Effects of Heat Stress on Follicular Physiology in Dairy Cows

Heat stress: a major threat to ruminant reproduction and mitigating strategies

Stress is an external event or condition that puts pressure on a biological system. Heat stress is defined as the combination of internal and external factors acting on an animal to cause an increase in body temperature and elicit a physiological response. Heat stress is a set of conditions caused by overexposure to or overexertion at excess ambient temperature and leads to the inability of animals to dissipate enough heat to sustain homeostasis. Heat exhaustion, heat stroke, and cramps are among the symptoms. For the majority of mammalian species, including ruminants, heat stress has a negative impact on physiological, reproductive, and nutritional requirements. Reproductive functions, including the male and female reproductive systems, are negatively affected by heat stress. It decreases libido and spermatogenic activity in males and negatively affects follicle development, oogenesis, oocyte maturation, fertilization, implantation, and embryo-fetal development in females. These effects lead to a decrease in the rate of reproduction and financial losses for the livestock industry. Understanding the impact of heat stress on reproductive tissues will aid in the development of strategies for preventing heat stress and improving reproductive functions. Modification of the microenvironment, nutritional control, genetic development of heat-tolerant breeds, hormonal treatment, estrous synchronization, timed artificial insemination, and embryo transfer are among the strategies used to reduce the detrimental effects of heat stress on reproduction. These strategies may also increase the likelihood of establishing pregnancy in farm animals.

Roe Deer Reproduction in Western Poland: The Late Autumn Rut Phenomenon

Roe deer (Capreolus capreolus L.) populations in Poland are characterized by low productivity, which is why their reproductive potential was investigated. The presence of corpora lutea (CL) on the ovaries of females hunted in autumn and winter in the years 2015 and 2016 was assessed. Most animals were post-rut and most often had multiple ovulations. However, in early autumn 2015, 60% of the females had no CL. Therefore, they did not participate in mating at the turn of July/August. Those that did ovulate were found to have one CL. In late autumn, 97% of the females were post-rut, mostly with twin ovulations. This confirmed the occurrence of a late autumn rut. This phenomenon had been suggested in studies but not confirmed. In contrast, almost 100% of roe deer ovulated in summer in 2016. It was hypothesized that the reason for the low mating activity of roe deer in the summer of 2015 was heat stress and limited food resources. The summer was exceptionally hot, with many days in July and August when temperatures exceeded 30 °C. The heat combined with low rainfall led to extreme drought. Meanwhile, July and August are the months of mating activity for roe deer. The late autumn rut allowed the roe deer-a monoestrous species-to limit the consequences of a decrease in mating activity or fertility during the hot summer. Global warming may affect roe deer reproduction, so climatic conditions should be considered in population studies, not only in terms of food availability.

Advances in Dairy Cattle Reproduction-A Foreword

Dairy cattle have played an important role in economic development since the beginning of agriculture [...].

Livestock-Forest integrated system attenuates deleterious heat stress effects in bovine oocytes

Global warming poses significant challenges to the fertility of tropical dairy cattle. One promising approach to mitigate heat stress effects on reproductive function and reduce the carbon footprint is the use of integrated livestock-forest (ILF) systems. The aim of this study was to investigate the effects of two different systems, namely Full Sun (FS) and ILF, on maternal hyperthermia and oocyte quality of Holstein and Girolando heifers during the tropical summer season. The temperature-humidity index (THI) data revealed intense heat stress during the experiment. Both the system (P<0.01) and the breed (P<0.01) factors had a significant impact on vaginal temperature, being hyperthermia more pronounced in the FS system and in the Holstein breed. Over the five time points collected at a 33-day interval, we observed distinct patterns for ILF (P=0.65) and FS (P<0.001) systems, suggesting an adaptive response in animals kept in FS systems. Furthermore, oocyte quality assessment revealed an effect of the system for oocyte diameter (P<0.001) and levels of IGFBP2 (P<0.001), and caspase 3 levels showed a decrease in ILF compared to FS for both Holstein (P<0.001) and Girolando (P<0.001) breeds. Collectively, these parameters indicate that oocyte quality during the summer months was superior in animals maintained in the ILF system. In conclusion, the ILF system demonstrated promising results in attenuating maternal hyperthermia and mitigating its effects on oocyte quality. Additionally, our observations suggest that animals in the FS system may exhibit an adaptive response to heat stress.

