Synchronization of ovulation in beef herds: improved conception rate after and interrupted course of progesterone administration

Administration of 50 mg progesterone in 6 im injections every 48 hours was used to synchronize the estrous cycles of 6 herds of cows and/or heifers. Because of a previous finding that cows in the last 3rd of their estrous cycles when progesterone treatment is begun exhibit lower fertility, such cows were excluded by presynchronization of the herd 8 days prior to the start of treatment by 4 48-hourly injections of 50 mg progesterone. 69%-86% of the animals became pregnant in 4 of the 6 herds during the synchronized estrus, which compares favorably with the average 1st insemination conception figure without synchronization (approximately 70%). Previous conception rates observed averaged approximately 47%.     

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.     

Effect of human chorionic gonadotrophin on dominant follicles in cows: formation of accessory corpora lutea, progesterone production and pregnancy rates.

Thirty-four lactating Holstein cows were randomly assigned to four groups for treatment with human chorionic gonadotrophin (hCG, 1000 iu) at insemination day 0 (n = 8) or 7 (n = 9) or 14 days (n = 9) after insemination or with no hCG treatment (control, n = 8). Ultrasound imaging of the ovaries and plasma progesterone measurements were carried out to determine follicular dynamics and corpus luteum growth and function. Rates of formation of accessory corpora lutea were higher among cows treated on days 0 (three cows), 7 (seven cows) or 14 (four cows) than in the controls (one cow). Total corpus luteum diameter was greater (P less than 0.01) among hCG-treated cows than in controls 7-42 days after insemination. Concentrations of progesterone in plasma were significantly (P less than 0.05) higher in cows treated with hCG on days 7 or 14 than in those treated on day 0 or in controls, at days 18, 35 or 42 after insemination. Seven of the cows treated on day 7 became pregnant, whereas four, four and three cows treated on days 0 or 14 and control cows became pregnant, respectively. The results suggest that hCG treatment at 7 days after insemination could be used to produce accessory corpora lutea, raise plasma progesterone concentration and hence reduce the incidence of early embryonic mortality in cattle.     

Exogenous glucagon effects on health and reproductive performance of lactating dairy cows with mild fatty liver

Severe fatty liver, a metabolic disease of dairy cows in early lactation, results in decreased health and reproductive performance, but can be alleviated by treatment with i.v. injections of glucagon. Mild fatty liver in cows effects on health and reproductive performance were determined by treatment with 14-day s.c. injections of glucagon at 7.5 or 15 mg/day. Multiparous Holstein cows (n=32) were grouped into Normal and Susceptible based on liver triacylglycerol concentrations (>1% liver tissue biopsy wet weight) at day 8 postpartum (day 0=day of parturition). Susceptible cows (n=24) were assigned randomly to three groups and s.c. injected with 0mg glucagon [60 ml 0.15M NaCl] [n=8] (same for Normal cows), 2.5 mg glucagon, or 5 mg glucagon every 8 h for 14 days, beginning day 8 postpartum. Mild fatty liver resulted in an increased number of days with elevated body temperature during the injection period, an increased incidence of mastitis after glucagon treatment, increased days to first estrus and insemination, increased days before conception occurred, and decreased conception rate. In cows with mild fatty liver, glucagon (15 mg/day) decreased the number of days with elevated body temperature and the incidence of mastitis after hormone treatment. From these results, we suggest that mild fatty liver is detrimental to health and reproduction of dairy cows and, furthermore, that exogenous glucagon decreases some of these detrimental effects.     

Conception rates and serum progesterone concentration in dairy cattle administered gonadotropin releasing hormone 5 days after artificial insemination

