Modified Ovsynch Protocol Research Articles

Comparison of Two Different Programmes of Ovulation Synchronization in the Treatment of Ovarian Cysts in Dairy Cows

It was the aim of this field study to evaluate two different protocols of ovulation synchronization for the treatment of ovarian cysts and their effect on reproductive performance in dairy cows. In addition, factors with a possible influence on treatment success and pregnancy outcome as well as costs per pregnancy were analysed. The study was performed with 130 German Holsteins with ovarian cysts diagnosed on days 55 to 60 postpartum. Cows belonging to group 1 (n = 65) received a modified ovsynch protocol [day 0: 0.15 mg cloprostenol (PGF) + 0.02 mg buserelin acetate (GnRH); day 14: PGF; day 16: GnRH]. Group 2 (n = 65) was treated with the conventional ovsynch protocol (day 0: GnRH; day 7: PGF; day 9: GnRH). Timed artificial insemination was performed 20 to 24 h later. Cows without ovarian cysts served as controls. Treatment success (disappearance of the ovarian cyst) after the first ovsynch cycle was higher in group 1 (66.2%) than in group 2 (23.1%, p < 0.05). Reproductive measures in group 1 were comparable with those of the control group and, compared with group 2, were conspicuously better (66.2%, 76.9%, 83.1%, 59.5% vs. 40.0%, 50.7%, 60.0%, 27.5% for cumulative pregnancy rate after treatment cycle 1 to 3 and second service conception rate, respectively, p < 0.05). Overconditioned cows and cows with larger ovarian cysts showed a diminished treatment and pregnancy success. In group 1, costs per pregnancy were only slightly higher than in the control group (group 1: €352.44, group 2: €484.59, control group: €333.77). In conclusion, our results suggest that ovsynch protocols can be used in the treatment of ovarian cysts. The modified ovsynch protocol led to a better cure rate as well as a better reproductive performance, and was economically beneficial compared with a conventional ovsynch protocol.

Fertility response in postpartum anoestrus buffaloes (Bubalus bubalis) using modified Ovsynch based timed insemination protocols.

Background and Aim:Suboestrus constitutes the largest factor responsible for poor reproductive efficiency in buffaloes. Therefore, oestrus synchronization (OS) and fixed-time artificial insemination (AI) is considered an alternative approach to enhance reproductive efficiency in buffaloes. Thus, the present study was carried out to study the efficacy of modified Ovsynch protocol with fixed time insemination in post-partum anoestrus buffaloes.Materials and Methods:Total 50 post-partum anoestrus dairy buffaloes were selected and randomly divided into 5 Groups, each comprising ten animals (n=10). Animals of Group I received buserelin acetate 10 μg(GnRH) at day 0 and 9, and prostaglandin F2α (PGF2α) at day 7; in Group II similar to Group I except double dose of Gn RH (20 μg) at day 0; in Group III, similar to Group I plus additional administration of insulin on day 0, 1 and 2; in Group IV, similar to Group II plus additional administration of insulin on day 0,1 and 2; in Group V similar to Group I except replacement of first Gn RH with insulin on day 0,1 and 2. Animal of all groups were inseminated at fixed time using frozen semen at 60 h and 72 h after PGF2α administration and confirmed for pregnancy at day 60 post-insemination.Results:The results revealed satisfactory and comparable synchronization of oestrus (60-80%) and conception rate (20-40%) among the various treatment groups in the present study. The better synchronization was observed in modified protocols. However, non-significantly higher conception rate was recorded in modified Ovsynch protocol (40% each in Group II-V) as compared to 20% in Group-I (p>0.05).Conclusion:In this study comparatively higher OS and conception following insulin modified Ovsynch based timed AI protocols in post-partum anoestrus buffaloes was found satisfactory and comparable.

First Results from Insemination With Sex-Sorted Semen in Dairy Heifers in Macedonia

