Effects of prenatal and postnatal melatonin supplementation on overall performance, male reproductive performance, and testicular hemodynamics in beef cattle.

Melatonin has been documented to alleviate compromised pregnancies and enhance livestock performance traits. The objective of this study was to determine the effect of prenatal and postnatal melatonin supplementation on overall calf performance and dam milking traits in relation to calves, molecular factors involved in growth and metabolism of calves, along with testicular physiology and fertility traits in subsequent bulls. On days 190, 220 and 250 of gestation, dams (N = 60) were administered either two subdermal ear melatonin implants (preMEL) or no implants (preCON). After parturition, birth weights were recorded and calves were blocked based on prenatal treatment and sex. Calves received either melatonin implants (posMEL) or none (posCON) on days 0, 30, and 60 of age. On day 60 of lactation, a subset of dams (N = 32) were selected based on age, weight, and calf sex for milk collection and analysis. At weaning, (day 210 postnatally) calf weight, morphometric data, liver samples, and loin samples were collected. At 12 mo of age, bull (N = 30) scrotal circumference, scrotal temperature, and testicular artery measurements were recorded. Milk yield and fat percent from dams tended to decrease in the preMEL group (P < 0.07) compared with preCON group. Prenatal melatonin administration did not affect (P = 0.95) calf birth weight and similarly calf weaning weight was unaffected (P < 0.10) by prenatal or postnatal melatonin supplementation. Blood analysis demonstrated that plasma concentrations of melatonin were not different (P = 0.12) in dams; however, an increase (P < 0.001) in plasma concentrations of melatonin was observed in posMEL vs. posCON calves. A tendency (P < 0.10) for decreased MYF5 and MYOD1 expression in loin muscle was observed in the posMEL calves. Prenatal and postnatal melatonin administration did not affect subsequent bull scrotal measurements or testicular hemodynamics (P ≥ 0.14). Administering supplemental melatonin via implants during the prenatal and postnatal phase did not alter performance characteristics in offspring. In this study, dams were implanted in winter months, whereas calves were implanted in the spring months. Seasonal differences involving photoperiod and ambient temperature might have attributed to a lack of differences in melatonin levels during the prenatal phase. In the postnatal period, the level of developmental plasticity appears to be too low for melatonin properties to be effective.

Previous studies have examined maternal melatonin implants in fall calving Mississippi cattle during the third trimester of pregnancy. These studies have shown increased maternal uterine blood flow without any change in calf birth weight when supplemented with melatonin implants. However, calf weaning weights were increased in calves born to melatonin supplemented dams vs. their control counterparts. The objective of this study was to examine offspring performance following maternal melatonin supplementation (prenatal) and/or postnatal calf melatonin supplementation in spring calving Montana cattle. Calf performance and weight at weaning were not affected by maternal or postnatal melatonin supplementation. However, dam milk yield and fat percent were decreased in the melatonin supplemented dams. Maternal and postnatal melatonin supplementation did not affect bull measurements of reproductive performance. Interestingly, maternal concentrations of melatonin were not different between dam treatment groups; however, postnatal melatonin supplementation increased calf concentrations of melatonin. In this study, dams were implanted in winter months, whereas calves were implanted in the spring months. Seasonal differences involving photoperiod and ambient temperature may attribute to a lack of differences in melatonin levels during the prenatal phase.

Modulation of expression of estrus, steroidogenesis and embryo development following peri-artificial insemination nutrient restriction in beef heifers.

