Pregnancy Rates In Dairy Cattle Research Articles

Impact of Accessory Corpus Luteum Induced by Gonadotropin-Releasing Hormone or Human Chorionic Gonadotropin on Pregnancy Rates of Dairy Cattle following Embryo Transfer: A META-Analysis.

The circulation of progesterone (P4) concentrations of recipients has positive correlations with embryo survival and pregnancy success of embryo transfer (ET) in dairy cows. One strategy to improve P4 concentration is the administration of gonadotropin-releasing hormone (GnRH) or human chorionic gonadotropin (hCG), thereby inducing the formation of accessory corpus luteum (CL). This study aimed at determining the efficacy of GnRH or hCG treatment regarding embryo transfer (ET) and providing a better clinical veterinary practice guidance. A meta-analysis was conducted on the data from 2048 treated recipient cows and 1546 untreated cows. By inducing the formation of accessory CL with GnRH (100 µg), GnRH analogue Buserelin (8-10 µg), or hCG (≥1500 IU) 5-11 days after synchronized ovulation, hCG alone achieved an improvement (RR = 1.39, p < 0.05), while GnRH and GnRH analogue did not result in significant changes (RR = 1.04, p = 0.26). Treatment with GnRH or hCG 5-7 days after synchronized ovulation was associated with increased chances of pregnancy compared with later treatment (11-14 days). Owing to the treatment, the pregnancy rate of cows with very poor fertility (<40%) was improved, while that of cows with good fertility (≥40%) was not affected. Treatment with GnRH or hCG greatly improved pregnancy rates of parous lactating cows (RR = 1.32, p < 0.05) compared with heifers (RR = 1.02, p > 0.05). Additionally, as indicated by pregnancy loss analysis, the treatment had no benefit on late embryo/early fetus survival at days 28-81. In conclusion, the induction of accessory CL with GnRH or hCG may benefit fertility and have important implications for the management of reproductive performance in the dairy industry.

Vaginal mucus resistivity evaluation in cattle: reliability of the swine estrous detector probe during 16th-hour insemination following ovulation synchronization

Accuracy of insemination timing is a key determinant of high pregnancy rates in livestock breeding stations. The estrous detector probes are a recent introduction into the Nigerian livestock farming sector. Many often these probes are species-labeled and they measure changes in the vaginal mucus resistivity (VMR) during the stages of the estrous cycle. With respect to size and shaft conformation, the Draminski ® swine estrous detector probe (sEDP) is quite similar to the bovine type. We investigated the reliability of the sEDP at insemination time on two farms designated as FM A and FM B. Cows (Bunaji, n=20 per farm) were evaluated for VMR at 16th hour post standard OvSynch protocol, with concurrent insemination on FM B only. The difference in the mean VMR between FM A (221(±)24.36) Ohms and FM B (254(±)35.59) Ohms was not significant (p>0.05). Sixteen cows (80%) at FM B were later (day 70) confirmed pregnant via rectal palpation and all calved at term. These findings suggest consistency in VMR evaluated with sEDP at insemination aswell as a high predictability for VMR associated with good pregnancy rates in dairy cattle. We conclude that Draminski ® swine estrous detector probe is reliable in determining time of insemination in cattle breeding stations. Keywords: Swine estrous probe, vaginal mucus resistivity, insemination, OvSynch protocol, cattle

PSVIII-42 Effect of presynchronization using prostaglandin F2α prior to a shortened progesterone exposure in fixed-timed AI of crossbred beef cows nursing calves