Abstract Science has been searching for a long time for a reliable method for controlling the sex of mammalian offspring. Recently, the application of specific modern cellular methodologies has led to the development of a flow cytometric system capable of differentiating and separating living X- and Y-chromosome-bearing sperm cells in amounts suitable for AI and therefore, commercialization of this sexing technology. The aim of this work was to present the first results of heifers that introduce bovine AI with sex sorted semen, for the first time in Macedonia. Insemination with sex sorted cryopreserved semen (2×106 spermatozoa per dose) imported from the USA was done at two dairy farms in ZK Pelagonija. In total, 74 heifers (Holstein Friesian) were inseminated. Inseminations were carried out in a timely manner following a modified OvSynch protocol. During the insemination, the sperm was deposited into the uterine horn ipsi lateral to the ovary where a follicle larger than 1.6 cm was detected by means of transrectal ultrasound examination. Pregnancy was checked by ultrasound on day 30 after the insemination. Overall, the average pregnancy rate in both farms was 43,24% (40,54% and 45,95%, for farm 1 and farm 2, respectively). All pregnant heifers delivered their calves following a normal gestation length (274,3 days in average) and of the 32 born calves, 30 (93,75%) were female. In conclusion, since the first results from inseminations with sex-sorted semen in dairy heifers in Macedonia are very promising, the introduction of this technique may bring much benefit to the local dairy sector. Average pregnancy rate seems similar with results obtained following ‘regular’ inseminations, notwithstanding the relatively low number of spermatozoa per insemination dose. Due to the latter, we however recommend inseminations only to be carried out by experienced technicians followinga TAI protocol and ultrasound examinations of the ovaries prior to insemination.

87 OVARIAN RESERVE, EMBRYO PRODUCTION, AND THEIR CORRELATION WITH ANTI-MÜLLERIAN HORMONE (AMH) IN HOLSTEIN COWS

Anti-Müllerian hormone (AMH) is a small peptide hormone that has been associated with ovarian follicular reserve in humans and in some animal species including bovine. Profiles of AMH, as well as the relationship between serum AMH to oocyte number and in vivo embryo production, were evaluated in Holstein cows. AMH levels were determined in 15 unstimulated cows at monthly intervals for 4 months and in 394 male and 399 female developing Holstein animals from birth to adulthood. Also, AMH was measured in 41 heifers at the time of ovum pick-up (OPU) and 125 heifers at the time of embryo flushing. Superovulation was induced before OPU or embryo flushing using a modified Ovsynch protocol with 4 days of decreasing FSH (Pluset H®, MOFA Global, Verona, WI, USA). Blood samples were collected using serum tubes and spun within 2 h. The samples were stored at –20°C until evaluated for AMH using the AMH-Bovine specific immunoassay® (MOFA Global). AMH levels in males and females peaked at 2 months of age and then decrease as they reached adulthood. The average AMH level of adult cows was stable for each of the 4 monthly measurements, with a high correlation between all values per animal (r2 = 0.9077; P &lt; 0.01), suggesting that AMH levels are consistent for at least 4 consecutive months. However, AMH levels were lowest during the summer months, suggesting a seasonal change in AMH secretion. Animals repeatedly ovarian stimulated showed decreasing AMH levels (509 ± 295, 299 ± 210, 211 ± 119) with subsequent stimulations. There was also a significant decrease in the number of embryos recovered (5.7 ± 4, 2.2 ± 1.9; P = 0.02); however, the number of oocytes was not altered by multiple stimulations (9.9 ± 9.8, 8.1 ± 6.2; P = 0.57). Because AMH and embryo numbers decreased after multiple stimulations, the first AMH value and results of the first OPU or embryo flush were used for the correlation of AMH to the number of oocytes or embryos. Animals were separated into 3 AMH categories: low (&lt;100), normal (100–400), and high (&gt;400 pg mL–1). High AMH OPU animals had significantly higher numbers of oocytes than the normal or low AMH groups (13.8 ± 9.2, 9.2 ± 5.3, 5.6 ± 3.9; P = 0.001). High AMH flushed animals had significantly higher numbers of embryos than animals with low AMH (10.9 ± 8.0, 5.7 ± 5; P = 0.002). Statistical analyses were performed using Statview 5. Differences were determined using Student's t-test; P &lt; 0.05 was considered significant. In conclusion, AMH serum concentrations are consistent over multiple months; however, blood should not be taken for animal selection by AMH after ovarian stimulations have begun and should be interpreted with caution during the summer months. AMH is highly associated with superovulation response and oocyte and embryo production and should improve efficiency of multiple-ovulation embryo transfer.

