GnRH treatment at artificial insemination in beef cattle fails to increase plasma progesterone concentrations or pregnancy rates

Impact of 17β-estradiol administration at the moment of timed-AI in Nelore cows with small dominant follicle or not showing estrus

We aimed to evaluate the effects of administering estradiol (E-17β) at the moment of timed-AI (TAI) on uterine gene expression, estrous expression rate (EER), and pregnancy rate (P/TAI) in Nelore cows with a small dominant follicle (DF) or not showing estrus at TAI. In Experiments 1 and 2 (Exp1, Exp2) cows were submitted to a P4/E-17β-based protocol (day 0) for synchronization of ovulation. On day 7, devices were removed, cows received 1 mg E-17β cypionate and 12.5 mg dinoprost. On day 9, cows with DF < 11.5 mm in diameter were split into different groups. In Exp1 (n = 16/group): Control (no treatment), E-2 (2 mg E-17β) and E-4 (4 mg E-17β). In Exp2: Control (n = 12); E-2 (n = 14); GnRH (0.1 mg gonadorelin acetate, n = 13); and E-2+GnRH (association of GnRH and E-17β, n = 13). Between days 9 and 11, endometrial thickness (ET), time of ovulation detection, and EER were recorded. In Exp1, a uterine cytological sample was collected 4 h after treatment to evaluate the transcript expression of receptors for E-17β (ESR1 and ESR2), oxytocin (OXTR), and P4 (PGR). In Experiment 3 (Exp3), 3829 suckled cows were submitted to a P4/E-17β-based protocol for TAI. On day 9, devices were removed and cows received 1 mg E-17β cypionate and 0.4 mg sodium cloprostenol. On day 11, TAI was performed and cows that did not demonstrate estrus received 0.1 mg gonadorelin acetate, and were allocated into two groups: GnRH (n = 368) and E-2+GnRH (2 mg E-17β; n = 363). In Exp1, plasma E-17β concentrations increased at 4 h after treatment in a dose-dependent manner but reduced at 12 h. The E-17β-treated cows had greater transcript abundance for OXTR and lesser for ESR1 and ESR2, and the ET was reduced 12 h after treatment (P < 0.05). No significant difference (P > 0.1) was observed between the E-17β doses in estrus or ovulation rate. In Exp2, the interval from treatment to ovulation was longer (P < 0.05) in the E-17β group. GnRH-treated cows showed higher ovulation rates (89 vs. 35 %) compared to cows not treated with GnRH, as E-17β-treated cows (P < 0.01) had a lower ovulation rate compared to those not receiving E-17β (44 vs. 78 %). In Exp3, P/TAI was 55 % for cows in estrus. For those not showing estrus, no difference (P > 0.1) in P/TAI was observed between GnRH (34 %) and E-2+GnRH (31 %) groups. Cows with a DF ≥ 11 mm (n = 192) had a greater (P < 0.05) P/TAI (49 %) than those with DF < 11 mm (n = 377; 29 %). In conclusion, E-17β administration in the moment of TAI modulates the mRNA expression of uterine receptors in cows with a small DF but does not impact the P/TAI compared with GnRH treatment in suckled Nelore not showing estrus previous to TAI.

Seasonal distribution of repeat breeder cows and evaluation of modified protocols for post AI treatment during summer

