and its calf before weaning, as well as main behaviors of the cows before and after weaning were recorded using 10 min scan sampling. BW and BC were greater in multiparous than in primiparous cows (P < 0.0001 for both). There was an interaction between parity (P = 0.004) and gender of the calf (P = 0.007): both BW and BC were greater in multiparous than primiparous cows, but multiparous cows with male calves had lower BW and BC than those with females (P < 0.001 for both variables). Milk yield was greater in multiparous than in primiparous cows (P = 0.02), and there was an interaction, with greatest milk yield in multiparous cows that calved males (P = 0.02). Total protein blood concentration increased with time (P < 0.0001), similarly in both groups. Albumin concentration profiles were similar despite parity, and decreased since 10 days after weaning (P < 0.0001). Basophil/neutrophil ratio was not affected by parity, and increased the day of weaning, remaining high until day 10 (P < 0.05). Frequency of standing decreased during the 2 days after weaning, with a greater decrease in multiparous than in primiparous cows (P < 0.0001). Pacing increased after weaning (P = 0.001), but increased significantly more in multiparous cows (P = 0.006), with a significant time after weaning by parity interaction (P = 0.01) reflected in greater frequencies in multiparous cows in the 2 days. Walking increased with time (P < 0.0001), and was more frequent in primiparous cows (P = 0.008). Ruminating, which was less frequent in multiparous cows (P = 0.014), decreased and remained low during the 3 days following weaning (P < 0.0001), but there was a significant time by parity interaction (P < 0.0001). The frequency of vocalizations increased dramatically after weaning (P < 0.0001), and was greater in multiparous cows (P = 0.006), and decreased more slowly during the 2nd and 3rd days after weaning (P = 0.005) in these animals. In conclusion, behavioral changes induced by weaning were greater in multiparous than primiparous cows, indicating a more intense response to cow-calf separation in the multiparous animals.     

Effects of an agonist of gonadotrophin releasing hormone in cattle. I. Hormone concentrations and oestrous cycle length

Four trials were completed to study the effects of a single intramuscular injection of 5 μg of an agonist of gonadotrophin releasing hormone (Hoe 766) on plasma concentrations of LH and progesterone, and on oestrous cycle length in normally cycling dairy cows.The first trial (four cows) showed that a mid-cycle injection of Hoe 766 temporarily increases plasma LH from less than 5 ng/ml to over 20 ng/ml within 2.5 h. Average plasma progesterone concentrations ranged from 4.8 to 7.0 ng/ml compared to 3.3 ng/ml in the control animal.The second trial (22 cows) showed that an injection of Hoe 766 on Cycle Day 3, 6 or 9 (Oestrus = Cycle Day 0) increased average plasma progesterone concentrations during Cycle Days 13, 14 and 15 by 1.2 ng/ml. Each of three cows injected on Cycle Day 16 maintained plasma concentrations above 3.9 ng/ml until Cycle Day 19 and corpus luteum (CL) size was maintained until Cycle Day 21. Except for the group of cows injected on Cycle Day 3, all other groups had temporarily reduced concentrations of plasma progesterone when sampled 24 h after Hoe 766 administration.The third trial (216 cows) showed that a single injection of Hoe 766 made between Cycle Day 1 and 10 did not alter oestrous cycle length (21.5 vs 21.3 days). In contrast, in the fourth trial (371 cows), a single injection of Hoe 766 between Cycle Days 12 to 16 altered the distribution of cycle lengths of 17–29 days, the average cycle length and the incidence of ovulation without detected oestrus. Compared to matched control cows, fewer Hoe 766-treated cows were detected in oestrus (73.9% vs 90%), or had cycle lengths of less than 20 days (4.7% vs 22.2%). These effects were most pronounced among cows injected on Cycle Day 16 when only 51.7% were detected in oestrus and their average cycle length was 24.1 days.These effects were not due to the formation of a secondary CL. Rather, the injection of Hoe 766 stimulated CL function and appeared to prevent or delay normal luteolysis when administered from Cycle Day 12.     