The objectives of this study were to determine the effect of administration of exogenous GnRH 5days after artificial insemination (AI) on ovarian structures, serum progesterone concentration, and conception rates in lactating dairy cows. In experiment 1, 23 Holstein cows were synchronized using the Ovsynch protocol. Five days after AI (day 0) cows were assigned randomly to receive either saline (saline; n=11) or 100microg GnRH (GnRH; n=12). To examine ovarian structures, ultrasonography was performed on day 1 and every other day beginning on day 5 until day 13. On days 5 and 13 blood samples were obtained to measure serum progesterone concentrations. All cows in the GnRH-treated group developed an accessory corpus luteum (CL), whereas cows in the saline group did not. Mean serum progesterone concentrations did not differ between GnRH and saline groups on day 5 (1.64+/-0.46ng/ml versus 2.04+/-0.48ng/ml). On day 13 serum progesterone concentrations were greater (P<0.05) in the GnRH group compared with saline (5.22+/-0.46ng/ml versus 3.36+/-0.48ng/ml). In experiment 2, 542 lactating cows, at two different commercial dairies, were used to test the effect of administering GnRH 5 days after AI on conception rates. Cows were synchronized and detected for estrus according to tail chalk removal. Cows detected in estrus received AI within 1h after detection of estrus. Five days after AI, cows were assigned randomly to receive either GnRH (n=266) or saline (n=276). Pregnancy status was determined by palpation per rectum of uterine contents approximately 40 days after AI. There was no effect of farm on conception rate. There was no effect of treatment as conception rates did not differ between GnRH and saline groups (26.7% GnRH versus 24.3% saline). Regardless of treatment, days in milk, parity, milk yield, and number of services had no effect on the odds ratio of pregnancy. In summary, the results of this study indicated that GnRH administered 5 days after AI increased serum progesterone by developing an accessory CL but did not improve conception rates in dairy cattle.     

Supplementing previously treated anestrous dairy cows with progesterone does not increase first-service conception rate

The objective of this study was to investigate the effect of supplementing previously treated anovulatory anestrous (AA) dairy cows with progesterone delivered intra-vaginally for 7 days, commencing 4 or 5 days after insemination, on first-service conception rate. A clinical trial, involving 990 AA dairy cows in 14 dairy herds, was conducted during the 2002/2003 breeding season. On Day -8, all cows were treated with a progesterone-containing intravaginal device (Cue-Mate). The devices were removed on Day -2; on Day -1, all cows were given 1mg of estradiol benzoate im. Cows in the control group (n = 499) received no further treatments. Cows in the treatment group (n = 491) that had been inseminated on Day 0 or 1 had a new device inserted (on Day 4 or 5), with removal of the device after 7 days. First-service conception rates for the control and treatment groups were not different (35.0% versus 36.7% respectively; P = 0.41). Furthermore, there was no difference in conception rates between cows that had devices inserted on Day 4 or 5 (31.3% versus 37.2% respectively; P = 0.45). In conclusion, supplementation of previously treated AA dairy cows with an intravaginal progesterone-releasing device for 7 days (commencing 4 or 5 days after insemination) did not significantly improve first-service conception rate.     

Plasma oestradiol and progesterone during early pregnancy in the cow and the effects of treatment with buserelin

We have monitored plasma concentrations of oestradiol and progesterone after insemination in dairy cows, and investigated the effects of injection with 10 μg of the gonadotrophin-releasing hormone (GnRH) analogue buserelin, a treatment known to result in an improvement in conception rate. Animals were injected intramuscularly on Day 12 after insemination with 2.5 ml of either control saline (n = 29) or buserelin (n = 26). Blood samples were collected from Day 8 to Day 17 and milk samples from the day of insemination until Day 30. On the basis of milk progesterone profiles, in the control group 15 cows remained pregnant (control pregnant) and 11 underwent luteolysis (control not pregnant), whereas in the treated group 13 cows remained pregnant (treated pregnant) and 13 underwent luteolysis (treated not pregnant). Three cows were excluded from the control group owing to high progesterone at insemination or failed ovulation. In both the control and treated groups mean plasma progesterone concentration was significantly (P < 0.05) lower in non-pregnant cows than in pregnant ones from Day 12 post-insemination. However, no significant effect of buserelin treatment on plasma progesterone concentration was detected. In the control group the plasma oestradiol concentration was similar in the pregnant and non-pregnant cows. In the treated group, plasma oestradiol concentration in the non-pregnant cows was similar to that in the control group, whereas in the treated pregnant cows the plasma oestradiol concentration showed a significant (P < 0.05) decline after treatment. As oestradiol is known to stimulate the development of the luteolytic mechanism at this time, we suggest that buserelin is acting to reduce the strength of the luteolytic drive in some cows, thus improving the chance of the embryo being able to prevent luteolysis.     