An important problem of dairy industry worldwide is repeat breeder cows (RB). In this study, one farm in Northern Greece was chosen on the basis of subfertility. First, the available reproductive data of the previous year were evaluated; then, the farmer was advised to use the most common treatments for RB [GnRH with the third artificial insemination (AI+GnRH) or ovsynch in cows found empty after the second AI], and their efficacy was monitored throughout the year. Cows (n = 147) 2.5 to 4.5 years old were included. During the next year, post AI treatments (meta-ovsynch and meta-AI) were used during summer; GnRH (100 μg of gonadorelin) alone or in combination with meloxicam (0.5 mg/kg BW) was used during summer. Meta-ovsynch protocols were administered to 29 cows detected as non-pregnant after 3 AIs; cows were divided randomly into 3 meta-ovsynch (OS) groups: group OS (controls, n = 10), standard ovsynch protocol; group OS+GnRH (n = 10), OS plus GnRH 10 days after AI; and group OS+GnRH+NSAID (n = 9), OS plus GnRH 10 days after AI plus meloxicam 15 days after AI. Meta-AI protocols were tested in 27 RB cows; cows were divided randomly into 3 AI groups: GnRH (n = 9), 1 dose of GnRH with AI; GnRH-GnRH (n = 9), two doses of GnRH, one with AI and another 10 days later; and GnRH-GnRH-NSAID (n = 9), two doses of GnRH as in GnRH-GnRH group and one dose of meloxicam 15 days after AI. Pregnancy diagnosis was performed by ultrasound 38 days after AI. During the 1st year of evaluation, 53.79% of cows had received ≥ 3 AIs; the highest percentage (38.48%) of RB cows was recorded during autumn. After treatment for RB, the lowest (P < 0.05) pregnancy rate (PR) was recorded in August (18.18%) compared to January (75.0%), May (53.33%), and December (50.0%, P < 0.1). The efficacy of ovsynch during winter was significantly higher compared to all the other seasons. The percentage of cows with increased estrous cycle duration (24-37 days) after RB treatment and AI was the highest (P < 0.05) in spring and summer after AI+GnRH and the lowest (P < 0.05) during winter after ovsynch. Concerning the post AI treatments during summer, significantly higher pregnancy rate was recorded in the OS+GnRH+NSAID group (55.55%) compared to OS+GnRH (10%) and OS (10%) groups. Pregnancy rates did not differ among AI groups. It seems that OS+GnRH+NSAID is a promising protocol for RB cows during summer although further research is needed to support the present results.

GnRH34 with or without estradiol cypionate in timed AI in Bos indicus beef cows

Two experiments were performed to evaluate the effects of GnRH treatment on the fertility of suckled Nelore beef cows treated with an estradiol/progesterone (E2/P4)-based protocol for timed artificial insemination (TAI). Experiment 1 focused on determining the effects of estradiol cypionate (EC) on ovulation in TAI cows treated with GnRH 34 h after removal of the intravaginal P4 device (IPD). Suckled cows (n = 26) were treated with 2 mg estradiol benzoate (EB) and IPD containing 1 g P4. After 8 days, IPDs were removed, and all cows were treated with 150 μg of d-cloprostenol (prostaglandin F2 alpha analog) and 300 IU of equine chorionic gonadotropin (eCG), then separated into two treatment groups consisting of cows who received 1) saline 0.9% i.m. (GnRH34 group) or 2) 0.6 mg i.m. of EC (EC-GnRH34 group). On day 9 (05:00 p.m.), all cows were given GnRH (10.5 μg of buserelin acetate) i.m. No differences were observed between the groups (P > 0.05) in the time of ovulation after IPD removal or in the proportion of cows ovulating. Experiment 2 focused on determining the effects of GnRH34 along with or in the absence of EC on day 8 on pregnancy per AI (P/AI) in postpartum beef cows. Cows (n = 981) were treated similarly to those in Experiment 1, but an additional group, the EC-GnRH48 group, was included, in which cows received EC on day 8 whereas those that did not show estrus received GnRH at TAI. Thus, in this experiment, groups consisted of GnRH34 (n = 322), EC-GnRH34 (n = 335), and EC-GnRH48 (n = 324). A higher rate of estrus expression was observed in cows treated with EC following IPD removal (EC-GnRH34: 69%, EC-GnRH48: 64.8%) than in cows in the GnRH34 group (45.6%). No difference in P/AI was observed between the treatment groups (P = 0.45), but P/AI in cows in the EC-GnRH34 group (64.2%) tended to be greater (P = 0.1) than in cows in the GnRH34 group (58%). In summary, although ovulation synchrony did not differ among the groups, P/AI in cows treated with EC and GnRH 34 h after IPD removal tended to be greater than in cows treated solely with GnRH; this was most likely due to a shorter proestrus/estrus period, considering the lower proportion of cows that displayed estrus in the GnRH34 group. Finally, given that P/AI did not differ between the EC-GnRH34 and EC-GnRH48 groups, our results suggest that, for cows not displaying estrus, administration of EC at the time of IPD removal followed by treatment with GnRH 48 h afterward represents the most cost-efficient TAI strategy for South American Zebu-based beef operations.