Nutritional changes immediately after insemination cause increased embryonic mortality, but the mechanisms controlling this are not well known. Our objective was to evaluate the impact of nutritional change on estrus expression, steroid concentrations, peripheral and uterine luminal fluid metabolites, and embryo quality in beef heifers. Heifers (n = 139) were assigned to one of two pre-artificial insemination (AI) dietary treatments: LOW (≤ 90% NEm) or HIGH (≥ 139% NEm). Heifers were on treatment for 33-36 days before AI (d0) when half of the heifers in each treatment were randomly reassigned to generate four treatments; HIGH-HIGH, HIGH-LOW, LOW-HIGH, and LOW-LOW. Heifers remained on treatments until embryo collection (d 6-8). Negative energy balance was achieved among LOW heifers as demonstrated by body weight loss and increased NEFA concentrations (P < 0.05). Pre-AI treatment influenced expression of estrus (P = 0.05; HIGH 80.4 ± 4.0% vs. LOW 69.4 ± 4.2%). Estradiol concentrations and interval to estrus were not affected by treatment (P > 0.55); however, progesterone concentrations were reduced among LOW compared to HIGH (3.57 ± 0.27, 4.64 ± 0.26 ng/mL, respectively; P = 0.004), and heifers maintained on the HIGH pre-AI diet had consistently greater concentrations of progesterone from d 0 to d 8 (P = 0.014). Pre-AI treatment influenced embryo stage (P = 0.05; HIGH 3.61 ± 0.32 vs. LOW 2.72 ± 0.30). Post-AI treatment affected embryo grade (P = 0.02; HIGH 1.78 ± 0.23 vs. LOW 2.64 ± 0.27). In summary, pre-AI nutrient restriction caused decreased expression of estrus, reduced progesterone concentrations after AI, and negatively impacted embryo development, while post-AI restriction hindered embryo quality.

Medroxyprogesterone acetate in reindeer bulls: testes histology, cfos activity in the brain, breeding success, and semen quality.

A previous study reported that a 400-mg dose of medroxyprogesterone acetate (MPA) reduced male reindeer aggression and blocked development of secondary sexual characteristics but did not completely impair fertility. Here we have repeated that protocol in two separate trials. In 2017, tissues and blood samples, collected from MPA and control (CTL) reindeer bulls, euthanized at 30 and 60 d post-treatment were used to evaluate testes histology and morphometrics, cfos activity in the brain and androgen levels. While testes weight tended to decline from August to September in both groups, indices of spermatogenesis remained high. By September, indices of spermatogenesis were declining in both groups with sperm density lower (P = 0.05) in MPA compared to CTL bulls. Aug CTL bulls had the highest concentrations of androstenedione (A4) (P = 0.009) and testosterone (T) (P = 0.08), whereas these androgens were baseline in Aug MPA bulls. By September, A4 and T levels in CTL bulls declined to levels measured in MPA bulls. Cfos activity had a greater number (P = 0.02) of cfos positive neurons in the central amygdala in MPA compared to CTL bulls, suggesting a heightened fear response among the MPA bulls. In the second trial (2019), MPA-treated bulls, with (E, n = 4) and without (IE, n = 4) breeding experience, were blood sampled at key points from July through September when they were put in individual harems with estrous-synchronized cows. Concentrations of T were greatest (P < 0.001) among E bulls prior to MPA treatment but 1 mo after treatment, both T and A4 were baseline in all eight reindeer. Semen collected by electroejaculation at 60 d post-MPA treatment revealed only minor differences in sperm abnormalities between E and IE bulls using both fresh and frozen/thawed semen. Only three bulls (2 E and 1 IE) sired offspring. Breeding success was not related to previous breeding experience, body weight, or bull age. The failure of some MPA bulls to breed appears to be a behavioral, not a physiological, limitation. Limited application of MPA is clearly a useful tool for managing rut-aggression in non-breeding reindeer. However, the possibility that semen could be collected from MPA-treated bulls using restraint and mild sedation rather than general anesthesia should be investigated. This could improve the quality of semen collection while enhancing the safety of both handlers and animals.

A single 400 mg dose of MPA given to reindeer bulls just before the onset of rut eliminates aggressive behavior and suppresses androgen concentrations without dramatic differences in the gross or histological structure of the testes within the first 30 d of treatment. By 60 d post-treatment, there is evidence of smaller testes size and decreased sperm density in treated bulls. However, if given the opportunity, some treated bulls can still successfully breed. Breeding success in MPA bulls was not solely related to previous breeding experience, body weight, or bull age. Androgen concentrations and semen characteristics did not vary with previous breeding experience. Failure of some treated bulls to breed appears to be a behavioral limitation. Differences in brain activity between control and treated bulls were few except for increased cfos activity in the central amygdala of MPA bulls, potentially increasing the fear response in these reindeer.

Impact of preovulatory estradiol concentrations on subsequent luteal function in beef cattle.