Hormonal interelationships and physiological responses of lactating dairy cows to a shade management system in a subtropical environment

Lactating cows (n = 64) were assigned randomly to shade or no shade treatments for a continuous trial (20 wk) during summer of 1976. Respirations/min and rectal temperatures were higher for no shade cows. Dry matter forage intake was 9.7% higher for shade cows, whereas water intake was 19% greater for no shade cows. Milk yield and conception rates were higher for shade managed cows. Mean plasma corticoid concentrations were high throughout the entire estrous cycle in no shade compared to shade cows (13.04 > 8.72 ng/ml). No shade cows had higher progesterone and LH concentrations, and a lower estradiol to progesterone ratio during the cycle. Results indicated that endocrine changes were indicative of recurring estrous cycles in thermal stressed lactating dairy cows. However, alterations in water intake and thermoregulation of stressed cows were associated with decreased fertility and milk production, and changes in steroid concentrations that may reduce uterine blood flow.     

Endocrine changes before and after weaning in response to boar exposure and altered suckling in sows

Eighteen sows (6 primiparous and 12 multiparous) were allotted randomly within parity to two lactational treatments: litter separation (LS; 6 h/day) plus boar exposure (BE; 1 h/day; N = 14) beginning 8 days before weaning (4 weeks) and no LS + no BE (controls; N = 4). Blood was collected from all sows via indwelling venous catheters at 20-min intervals for 5 h on Days -1, 0, 1, 2 and 3 from start of treatment. Control sows and those exposed to LS + BE not exhibiting oestrus during lactation were resampled on Days -1, 0, 1 and 2 from weaning. All 10 multiparous sows receiving LS + BE exhibited oestrus during lactation, whereas none of the 4 primiparous sows exposed to LS + BE or the 2 control multiparous and 2 control primiparous sows exhibited lactational oestrus. Overall concentrations of LH in serum were higher (P less than 0.05) in sows receiving LS + BE than in control sows during lactation, whereas overall FSH was higher (P less than 0.05) in primiparous than multiparous sows. Number and amplitude of pulses of LH were greater (P less than 0.05) for treated primiparous than multiparous sows during lactation. Oestradiol-17 beta increased (P less than 0.05) in sows during LS + BE and was higher (P less than 0.01) in multiparous sows of this group than control multiparous or treated primiparous sows. Preweaning concentrations of cortisol and progesterone in serum were higher (P less than 0.05) in treated than control sows for multiparous and primiparous animals. In sows resampled at weaning, the number of pulses of LH was greater (P less than 0.05) in treated primiparous than in control sows. Postweaning concentrations of FSH in serum were unaffected by preweaning treatments. It was concluded that (1) litter separation and boar exposure increased basal and pulsatile secretion of LH in multiparous and primiparous sows; (2) lack of ovarian follicular development and oestradiol secretion may preclude expression of oestrus in primiparous sows during lactation, despite elevated concentrations of FSH and LH in serum; and (3) if elevated concentrations of cortisol and progesterone inhibit the onset of oestrous cycles, in response to litter separation and boar exposure during lactation, the effect is limited to primiparous sows.     

Gonadal steroid hormone level in blood plasma and milk of primiparous and multiparous nonpregnant and pregnant buffaloes

Changes in the concentration of progesterone and estradiol-17beta were measured by radioimmunoassay in 11 primiparous and 17 multiparous buffaloes at estrus and daily post insemination and in 6 nonbred buffaloes at 6 hour intervals from 4 days before expected estrus to one day after estrus. Plasma progesterone concentration at estrus was 0.1 ng/ml which rose to a peak level of 3.47 ng/ml on day 17. It fluctuated around this level in those animals which conceived, but followed a declining trend in those which failed to do so and attained lowest values on the day of next estrus. Temporal changes of the hormone revealed that the occurrence of major decline varjed between 16 and 62 h before estrus. The average concentration in milk was about three to four times higher than in plasma. The concentration of estradiol-17beta about 23.50 pg/ml at estrus and fluctuated around 10 pg/ml in animals that returned to estrus with a peak around estrus. Temporal changes of hormone revealed that peak level occurred 8-17 h before estrus. The concentration of estradiol in pregnant animals fluctuated around 10 pg/ml. The concentration in milk was about 2-3 times higher than in plasma. There was no significant (P > 0.05) difference in the concentrations of progesterone and estradiol-17beta between primiparous and multiparous animals.     