Abstract Presynchronization has been shown to improve AI pregnancy rates in dairy cattle, by improving the effectiveness of the initial GnRH treatment required for follicular control in estrous synchronization protocols. An experiment was designed to evaluate the effect of prostaglandin presynchronization in a fixed-timed AI (FTAI) protocol with a shortened progesterone exposure in beef cows. A total of 135 fall calving (September-November) crossbred beef cows from two locations were randomized to receive either: A) prostaglandin (25mg; Lutalyse, Zoetis, Kalamazoo, MI) administered two days prior to synchronization with GnRH (100mg; Factrel, Zoetis) and progesterone (1.38g P4; CIDR, Zoetis) for 6 days, or B) Control, the same synchronization protocol without Lutalyse pretreatment. On day 6 (Day 0: CIDR insertion) CIDR was removed and 25mg of prostaglandin was administered and 72 hours later fixed-timed AI with concurrent injection of 100mg of GnRH. Conventional frozen semen from Simmental-Angus sires was used for AI and fertile Angus bulls were used for pasture exposure. Pregnancy was determined by ultrasound 30 days after AI and again 30 days after clean-up bull exposure for 30 days. Statistical analysis was performed using the frequency procedure of SAS (Cary, NC) and significance was set at X2 &amp;lt; 0.05. Pregnancy rate to timed AI was 71.4% for Treatment A compared to 80.3% for Treatment B (Control). Although these results were greater than previously reported at these research stations, there was no statistical difference (X2=0.23) between treatments. Overall pregnancy rates tended to be different (X2=0.06) and were potentially influenced by treatment or shortened exposure to progesterone in synchronization. Research supported by University of Arkansas Division of Agriculture.

Effect of chronic administration of a gonadotropin-releasing agonist on luteal function and pregnancy rates in dairy cattle.

Increased embryonic losses may be associated with inadequate progesterone (P4) concentrations in high-producing lactating dairy cattle. The objectives of the present studies were to determine if chronic administration of a gonadotropin-releasing hormone (GnRH) agonist, Deslorelin, would increase circulating P4 concentrations and subsequently increase pregnancy rates in dairy cattle. Administration of Deslorelin for 12days increased (p<.05) luteal volume and circulating P4 concentrations in primiparous lactating dairy cows, but increased only luteal volumes in multiparous cows. Treatment with Deslorelin increased Day 45 pregnancy rates in cows as compared to untreated controls. Chronic treatment with Deslorelin in dairy cattle; (a) increased luteal volume of the primary CL, (b) induced accessory CL, (c) increased circulating P4 concentration in primiparous cows only, (d) did not lengthen the estrous cycle upon removal of treatment, and (e) increased pregnancy rates. Although luteal volume was increased in multiparous cows and circulating P4 concentrations were not with Deslorelin treatment, there was an apparent effect on pregnancy rates. This hormonal strategy may represent a suitable model to address local effects of P4 and GnRH/luteinizing hormone on uterine environment and subsequent embryonic survival.

乳牛の受胎向上を目的とする人工授精（AI）5日後のホルモン製剤投与効果に関する一考察

乳牛の受胎向上を目的とする人工授精（AI）5日後のホルモン製剤投与効果に関する一考察

Application of recombinant bovine somatotropin (rbST) in FTAI protocols and its effect on estrus detection and pregnancy rates in dairy cattle in a humid tropical environment

Application of recombinant bovine somatotropin (rbST) in FTAI protocols and its effect on estrus detection and pregnancy rates in dairy cattle in a humid tropical environment

Effect of heat stress on reproductive performances of dairy cattle and buffaloes: A review.

Heat stress has adverse effects on the reproductive performances of dairy cattle and buffaloes. The dairy sector is a more vulnerable to global warming and climate change. The temperature humidity index (THI) is the widely used index to measure the magnitude of heat stress in animals. The objective of this paper was to assess the decline in performances of reproductive traits such as service period, conception rate and pregnancy rate of dairy cattle and buffaloes with respect to increase in THI. The review stated that service period in cattle is affected by season of calving for which cows calved in summer had the longest service period. The conception rate and pregnancy rate in dairy cattle were found decreased above THI 72 while a significant decline in reproductive performances of buffaloes was observed above threshold THI 75. The non-heat stress zone (HSZ) (October to March) is favorable for optimum reproductive performance, while fertility is depressed in HSZ (April to September) and critical HSZ (CHSZ) (May and June). Heat stress in animals has been associated with reduced fertility through its deleterious impact on oocyte maturation and early embryo development. The management strategies viz., nutrition modification, environment modification and timed artificial insemination protocol are to be strictly operated to ameliorate the adverse effects of heat stress in cattle and buffaloes during CHSZ to improve their fertility. The identification of genes associated with heat tolerance, its incorporation into breeding program and the inclusion of THI covariate effects in selection index should be targeted for genetic evaluation of dairy animals in the hot climate.

