Letrozole: A Steroid-Free Estrous Synchronization Method

Most bovine estrous synchronization protocols utilize progesterone plus estrogen to control ovulation timing. A drug that inhibits estrogen production (Letrozole) may be an alternative, steroid-free synchronization method (not yet commercially available). However, low estrogen can negatively affect the health of follicles/oocytes and impact fertility. To determine its effects, Letrozole was administered intramuscularly while tracking follicle growth and circulating hormones. Letrozole response was variable. Two of three cows experienced delayed luteolysis/ovulation and extended progesterone production. This preliminary data indicates that Letrozole treatment allows normal follicle progression but drug response may vary and little is known about effects on oocyte quality

Attainment and maintenance of pubertal cyclicity may predict reproductive longevity in beef heifers

We hypothesized the manner that heifers achieve puberty may indicate their future reproductive longevity. Heifers with discontinued or delayed cyclicity during puberty attainment may have irregular reproductive cycles, anovulation, and infertility in their first breeding season contributing to a shorter reproductive lifespan. Therefore, plasma progesterone (P4) was measured from weaning to breeding on 611 heifers born 2012–2017 and four pubertal classifications were identified: (1) Early; P4 ≥ 1 ng/ml \u3c March 12 with continued cyclicity, (2) Typical; P4 ≥ 1 ng/ml ≥ March 12 with continued cyclicity, (3) Start-Stop; P4 ≥ 1 ng/ml but discontinued cyclicity, and (4) Non-Cycling; no P4 ≥ 1 ng/ml. Historical herd records indicated that 25% of heifers achieved puberty prior to March 12th in the 10 years prior to the study. Start-Stop and Non-Cycling yearling heifers were lighter indicating reduced growth and reproductive maturity traits compared with Early/Typical heifers. In addition, Non-Cycling/Start-Stop heifers were less responsive to prostaglandin F2 alpha (PGF2α) to initiate estrous behavior and ovulation to be artificially inseminated. Non-Cycling heifers had fewer reproductive tract score-5 and reduced numbers of calves born in the first 21-days-ofcalving during their first breeding season. Within the Start-Stop classification, 50% of heifers reinitiated cyclicity with growth traits and reproductive parameters that were similar to heifers in the Early/Typical classification while those that remained non-cyclic were more similar to heifers in the Non-Cycling group. Thus, heifers with discontinued cyclicity or no cyclicity during puberty attainment had delayed reproductive maturity resulting in subfertility and potentially a shorter reproductive lifespan

ENDOCRINE HORMONE PROFILES AND OVARIAN MORPHOLOGY AS MARKERS OF PUBERTAL STATUS AND FERTILITY IN HEIFERS

Two studies were conducted to determine: 1) endocrine profiles during puberty attainment to predict populations of females that may develop irregular estrous cycles, altered endocrine hormone secretion affecting reproductive performance; 2) whether theca cells from heifers with High or Low antral follicles counts (AFC) had altered VEGFA signal transduction which may predict changes in angiogenesis and follicle development affecting antral follicle size and fertility. We previously identified a population of cows with increased androgens in follicular fluid (High A4), which has reduced SHBG in plasma and displays irregular cycles. High A4 females attain puberty 45 days earlier than control cows. Further investigation into this phenotype led to a three year study to characterize pubertal onset in heifers of the UNL Physiology herd. We hypothesized that heifers with earlier puberty may be predisposed to aberrant endocrine profiles and abnormal cyclicity due to inappropriate establishment of the hypothalamic-pituitary-gonadal axis, resulting in reduced fertility. Study 1 culminated in the discovery of four classifications of heifers depending on pubertal onset (Early, Typical, Start-Stop and Non-Cycling). Progesterone was reduced in Start-Stop and Non-Cycling females when compared with Early and Typical females. Furthermore, SHBG was reduced in the Start-Stop females in comparison to the Early females. Start-Stop heifers identified in this study may become the High A4 females of the herd as mature cows. In study 2, theca cells from High AFC and Low AFC heifers were analyzed for differential expression in VEGFA signal transduction. High antral follicle counts are indicative of a larger ovarian reserve and increased fertility. Preliminary analyses revealed decreased expression of genes in the VEGFA signaling pathway, and increased instances of apoptosis and cell death in the theca cells of the High AFC heifers. This study is ongoing, in an effort to investigate the molecular mechanisms and gene networks that play central roles in establishment of the ovarian reserve and formation of antral follicles, which both are reliable predictors of a young female’s reproductive potential. Advisors: Andrea S. Cupp and Robert A. Cushma