It has been hypothesized that circulating concentrations of estradiol during the preovulatory period, can impact subsequent progesterone concentrations. Ovulation was synchronized in nonlactating beef cows (n = 53). Cows that exhibited estrus before gonadotrophin-releasing hormone (GnRH)-induced ovulation (d 0) had greater (p<.01) peak concentrations of estradiol compared with cows that did not express estrus (11.5 ± 0.8 vs. 6.2 ± 0.6 pg/mL), respectively, but there was no difference in ovulatory follicle size (p= .80) or interval from GnRH2 to ovulation (p=.23). Circulating concentrations of progesterone during luteal formation (d 3-7; p=.70 and p=.77) or mid-luteal phase (d 8-14; p=.39 and p=.12) were not affected by elevated periovulatory estradiol or an interaction with day. To investigate the direct influence of estradiol on luteal function, ovulation (d 0) was synchronized in nonlactating beef cows and cows were allocated to three groups (control, n = 5; vehicle injection, n = 4; or an estradiol antagonist (Fulvestrant; ICI 182,780), n = 4. Intrafollicular injection of vehicle (100 µL) or an estradiol antagonist (25 µg Fulvestrant in 100 µL) into the largest follicle occurred on d -2. Concentrations of estradiol increased (p<.0001) from d -2 to 0 but did not differ among groups (p>.50). Furthermore, plasma concentrations of progesterone on d 0 through 20 were not affected by treatment (p=.86). These results indicate that elevated preovulatory estradiol before ovulation was not required to prepare granulosa cells for luteinization or subsequent luteal progesterone secretion but did tend to impact luteal lifespan.

Differential Transcript Profiles in Cumulus-Oocyte Complexes Originating from Pre-Ovulatory Follicles of Varied Physiological Maturity in Beef Cows.

Small dominant follicle diameter at induced ovulation, but not at spontaneous ovulation, decreased pregnancy rate, fertilization rate, and day seven embryo quality in beef cows. We hypothesized that the physiological status of the follicle at GnRH-induced ovulation has a direct effect on the transcriptome of the Cumulus-Oocyte complex, thereby affecting oocyte competence and subsequent embryo development. The objective of this study was to determine if the transcriptome of oocytes and associated cumulus cells (CC) differed among small (≤11.7 mm) and large follicles (≥12.7 mm) exposed to a GnRH-induced gonadotropin surge and follicles (11.7-14.0 mm) exposed to an endogenous gonadotropin surge (spontaneous follicles). RNA sequencing data, from pools of four oocytes or their corresponding CC, revealed 69, 94, and 83 differentially expressed gene transcripts (DEG) among oocyte pools from small versus large, small versus spontaneous, and large versus spontaneous follicle classifications, respectively. An additional 128, 98, and 80 DEG were identified among small versus large, small versus spontaneous, and large versus spontaneous follicle CC pools, respectively. The biological pathway "oxidative phosphorylation" was significantly enriched with DEG from small versus spontaneous follicle oocyte pools (FDR < 0.01); whereas the glycolytic pathway was significantly enriched with DEG from CC pools obtained from large versus small follicles (FDR < 0.01). These findings collectively suggest that altered carbohydrate metabolism within the Cumulus-Oocyte complex likely contributes to the decreased competency of oocytes from small pre-ovulatory follicles exposed to an exogenous GnRH-induced gonadotropin surge.

Analysis of the uterine lumen in fertility-classified heifers: II. Proteins and metabolites†.

Survival and growth of the bovine conceptus is dependent on endometrial secretions or histotroph. Previously, serial blastocyst transfer was used to classify heifers as high fertile (HF), subfertile (SF), or infertile (IF). Here, we investigated specific histotroph components (proteins and metabolites) in the uterine lumen of day 17 fertility-classified heifers. Interferon tau (IFNT) was more abundant in uterine lumenal fluid (ULF) of pregnant HF than SF animals as the conceptus was longer in HF heifers. However, no differences in endometrial expression of selected classical and nonclassical interferon-stimulated genes (ISGs) were observed, suggesting that IFNT signaling in the endometrium of pregnant HF and SF heifers was similar. Pregnancy significantly increased the abundance of several proteins in ULF. Based on functional annotation, the abundance of a number of proteins involved in energy metabolism, oxidative stress, amino acid metabolism, and cell proliferation and differentiation were greater in the ULF of pregnant HF than SF heifers. Metabolomics analysis found that pregnancy only changed the metabolome composition of ULF from HF heifers. The majority of the metabolites that increased in the ULF of pregnant HF as compared to SF heifers were associated with energy and amino acid metabolism. The observed differences in ULF proteome and metabolome are hypothesized to influence uterine receptivity with consequences on conceptus development and survival in fertility-classified heifers.