Cervix-Deep Rectal Temperature Differential on the Day of Ovulation is Correlated With Embryo Recovery Results in Mares

Variations in temperature throughout the reproductive tract have been noted in many species. A recent study found the cervix-rectum temperature differential (CR-TD) in cattle was related to fertility. The present study aimed to assess the CR-TD in mares around the time of ovulation and relate it to embryo recover. Over 52 cycles, mares were inseminated with a fertile stallion and embryo recovery was undertaken on Day 7 post ovulation. Further 10 control mares were not inseminated. Rectal and cervical temperatures were measured using a precision thermometer on four or five occasions: the day of deslorelin administration and insemination, the day before ovulation, the day of ovulation (Day 0), the day after ovulation and, for inseminated mares, before embryo recovery on Day 7. One-way ANOVA showed that the CR-TD was significantly lower on the day of ovulation in the 36 positive cycles, in which an embryo was recovered, versus the 16 in which the embryo flush was negative (0.21 ± 0.17 vs. 0.40 ± 0.09°C; p < 0.001). Control cycles showed equivalent CR-TD to positive (0.13 ± 0.22 vs. 0.21 ± 0.17°C; p = 0.196) but not negative cycles (0.13 ± 0.22 vs. 0.40 ± 0.09°C; p < 0.001). A positive embryo recovery was associated with lower CR-TDs from the time of insemination and deslorelin to the day after ovulation compared to the day of embryo flushing (RM ANOVA; p < 0.001; Pairwise comparisons; p ≤ 0.01, in all cases). Rectal or cervical temperatures per se showed no significant differences between positive, negative or control cycles at any time point. In conclusion, a thermoregulatory process occurs close to ovulation which results in a lower CR-TD in cycles that produced an embryo versus those which did not. Further characterisation of TDs within the reproductive tract of the mare would increase our understanding of the conditions required for optimum fertility.

Impact of Heat Stress on Oocyte Developmental Competence and Pre-Implantation Embryo Viability in Cattle

Rectal and vaginal temperatures are utilised in both in vivo and in vitro models to study the effects of heat stress on oocyte competence and embryo viability in cattle. However, uterine temperature increases by only 0.5 °C in heat-stressed cows, significantly lower than simulated increases in in vitro models. Temperature variations within oviducts and ovarian follicles during heat stress are poorly understood or unavailable, and evidence is lacking that oocytes and pre-implantation embryos experience mild (40 °C) or severe (41 °C) heat stress inside the ovarian follicle and the oviduct and uterus, respectively. Gathering detailed temperature data from the reproductive tract and follicles is crucial to accurately assess oocyte competence and embryo viability under realistic heat stress conditions. Potential harm from heat stress on oocytes and embryos may result from reduced nutrient availability (e.g., diminished blood flow to the reproductive tract) or other unidentified mechanisms affecting tissue function rather than direct thermal effects. Refining in vivo stress models in cattle is essential to accurately identify animals truly experiencing heat stress, rather than assuming heat stress exposure as done in most studies. This will improve model reliability and aid in the selection of heat-tolerant animals.

Interval from Oestrus to Ovulation in Dairy Cows-A Key Factor for Insemination Time: A Review

This review describes the oestrus-to-ovulation interval, the possibility of predicting the time of ovulation, and the optimum time for insemination relative to oestrus in dairy cows. The duration of oestrus in dairy cows is approximately 8-20 h, with differences possibly related to the methods of oestrus detection and the frequency of observations. Most cows ovulate approximately 24-33 h after the onset of oestrus and 15-22 h after the end of oestrus. The interval from the preovulatory luteinising hormone (LH) surge to ovulation is approximately 4-30 h. Ovulation occurs when follicle diameter averages 18-20 mm. When it is possible to correctly determine the beginning of oestrus, artificial insemination can be performed utilizing the "a.m.-p.m. rule", and only one insemination may be applied. In cows with too long or too short oestrus-to-ovulation intervals, fertility can be compromised. One important factor that can alter the oestrus-to-ovulation interval is acute or chronic heat stress during the warm season. When there is a risk that insemination may occur too early or too late with respect to the time of ovulation, GnRH administration can be considered.

Advancements in Genetic Biomarkers and Exogenous Antioxidant Supplementation for Safeguarding Mammalian Cells against Heat-Induced Oxidative Stress and Apoptosis

Heat stress represents a pervasive global concern with far-reaching implications for the reproductive efficiency of both animal and human populations. An extensive body of published research on heat stress effects utilizes controlled experimental environments to expose cells and tissues to heat stress and its disruptive influence on the physiological aspects of reproductive phenotypic traits, encompassing parameters such as sperm quality, sperm motility, viability, and overall competence. Beyond these immediate effects, heat stress has been linked to embryo losses, compromised oocyte development, and even infertility across diverse species. One of the primary mechanisms underlying these adverse reproductive outcomes is the elevation of reactive oxygen species (ROS) levels precipitating oxidative stress and apoptosis within mammalian reproductive cells. Oxidative stress and apoptosis are recognized as pivotal biological factors through which heat stress exerts its disruptive impact on both male and female reproductive cells. In a concerted effort to mitigate the detrimental consequences of heat stress, supplementation with antioxidants, both in natural and synthetic forms, has been explored as a potential intervention strategy. Furthermore, reproductive cells possess inherent self-protective mechanisms that come into play during episodes of heat stress, aiding in their survival. This comprehensive review delves into the multifaceted effects of heat stress on reproductive phenotypic traits and elucidates the intricate molecular mechanisms underpinning oxidative stress and apoptosis in reproductive cells, which compromise their normal function. Additionally, we provide a succinct overview of potential antioxidant interventions and highlight the genetic biomarkers within reproductive cells that possess self-protective capabilities, collectively offering promising avenues for ameliorating the negative impact of heat stress by restraining apoptosis and oxidative stress.