Milk somatic cell counts and pregnancy rates in dairy cattle

OPTIMISING dairy herd fertility and productivity depends on maximising the pregnancy rate (the proportion of inseminations resulting in a pregnancy). However, pregnancy rates in dairy cattle are at an all-time...

12 EFFECT OF HUMAN CHORIONIC GONADOTROPIN (hCG) ADMINISTRATION ON DAY 2 OR DAY 5 AFTER OESTRUS ON PREGNANCY RATE IN HIGH-YIELDING DAIRY COWS

In cattle, ~40% of embryonic loss occurs in the period from Day 8 to Day 16 of pregnancy. A significant proportion of embryo loss may be due to inadequate circulating progesterone (P4) concentrations. Low P4 concentrations have also been implicated as a causative factor in the low pregnancy rates (PR) observed in high-yielding dairy cows. Administration of hCG during the early luteal phase stimulates hypertrophy of the original corpus luteum (CL) and, depending on the day of administration, induces ovulation of the first-wave dominant follicle and formation of a functional accessory CL, which increases circulating P4 concentrations. The aim of this study was to examine whether administration of hCG on Day 2 or Day 5 after oestrus after timed AI (TAI) would lead to an increase in pregnancy rates in dairy cattle. Lactating Holstein cows (n = 194) from 12 commercial dairy herds in Southern Spain (37.8833° N, 4.7667° W) with an average milk production at 37.8 L/cow per day and typically with a PR to first AI of ~30% were randomly assigned based on their body condition score (2.65 ± 0.05; mean ± SEM), parity (2.60 ± 0.09), and days in milk (75.06 ± 0.63) to 1 of 3 treatments and administered a single intramuscular injection of 3000 IU of hCG (4 mL of Veterin Corion) either (1) on Day 2 = 36 h after TAI (n = 65; hCG2 group), (2) Day 5 = 108 h after TAI (n = 64; hCG5 group), or (3) 4 mL of saline on Day 2 = 36 h after TAI (n = 65; control group). Cows were synchronized using a 7-day Ovsynch TAI protocol that included a P4-releasing intravaginal device (PRID DELTA 1.55 g). First, gonadotropin-releasing hormone (Cystorelin 100 mg) treatment was administered at PRID insertion (Day 0) followed by 25 mg Dinoprost (prostaglandin F2α: Enzaprost T) on Day 7 at PRID withdrawal. Then, 56 h later, the second gonadotropin-releasing hormone (100 mg) treatment was administered and all cows were inseminated 16 h later. Pregnancy was diagnosed by ultrasonography 28 to 32 days after TAI. Logistic regression model and chi-squared test were used to analyse data. Pregnancy rate to AI was significantly higher in the hCG2 and hCG5 groups than in the control group (43.1 and 45.3%, v. 27.7%; P &lt; 0.05). A treatment-by-parity interaction was observed; while pregnancy rate for primiparous cows was not affected by treatment, multiparous cows from the hCG2 group had greater pregnancies per AI than those in the control group (47.2% v. 21.1%, respectively; P &lt; 0.05). In conclusion, these preliminary results suggest that hCG administration on Day 2 and 5 after oestrus increases PR at first postpartum AI in Holstein cows. In addition, hCG on Day 2 increases the fertility in multiparous cows.This study was funded by the Spanish Ministry of Science and Innovation (AGL2012–37510) and partially supported by Ceva Salud Animal S.A., Spain – synchronization protocol and DFV, Spain – hCG treatments.