Fixed-time AI protocols replacing eCG with a single dose of FSH were less effective in stimulating follicular growth, ovulation, and fertility in suckled-anestrus Nelore beef cows

The aim of the present study was to evaluate the effects of a single treatment with FSH on diameter of the largest follicle and on conception rates of suckled Bos indicus beef cows submitted to timed artificial insemination (TAI). Four hundred fifty-six suckled anestrous Nelore beef cows at 30-60 days postpartum were assigned to treatments. At the first day of the estrous synchronization protocol (Day 0), all cows received a progesterone-releasing intravaginal device plus 2mg of estradiol benzoate. On Day 8, cows were assigned to blocks according to the diameter of the largest follicle and then allocated to one of three treatment groups (Control, FSH, or eCG) within each block. Simultaneously to progesterone device withdrawal on Day 8, cows in the eCG treatment group (n=150) received 300 IU of eCG and cows in FSH treatment group (n=153) received 10mg of FSH, and Control cows (n=153) did not receive any additional treatment. Additional treatments with 150 μg of cloprostenol and 1mg of estradiol cypionate (EC) were also administered concurrently to progesterone device removal in all cows on Day 8. Two days later (D10), TAI and ovarian ultrasonic examinations to evaluate follicle size were performed in all cows. On Day 12, a subset of cows (n=389) were submitted a second ultrasonic exam to confirm ovulation. Final follicular growth (mm/day) was less (P=0.006) in both Control (0.95±0.11) and in FSH-treated cows (0.90±0.10) than in eCG-treated cows (1.40±0.13). Interestingly, there was a treatment-by-BCS interaction in ovulation results (P=0.03), in which, eCG treatment increased percentage of cows having ovulations with a lesser BCS. Similarly, there was a treatment-by-BCS interaction for conception (P=0.04), where the eCG treatment increased fertility in cows with a lesser BCS. In conclusion, FSH failed to stimulate final follicular growth, ovulation, and conception rate in sucked-anestrous beef cows submitted to TAI as effectively as eCG. However, physiological effects of eCG seem to be more evident in cows with a lesser BCS.     

Blood levels of progesterone and 15-Keto-13, 14-dihydro prostaglandin F(2alpha) during the estrous cycle of oxytocin-treated cows

Six non-lactating Holstein cows were injected with 230 iu oxytocin subcutaneously twice daily from days 2 through 6 of the cycle. Controls (n=6) were given saline injections using the same schedule. Blood samples were collected at frequent intervals before and after each saline or oxytocin injection. Progesterone and 15-Keto-13, 14-dihydro prostaglandin F(2alpha) (PGFM), the major metabolite of prostaglandin F(2alpha), were analysed by radio-immunoassay. Oxytocin injections significantly increased plasma prostaglandin concentrations on days 2 and 3 when compared with the controls. In two oxytocin-treated cows, the cycle was shortened to 10 and 12 days. Estrus was preceded by a PGF(2alpha) release very similar to that preceding spontaneous estrus. Two of the oxytocin-treated cows showed estrus on day 21 and 22 preceded by luteolytic release of PGF(2alpha). Two oxytocin-treated cows developed cystic corpora lutea and had not shown heat when the ovaries were removed four weeks later. All oxytocin-treated cows showed a slower progesterone increase through day 8 than the controls. The study shows that endocrine events preceding cycle alterations in oxytocin-treated cows involve release of PGF(2alpha) and lowered levels of progesterone.     

Effect of short-term treatment with bovine somatotropin at estrus on conception rate and luteal function of repeat-breeding dairy cows.

We studied the effect of recombinant bovine somatotropin (rbST) at the time of estrus on progesterone concentrations and conception rates of repeat-breeding Holstein cows. We used repeat-breeding cows of varied parity (n = 510). All the animals were clinically healthy and had had at least three unsuccessful services before entering the study. After detection of estrus, the cows were randomly assigned to either a treated (n = 201) or a control (n = 309) group. The animals in the treated group were given rbST (500 mg s.c.) at the time of estrus and again 10 d later. Artificial insemination was performed 12 h after the first detection of estrus. In order to evaluate the effect of rbST on luteal function, blood samples were taken from 10 cows in each group every 3 d for 18 d, starting on the day of insemination (Day 0) to determine progesterone concentrations. Conception rates were significantly higher (P < 0.05) in the cows treated with rbST (29.3%) than in the control cows (16.9%). The effects of rbST were maximal in cows with 8 or more previous unsuccessful services and in cows with 2 to 4 calvings. Progesterone concentrations tended to be higher in nonpregnant cows that were treated with rbST than in those that were not treated. The difference between groups was significant (p < 0.05) on Day 18 after insemination. In pregnant cows there were no significant differences in progesterone concentrations between treated and nontreated animals at any time. Treatment with rbST at estrus improved the conception rate of repeat-breeding Holstein cows. This effect was associated with an increase in circulating progesterone concentrations on Day 18.     