Peripheral inhibin levels in relation to climatic variations and stage of estrous cycle in buffalo (Bubalus bubalis )

The present study investigated the peripheral plasma inhibin levels in relation to 1) the stage of estrous cycle and the effect of climatic variations. Blood samples were collected from cyclic buffalo (n=5) once daily for 32 consecutive days during the tropical hot humid (summer) and cold (winter) seasons. Estrus was recorded by parading a vasectomized bull as well as by plasma progesterone determination. In the winter season, peripheral inhibin concentrations which were lowest (0.35 +/- 0.02 ng/ml) during the mid-luteal phase of estrous cycle (Day 6 to Day 14, Day 0 = day of estrus) increased significantly (P < 0.02) to 0.47 +/- 0.04 ng/ml during the late luteal phase (Day -4 to Day -2) and then further to 0.52 +/- 0.03 ng/ml (P< 0.02) during the periestrus phase (Day -1 to Day 1). Inhibin concentrations then decreased significantly (P < 0.02) to 0.40 +/- 0.03 ng/ml during the early luteal phase (Day 2 to Day 5). In the summer season the differences in peripheral inhibin concentrations among different phases of estrous cycle were found to be nonsignificant. A comparison of the circulating inhibin concentrations between the two seasons indicated that inhibin concentrations were significantly higher in the late luteal phase (P < 0.01) and periestrus phase (P < 0.05) during the winter season compared with corresponding periods during the summer season. The present study suggests that peripheral inhibin concentrations change in the estrous cycle during cooler breeding season and that environmental heat stress can cause a reduction in peripheral inhibin concentrations.     

Use of Ovsynch in dairy herds--differences between primiparous and multiparous cows

Ovsynch protocols are used to increase service rate and decrease days open and cullings for infertility. Recent reports have indicated better results after Ovsynch in primiparous than in older cows. However, this was not observed in all investigations on the subject. The objective of the study was to evaluate differences between primiparous and multiparous cows after synchronization of ovulation with an Ovsynch protocol in six trials. A total of 1584 cows (583 primiparous and 1001 multiparous cows, respectively) on three dairy farms were synchronized with an Ovsynch protocol consisting of a GnRH-analogue at Days 0 and 9, and a prostaglandin F(2alpha) analogue on Day 7. AI was carried out in all cows 16-20 h after the last treatment. Cows were categorized into primiparous and multiparous cows for analysis. Conception rate (CR) to timed AI, to further AI, overall conception rate and proportion of cows pregnant by 200 days in milk were compared between the age groups. Finally, two logistic regression models were calculated with conception to first service and conception by 200 DIM as the outcome variables. Independent variables were trial (categorical) and age group (primiparous versus multiparous). Conception rates to TAI were higher in primiparous than in older cows (37.9% versus 31.6%, P=0.015). Likewise pregnancy rates by 200 DIM were higher in primiparous cows (81.8% versus 75.4%, P=0.003). However, the extent of the difference varied between trials. Results indicate that Ovsynch protocols are more effective in primiparous than in older cows.     

Relationships between calf birth weight, prepartum concentrations of plasma energy metabolites and resumption of ovulation postpartum in Limousine suckled beef cows

The aim of this study was to determine the relationship between energy status before calving and calf birth weight and their potential effects on interval between calving and first ovulation. Sixty-nine Limousine, suckled beef cows were sampled weekly over a 3-yr period during the last 2 m.o. of pregnancy to determine the concentrations of nonesterified fatty acids (NEFA), beta-3-hydroxybutyrate (beta-OHB), glucose and glycerol. After parturition, progesterone concentrations were measured weekly to determine time of resumption of ovulation. Cows were allotted to 3 groups according to calf birth weight (Heavy: > 44 kg, n = 37; Medium: 39 to 43 kg, n = 56; and Light: < 38 kg, n = 45) and to postpartum ovarian resumption of cyclicity (Late: > 11 wk, n = 41; Mid: 7 to 10 wk, n = 57; and Early: < 6 wk, n = 40). Puerperium glycaemia of the dams was steady state (0.66 +/- 0.03 g/L) and was not related to calf birth weight. Plasma NEFA, beta-OHB and glycerol values were higher (P < 0.05) in Heavy than in Medium and Light group dams during the last 4 wk of pregnancy. Interval between calving and first ovulation was significantly longer for primiparous than for multiparous cows (respectively, 9.9 +/- 2.0 and 7.7 +/- 1.4 wk; P < 0.05). Calf birth weight was not related to time of first ovulation. Late primiparous cows had higher NEFA plasma concentrations than Mid and Early group primiparous cows during the last 4 wk of pregnancy, whereas NEFA plasma concentrations were not related to interval between calving and first ovulation in multiparous cows. Thus, lipomobilization increased with calf birth weight during the last 4 wk of pregnancy. High level of body reserves mobilization was associated with delayed first ovulation in primiparous but not in multiparous cows.     