Photoperiod length and the estrus synchronization protocol used before AI affect the twin pregnancy rate in dairy cattle

This study addresses potential management risk factors affecting the incidence of twin pregnancies in high-producing dairy cows. Special attention was paid to the estrus synchronization protocol used before the AI resulting in pregnancy. Possible factors affecting the twin pregnancy rate were analyzed through binary logistic regression procedures on 2015 pregnant cows from July 2010 to July 2011. Twin pregnancy was recorded in 361 of the 2015 pregnancy diagnoses made (17.9%). Twin pregnancy rates differed among herds (P < 0.001) and ranged from 12.4% to 23.9%. Based on the odds ratios, the risk of twin pregnancy was reduced by factors of 0.65 or 0.71 when AI was performed during the warm season or an increasing photoperiod, respectively and increased by a factor of 1.11 for each unit increase in lactation number; by factors of 4.57 or 6.33 in cows that received a progesterone-releasing intravaginal device (PRID) plus 500 or 750 IU of equine chorionic gonadotropin (eCG) 28 days before the pregnancy AI, respectively; by a factor of 2.39 in cows with an ovarian cyst diagnosed in the 14 days prior to AI and treated with prostaglandins (PG); by factors of 1.94 or 3.91 in cows that received two PG doses during the 14 days prior to AI or cows that following failed PRID treatment had received PG started over the 28 days prior to AI, respectively; and by a factor of 2.58 in cows that had previously delivered twins compared to cows delivering singletons. Our results indicate that cow factors, such as lactation number and previous twining, as well as environmental factors, such as photoperiod and season and management related to synchronization protocols affect significantly the incidence of twin pregnancies.

Effect of Acute Bull Exposure Around the Time of Artificial Insemination on Serum Oxytocin and Progesterone Concentrations and Pregnancy Rates in Dairy Cows

This study examined the effect of acute bull exposure around the time of artificial insemination (AI) on oxytocin and progesterone concentrations, and pregnancy rates in dairy cows. Ninety six dairy cows, stratified according to parity into primiparous and pluriparous, were divided into three groups; short bull exposure (SBE; 10 min, n = 32), long bull exposure (LBE; 4 h, n = 32) or no bull exposure (NBE; n = 32). On day 45 post-partum, all cows were treated with PGF2α on three occasions 11-14 days apart to synchronize oestrus. They were submitted to fixed time AI 80 h after the third PGF2α injection. Cows in the SBE and LBE groups were artificially inseminated 5 min after the introduction of the bull. From a subset of cows (n = 6 per group; three primiparous and three pluriparous), blood samples were collected once every 5 min starting 15 min before AI until 15 min after AI and analysed for oxytocin concentrations. Additional blood samples were collected for measurements of progesterone (P4) concentrations once daily for 4 days starting on the day of AI and once every 3 days thereafter until day 22. The effects of bull exposure, time, parity, difficulty of AI, and pregnancy on oxytocin and P4 concentrations were analysed using the mixed linear model procedure. Mean oxytocin concentrations or change in oxytocin concentrations after bull exposure or AI were not different among groups. Pregnancy rates for the NBE, SBE and LBE groups were 55.5%, 33.3% and 44.4%, respectively, and were not different among groups. In conclusion, acute bull exposure around the time of AI did not affect oxytocin and progesterone concentrations and did not improve pregnancy rates in dairy cattle under these farms conditions.