GnRH34: An alternative for increasing pregnancy in timed AI beef cows

Two experiments evaluated the effects of gonadotropin releasing hormone (GnRH) treatment on fertility of suckled Nelore beef cows treated with an estradiol/progesterone (P4)-based protocol for timed artificial insemination (TAI). Experiment 1 was designed to determine the effect of GnRH administered 34 h after P4 insert removal (GnRH34) on time of ovulation. Suckled cows (n = 34) were treated with 2 mg estradiol benzoate (EB) and an intravaginal insert containing 1.9 g of P4. Eight days later, P4 inserts were removed, and all cows received 150 μg of d-cloprostenol (prostaglandin F2 alpha analogue), 300 IU of eCG, and 1 mg of estradiol cypionate (ECP). On Day 9 (05:00 p.m.), cows were randomly distributed, according to the diameter of the pre-ovulatory follicle, in two treatments: 1) GnRH (n = 17) cows that received 10.5 μg of buserelin acetate, or 2) no further treatment (control, n = 17). Cows treated with GnRH 34 h after P4 insert removal ovulated earlier (P = 0.02) than control cows (66 ± 0.0 and 77.2 ± 4.3 h). Experiment 2 was designed to determine the effect of GnRH34 on the fertility of suckled beef cows. Nelore cows (n = 506) were treated as Experiment 1. On Day 8, cows were painted in the sacrocaudal region to identify cows that displayed estrus. On Day 9 (05:00 p.m.), cows were randomized to receive same treatment as Experiment 1, control (n = 252), or GnRH (n = 254). All cows were TAI 48 h after P4 insert removal. At TAI, estrus was evaluated, and deemed to have occurred in cows without a tail-head chalk mark (>75% paint loss). Cows treated with GnRH 34 h after P4 insert removal had greater (P = 0.01) pregnancy per AI (P/AI) than cows that only received ECP (63.0% and 50.4%). No difference (P = 0.5) was observed in the proportion of cows that displayed estrus between treatments. Furthermore, cows that displayed estrus had greater (P < 0.01) P/AI than cows that did not. Treatment with GnRH, given at 34 h after P4 insert removal, increased (P < 0.05) P/AI in cows that did not show estrus at TAI. In summary, treatment with GnRH 34 h after P4 insert removal was associated with earlier ovulation and resulted in greater P/AI in suckled Nelore cows treated with an estradiol/P4-based protocol for TAI.

Timed artificial insemination plus heat II: gonadorelin injection in cows with low estrus expression scores increased pregnancy in progesterone/estradiol-based protocol

The use of tail chalk and estrus/heat expression scores (HEATSC) evaluation is instrumental in identifying cows with greater estrus expression and greater artificial insemination pregnancy rates (P/AI) in cows submitted to timed artificial insemination (TAI), and cows with low or no estrus expression present lower P/AI. It was intended in this study to improve the pregnancy rates in TAI for Bos indicus beef cows, and gonadotrophin-releasing hormone (GnRH) injection was hypothesized to increase pregnancy rates in a TAI program for cows submitted to progesterone-estradiol-based protocols with low or no estrus expression, evaluated by HEATSC. Cows (n= 2284) received a progesterone device and 2 mg estradiol benzoate, after 8 days the device was removed and 1 mg estradiol cypionate, 150 μg of d-cloprostenol and 300 IU equine chorionic gonadotropin was administered. All cows were marked with chalk and HEATSC evaluated (scales 1 to 3) at TAI performed on day 10. Animals with HEATSC1 and HEATSC2 (n= 937) received 100 μg de gonadorelin (GNRH group; n= 470), or 1 ml saline (Control group; n= 467), and cows with HEATSC3 (named HEAT group; n= 1347) received no additional treatment. The larger dominant follicle, evaluated on day 8and at TAI (day 10), was greater in HEAT group (P= 0.0145 and P <0.001, respectively). Corpus luteum (CL) area and progesterone concentration was evaluated on day 17, and CL area was larger in HEAT group, intermediary in Control and lower in GnRH group (Control= 2.68 cm2, GnRH= 2.37 cm2, HEAT group= 3.07 cm2, P <0.001). Greater progesterone concentrations were found in HEAT group than in Control and GnRH groups (Control= 4.74 ng/ml, GnRH= 4.29 ng/ml, HEAT group= 6.08 ng/ml, P<0.001). There was a difference in ovulation rate, greater in HEAT group than GnRH and Control groups (Control= 72.5%; GnRH= 81.25%; HEAT group= 90.71%; P= 0.0024). Artificial insemination pregnancy rates was greater in HEAT group (57.09% (769/1347) than in Control and GNRH groups, with positive effect of GnRH injection at the time of TAI in P/AI (Control= 36.18% (169/467), GnRH= 45.95% (216/470); P<0.0001). In conclusion, GnRH application in cows with low HEATSC (1 and 2) is a simple strategy, requiring no changes in TAI management to increase pregnancy rates in postpartum beef cows submitted to progesterone-estradiol-based TAI protocols, without reaching, however, the pregnancy rates of cows that demonstrate high estrus expression at the TAI.