Analysis of the uterine lumen in fertility-classified heifers: I. Glucose, prostaglandins, and lipids†.

Survival and growth of the bovine conceptus (embryo and associated extraembryonic membranes) are dependent on endometrial secretions or histotroph found in the uterine lumen. Previously, serial embryo transfer was used to classify heifers as high fertile (HF), subfertile (SF), or infertile (IF). Here, we investigated specific histotroph components [glucose, prostaglandins (PGs), and lipids] in the uterine lumen of day 17 pregnant and open fertility-classified heifers. Concentrations of glucose in the uterine lumen were increased by pregnancy but did not differ among fertility-classified heifers. Differences in expression of genes encoding glucose transporters and involved with glycolysis and gluconeogenesis were observed between conceptuses collected from HF and SF heifers. In the uterine lumen, PGE2 and PGF2α were increased by pregnancy, and HF heifers had higher concentrations of PGE2, PGF2α, and 6-keto-PFG1α than SF heifers. Differences were found in expression of genes regulating PG signaling, arachidonic acid metabolism, and peroxisome proliferator-activated receptor signaling among conceptuses and endometrium from fertility-classified heifers. Lipidomics was conducted exclusively in samples from HF heifers, and phosphatidylcholine was the main lipid class that increased in the uterine lumen by pregnancy. Expression of several lipid metabolism genes differed between HF and SF conceptuses, and a number of fatty acids were differentially abundant in the uterine lumen of pregnant HF and SF heifers. These results support the ideas that uterine luminal histotroph impacts conceptus survival and programs its development and is a facet of dysregulated conceptus-endometrial interactions that result in loss of the conceptus in SF cattle during the implantation period of pregnancy establishment.

Uterine influences on conceptus development in fertility-classified animals.

A major unresolved issue is how the uterus influences infertility and subfertility in cattle. Serial embryo transfer was previously used to classify heifers as high-fertile (HF), subfertile (SF), or infertile (IF). To assess pregnancy loss, two in vivo-produced embryos were transferred into HF, SF, and IF heifers on day 7, and pregnancy outcome was assessed on day 17. Pregnancy rate was substantially higher in HF (71%) and SF (90%) than IF (20%) heifers. Elongating conceptuses were about twofold longer in HF than SF heifers. Transcriptional profiling detected relatively few differences in the endometrium of nonpregnant HF, SF, and IF heifers. In contrast, there was a substantial difference in the transcriptome response of the endometrium to pregnancy between HF and SF heifers. Considerable deficiencies in pregnancy-dependent biological pathways associated with extracellular matrix structure and organization as well as cell adhesion were found in the endometrium of SF animals. Distinct gene expression differences were also observed in conceptuses from HF and SF animals, with many of the genes decreased in SF conceptuses known to be embryonic lethal in mice due to defects in embryo and/or placental development. Analyses of biological pathways, key players, and ligand-receptor interactions based on transcriptome data divulged substantial evidence for dysregulation of conceptus-endometrial interactions in SF animals. These results support the ideas that the uterus impacts conceptus survival and programs conceptus development, and ripple effects of dysregulated conceptus-endometrial interactions elicit loss of the postelongation conceptus in SF cattle during the implantation period of pregnancy.

Loci and pathways associated with uterine capacity for pregnancy and fertility in beef cattle.