Metabolic variables and plasma leptin concentrations in dairy cows exhibiting reproductive cycle abnormalities identified through milk progesterone monitoring during the post partum period

We have used milk progesterone analysis to monitor reproductive function in lactating dairy cows and have then related this reproductive function to a variety of metabolic variables. Monitoring of cows (n = 41) during the period of onset of luteal function (first milk progesterone reading>5 ng/ml) revealed that delayed onset was associated with increased milk yield and greater loss of body weight and body condition but was not related to plasma metabolite or leptin concentrations. Further monitoring of reproductive function in these 41 cows and an additional 33 cows (total n = 74) during the mating period (from weeks 6 to 14 post partum) identified reproductive cycle abnormalities in 29 (39.2% of animals). The occurrence of cycle abnormalities was associated with increased milk yield (P < 0.05), elevated plasma beta hydroxybutyrate (P < 0.05) and reduced plasma leptin (P < 0.01) concentrations as well as a lower (P < 0.05) rate of gain of body weight and condition score but was not associated with plasma urea or glucose concentrations. Furthermore, cows exhibiting cycle abnormalities had a longer (P < 0.01) interval to first service and a smaller percentage had conceived by 100 days post partum (34.5% versus 66.7%; P < 0.01). These results provide further evidence that impaired reproductive function during the post partum period in dairy cows is caused by a poor energy status and not elevated urea concentrations. Reduced plasma leptin concentrations in animals suffering reproductive dysfunction further supports this view.     

Effect of climate on the response to three oestrous synchronisation techniques in lactating dairy cows

The reproductive efficiency of Friesian dairy cows was investigated in a three (oestrous synchronisation technique) x two (seasons of the year) factorial design. The 90 primiparous and multiparous cows (winter, n=42; summer, n=48) were allocated at random to three synchronisation treatments (n=30 cows per treatment). In treatment 1 (GPG), the cows were administered 15 mg PGF(2alpha) i.m. at 30 +/- 3 days postpartum, 100 microg GnRH i.m. at 51 +/- 3 days and 15 mg PGF(2alpha) 7 days later. A second 100 microg dose of GnRH was given after, further 2 days and fixed time AI occurred 16-20 h later. In treatment 2 (PG-PG), 15 mg PGF(2alpha) was administered i.m. to each cow on three occasions at successive 14 days interval starting at 30 +/- 3 days postpartum and the cows were inseminated at observed oestrus following the third dose of PGF(2alpha). Cows in treatment 3 (PG) had a single administration of 15 mg PGF(2alpha) i.m. at 57+/-3days postpartum and were inseminated as in treatment 2. Mean daily ambient temperature was 10.9 degrees C in winter (November-March) and 20.2 degrees C in summer (June-October). The cows were confined in an open-fronted shed and had ad libitum access to a complete diet with a 37:63 forage to concentrate ratio. Body condition score was assessed at 57 +/- 3 days postpartum. Cow rectal temperature at insemination, milk yield, reproductive data and climatic variables were recorded. Blood samples were collected for progesterone assay on days 4, 11, 18, 25, 32, 39 and 46 post-AI from 54 of the cows (19 GPG; 17 PG-PG; 18 PG). Pregnancy rate to first AI was 36.7% (11/30) for GPG and 16.7% (5/30) for both PG-PG and PG treatments. The difference was not significant. The cumulative pregnancy rate after third AI were GPG 83.3% (25/30), PG-PG 60.0% (18/30) and PG 60.0% (18/30; P<0.057). The cumulative pregnancy rate for cows inseminated in the winter (81.0%; 34/42) was higher (P<0.01) than for those inseminated in the summer (56.3%; 27/48). The interval from calving to first service was shorter (P<0.05) in treatment PG-PG (65.4+/-1.3 days) than in PG (69.2+/-1.3 days). Mean plasma progesterone concentrations post-AI of pregnant cows were higher (P<0.001) for GPG cows than those for PG-PG and PG cows. Plasma progesterone levels of pregnant cows tended to be higher (P=0.087) in winter than in summer. In conclusion, although the cumulative pregnancy rate was higher for GPG cows, it may be appropriate to correct the nutrition and management of the herd before resorting to synchronisation techniques to improve animal reproductive performances.     