Presynchronization using a modified Ovsynch protocol or a single gonadotropin-releasing hormone injection 7 d before an Ovsynch-56 protocol for submission of lactating dairy cows to first timed artificial insemination

Presynchronization strategies, such as Presynch-Ovsynch and Double-Ovsynch, increase fertility to timed artificial insemination (TAI) compared with Ovsynch alone; however, simpler presynchronization strategies could reduce costs and simplify reproductive management. Lactating Holstein cows (n=601) were randomly assigned to 1 of 2 presynchronization treatments before beginning an Ovsynch-56 protocol (GnRH at 70±3 DIM, PGF2α 7 d later, GnRH 56h after PGF2α, and TAI 16h later at 80±3 DIM) for first TAI. Cows (n=306) in the first treatment (Double-Ovsynch; DO) were presynchronized using a modified Ovsynch protocol (GnRH at 53±3 DIM, 7 d later PGF2α, and GnRH 3 d later) ending 7 d before the first GnRH injection (G1) of an Ovsynch-56 protocol. Cows (n=295) in the second treatment (GGPG) were presynchronized using a single injection of GnRH 7 d before G1 of an Ovsynch-56 protocol at 63±3 DIM. Blood samples were collected at G1 and the PGF2α injections of the Ovsynch-56 protocol to determine progesterone (P4) concentrations. Pregnancy diagnosis was performed using ultrasonography 32 d after TAI, and pregnant cows were reexamined 46 and 70 d after TAI. Overall, DO cows had more pregnancies per artificial insemination (P/AI) compared with GGPG cows 32 d after TAI (53 vs. 43%). Overall, P/AI did not differ by parity (primiparous vs. multiparous), and pregnancy loss did not differ between treatments or parities. More DO cows had P4 in a medium range (>0.5 to <4ng/mL) at G1 of the Ovsynch-56 protocol compared with GGPG cows (82 vs. 50%), and more DO cows had high P4 (>4ng/mL) at the PGF2α injection of the Ovsynch-56 protocol compared with GGPG cows (67 vs. 36%). Thus, presynchronization with a modified Ovsynch protocol increased P/AI after TAI at first AI by increasing synchrony to the Ovsynch-56 protocol compared with presynchronization using a single injection of GnRH.

314 RELATIONSHIP BETWEEN SERUM ANTI-MULLERIAN HORMONE (AMH), OVARIAN RESERVE, AND EMBRYO PRODUCTION IN SUPEROVULATED HOLSTEIN COWS

The relations between serum anti-Mullerian hormone (AMH), oocyte numbers, and in vivo embryo production in Holstein heifers were evaluated. The AMH levels of 15 unstimulated cows were followed at weekly intervals during their oestrous cycles and monthly for 4 months. Forty-one superovulated heifers were evaluated at ovum pick-up (OPU) performed 20 h after cystorelin administration, and 125 others were evaluated at embryo recovery. Animals were followed over 3 consecutive cycles induced using a modified Ovsynch protocol with 4 days of FSH (Pluset H, Minitube of America, Verona, WI, USA). Blood samples were collected in serum tubes and spun within 2 h. The samples were stored at –20°C until evaluation using the Minitube of America AMH-bovine specific immunoassay (AMH Fertility Assay™). The statistical analyses (ANOVA and data correlation) were performed using Statview 5 with P &lt; 0.05. Serum AMH ranged from 43 to 960 pg mL–1. The average AMH level of all cows was stable during the oestrous cycle and for each of the 4 monthly measurements. There was a high correlation between all values per animal (r2 = 0.9077; P &lt; 0.01), suggesting that AMH levels are consistent during the cycle and for at least 4 consecutive months. Animals that were repeatedly stimulated showed decreasing AMH levels (509 ± 295, 299 ± 210, 211 ± 119) and a decrease in recovered embryos (7.4 ± 4, 5.6 ± 3.8, 4.2 ± 3.2; P = 0.02). The number of oocytes was not altered by multiple stimulations (10.4 ± 9.8, 11.3 ± 6.2, 8.5 ± 7.6; P = 0.75). As AMH and embryo numbers decreased after multiple stimulations, only the first AMH value and results of the first OPU or flush were used to establish following correlation. Serum AMH showed a positive correlation to the number of oocytes (r2 = 0.245) and embryos collected (r2 = 0.27).When separated into AMH categories, low (&lt;100), normal (100–400), and high (&gt;400 pg mL–1), high-AMH OPU animals yielded significantly higher numbers of oocytes than the animals in the normal or low AMH groups (13.8 ± 9.2 v. 9.2 ± 5.2 and 5.6 ± 3.9; P = 0.001). Flushed animals with high AMH levels had significantly higher numbers of embryos than those with low AMH (10.9 ± 7.9 v. 5.7 ± 5; P = 0.002). Comparison of the first AMH value to the average number of oocytes or embryos collected over the course of 3 collections/animal showed a positive correlation to the average number of oocytes/collection from individual OPU donors (r2 = 0.436) and a positive correlation to the average number of embryos/collection from individual donors (r2 = 0.176). When separated into AMH groups, high-AMH flushed animals had significantly higher numbers of embryos than the normal- or low-AMH animals (9.3 ± 3.1 v. 5.7 ± 3.4 and 4.5 ± 2; P = 0.0001). As OPU animals with low AMH were used only once, average oocyte/collection data was not available for this category. A significant difference was observed between the high- and normal-AMH categories (12 ± 3.6 v. 7 ± 2; P = 0.0001). Circulating AMH is stable over time in unstimulated animals but decreases in repetitively stimulated animals. Anti-Mullerian hormone is highly associated with superovulation response and oocyte and embryo production, and its use should improve animal selection to achieve improve efficiency of multiple-ovulation embryo transfer.