Infertility and subfertility negatively impact the economics and reproductive performance of cattle. Of note, significant pregnancy loss occurs in cattle during the first month of pregnancy, yet little is known about the genetic loci influencing pregnancy success and loss in cattle. To identify quantitative trait loci (QTL) with large effects associated with early pregnancy loss, Angus crossbred heifers were classified based on day 28 pregnancy outcomes to serial embryo transfer. A genome wide association analysis (GWAA) was conducted comparing 30 high fertility heifers with 100% success in establishing pregnancy to 55 subfertile heifers with 25% or less success. A gene set enrichment analysis SNP (GSEA-SNP) was performed to identify gene sets and leading edge genes influencing pregnancy loss. The GWAA identified 22 QTL (p < 1 x 10-5), and GSEA-SNP identified 9 gene sets (normalized enrichment score > 3.0) with 253 leading edge genes. Network analysis identified TNF (tumor necrosis factor), estrogen, and TP53 (tumor protein 53) as the top of 671 upstream regulators (p < 0.001), whereas the SOX2 (SRY [sex determining region Y]-box 2) and OCT4 (octamer-binding transcription factor 4) complex was the top master regulator out of 773 master regulators associated with fertility (p < 0.001). Identification of QTL and genes in pathways that improve early pregnancy success provides critical information for genomic selection to increase fertility in cattle. The identified genes and regulators also provide insight into the complex biological mechanisms underlying pregnancy establishment in cattle.

Identification of Beef Heifers with Superior Uterine Capacity for Pregnancy.

Infertility and subfertility represent major problems in domestic animals and humans, and the majority of embryonic loss occurs during the first month of gestation that involves pregnancy recognition and conceptus implantation. The critical genes and physiological pathways in the endometrium that mediate pregnancy establishment and success are not well understood. In study one, predominantly Angus heifers were classified based on fertility using serial embryo transfer to select animals with intrinsic differences in pregnancy loss. In each of the four rounds, a single in vitro-produced, high-quality embryo was transferred into heifers on Day 7 postestrus and pregnancy was determined on Days 28 and 42 by ultrasound and then terminated. Heifers were classified based on pregnancy success as high fertile (HF), subfertile (SF), or infertile (IF). In study two, fertility-classified heifers were resynchronized and bred with semen from a single high-fertility bull. Blood samples were collected every other day from Days 0 to 36 postmating. Pregnancy rate was determined on Day 28 by ultrasound and was higher in HF (70.4%) than in heifers with low fertility (36.8%; SF and IF). Progesterone concentrations in serum during the first 20 days postestrus were not different in nonpregnant heifers and also not different in pregnant heifers among fertility groups. In study three, a single in vivo-produced embryo was transferred into fertility-classified heifers on Day 7 postestrus. The uteri were flushed on Day 14 to recover embryos, and endometrial biopsies were obtained from the ipsilateral uterine horn. Embryo recovery rate and conceptus length and area were not different among the heifer groups. RNA was sequenced from the Day 14 endometrial biopsies of pregnant HF, SF, and IF heifers (n = 5 per group) and analyzed by edgeR-robust analysis. There were 26 differentially expressed genes (DEGs) in the HF compared to SF endometrium, 12 DEGs for SF compared to IF endometrium, and three DEGs between the HF and IF endometrium. Several of the DEG-encoded proteins are involved in immune responses and are expressed in B cells. Results indicate that preimplantation conceptus survival and growth to Day 14 is not compromised in SF and IF heifers. Thus, the observed difference in capacity for pregnancy success in these fertility-classified heifers is manifest between Days 14 and 28 when pregnancy recognition signaling and conceptus elongation and implantation must occur for the establishment of pregnancy.

Effects of preovulatory estradiol on embryo survival and pregnancy establishment in beef cows.