Progesterone concentrations during estrous cycle of dairy cows exposed to electric and magnetic fields

Sixteen multiparous nonpregnant lactating Holstein cows (each weighing 662 ± 65 kg in 150.4 ±40 day of lactation) were confined to wooden metabolic cages with 12:12 h light:dark cycle during the experiment. The cows were divided into two sequences of eight cows each and exposed to electric and magnetic fields (EMF) in an exposure chamber. This chamber produced a vertical electric field of 10 kV/m and a uniform horizontal magnetic field of 30 μT at 60 Hz. One sequence was exposed for three estrous cycles of 24 to 27 days. During the first estrous cycle, the electric and magnetic fields were off; during the second estrous cycle, they were on; and during the third estrous cycle, they were off. The second sequence was also exposed for three 24 to 26 days estrous cycles, but the exposure to the fields was reversed (first estrous cycle, on; second estrous cycle, off; third estrous cycle, on). The length of each exposure period (21 to 27 days) varied according to the estrous cycle length. No differences were detected in plasma progesterone concentrations and area under the progesterone curve during estrous cycles between EMF nonexposed and exposed periods (2.28 ±0.17 and 2.25 ± 0.17; and 24.5 ± 1.9 vs. 26.4 ± 1.9 ng/ml, respectively). However, estrous cycle length, determined by the presence of a functional corpus luteum detected by concentrations of progesterone equal to or more than 1 ng/ml plasma, was shorter in nonexposed cows than when they were exposed to EMF (22.0 ± 0.9 vs. 25.3 ± 1.4 days). Bioelectromagnetics 19:438–443, 1998. © 1998 Wiley-Liss, Inc.     

Follicular growth and ovulation in postpartum beef cows following calf removal and GnRH treatment

This study investigated the effects of calf removal (CR) and gonadotrophin releasing hormone (GnRH) administration on the duration of the postpartum anoestrous period in suckled beef cows. Experiment 1 involved 20 multiparous suckled cows that were assigned to each of two treatments on Day 61 postpartum: (i) unlimited access to their calves (C; n=10) and (ii) calf removal for a period of 96 h (CR96, n=10). Experiment 2 involved 24 multiparous cows that were assigned to each of two treatments on Day 63 postpartum: (i) CR96 (n=12); and (ii) CR96 plus 250 microg of GnRH administered on the day before calf return (CR96+GnRH, n=12). Experiment 3 was a 3x2 factorial experiment, involving 48 multiparous cows assigned to the experiment on Day 58 postpartum. The factors were C, CR96 and calf removal for 144 h (CR144), and 0 or 250 microg GnRH administered on the day prior calf return. In Experiment 1, the number of cows that ovulated within 12 days of calf removal was higher (P<0.05) in CR96 group (3/9) compared to the C group (0/10). In Experiment 2, all 12 cows in the CR96+GnRH group ovulated. In contrast only 4/12 cows in the CR96 group ovulated in response to calf removal. The diameter of the ovulatory follicle tended (P=0.06) to be smaller in CR96+GnRH cows (9.8 +/- 0.3 mm) than in CR96 cows (11.3 +/- 0.9 mm). The maximum diameter attained by the corpus luteum (CL) also tended (P=0.08) to be smaller for cows in the CR96+GnRH than for cows in the CR96 group (12.1 +/- 2.4 mm versus 16.7 +/- 7.5 mm, respectively). Plasma progesterone concentrations 12 days after calf removal tended (P=0.06) to be lower in CR96+GnRH cows than in CR96 cows (0.66 +/- 0.1 ng/ml versus 2.00 +/- 1.1 ng/ml, respectively). Few cows in the CR96+GnRH group regained normal cyclical activity and the interval from onset of calf removal to conception was longer (P<0.05) compared to cows in the CR group (52.2 +/- 5.7 days versus 20.0 +/- 6.6 days). In Experiment 3, 5/8 cows on the CR144 group and all 8 cows in the CR144+GnRH group ovulated. However, the interval from CR to conception was similar for all treatments. Temporary (96-144 h) calf removal, particularly in combination with GnRH treatment, can induce a high proportion of beef cows to ovulate, but the restoration of oestrous cycles may not be achieved.     