Combined use of Ovsynch and progesterone supplementation after artificial insemination in dairy cattle

The objective of this study was to evaluate the effects of different Ovsynch protocols combined with progesterone (P4) supplementation after artificial insemination (AI) of Holstein-Friesian cows. Cows were randomly synchronized at 52 to 63 d after parturition with either the classical Ovsynch protocol (GnRH on d 0, PGF2α on d 7, GnRH 48h after PGF2α) or with a modified Ovsynch protocol (second GnRH 60h after PGF2α). On d 4 after timed AI (TAI), the cows were blocked by parity and randomly divided into 2 groups. Half of the cows were supplemented with P4 (P4+) by applying a P4-releasing intravaginal device intravaginally for 14 d, whereas the other half remained untreated (P4−). In 50% of randomly chosen cows, plasma P4 was measured on d 4, 5, and 18 after TAI. Sonographic pregnancy diagnosis was performed on d 33 after TAI in a total of 398 cows. Health status and body condition score (BCS) of all cows were examined at several stages of the study. Cows in the modified Ovsynch protocol tended to have higher P4 values on d 4 after TAI than cows in the classical Ovsynch protocol (2.1±0.2 vs. 1.6±0.2ng/mL), but no difference in pregnancy per AI (P/AI) was observed between the 2 Ovsynch protocols (38.4% vs. 44.1%). Independent of the Ovsynch protocols, P4+ cows tended to have higher P/AI compared with P4− cows (44.4% vs. 38.1%). The retention of fetal membranes and BCS at the time of insemination affected P/AI. Moreover, an interaction between BCS at the time of insemination and P4 supplementation was apparent; that is, the difference in P/AI between P4+ and P4− cows was significant in cows with BCS ≥3.25. Progesterone-supplemented cows showed higher P4 values on d 5 (4.9±0.2 vs. 2.6±0.2) and d 18 (7.8±0.2 vs. 6.3±0.2) after TAI, respectively. In conclusion, the elongation of the time interval between the injections of PGF2α and the second GnRH from 48 to 60h had no effect on P/AI. Progesterone supplementation after insemination improved the P/AI of the Ovsynch protocols, but this effect was more apparent in cows with BCS ≥3.25.

Fixed-time artificial insemination in beef cattle

BackgroundThe study was designed to test the effect of fixed-time artificial insemination (fixed-AI) after the slightly modified Ovsynch protocol on the pregnancy rate in beef cattle in Finnish field conditions. The modification was aimed to optimize the number of offsprings per AI dose.MethodsNinety Charolais cows and heifers were entered into the program an average of 1.8 times. Thus, 164 animal cases were included. Animals were administered 10-12 μg of buserelin. Seven days later animals without a corpus luteum (CL) were rejected (20.7%) while the remaining 130 cases with a CL were administered prostaglandin F2α, followed 48 h later with a second injection of buserelin (8-10 μg). Fixed-AI was performed 16-20 hours after the last injection.ResultsThe pregnancy rate was 51.5% (67/130). The pregnancy rate after a short interval (50-70 d) from calving to entering the program was significantly higher than that after a long interval (>70 d).ConclusionThis protocol seems to give acceptable pregnancy results in beef herds and its effect on saving labour is notable.