Negative photoperiod induces an increase in the number of ovulations in dairy cattle

This study sought to examine the impact of negative photoperiod on the incidence of multiple ovulations and pregnancies in dairy cattle. The study population consisted of 5,373 pregnant cows in their third or greater lactation that experienced their first post-partum pregnancy after spontaneous estrus. The positive photoperiod (increasing day-length) extends from December 22 to June 21, whereas the negative photoperiod (decreasing day-length) extends from June 22 to December 21. The odds ratios (ORs) for multiple ovulations and pregnancies in cows that became pregnant during the negative photoperiod and the remaining cows that became pregnant during the positive photoperiod were 1.4 and 1.3 (P < 0.0001), respectively. The ORs for cows that became pregnant ≥ 90 days in milk and the remaining cows that became pregnant < 90 days in milk were 4.3 and 4.1 (P < 0.0001), respectively. No significant differences were detected in the monthly rates of multiple ovulations or pregnancies during positive and negative photoperiods. Thus, the present study demonstrates that the ovarian function in cows is related to changes in day-length, with decreasing day-length being associated with greater multiple ovulation and pregnancy rates. The present study also shows that positive and negative photoperiods exhibit different trends. The results of this study are consistent with a growing body of work demonstrating the effects of photoperiod patterns on the reproductive physiology of cows, with clear implications for twin pregnancy prevention.

The Anti-Müllerian Hormone as Endocrine and Molecular Marker Associated with Reproductive Performance in Holstein Dairy Cows Exposed to Heat Stress

Anti-Müllerian hormone (AMH) is proposed as a biomarker for fertility in cattle, yet this associative relationship appears to be influenced by heat stress (HS). The objective was to test serum AMH and AMH-related single nucleotide polymorphisms (SNPs) as markers potentially predictive of reproductive traits in dairy cows experiencing HS. The study included 300 Holstein cows that were genotyped using BovineSNP50 (54,000 SNP). A genome-wide association study was then executed. Nine intragenic SNPs within the pathways that influence the AMH gene were found important with multiple comparisons adjustment tests (p < 1.09 × 10-6). A further validation study was performed in an independent Holstein cattle population, which was divided into moderate (MH; n = 152) and severe heat-stressed (SH; n = 128) groups and then subjected to a summer reproductive management program. Serum AMH was confirmed as a predictor of fertility measures (p < 0.05) in MH but not in the SH group. Cows were genotyped, which revealed four SNPs as predictive markers for serum AMH (p < 0.01), reproductive traits (p < 0.01), and additional physiological variables (p < 0.05). These SNPs were in the genes AMH, IGFBP1, LGR5, and TLR4. In conclusion, serum AMH concentrations and AMH polymorphisms are proposed as predictive markers that can be used in conjunction with genomic breeding value approaches to improve reproductive performance in Holstein cows exposed to summer HS conditions.

Heat Stress as a Barrier to Successful Reproduction and Potential Alleviation Strategies in Cattle

In recent decades, the adverse effects of global warming on all living beings have been unanimously recognized across the world. A high environmental temperature that increases the respiration and rectal temperature of cattle is called heat stress (HS), and it can affect both male and female reproductive functions. For successful reproduction and fertilization, mature and healthy oocytes are crucial; however, HS reduces the developmental competence of oocytes, which compromises reproduction. HS disturbs the hormonal balance that plays a crucial role in successful reproduction, particularly in reducing the luteinizing hormone and progesterone levels, which leads to severe problems such as poor follicle development with a poor-quality oocyte and problems related to maturity, silent estrus, abnormal or weak embryo development, and pregnancy loss, resulting in a declining reproduction rate and losses for the cattle industry. Lactating cattle are particularly susceptible to HS and, hence, their reproduction rate is substantially reduced. Additionally, bulls are also affected by HS; during summer, semen quality and sperm motility decline, leading to compromised reproduction. In summer, the conception rate is reduced by 20-30% worldwide. Although various techniques, such as the provision of water sprinklers, shade, and air conditioning, are used during summer, these methods are insufficient to recover the normal reproduction rate and, therefore, special attention is needed to improve reproductive efficiency and minimize the detrimental effect of HS on cattle during summer. The application of advanced reproductive technologies such as the production of embryos in vitro, cryopreservation during the hot season, embryo transfer, and timed artificial insemination may minimize the detrimental effects of HS on livestock reproduction and recover the losses in the cattle industry.