The role of preovulatory estradiol on post-fertilization embryo survival and pregnancy establishment has not been well characterized in beef cows. We hypothesized that preovulatory estradiol is important for embryo survival and pregnancy establishment in beef cows. Twenty-four ovariectomized multiparous cows were used in a replicated 3×3 Latin Square design. Cows received estradiol cypionate (ECP) 36h, estradiol benzoate (EB) 12h, or no estradiol (CON) before a gonadotropin-releasing hormone (d 0) induced LH surge. Luteal phase progesterone was mimicked with twice daily progesterone injections from d 3 to 6. On d 7 cows received one embryo and progesterone was supplemented with progesterone-releasing devices (CIDR). Expression of interferon stimulated genes, ISG15, MX2, and OAS1, in leukocytes was determined on d 17, 19, 21, and 28 to determine capability of embryonic signaling. Pregnancy specific protein B concentrations were measured in serum samples from d 17 through 29 to determine embryonic attachment. Transrectal ultrasonography was performed on d 29 and 32 to determine pregnancy viability (heartbeat). Serum estradiol profiles during simulated proestrus/estrus were different (P<0.001) between treatments. Mean serum progesterone concentrations from d 17 to 24 were decreased (P=0.05) in EB and ECP cows compared to CON. Transrectal ultrasonography indicated that fewer CON (4%) cows had a viable embryo present compared to estradiol treated cows (25%). Embryonic loss in cows that did not receive estradiol during the simulated preovulatory period occurred following maternal recognition of pregnancy, indicating that its impact was likely on uterine receptivity and embryonic attachment.

Increased conception rates in beef cattle inseminated with nanopurified bull semen.

Aberrant sperm phenotypes coincide with the expression of unique sperm surface determinants that can be probed by objective, biomarker-based semen analysis and targeted as ligands for semen purification. This study evaluated a nanoparticle-based magnetic purification method that removes defective spermatozoa (∼30% of sample) from bull semen and improves sperm sample viability and fertilizing ability in vitro and in vivo. Two types of nanoparticles were developed: a particle coated with antibody against ubiquitin, which is present on the surface of defective spermatozoa, and a particle coated with the lectin peanut agglutinin, which binds to glycans exposed by acrosomal damage. In a 2 yr artificial insemination field trial with 798 cows, a conception rate of 64.5% ± 3.7% was achieved with a 10 × 10(6) sperm dose of peanut agglutinin-nanopurified spermatozoa, comparable to a control nonpurified full dose of 20 × 10(6) spermatozoa per dose (63.3% ± 3.2%) and significantly higher than a 10 × 10(6) sperm dose of nonpurified control semen (53.7% ± 3.2%; P < 0.05). A total of 466 healthy calves were delivered, and no negative side effects were observed in the inseminated animals or offspring. Because the method is inexpensive and can be fully integrated in current protocols for semen cryopreservation, it is feasible for use in the artificial insemination industry to improve fertility with reduced sperm dosage inseminations. Spermatology will benefit from nanopurification methodology by gaining new tools for the identification of candidate biomarkers of sperm quality such as binder of sperm protein 5 (BSP5), described in the present study.

Effects of fertility on gene expression and function of the bovine endometrium.

Infertility and subfertility are important and pervasive reproductive problems in both domestic animals and humans. The majority of embryonic loss occurs during the first three weeks of pregnancy in cattle and women due, in part, to inadequate endometrial receptivity for support of embryo implantation. To identify heifers of contrasting fertility, serial rounds of artificial insemination (AI) were conducted in 201 synchronized crossbred beef heifers. The heifers were then fertility classified based on number of pregnancies detected on day 35 in four AI opportunities. Heifers, classified as having high fertility, subfertility or infertility, were selected for further study. The fertility-classified heifers were superovulated and flushed, and the recovered embryos were graded and then transferred to synchronized recipients. Quantity of embryos recovered per flush, embryo quality, and subsequent recipient pregnancy rates did not differ by fertility classification. Two in vivo-produced bovine embryos (stage 4 or 5, grade 1 or 2) were then transferred into each heifer on day 7 post-estrus. Pregnancy rates were greater in high fertility than lower fertility heifers when heifers were used as embryo recipients. The reproductive tracts of the classified heifers were obtained on day 14 of the estrous cycle. No obvious morphological differences in reproductive tract structures and histology of the uterus were observed in the heifers. Microarray analysis revealed differences in the endometrial transcriptome based on fertility classification. A genome-wide association study, based on SNP genotyping, detected 7 moderate associations with fertility across 6 different chromosomes. Collectively, these studies support the idea that innate differences in uterine function underlie fertility and early pregnancy loss in ruminants. Cattle with defined early pregnancy success or loss is useful to elucidate the complex biological and genetic mechanisms governing endometrial receptivity and uterine competency for pregnancy.

Follicular determinants of pregnancy establishment and maintenance.