142 EFFECT OF GRANULOCYTE-MACROPHAGE COLONY-STIMULATING FACTOR AND INSULIN-LIKE GROWTH FACTOR-1 ON DEVELOPMENT AND POST-TRANSFER SURVIVAL OF BOVINE EMBRYOS PRODUCED IN VITRO

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a multifunctional cytokine that has been implicated in the regulation of preimplantation embryonic development in several species. Insulin-like growth factor-1 (IGF-1) can increase the proportion of bovine embryos that develop to the blastocyst stage and increase embryo survival following transfer to heat-stressed, lactating dairy cows (Block J et al. 2003 J. Anim. Sci. 81, 1590–1602; Theriogenology 67, 1518–1529). The current objective was to test whether addition of GM-CSF and IGF-1 to culture medium enhances development and post-transfer survival of in vitro-produced bovine embryos. Holstein COC shipped overnight in a portable incubator in oocyte maturation medium were fertilized with X-chromosome selected sperm from Holstein bulls. X-selected sperm were used because females were preferred. Morulae and blastocysts were collected at Day 7 after insemination and transferred at Day 7 after ovulation to lactating dairy cows subjected to a modified OvSynch protocol. In Experiment 1, conducted from June 29 to August 31, embryos were cultured in KSOM-BE2 alone, KSOM-BE2 with 100 ng mL–1 of Arg3-IGF-1 or KSOM-BE2 with 10 ng mL–1 of recombinant BoGM-CSF. Treatments were added at Day 1 after insemination. As compared to control embryos (17 ± 2%), the percentage of cleaved embryos that became transferable morulae or blastocysts at Day 7 was increased (P &lt; 0.05) by GM-CSF (25 ± 2%) but not by Arg3-IGF-1 (18 ± 2%). There was no significant effect of treatment on pregnancy rate at Day 30 to 35 [34% (n = 52), 35% (n = 51), and 43% (n = 55) for control, GM-CSF, and IGF-1, respectively] or calving rate (27, 35, and 40%) although values were numerically greater for cows receiving IGF-1 treated embryos. In experiment 2, conducted from September 7 to February 1, embryos were cultured in KSOM-BE2 alone, KSOM-BE2 with 100 ng mL–1 Arg3-IGF-1 added at Day 1 after insemination, or KSOM-BE2 with 10 ng mL–1 recombinant BoGM-CSF added at Day 5 after insemination. GM-CSF, but not IGF-1, increased the percentage of oocytes (P &lt; 0.03) and the percentage of cleaved embryos (P = 0.05) that became transferable morulae or blastocysts at Day 7. The percentage of cleaved embryos becoming blastocysts was 14 ± 1% for GM-CSF, 14 ± 2% for Arg3-IGF-1 (P = 0.11), and 10 ± 1% for controls. Treatment with GM-CSF increased (P = 0.056) the percentage of cows pregnant at Day 30 to 35 [34% (n = 79), 43% (n = 107), and 27% (n = 44) for control, GM-CSF, and IGF-1, respectively]. Data on calving rate are currently being collected; to date, 86% of calves were female. Results indicate that embryo competence for post-transfer survival can be enhanced by treatment with GM-CSF at Day 5 after fertilization. Research supported by USDA Grant 2006-55203-17390, BARD Grant US-3986-07 and the Southeast Milk Dairy Checkoff Program. B.L. was supported by a CAPES (Brazil)/Fulbright Fellowship.

Evaluation of the Mechanism of Action of Conjugated Linoleic Acid Isomers on Reproduction in Dairy Cows

The objective of this study was to evaluate the mechanism of action through which conjugated linoleic acid (CLA) beneficially affects reproduction. Lactating Holstein cows (n=45, 20±1 DIM) were assigned to 1 of 3 treatments: 70g/d of Ca salts of tallow (control); 63g/d of lipid-encapsulated CLA providing 7.1g/d of cis-9, trans-11 CLA and 2.4g/d of trans-10, cis-12 CLA (CLA 75:25); or 76g/d of lipid-encapsulated CLA providing 7.1g/d each of cis-9, trans-11 and trans-10, cis-12 CLA (CLA 50:50). Supplements were top-dressed for 37 d, milk production and DMI were recorded daily, and blood samples were taken 3 times per week. At 30±3 DIM, ovulation was synchronized in all cows with a modified Ovsynch protocol, and on d 15 of the cycle cows received an oxytocin injection; blood samples were obtained frequently to measure 13,14 dihydro, 15-keto PGF2α. On d 16 of the cycle cows received a PGF2α injection and ovarian follicular aspiration was performed 54h later. Follicular fluid was analyzed for fatty acids, progesterone, and estradiol. Endometrial biopsies were taken before and again near the end of the supplementation period for fatty acid analysis. The CLA resulted in decreased milk fat content of 14.1 and 6.1% at wk 5 of treatment of CLA 50:50 and CLA 75:25, respectively. There were no differences in energy balance or plasma nonesterified fatty acids; however, plasma IGF-I was greater in cows supplemented with CLA 50:50. The CLA isomers were not detectable in endometrial tissue, but cis-9, trans-11 CLA tended to be greater in follicular fluid of supplemented cows. Response to the oxytocin challenge was not different among treatments. Progesterone during the early luteal phase and the estradiol:progesterone ratio in follicular fluid tended to be greater in cows supplemented with CLA 50:50. Overall, these results indicate that short periods of CLA supplementation do not alter uterine secretion of PGF2α. The mechanism through which CLA affects reproduction may involve improved ovarian follicular steroidogenesis and increased circulating concentrations of IGF-I.