Effect of repeated administration of PGF2alpha in the early post partum period on the prevalence of clinical endometritis and probability of pregnancy at first insemination in lactating dairy cows

Two experiments were performed to determine the effects of repeated administration of PGF2alpha in the immediate post partum period on the prevalence of clinical endometritis at 22 and 58 days post partum, and the probability of pregnancy at first insemination, in post partum dairy cows. In Experiment 1, 228 cows on day 7 post partum were used. Cows in Group 1 (n = 114) were treated twice with PGF2alpha (25 mg, im) 8 h apart on days 7 and 14 post partum, and only once on days 22 and 35 post partum. Cows in Group 2 (n = 114) served as untreated controls. Vaginoscopy and transrectal palpation of the genital tract were done on days 22 and 58 post partum. Cows in both groups were inseminated at estrus after a voluntary waiting period of 100 days. Pregnancy was determined by transrectal palpation between 45 and 50 days after insemination. Repeated administration of PGF2alpha in the immediate post partum period did not reduce the prevalence of clinical endometritis on days 22 or 58 post partum (65 versus 62% on day 22 and 28.3 versus 35.2% on day 58 in Groups 1 and 2, respectively). There was no significant difference in the probability of pregnancy at first insemination between cows in both groups. In Experiment 2, 418 cows on day 7 post partum were used. Cows in Group 1 (n = 209) were treated twice with PGF2alpha (25 mg, im) 8h apart on days 7 and 14 post partum, and only once on days 22 and 35 post partum. Cows in Group 2 (n = 209) served as untreated controls. Cows in both groups were subjected to the Presynch and Ovsynch protocols on days 49 and 75 post partum, respectively. Pregnancy was determined by transrectal ultrasonography between days 29 and 32 after timed insemination. There was no significant difference in the probability of pregnancy at first insemination between cows in both groups. In conclusion, repeated administration of PGF2alpha to cows in this herd had no effect on the prevalence of clinical endometritis at 22 and 58 days post partum, and that there was no effect on the probability of pregnancy after insemination at estrus among cows with a voluntary waiting period of > 100 days, or at timed AI at 85 days in milk when Presynch was employed.     

Fertility of swamp buffalo following the synchronization of ovulation by the sequential administration of GnRH and PGF2alpha combined with fixed-timed artificial insemination

This study evaluated fertility in swamp buffalo after synchronization of ovulation combined with fixed time artificial insemination. At the start of the study, designated day 0, from a group of 98 female Thai swamp buffalo, 55 buffalo (heifers n° = 20 and cows n° = 35) were selected to be synchronized with GnRH (Day 0) followed by PGF₂alpha (Day 7) and a second treatment with GnRH (Day 9). All buffalo were inseminated at two fixed times 12 h and 24 h after the second injection of GnRH (Ovsynch+TAI group); a second group of 43 buffalo (heifers n° = 19 and cows n° = 24) were not treated and were artificially inseminated (AI) at natural estrus (AI group). Blood samples were taken 22 days after insemination to evaluate progesterone plasma levels. In the Ovsynch+TAI group, overall conception rate (CR; i.e. the number of cows with progesterone >4.0 ng/ml on day 22 after AI divided by the number of animals inseminated), was 38.1% and overall pregnancy rate (PR; i.e. the number of cows that were pregnant at day 50-60 after insemination divided by the number of animals inseminated), was 32.7%. In the AI group overall CR and PR was 34.9%.Within the Ovsynch+TAI group, CR and PR were reduced (P < 0.05) in heifers compared with cows (CR 15.0% vs. 51.4% for heifers and cows, respectively; PR 15.0% vs. 42.9% for heifers and cows, respectively). Within the AI group the efficacy of treatment was similar between heifers and cows (CR and PR 31.6% for heifers and 37.5% for cows).In conclusion, this study indicates that in swamp buffalo it is possible to synchronize ovulation and use timed artificial insemination with the Ovsynch+TAI protocol.     

Induced abortion in cattle

Several procedures were used to abort cattle during the second and third trimesters of gestation. The treatment to abortion interval was better (P<0.05) when dexamethasone trimethyiacetate (DTMA) injections repeated at either 6 or 4 day intervals than when a single injection of DTMA was followed 6 days later by the administration of stilboestrol. The treatment to abortion interval was not significantly shorter when DTMA was repeated after 4 days rather than 5 days (0.10<P>0.05). Prostaglandin F_2α produced abortion 1 to 4 days following direct administration into the foetai fluids.Peripheral plasma progesterone concentration had a tendency to rise immediately following the second injection of DTMA given at a 6 day interval. This was followed by a decline. Two injections of DTMA given at 4 day intervals resuited in a decline in progesterone concentration. Abortion occurred when plasma progesterone concentrations were about 1 ng/ml in cows treated with DTMA. In cows treated with prostagiandin F_2α the plasma progesterone concentration fell rapidly within one day of administration to approximately 2 ng/ml, at which concentration abortion took place.     