Plasma concentrations of inhibin a and follicle-stimulating hormone differ between cows with two or three waves of ovarian follicular development in a single estrous cycle

Patterns of ovarian follicle development were monitored daily in Holstein-Friesian cows that had two (n = 4) or three (n = 4) waves of ovarian follicle development during a single estrous cycle. The plasma from daily blood samples was used in assays for inhibin A, FSH, progesterone, and estradiol-17beta. Mean cycle lengths for cows with two and three waves were 21.8 and 25.3 days, respectively (P < 0.02). Although the average number of follicles >3-mm diameter on each pair of ovaries was similar for two- and three-wave cows on Days 2, 3, and 4 (Day 0 = day of ovulation; 8.6 vs. 9.6 follicles), there were more follicles >6-mm diameter on the ovaries of cows with two waves on Days 3 and 4. This difference was associated with a shorter interval from wave emergence to peak concentrations of inhibin A during the first wave in two-wave cows (2.0 vs. 3.8 days; P = 0.03) and with higher peak concentrations (474 vs. 332 pg/ml; P = 0.03). Differences in peak FSH concentrations were not significant (1.7 vs. 1.3 ng/ml; P = 0.10) and were inversely related to inhibin A concentrations. The peak concentrations of inhibin A and FSH in the second nonovulatory wave in the three-wave cows were similar to the low concentrations measured in the first wave (292 vs. 332 pg/ml of inhibin A, 1.3 vs. 1.3 ng/ml of FSH; P > 0.20). Average peak concentrations of inhibin A and FSH were similar during the ovulatory wave for cows with either two or three waves in a cycle (432 vs. 464 pg/ml of inhibin A, 2.3 vs. 2.1 ng/ml of FSH; P > 0.3). The lower concentrations of FSH during the emergence of the first follicular wave in cows with three-wave cycles may have reduced the rate of development of some of the follicles and reduced the concentrations of inhibin A. This pattern of lower concentrations of FSH and inhibin A was repeated in the second nonovulatory wave but not in the ovulatory wave. Subtle differences in the concentrations of these two hormones may underlie the mechanism that influences the number of waves of ovarian follicle development that occur during the bovine estrous cycle.     

Reproductive hormones associated with the ovarian cyst response to GnRH

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

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

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

Extension of oestrous cycles and prolonged secretion of progesterone in non-pregnant cattle infused continuously with oxytocin

The experimental objective was to evaluate how continuous infusion of oxytocin during the anticipated period of luteolysis in cattle would influence secretion of progesterone, oestradiol and 13,14-dihydro-15-keto-prostaglandin F-2 alpha (PGFM). In Exp. I, 6 non-lactating Holstein cows were infused with saline or oxytocin (20 IU/h, i.v.) from Day 13 to Day 20 of an oestrous cycle in a cross-over experimental design (Day 0 = oestrus). During saline cycles, concentrations of progesterone decreased from 11.0 +/- 2.0 ng/ml on Day 14 to 2.0 +/- 1.3 ng/ml on Day 23; however, during oxytocin cycles, luteolysis was delayed and progesterone secretion remained near 11 ng/ml until after Day 22 (P less than 0.05). Interoestrous interval was 1.6 days longer in oxytocin than in saline cycles (P = 0.07). Baseline PGFM and amplitude and frequency of PGFM peaks in blood samples collected hourly on Day 18 did not differ between saline and oxytocin cycles. In Exp. II, 7 non-lactating Holstein cows were infused with saline or oxytocin from Day 13 to Day 25 after oestrus in a cross-over experimental design. Secretion of progesterone decreased from 6.8 +/- 0.7 ng/ml on Day 16 to less than 2 ng/ml on Day 22 of saline cycles; however, during oxytocin cycles, luteolysis did not occur until after Day 25 (P less than 0.05). Interoestrous interval was 5.9 days longer for oxytocin than for saline cycles (P less than 0.05). In blood samples taken every 2 h from Day 17 to Day 23, PGFM peak amplitude was higher (P less than 0.05) in saline (142.1 +/- 25.1 pg/ml) than in oxytocin cycles (109.8 +/- 15.2 pg/ml). Nevertheless, pulsatile secretion of PGFM was detected during 6 of 7 oxytocin cycles. In both experiments, the anticipated rise in serum oestradiol concentrations before oestrus, around Days 18-20, was observed during saline cycles, but during oxytocin cycles, concentrations of oestradiol remained at basal levels until after oxytocin infusion was discontinued. We concluded that continuous infusion of oxytocin caused extended oestrous cycles, prolonged the secretion of progesterone, and reduced the amplitude of PGFM pulses. Moreover, when oxytocin was infused, pulsatile secretion of PGFM was not abolished, but oestrogen secretion did not increase until oxytocin infusion stopped.     