Efeito do intervalo entre recrutamentos foliculares na superovulação de vacas da raça Holandesa não-lactantes

Vacas da raça Holandesa não-lactantes, distribuídas em dois grupos, foram sincronizadas com o protocolo Ovsynch modificado. No dia sete (dia 0 = dia do segundo GnRH), o grupo 7 (G-7; n=19) recebeu CIDR usado previamente por cinco dias e 100mcg de GnRH, e o grupo 14 (G-14; n=21), CIDR e 25mg de PGF2alfa. No dia 14 foi aspirado o folículo dominante (FD), trocado o CIDR usado por um novo e foram aplicados 25mg de PGF2alfa. Iniciou-se o tratamento com FSH 36h depois, removeu-se o CIDR com o sétimo FSH e aplicou-se GnRH 36h depois. As inseminações foram feitas 12 e 24h depois. Recuperaram-se os embriões sete dias depois da inseminação artificial. O diâmetro do FD no G-7 foi 13,1±0,57mm no dia sete e 11,2±0,57mm no dia 14. O diâmetro FD persistente no G-14 aumentou de 12,6±0,55mm no dia sete para 16,4±0,55mm no dia 14 (P&lt;0,001). O número de folículos &gt;8mm, 48h após o início do tratamento com FSH, foi maior (P&lt;0,05) no G-7 (15,6±0,05) que no G-14 (12,5±0,05). Não foi detectado efeito de tratamento sobre o número de corpos lúteos e de embriões. O menor intervalo entre recrutamentos foliculares aumentou o número de folículos recrutados, porém não alterou a quantidade e a qualidade dos embriões produzidos.

Fertility of Lactating Dairy Cows Administered Recombinant Bovine Somatotropin During Heat Stress

Administration of recombinant bovine somatotropin (bST) to lactating dairy cows during heat stress increases milk yield, but it also can increase body temperature and may therefore compromise fertility. However, it is possible that bST treatment could increase fertility during heat stress because it has been reported to increase fertility in lactating cows. In addition, bST increases secretion of insulin-like growth factor-I (IGF-I) that promotes embryo survival. The purpose of this study was to determine effects of bST on reproductive function in lactating dairy cows during heat stress. The experiment was conducted in southern Georgia from July to November 2005 using lactating Holstein cows (n=276 for reproductive traits). For first service timed artificial insemination (TAI), cows were presynchronized with 2 injections of PGF2α given 14 d apart followed by a modified Ovsynch protocol (GnRH and insemination at 72h following PGF2α). Pregnancy was diagnosed by using ultrasonography on d 29 and reconfirmed by palpation between d 45 and 80 post-TAI. Nonpregnant cows were resynchronized with the modified Ovsynch protocol and received a second TAI. Treatment with bST started 1 wk before the start of Ovsynch and continued at 2-wk intervals. Blood samples were collected from a subset of cows to determine IGF-I profiles immediately before the first bST injection, 1 wk later, and at d 35 of bST treatment. Rectal temperatures were assessed on d 29 of bST treatment. Pregnancy rates (d 45 to 80 post-TAI) did not differ between bST and control cows for first- (16.7 vs. 15.2%) or second-service TAI (14.8 vs. 17.2%). Plasma concentrations of IGF-I and milk yield were greater for bST-treated cows following the initiation of bST treatment and bST increased rectal and vaginal temperatures. Body condition score was less for bST-treated cows. In conclusion, treatment with bST during heat stress increased IGF-I concentrations, milk yield over time, and rectal and vaginal temperatures without affecting first- or second-service pregnancy rates. Thus, at least under certain housing conditions, bST can be used to improve milk yield during heat stress without compromising fertility.