Synchronization of dominant follicle development and control of ovulation/oocyte retrieval are commonly used assisted reproductive technologies in both cattle and humans. The final maturation of the dominant follicle is intimately tied to the final maturation of the oocyte, preovulatory secretion of estradiol, preparation of follicular cells for luteinization, postovulatory secretion of progesterone and endocrine control of the oviductal and uterine environment for gamete and embryo development. The physiological maturity of a dominant/ovulatory follicle can affect the establishment and maintenance of pregnancy. Premature induction of the ovulatory process can reduce pregnancy rates and increase late embryonic/fetal mortality in cattle, which is likely mediated through inadequate oocyte competence and a compromised maternal environment. Oocyte competence increases with follicular maturity and is dependent upon acquisition of a complete complement of mRNA transcripts and establishment of the appropriate epigenetic marking of the oocyte genome before the preovulatory gonadotropin surge. Preovulatory secretion of estradiol is a reflection of follicular maturity and affects the oocyte, follicular cells, oviduct and uterus. The corpus luteum is a continuation of follicular maturation and rate of progesterone secretion following ovulation is linked to fertility. Advancements in our understanding of how the follicular microenvironment affects pregnancy establishment and maintenance will improve the efficiency of assisted reproductive technologies in all species. The purpose of this review is to discuss how follicular microenvironment, oocyte competence, preovulatory secretion of estradiol and postovulatory secretion of progesterone can affect pregnancy establishment and embryo/fetal survival, with an emphasis on cattle.

Effect of reduced heifer nutrition during in utero and post-weaning development on glucose and acetate kinetics.

Energetic efficiency was evaluated in composite bred heifers born from dams receiving 1·8 or 1·2 kg/d winter supplementation for approximately 80 d before parturition. Heifers were then developed post-weaning and randomly assigned to heifer development treatments of either control (100 %; ad libitum; n 8/year) or restricted (80 %; fed 80 % of supplementation fed to controls adjusted to a common body weight: n 8/year) in a 2-year study. A glucose tolerance test (GTT) and acetate irreversible loss test (AILT) were administered to heifers at the termination of a 140 d development period when the heifers were approximately 403 d of age and consumed a silage-based diet, and again at 940 d of age when pregnant with their second calf and grazing dormant forage. No differences were measured (P>0·08) for dam winter nutrition or heifer development treatment for baseline serum metabolites or measures in either the GTT or the AILT. However, changes in baseline serum concentrations (P>0·05) were different between metabolic challenges, which occurred at different stages of development. No difference in acetate disappearance (P = 0·18) and half-life (P = 0·66) was measured between the two metabolic challenges. A trend for glucose half-life to be shorter in heifers born from dams receiving in utero winter treatments that supplied 1·2 kg/d of winter supplementation was observed (P = 0·083). Heifers developed with lower total DM intake during a 140 d development period had similar glucose and acetate incorporation rates as ad libitum-fed heifers when evaluated at two different production stages.

Relationship between follicle size at insemination and pregnancy success.

Administration of gonadotropin-releasing hormone (GnRH) induces a surge of luteinizing hormone and ovulation in a variety of species, including human beings. Our objectives were to determine the effect of follicle size at the time of ovulation on corpus luteum function and establishment and maintenance of pregnancy in cows in which ovulation was either spontaneous or induced with GnRH. GnRH-induced ovulation of follicles < or approximately = 11 mm in diameter resulted in decreased pregnancy rates and increased late embryonic mortality. This decrease in fertility was associated with lower circulating concentrations of estradiol on the day of insemination, a decreased rate of increase in progesterone after insemination, and, ultimately, decreased circulating concentrations of progesterone. In contrast, ovulatory follicle size had no apparent effect on fertility when ovulation occurred spontaneously. Follicles undergoing spontaneous ovulation do so at a wide range of sizes when they are physiologically mature. Therefore, administration of GnRH to induce ovulation likely initiates a preovulatory gonadotropin surge before some dominant follicles attain physiological maturity. GnRH-induced ovulation of follicles that are physiologically immature has a negative impact on pregnancy rates and late embryonic/fetal survival. These observations in cattle may have implications for assisted reproductive procedures in human beings.