Observations on the use of human chorionic gonadotrophin (HCG) during the post-insemination period on conception rates in synchronized beef cows with sub-optimum reproductive performances

Oestrus was synchronized in 46 Afrikaner and Mashona beef cows by two injections of cloprostenol 11 days apart. All cows had a history of sub-optimum reproductive performance. Cows were inseminated up to three times after the second cloprostenol injection on the basis of observed oestrus and changes in the conductivity of cervical mucus. Half the cows received daily injections of 1000 i.u. HcG from days four to 19 after their last insemination; the other half received daily injections of 2 ml saline over the same period. Concentrations of progesterone in plasma was determined from samples taken on days 6, 8 and 22 after the last insemination. Treatment did not significantly affect conception rate and overall conception rate was 39 per cent. On day 8 after insemination none of the 8 cows that had progesterone levels of less than 1 ng/ml were pregnant when examined at day 70. Mean progesterone concentrations were not significantly different between treated and control cows on days 6 and 8, but were significantly higher (P<0,05) in treated cows by day 22. The practical significance of using HcG to stimulate luteal function in the early post-inseminaion period is discussed.     

Conception rates in dairy cows after timed-insemination and simultaneous treatment with gonadotrophin releasing hormone and/or prostaglandin F2 alpha

This study was designed to determine conception rates in dairy cows after timed-insemination and simultaneous treatment with gonadotrophin releasing hormone (GnRH) and/or prostaglandin F2 alpha (PGF2alpha). A total of 2352 cows was randomly assigned to six groups. Cows in Groups 1 to 5 were palpated per rectum to determine the presence of a corpus luteum (CL) on the ovary, and blood samples were obtained for the determination of plasma progesterone (P4) concentrations. Cows with a CL and P4 concentrations >1 ng/ml were treated (Day 0) with PGF2alpha (25 mg, i.m.) and were observed for estrus. Cows in estrus prior to 72 hours after treatment (Group 5, n = 106) were bred, but were not treated. Cows not observed in estrus by 72 hours were divided into four remaining groups, were bred between 72 and 80 hours and were assigned as follows: Cows in Group 1 (n = 203) were not treated; Cows in Group 2 (n = 200) were treated with GnRH (100 ug, i.m.); Cows in Group 3 (n = 201) were treated with PGF2alpha (25 mg, i.m.); and cows in Group 4 (n = 202) were treated with both GnRH and PGF2alpha. Cows in Group 6 (n = 1440) were not treated with PGF2alpha on Day 0 and were estrual cows that were bred on days when cows in Groups 1 to 5 were time-inseminated. The percentage of cows in all groups pregnant at 45 to 50 days after one insemination was compared using analysis of variance (P<0.05). The conception rate of cows in Group 2 was significantly higher than that of cows in Groups 1 to 4. There was a significant group-by-season interaction. Cows treated with GnRH during the spring had a higher conception rate than at other times of the year. Conception rates of cows in Groups 1 to 4 that were inseminated during the summer were low and not significantly different from each other. Conception rates of cows in Groups 5 and 6 inseminated during the summer were not significantly different from each other, but were significantly higher than that of cows in Groups 1 to 4 that were inseminated during the summer.