Plasma gonadotrophin responses in beef cows two progesterone plus prostaglandin treatment

Progesterone Releasing Intravaginal Devices (Prids) were inserted into six post-partum beef cows for nine days and 0.5 mg cloprostenol was injected i m on day eight. Blood samples were taken via jugular venous catheters at frequent intervals for seven days after Prid removal and assayed for LH, FSH and progesterone. The induced pre-ovulatory type LH and FSH surges occurred between 35 and 123h after Prid withdrawal in five of the cows. In four cows which underwent surges during the time of most intensive sampling, LH levels were significantly higher during the 30h period prior to the LH surge than during the 30h period after the surge. FSH values were low for the 30h period preceding and the 14h period following the time of maximum FSH/LH concentrations. 16 - 30h after the FSH and LH surges, FSH values were again significantly raised compared with the period immediately after the surge. Despite the success of this Prid/PG regime in inducing ovulation, the variability in time between progestagen withdrawal and the LH surge and ovulation is such that the use of fixed time artificial insemination may give poor results.     

Cloprostenol-induced luteolysis in the marmoset monkey (Callithrix jacchus)

A single intramuscular injection of 0.5 micrograms cloprostenol was not luteolytic on Day 6 or 7 of the ovarian cycle (N = 3), but was luteolytic in some animals (3/5) on Day 8 and 9 and luteolytic in all 23 animals treated between Days 10 and 17 of the ovarian cycle, and in 7 animals treated between Days 19 and 43 of pregnancy. Luteal function was monitored by measurement of progesterone in peripheral blood using a simple and rapid non-extraction assay. There was a dramatic fall in peripheral blood progesterone to less than 10 ng/ml within 24 h of cloprostenol injection; progesterone remained at this low level until the day after post-treatment ovulation. The interval from cloprostenol injection to ovulation in animals treated between Days 8 and 17 was 10.7 +/- 0.3 days. A similar interval was found in pregnant animals. Embryos recovered from the uterus after cloprostenol treatment were morphologically normal (23/24).     

Compared to dinoprost tromethamine, cloprostenol sodium increased rates of estrus detection, conception and pregnancy in lactating dairy cows on a large commercial dairy

Using two PGF_2α treatments 14 days apart as a way to enhance estrus detection rate following the 2nd treatment is a reproductive management tool that continues to be used on large dairy farms. In one study, in cows with a functional CL and a dominant follicle, treatment with cloprostenol vs. dinoprost resulted in greater peripheral estradiol concentrations. The objective of the present study was to determine if cloprostenol could enhance pregnancy rates of cows in a large dairy herd using a PGF_2α program for 1st artificial insemination (AI). Lactating dairy cows (n = 4549) were randomly assigned to receive two treatments of either 500 μg cloprostenol or 25 mg dinoprost 14 days apart, with the 2nd treatment on the 1st day of the voluntary waiting period (57 DIM). Cows detected in estrus within 5 days after the 2nd treatment were inseminated. There was no effect of treatment on day of estrus detection, with 78% of cows inseminated on Days 3 or 4 following treatment. Cloprostenol increased (P < 0.01) estrus detection rates in 1st parity cows compared to dinoprost, 42.4 vs. 34.0%. In cows inseminated on Days 3 or 4 after treatment, cloprostenol increased (P = 0.05) conception rates compared to dinoprost, 38.3 vs. 34.4%. When treatments and parities were combined, conception rates increased (P < 0.02) with interval after treatment (27.0, 36.4, and 44.5% for Days 1 or 2, Days 3 or 4, and Day 5, respectively). Cloprostenol increased (P = 0.02) overall pregnancy rate compared to dinoprost, 14.4 vs. 12.2%. In summary, cloprostenol increased fertility in 1st parity cows inseminated on Days 3 or 4 following treatment and subsequently enhanced pregnancy rates of 1st parity lactating dairy cows compared to dinoprost. Fertility appeared greater in cows expected to have had a young antral ovarian follicle at treatment.     