Effect of flunixin meglumine and carprofen on pregnancy rates in dairy cattle

Embryonic losses contribute considerably to low pregnancy rates. Between d 8 and 17 after breeding, the conceptus secretes interferon-τ as a mechanism for maternal recognition of pregnancy and maintenance of the corpus luteum. Nonsteroidal antiinflammatory drugs inhibit the synthesis of prostaglandin F2α by suppressing the enzyme cyclooxygenase. Flunixin meglumine (FM) has been demonstrated to delay luteolysis and to support embryonic survival. The objective of this study was to evaluate the effect of FM and carprofen on conception rates in dairy heifers and cows, respectively. In experiment 1, the effect of FM on pregnancy rates and progesterone concentrations in dairy heifers was tested. A total of 391 heifers were randomly assigned to 1 of 2 groups. Heifers in the treatment group (n=197) received 2.2 mg of FM i.m./kg of body weight twice on d 14/15 and 15/16 after insemination, whereas heifers in the control group (n=194) remained untreated. Blood samples from 388 heifers were taken on d 14/15 and 21/22 after artificial insemination and analyzed for progesterone. Pregnancy rates were 58.2 and 54.8% for the control and treatment groups, respectively. Mean progesterone concentrations were not affected by treatment and number of artificial insemination service (first or second artificial insemination service), but were affected by time and time × pregnancy status. In experiment 2, the objective was to verify the effects of carprofen, a longer acting nonsteroidal antiinflammatory drug and to evaluate its effect on conception rate to first service in dairy cows. A total of 380 cows were randomly assigned to 1 of 2 treatment groups. Cows in the treatment group (n=194) received 1.4mg of carprofen s.c./kg of body weight on d 15 after insemination, whereas cows in the control group (n=186) remained untreated. Pregnancy was diagnosed between d 40 and 47 after insemination. Conception rates to first service were 35.5 and 33.0% in the control and treatment groups, respectively. Neither flunixin meglumine nor carprofen improved conception rates to first service in dairy cattle in the dosage and administration schedule tested.

Postinsemination treatment of primiparous and multiparous cows with cloprostenol failed to affect ovulation and pregnancy rate in dairy cattle

Previous studies have demonstrated the facilitating effect of a single cloprostenol treatment postinsemination on ovulation and pregnancy rate in dairy cattle and Italian buffalos. This study was conducted to test the effects of 500 μg cloprostenol given intramuscularly immediately postinsemination to 55 primiparous and 60 multiparous Holstein cows ( Bos taurus; TR) of a typical dairy operation in East Germany and to compare them with 57 primiparous and 48 multiparous saline-treated cows (CON). Animals of the TR and CON groups did not differ or only differed marginally for age at treatment, interval calving–treatment, lactation number, milk production on the day of treatment, body condition score, and their peri- and postparturient case histories. All animals were clinically and reproductively healthy on the day of treatment. They were inseminated once 12 h after the onset of estrus, scanned 24 h after insemination to confirm ovulation, and tested for pregnancy by transrectal palpation between Days 42 and 48 postinsemination. Treatment did not affect the number of cows ovulating. The overall combined pregnancy rate (PR) was 47.3%. Pregnancy rate did not differ statistically between TR and CON (46.1% vs. 48.6%; P > 0.05). In conclusion, this study failed to demonstrate beneficial effects of a postinsemination treatment of primiparous and multiparous cows with cloprostenol on ovulation and PR in a typical dairy cattle operation in East Germany.

Low Pregnancy Rate in Dairy Cattle after Fixed time Artificial Insemination using Norgestomet + PGF2α + eCG Program During the Hot and Humid Months in Thailand

Low Pregnancy Rate in Dairy Cattle after Fixed time Artificial Insemination using Norgestomet + PGF2α + eCG Program During the Hot and Humid Months in Thailand

Resynchronized pregnancyrates in dairy cattle: timingof gonadotropin-releasinghormone injection before timedartificial insemination (2008)