Using estrus detection patches to optimally time insemination improved pregnancy risk in suckled beef cows enrolled in a fixed-time artificial insemination program

A multilocation study examined pregnancy risk (PR) after delaying AI in suckled beef cows from 60 to 75 h when estrus had not been detected by 60 h in response to a 7-d CO-Synch + progesterone insert (CIDR) timed AI (TAI) program (d -7: CIDR insert concurrent with an injection of GnRH; d 0: PGF injection and removal of CIDR insert; and GnRH injection at TAI [60 or 75 h after CIDR removal]). A total of 1,611 suckled beef cows at 15 locations in 9 states (CO, IL, KS, MN, MS, MT, ND, SD, and VA) were enrolled. Before applying the fixed-time AI program, BCS was assessed, and blood samples were collected. Estrus was defined to have occurred when an estrus detection patch was >50% colored (activated). Pregnancy was determined 35 d after AI via transrectal ultrasound. Cows ( = 746) detected in estrus by 60 h (46.3%) after CIDR removal were inseminated and treated with GnRH at AI (Control). Remaining nonestrous cows were allocated within location to 3 treatments on the basis of parity and days postpartum: 1) GnRH injection and AI at 60 h (early-early = EE; = 292), 2) GnRH injection at 60 h and AI at 75 h (early-delayed = ED; = 282), or 3) GnRH injection and AI at 75 h (delayed-delayed = DD; = 291). Control cows had a greater ( < 0.01) PR (64.2%) than other treatments (EE = 41.7%, ED = 52.8%, DD = 50.0%). Use of estrus detection patches to delay AI in cows not in estrus by 60 h after CIDR insert removal (ED and DD treatments) increased ( < 0.05) PR to TAI when compared with cows in the EE treatment. More ( < 0.001) cows that showed estrus by 60 h conceived to AI at 60 h than those not showing estrus (64.2% vs. 48.1%). Approximately half (49.2%) of the cows not in estrus by 60 h had activated patches by 75 h, resulting in a greater ( < 0.05) PR than their nonestrous herd mates in the EE (46.1% vs. 34.5%), ED (64.2% vs. 39.2%), and DD (64.8% vs. 31.5%) treatments, respectively. Overall, cows showing estrus by 75 h (72.7%) had greater ( < 0.001) PR to AI (61.3% vs. 37.9%) than cows not showing estrus. Use of estrus detection patches to allow for a delayed AI in cows not in estrus by 60 h after removal of the CIDR insert improved PR to TAI by optimizing the timing of the AI in those cows.

Plasma progesterone profile and conception rate following exogenous supplementation of gonadotropin-releasing hormone, human chorionic gonadotropin, and progesterone releasing intra-vaginal device in repeat-breeder crossbred cows

AIM

This study was designed to evaluate the effect of gonadotropin-releasing hormone (GnRH), human chorionic gonadotropin (hCG), and progesterone impregnated intra-vaginal device on progesterone profile and conception rate in repeat-breeding crossbred cows.

MATERIALS AND METHODS

Repeat-breeding crossbred cows aged 3-8 years (n=32), lactating and negative to white side test were randomly divided into four groups: Group 1 (Control, n=8), Group 2 (GnRH at 10 µg i.m, n=8), Group 3 (hCG at 1500 IU i.m., n=8), and Group 4 (progesterone impregnated intra-vaginal device at 958 mg, n=8). All the treatme nts were given on 5(th) daypostbreeding and in Group 4 intra-vaginally implanted device was withdrawn on 9(th) day (i.e., implant inserted for total 4 days) of the estrous cycle. Blood samples were collected on day 0, 5, 10, 15, and day 20 of estrous cycle, and plasma was separated for progesterone estimation.