Effect of one spontaneous estrus cycle (after synchronization with PGF2alpha) on reproductive performance in dairy cows

The objective of the study was to analyze the effect of a spontaneous estrus cycle after synchronization of estrus with prostaglandin F2alpha (PGF2alpha) in dairy cows on the degree of synchronization and reproductive performance. We assigned 557 Holstein cows to two treatment groups. In Group 1 estrus was synchronized by two treatments with 25 mg of Dinoprost-Trometamol in 14-day intervals. Cows were treated 27 to 33 days postpartum (dpp) and 41 to 47 dpp, respectively. Cows in Group 2 were treated with 25 mg of Dinoprost-Trometamol three times in 14-day intervals, starting at 34 to 40 dpp. The second and third injections were administered at 48 to 54 dpp and 62 to 68 dpp, respectively. All cows were inseminated on observed estrus after a voluntary waiting period of 65 days post partum. Thus cows in Group 1 were inseminated on spontaneous estrus and cows in Group 2 on induced estrus. Cows not inseminated at 80 days post partum were palpated per rectum and treated according to a predefined protocol. Herd reproductive performance measures did not differ significantly between groups. The proportion of cows with low serum progesterone levels was significantly higher 3 days after synchronization than 24 days after synchronization (97% vs 39%). The first-service conception rate was 34.8% in Group 1 and 30.7% in Group 2 (P > 0.05). Days open were 113.5 in Group 1 and 110.9 in Group 2 (P > 0.05). It is concluded that postponing artificial insemination for one spontaneous estrus cycle after synchronization decreased the degree of synchronization. This procedure, however, had no effect on herd reproductive performance compared to insemination on first observed estrus after synchronization.     

Plasma growth hormone, insulin-like growth factor-I, and milk production responses to exogenous human growth hormone-releasing factor analogs in dairy cows

Responses of plasma growth hormone (GH) and insulin-like growth factor-I (IGF-I), and milk production to subcutaneous (sc) injection(s) of two synthetic human growth hormone-releasing factor (hGRF) analogs were studied in dairy cows. Two mg of each hGRF analog dissolved in 5 ml saline per cow were injected into the shoulder area of each experimental animal, and jugular venous blood samples were collected via an indwelling catheter or by venipuncture. Plasma GH and IGF-I concentrations were measured by radioimmunoassay methods. In dry cows, the mean concentration of plasma GH after a single sc injection of hGRF analogs rose to 22.0-28.3 ng/ml at about 5 h from 1.4-1.7 ng/ml at 0 h (just before injection), and returned to the level before injection after 10-12 h. On the other hand, the plasma IGF-I began to increase after a lag of 4-6 h following a single injection of hGRF analogs, and reached maximum values of 71.1-89.4 ng/ml at 20 h from 43.7-46.4 ng/ml at 0 h. The IGF-I concentration at 24 h after a single injection of hGRF analogs was still higher than the value for the dry cows given saline. In lactating cows, the plasma concentration of GH at 2 h after daily sc injections of hGRF analogs during 14 consecutive days (an injection period) was higher than those for the lactating cows which received saline. Also, during the injection period, the concentration of IGF-I was higher in the lactating cows which received hGRF analog injections than in the cows which received saline injections. During the last 7 days of the injection period, the administration of hGRF analogs increased the mean milk yield by 11-19% in comparison with those for the saline injected cows. A positive correlation was observed between the mean plasma IGF-I concentration and the mean milk yield in the lactating cows treated with hGRF analogs throughout the injection and a postinjection (11 consecutive days after cessation of hGRF analog injection) periods. The results demonstrate that a single sc injection of hGRF analogs stimulates both GH release and the circulating level of IGF-I in dry cows, and that daily sc injections of hGRF analogs over 14 days enhance milk production, and plasma GH and IGF-I levels in lactating cows.     

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) ? 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.     

Seasonal variations in post-partum plasma progesterone levels and conception in primiparous and multiparous dairy cows

Progesterone concentrations in peripheral plasma of 18 primiparous and 34 multiparous dairy cows were determined by radioimmunoassay every 4 days, from 10 days post partum until the next conception. The interval from parturition to the first progesterone peak (greater than 4 ng/ml plasma) was significantly longer (P less than 4 ng/ml plasma) was significantly longer (P less than 0.01) in primiparous than in multiparous cows. The progesterone concentrations on Days 4-15 of the oestrous cycle were significantly lower (P less than 0.025) during the summer than in cycles occurring in winter. The conception rate during the summer was lower than in winter. In cows inseminated in summer, conception was related to the shape of the progesterone curve in the cycle preceding insemination.