Lactating dairy cows and replacement virgin heifers of unknown pregnancy status were treated with either gonadotropin-releasing hormone (GnRH) or saline to initiate a resynchronization program that was continued 7 days later when a not-pregnant diagnosis was determined. Nonpregnant cattle were administered prostaglandin F2α and then either injected with GnRH 56 hours later and artificially inseminated (AI) by appointment at 72 hours or injected and inseminated concurrently at 72 hours. Injection of GnRH at 56 hours produced more pregnancies than injection of GnRH at 72 hours when AI was administered at 72 hours in both treatments (30.9 vs. 15.2%). Further, starting the resynchronization with GnRH was beneficial to resulting pregnancy rates but was timing dependent. When a not-pregnant status was determined between day 30 and 36 after AI, upfront GnRH injection (7 days before pregnancy diagnosis) may not be necessary because stage of cycle is 1 to 7 days (days 3 to 4 in 71% of cattle) and resulting pregnancy rates after GnRH or saline did not differ (27.5 vs. 26.6 %, respectively). In contrast, when pregnancy status was determined after day 36 (days 37 to 43; cycle days 10 to 11 in 71% of cattle), upfront GnRH as part of the resynchronization protocol nearly doubled the number of pregnancies compared with saline (31.0 vs. 15.1%).

Effect of transfer of one or two in vitro-produced embryos and post-transfer administration of gonadotropin releasing hormone on pregnancy rates of heat-stressed dairy cattle

Pregnancy rates following transfer of an in vitro-produced (IVP) embryo are often lower than those obtained following transfer of an embryo produced by superovulation. The purpose of the current pair of experiments was to examine two strategies for increasing pregnancy rates in heat stressed, dairy recipients receiving an IVP embryo. One method was to transfer two embryos into the uterine horn ipsilateral to the CL, whereas the other method involved injection of GnRH at Day 11 after the anticipated day of ovulation. In Experiment 1, 32 virgin crossbred heifers and 26 lactating crossbred cows were prepared for timed embryo transfer by being subjected to a timed ovulation protocol. Those having a palpable CL were randomly selected to receive one ( n = 31 recipients) or two ( n = 27 recipients) embryos on Day 7 after anticipated ovulation. At Day 64 of gestation, the pregnancy rate tended to be higher ( P = 0.07) for cows than for heifers. Heifers that received one embryo tended to have a higher pregnancy rate than those that received two embryos (41% versus 20%, respectively) while there was no difference in pregnancy rate for cows that received one or two embryos (57% versus 50%, respectively). Pregnancy loss between Day 64 and 127 only occurred for cows that received two embryos (pregnancy rate at Day 127 = 17%). Between Day 127 and term, one animal (a cow with a single embryo) lost its pregnancy. There was no difference in pregnancy rates at Day 127 or calving rates between cows and heifers, but females that received two embryos had lower Day-127 pregnancy rates and calving rates than females that received one embryo ( P < 0.03). Of the females receiving two embryos that calved, 2 of 5 gave birth to twins. For Experiment 2, 87 multiparous, late lactation, nonpregnant Holstein cows were synchronized for timed embryo transfer as in Experiment 1. Cows received a single embryo in the uterine horn ipsilateral to the ovary containing the CL and received either 100 μg GnRH or vehicle at Day 11 after anticipated ovulation (i.e. 4 days after embryo transfer). There was no difference in pregnancy rate for cows that received the GnRH or vehicle treatment (18% versus 17%, respectively). In conclusion, neither unilateral transfer of two embryos nor administration of GnRH at Day 11 after anticipated ovulation improved pregnancy rates of dairy cattle exposed to heat stress.

Effect of a single growth hormone (rbST) treatment at breeding on conception rates and pregnancy retention in dairy and beef cattle