RESULTS

Accessory corpus luteum was not formed in crossbred cows of Group4 and control group. However, total 6 and 8 accessory corpora lutea were found in Group 2 and Group 3, respectively. In pregnant cows, the plasma progesterone concentration increased continuously from day 0 to day 20. In non-pregnant cows, it increased from day 0 to day 15 and then declined. The conception rate on day 60 in Group 1, Group 2, Group 3, and Group 4 was 37.5%, 50%, 75%, and 37.5%, respectively.

CONCLUSIONS

Treating repeat-breeder cows with hCG is effective in increasing conception rate by developing accessory corpora lutea and higher progesterone level.

Impact of Buserelin Acetate or hCG Administration on the Day of First Artificial Insemination on Subsequent Luteal Profile and Conception Rate in Murrah Buffalo (Bubalus bubalis)

This study was designed to investigate the impact of buserelin acetate (BA) or human chorionic gonadotropin (hCG) administration on the day of first artificial insemination (AI) on subsequent luteal profile (diameter of corpus luteum (CL) and plasma progesterone) and conception rate in Murrah buffalo. The present experiment was carried out at two locations in 117 buffalo that were oestrus-synchronized using cloprostenol (500 μg) administered (i.m.) 11 days apart followed by AI during standing oestrus. Based on treatment (i.m.) at the time of AI, buffalo were randomly categorized (n = 39 in each group) into control (isotonic saline solution, 5 ml), dAI-BA (buserelin acetate, 20 μg) and dAI-hCG (hCG, 3000 IU) group. Out of these, 14 buffalo of each group were subjected to ovarian ultrasonography on the day of oestrus to monitor the preovulatory follicle and on days 5, 12, 16 and 21 post-ovulation to monitor CL diameter. On the day of each sonography, jugular vein blood samples were collected for the estimation of progesterone concentrations. All the buffalo (n = 117) were confirmed for pregnancy on day 40 post-ovulation. The conception rate was better (p < 0.05) in dAI-BA (51.3%) and dAI-hCG (66.7%) groups as compared to their control counterparts (30.8%). Furthermore, the buffalo of dAI-hCG group had improved (p < 0.05) luteal profile, whereas the buffalo of dAI-BA group failed (p > 0.05) to exhibit stimulatory impact of treatment on luteal profile when compared to control group. In brief, buserelin acetate or hCG treatment on the day of first AI leads to an increase in conception rate; however, an appreciable impact on post-ovulation luteal profile was observed only in hCG-treated Murrah buffalo.

Influence of standing estrus before an injection of GnRH during a beef cattle fixed-time AI protocol on LH release, subsequent concentrations of progesterone, and steriodogenic enzyme expression

Beef cows that exhibit estrus before fixed-time AI have been reported to have increased pregnancy success and increased concentrations of progesterone during the subsequent estrous cycle. Therefore, these experiments were conducted to evaluate if initiation of standing estrus before an injection of GnRH during a fixed-time AI protocol affected LH pulses, subsequent concentrations of progesterone, and luteal steroidogenic enzyme expression. In Experiments 1 and 2, cows were treated with the CO-Synch protocol (100 μg GnRH day -9, 25 mg PGF(2α) day -2, and 100 μg GnRH day 0) and allotted to one of two treatments: 1) cows that initiated estrus before GnRH on day 0 (estrus; n = 5) or 2) cows that did not initiate estrus and were induced to ovulate by the GnRH on day 0 (no estrus; n = 5). In Experiment 1, blood samples were collected at 15-min intervals from 0 to 6 (bleed 1), 12 to 20 (bleed 2), 26 to 34 (bleed 3), and 40 to 48 (bleed 4) h after GnRH. Daily blood samples were collected for 17 d. Initiation of estrus before the GnRH injection had no effect on LH release or the pattern of progesterone increase; however, cows detected in estrus had overall increased (P = 0.002) concentrations of progesterone compared with cows not in estrus. In Experiment 2, estrus was detected with the HeatWatch system. Location and size of the ovulatory follicle was determined on day 0 by transrectal ultrasonography at time of injection with GnRH. Blood samples were collected on days 3, 4, 5, 7, and 9; luteal tissue was collected on day 10 (n = 4 estrus and n = 9 no estrus) from corpus luteum (CL) originating from similar-sized follicles (13.0 to 16.0 mm). Total cellular RNA was extracted, and relative mRNA levels were determined by real-time reverse transcription PCR and corrected for GAPDH. There was no effect of estrus on CL weight or concentrations of progesterone. In addition, there was no effect of estrus, follicle size, or CL weight on luteal expression of LH receptor, StAR, CYP11A1, or 3βHSD. However, there was a correlation between follicle size and CL weight (P = 0.01; R(2) = 0.43); for every increase of 1 mm in follicle size, CL weight increased by 1.5 g. In summary, estrus did not influence release of LH, CL weight, progesterone concentrations, or expression of steriodogenic enzymes. However, as follicle size increased, CL weight increased; therefore, both follicle size and CL weight were associated with progesterone concentrations.