ENDOCRINE HORMONE PROFILES AND OVARIAN MORPHOLOGY AS MARKERS OF PUBERTAL STATUS AND FERTILITY IN HEIFERS

Two studies were conducted to determine: 1) endocrine profiles during puberty attainment to predict populations of females that may develop irregular estrous cycles, altered endocrine hormone secretion affecting reproductive performance; 2) whether theca cells from heifers with High or Low antral follicles counts (AFC) had altered VEGFA signal transduction which may predict changes in angiogenesis and follicle development affecting antral follicle size and fertility. We previously identified a population of cows with increased androgens in follicular fluid (High A4), which has reduced SHBG in plasma and displays irregular cycles. High A4 females attain puberty 45 days earlier than control cows. Further investigation into this phenotype led to a three year study to characterize pubertal onset in heifers of the UNL Physiology herd. We hypothesized that heifers with earlier puberty may be predisposed to aberrant endocrine profiles and abnormal cyclicity due to inappropriate establishment of the hypothalamic-pituitary-gonadal axis, resulting in reduced fertility. Study 1 culminated in the discovery of four classifications of heifers depending on pubertal onset (Early, Typical, Start-Stop and Non-Cycling). Progesterone was reduced in Start-Stop and Non-Cycling females when compared with Early and Typical females. Furthermore, SHBG was reduced in the Start-Stop females in comparison to the Early females. Start-Stop heifers identified in this study may become the High A4 females of the herd as mature cows. In study 2, theca cells from High AFC and Low AFC heifers were analyzed for differential expression in VEGFA signal transduction. High antral follicle counts are indicative of a larger ovarian reserve and increased fertility. Preliminary analyses revealed decreased expression of genes in the VEGFA signaling pathway, and increased instances of apoptosis and cell death in the theca cells of the High AFC heifers. This study is ongoing, in an effort to investigate the molecular mechanisms and gene networks that play central roles in establishment of the ovarian reserve and formation of antral follicles, which both are reliable predictors of a young female’s reproductive potential. Advisors: Andrea S. Cupp and Robert A. Cushma

Letrozole: A Steroid-Free Estrous Synchronization Method

Most bovine estrous synchronization protocols utilize progesterone plus estrogen to control ovulation timing. A drug that inhibits estrogen production (Letrozole) may be an alternative, steroid-free synchronization method (not yet commercially available). However, low estrogen can negatively affect the health of follicles/oocytes and impact fertility. To determine its effects, Letrozole was administered intramuscularly while tracking follicle growth and circulating hormones. Letrozole response was variable. Two of three cows experienced delayed luteolysis/ovulation and extended progesterone production. This preliminary data indicates that Letrozole treatment allows normal follicle progression but drug response may vary and little is known about effects on oocyte quality

Cows with Excess Androgen are Anovulatory and Have Differing Patterns of Progesterone Secretion

Within the physiology herd, a group of cows that have excess androgen (androstenedione, A4) in the dominant follicle and a 17% reduction in calving rate have being identified. Thus, our objective was to determine follicular dynamics (follicle growth) and progesterone (P4) concentrations in High A4 cows to determine if they were anovulatory. High A4 cows had more persistent dominant follicles and either did not display estrus and ovulated at an inappropriate time or did not ovulate compared with Low A4 cows (Controls). Furthermore, P4 concentrations had reduced peak values and were maintained longer in High vs Low A4 cows which may contribute to their failure to ovulate

Maternal age influences the number of primordial follicles in the ovaries of yearling Angus heifers

The number of antral follicles detectable by ultrasonography in heifers is influenced by age of the dam, because daughters of primiparous cows have fewer antral follicles than daughters of mature cows. We, therefore, hypothesized that heifers with primiparous dams would have fewer primordial follicles in their ovaries than heifers born to mature (4+ y) cows. Angus heifers (n=464) were submitted for ultrasonographic evaluation of antral follicle number at 325, 355, and 385 d of age. Ovaries were collected from a random subset of heifers (n=79) and processed for histological evaluation to determine number of primordial follicles. A greater percentage of heifers with primiparous dams had a corpus luteum at first ultrasonographic examination; however, a greater percentage of heifers with multiparous dams had ovulated by the start of breeding (P \u3c 0.01). Heifers with primiparous dams had fewer antral follicles detectable by ultrasonography (P \u3c 0.01). Heifers with primparous dams had fewer surface antral follicles on their ovaries (P \u3c 0.01), and the number of primordial follicles per histological section was less for heifers with primiparous dams (P= 0.02). These data indicate that the lesser number of antral follicles detectable by ultrasonography in heifers with primparous dams is due to less ovarian follicle reserves. Selecting replacement heifers from mature dams may result in daughters with greater fertility and reproductive longevity; however, further research is necessary to determine if interactions between size of the ovarian follicle reserve and age at puberty influence fertility and reproductive longevity in replacement heifers

Human and biophysical legacies shape contemporary urban forests: a literature synthesis

Understanding how urban forests developed their current patterns of tree canopy cover, species composition,and diversity requires an appreciation of historical legacy effects. However, analyses of current urban forestcharacteristics are often limited to contemporary socioeconomic factors, overlooking the role of history. Theinstitutions, human communities, and biophysical conditions of cities change over time, creating layers of legacieson the landscape, shifting urban forests through complex interactive processes and feedbacks. Urbangreen spaces and planted trees can persist long after their establishment, meaning that today’s mature canopyreflects conditions and decisions from many years prior. In this synthesis article, we discuss some of the majorhistorical human and biophysical drivers and associated legacy effects expressed in present urban forest patterns,highlighting examples in the United States and Canada. The bioregional context – native biome, climate, topography,initial vegetation, and pre-urbanization land use – represents the initial conditions in which a cityestablished and grew, and this context influences how legacy effects unfold. Human drivers of legacy effects canreflect specific historical periods: colonial histories related to the symbolism of certain species, and the urbanparks and civic beautification movements. Other human drivers include phenomena that cut across time periodssuch as neighborhood urban form and socioeconomic change. Biophysical legacy effects include the consequencesof past disturbances such as extreme weather events and pest and disease outbreaks. Urban treeprofessionals play a major role in many legacy effects by mediating the interactions and feedbacks betweenbiophysical and human drivers. We emphasize the importance of historical perspectives to understand past drivers that have produced current urban forest patterns, and call for interdisciplinary and mixed methodsresearch to unpack the mechanisms of long-term urban forest change at intra- and inter-city scales