Initiation of long-term treatment with rbST (Posilac ®, Monsanto, St. Louis, MO) coincident with first insemination increased pregnancy rates in dairy cattle, but neither the efficacy of using only the initial injection, nor its effects on retention of pregnancy are known. Lactating dairy cows, dairy heifers, and lactating beef cows were assigned at random to treatment (rbST) or control. Dairy cows, dairy heifers, and beef cows received 500 mg rbST ( n = 48, 35, 137 inseminations, respectively) at artificial insemination or were left untreated ( n = 62, 33, 130 inseminations, respectively). Pregnancy was diagnosed by ultrasonography at 28–36 days. Treatment with rbST at insemination improved conception rates in dairy cows (60.4% versus 40.3%; P < 0.05), but not in dairy heifers or beef cows. Conception rates did not differ in dairy cows at ≤100 days in milk (DIM), but were improved in cows treated with rbST after 100 DIM (64.3% versus 25.8%; P < 0.05). Retention of pregnancy to ∼60 days and sizes of CL, diameter of follicles ≥5 mm, and crown-rump lengths of embryos were not affected by treatment. The second objective was to examine the effects of rbST at insemination on birth weight and post-natal calf growth in beef cows. However, birth and weaning weights of beef calves were not affected by treatment. In conclusion, a single treatment with rbST at insemination increased conception rates in dairy cows, specifically in those >100 DIM.

189 PREGNANCY RATES IN HEAT-STRESSED DAIRY CATTLE RECEIVING ONE OR TWO IN VITRO-PRODUCED EMBRYOS IN A TIMED EMBRYO TRANSFER PROGRAM

The objective was to determine whether transfer of two embryos would increase pregnancy rates in heat-stressed dairy recipients receiving an in vitro-produced embryo transferred into the uterine horn ipsilateral to the corpus luteum (CL). Such a treatment would increase the likelihood that the cow receives at least one embryo competent for sustained development. In addition, transfer of two embryos into the ipsilateral uterine horn is likely to increase the amounts of interferon-tau and other embryonic-signaling molecules in the uterus. A total of 32 virgin crossbreed heifers and 26 lactating crossbreed cows were used as timed embryo transfer recipients after being subjected to an ovulation synchronization protocol as follows: GnRH (100 �g) and insertion of previously used progesterone-containing CIDR on Day -10, prostaglandin F2� and CIDR removal on Day -3, and GnRH (100 �g) on Day 0 (day of anticipated ovulation). All recipients had a palpable CL on Day 6 and were randomly selected to receive one (n = 31 recipients) or two (n = 27) embryos on Day 7. At Day 64, the pregnancy rate tended to be higher (P = 0.07) for cows than for heifers. While not significant, heifers that received two embryos tended to have lower pregnancy rates than those that received a single embryo (20% vs. 41%); there was no difference in pregnancy rate in cows (50% for two embryos vs. 57% for one embryo). Pregnancy losses between Day 64 and Day 127 occurred in one group only cows receiving two embryos. In that group, pregnancy rate was 50% at Day 64 but 17% at Day 127. Overall, there was no difference in pregnancy rates at day 127 between cows and heifers, but recipients that received two embryos (17% for cows and 20% for heifers) had lower pregnancy rates (P &lt; 0.03) than recipients that received one embryo (57% for cows and 41% for heifers). Only one animal, a cow, had twin fetuses at day 127. In conclusion, unilateral transfer of two embryos failed to improve pregnancy rates of dairy cattle exposed to heat stress. The fact that fetal loss occurred sooner for heifers than for cows points out the importance of uterine capacity as a limiting factor for maintenance of fetal development of two conceptuses. The suitability of timed embryo transfer was evident from the high pregnancy rates achieved with crossbreed females that received a single embryo. This work was supported by BARD Grant No. US-1551-14, USDA TSTAR Grant No. 2004-14135-14715, Grant No. 2001-12101-11318 from the USDA-IFAFS Program, and CAPES Grant No. 134202-1).