Comparison of the effect of a CIDR-Select Synch versus a long-term CIDR based AI protocol on reproductive performance in multiparous dairy cows in Swiss dairy farms

BACKGROUND

Synchronization programs have become standard in the dairy industry in many countries. In Switzerland, these programs are not routinely used for groups of cows, but predominantly as a therapy for individual problem cows. The objective of this study was to compare the effect of a CIDR-Select Synch and a 12-d CIDR protocol on the pregnancy rate in healthy, multiparous dairy cows in Swiss dairy farms.

METHODS

Cows (N = 508) were randomly assigned to CIDR-Select Synch (N = 262) or 12-d CIDR (N = 246) protocols. Cows in the CIDR-Select Synch group received a CIDR and 2.5 ml of buserelin i.m. on d 0. On d 7, the CIDR insert was removed and 5 ml of dinoprost was administered i.m.. Cows in the 12-d CIDR group received the CIDR on d 0 and it was removed on d 12 (the routine CIDR protocol in Swiss dairies). On d 0 a milk sample for progesterone analysis was taken. Cows were inseminated upon observed estrus. Pregnancy was determined at or more than 35 days after artificial insemination. As a first step, the two groups were compared as to indication for treatment, breed, stud book, stall, pasture, and farmer's business using chi square tests or Fisher's exact test. Furthermore, groups were compared as to age, DIM, number of AI's, number of cows per farm, and yearly milk yield per cow using nonparametric ANOVA. A multiple logistic model was used to relate the success of the protocols to all of the available factors; in particular treatment (CIDR-Select Synch/12-d CIDR), milk progesterone value, age, DIM, previous treatment of the uterus, previous gynecological treatment, and number of preceding inseminations.

RESULTS

The pregnancy rate was higher in cows following the CIDR-Select Synch compared to the 12-d CIDR protocol (50.4% vs. 22.4%; P < 0.0001).

CONCLUSION

The CIDR-Select Synch protocol may be highly recommended for multiparous dairy cows. The reduced time span of the progesterone insert decreased the number of days open, improved the pregnancy rate compared to the 12-d CIDR protocol and the cows did not to have to be handled more often.

Effects of GnRH treatment on initiation of pulses of LH, LH release, and subsequent concentrations of progesterone

Progesterone is essential for establishment and maintenance of pregnancy. One proposed method to increase progesterone is administering GnRH at insemination. However, this method has resulted in conflicting results. Therefore, 2 experiments were conducted to evaluate how administering GnRH at insemination affected pulses of luteinizing hormone (LH) and subsequent progesterone. In Experiment 1, cows were allotted to 2 treatments: (1) GnRH (100 microg) given approximately 12h after initiation of estrus (n=5); and (2) Control (n=5). Blood samples were collected at 15-min intervals for 6h at 12 (blood sampling period 1), 26 (blood sampling period 2), 40 (blood sampling period 3), 54 (blood sampling period 4), and 68 (blood sampling period 5) h after onset of estrus. Daily blood samples were collected for 17 d. In Experiment 2, cows were allotted into 2 treatments: GnRH administered 10 to 11h (n=10) or 14 to 15 h (n=10) after onset of estrus. Daily blood samples were collected for 17 d. Cows treated with GnRH tended (P<or=0.075) to have greater LH release during blood sampling period 1, tended (P=0.095) to have fewer pulses during blood sampling period 2, tended (P=0.067) to have greater concentrations of progesterone, and had an earlier (P=0.05) increase in progesterone than control cows. Cows treated with GnRH 10 to 11h after onset of estrus had greater (P=0.01) progesterone and an earlier (P=0.04) increase in progesterone than cows treated 14 to 15 h. In conclusion, timing of GnRH treatment following onset of estrus influenced pulses of LH and subsequent progesterone.