The use of hormonal treatments to improve the reproductive performance of lactating dairy cows in feedlot or pasture-based management systems

Hormonal interventions have been used to increase the probability of estrous detection and insemination, and to increase pregnancy rates of dairy cattle under a variety of management systems. The present review addresses the basic principles of hormonal intervention and presents typical examples that illustrate the methodology. The hormones used to control the estrous cycle mimic the reproductive hormones found within the normal cow. Most estrous synchronization systems employ a method for controlling follicular wave development, promoting ovulation in anestrous cows, regressing the corpus luteum in cyclic cows, and synchronizing estrus and (or) ovulation at the end of treatment. A wide range of reproductive systems are in place on dairy farms. In most herds, a non-intervention period is practiced where postpartum cows are observed estrus estrus. Cows not observed in estrus are then treated. A number of studies in pasture-based and confinement systems have demonstrated net benefits of whole-herd synchronization. Despite the advantages of whole-herd reproductive programs, their uptake has been inconsistent globally. The benefits of a timed artificial insemination (AI) system increase under conditions of poor estrous detection rate and poor conception rate. The unpopular nature of timed AI programs in pasture-fed cows relates to high rates of estrous detection and conception for pasture-based dairying. Regardless of production system, some cows must be re-inseminated because they are not pregnant after first insemination. The presence of “phantom cows” (non-pregnant cows that do not return to estrus) creates a serious reproductive challenge for both pasture-based and confinement-style operations. Early pregnancy diagnosis and second insemination timed AI may reduce the effects of phantom cows on dairy herds. Fundamental research into anestrous, the hormonal control of the estrous cycle, and early pregnancy detection should elucidate new methods that can be used to strengthen reproductive programs on dairy farms.

181COMPARISON OF THE PREGNANCY RATES AFTER SYNCHRONIZATION OF OVULATION USING GnRH AND PGF2± IN RECIPIENT DAIRY CATTLE

Treatments with GnRH and PGF2α for synchronization of ovulation has resulted in acceptable pregnancy rates after fixed-time artificial insemination in dairy cows without estrus detection. The objective of the present study was to evaluate the practicability of ovulation synchronization (Ovsynch, Pursley JR et al. 1995 Theriogenology 44, 915–923) in dairy cattle using GnRH and PGF2α for the embryo transfer recipients. Dairy cattle (cows; n=100, heifers; n=88) were randomly allocated to one of two groups. The control group (cows; n=45, heifers; n=37) was composed of cows in natural estrus. The ovulation synchronization group (cows; n=55, heifers; n=51) was treated with an intramuscular injection of 100μg of GnRH at a random stage of the estrous cycle. Seven days later, the cattle received PGF2α (Cows; 25–30mg) or PGF2α analog (Heifers; 0.5mg) in order to regress the corpora lutea (CL). Forty-eight hours later, cows and heifers received a second injection of 100μg GnRH. Embryo transfer was carried out 7 days after the second injection of GnRH in the ovsynch group and 7 days after estrus in the control group. The cattle judged to have CL 17mm were classified as acceptable recipients. The size of the follicles and the CL were determined to be of estrus stage and embryo transfer by means of ultrasonography. The mean numbers of follicles and CL were analyzed by ANOVA, while pregnancy rates were analyzed by chi-square test. The results are presented in the Table. The proportion of cows and heifers determined to be acceptable embryo transfers was not different between the control group and the ovsynch group. There were no differences in the proportion of acceptable embryo transfers between the control group and the ovsynch group. Follicle diameter at the time of estrus in the control group (cows; 20.7±0.7mm, heifers; 16.8±0.5mm) were significantly larger than that of the ovsynch group (cows; 18.0±1.0mm, heifers; 14.7±0.2mm) (P&amp;lt;0.05). Although CL diameter at the time of embryo transfer in heifers showed no differences between the control group and the ovsynch group (25.0±1.0mm v. 22.8±1.5mm), The CL diameter of the control cow group was larger than that of the ovsynch group (29.8±0.7mm v. 26.1±1.0mm, P&amp;lt;0.05). However, no differences in pregnancy rate were seen between the control group and the ovsynch group. These results suggest that ovsynch can be effectively applied in an embryo transfer program for cattle. Table 1 Proportion of acceptable embryo transfer recipients and pregnancy rate in dairy cattle in the control ovsynch groups