Heat Stress: A Serious Disruptor of the Reproductive Physiology of Dairy Cows

Global warming is a significant threat to the sustainability and profitability of the dairy sector, not only in tropical or subtropical regions but also in temperate zones where extreme summer temperatures have become a new and challenging reality. Prolonged exposure of dairy cows to high temperatures compromises animal welfare, increases morbidity, and suppresses fertility, resulting in devastating economic losses for farmers. To counteract the deleterious effects of heat stress, cattl e employ various adaptive thermoregulatory mechanisms including molecular, endocrine, physiological, and behavioral responses. These adaptations involve the immediate secretion of heat shock proteins and cortisol, followed by a complex network of disrupted secretion of metabolic and reproductive hormones such as prolactin, ghrelin, ovarian steroid, and pituitary gonadotrophins. While the strategic heat stress mitigation measures can restore milk production through modifications of the microclimate and nutritional interventions, the summer fertility records remain at low levels compared to those of the thermoneutral periods of the year. This is because sustainment of high fertility is a multifaceted process that requires appropriate energy balance, undisrupted mode of various hormones secretion to sustain the maturation and fertilizing competence of the oocyte, the normal development of the early embryo and unhampered maternal-embryo crosstalk. In this review, we summarize the major molecular and endocrine responses to elevated temperatures in dairy cows, as well as the impacts on maturing oocytes and early embryos, and discuss the consequences that heat stress brings about in dairy cattle fertility.

Effects of Heat Stress in Dairy Cows Raised in the Confined System: A Scientometric Review

Due to climate change, heat stress is a growing problem for the dairy industry. Based on this, annual economic losses in the dairy sector are verified mainly on a large scale. Despite several publications on thermal stress in lactating dairy cows in confinement systems, there need to be published reviews addressing this issue systematically. Our objective was to scientometrically analyze the effects of heat stress in dairy cows managed in a confinement system. Based on PRISMA guidelines, research articles were identified, screened, and summarized based on inclusion criteria for heat stress in a confinement system. Data was obtained from the Web of Science. A total of 604 scientific articles published between 2000 and April 2022 were considered. Data was then analyzed using Microsoft Excel and CiteSpace. The results pointed to a significant increase in studies on heat stress in lactating cows housed in confinement systems. The main research areas were Agriculture, Dairy Animal Science and Veterinary Sciences. The USA showed the highest concentration of studies (31.12%), followed by China (14.90%). Emerging themes included heat stress and behavior. The most influential journals were the Journal of Dairy Science and the Journal of Animal Science. The top authors were L. H. Baumgard and R. J. Collier. The leading institutions were the Chinese Academy of Agricultural Sciences, followed by the State University System of Florida and the University of Florida. The study maps the significant research domains on heat stress of lactating cows in confinement systems, discusses implications and explanations and highlights emerging trends.

Effects of Heat Stress on Bovine Oocytes and Early Embryonic Development-An Update

Heat stress is a major threat to cattle reproduction today. It has been shown that the effect of high temperature not only has a negative effect on the hormonal balance, but also directly affects the quality of oocytes, disrupting the function of mitochondria, fragmenting their DNA and changing their maternal transcription. Studies suggest that the induction of HSP70 may reduce the apoptosis of granular layer cells caused by heat stress. It has been shown that the changes at the transcriptome level caused by heat stress are consistent with 46.4% of blastocyst development disorders. Cows from calves exposed to thermal stress in utero have a lower milk yield in their lifetime, exhibit immunological disorders, have a lower birth weight and display a shorter lifespan related to the expedited aging. In order to protect cow reproduction, the effects of heat stress at the intracellular and molecular levels should be tracked step by step, and the impacts of the dysregulation of thermal homeostasis (i.e., hyperthermy) should be taken into account.

Effects of different short-chain fatty acids (SCFA) on gene expression of proteins involved in barrier function in IPEC-J2

Background

Gut microbial anaerobic fermentation produces short-chain fatty acids (SCFA), which are important substrates for energy metabolism and anabolic processes in mammals. SCFA can regulate the inflammatory response and increase the intestinal barrier integrity by enhancing the tight junction protein (TJp) functions, which prevent the passage of antigens through the paracellular space. The aim of this study was to evaluate the effect of in vitro supplementation with SCFA (acetate, propionate, butyrate, and lactate) at different concentrations on viability, nitric oxide (NO) release (oxidative stress parameter) in cell culture supernatants, and gene expression of TJp (occludin, zonula occludens-1, and claudin-4) and pro-inflammatory pathway-related mediators (β-defensin 1, TNF-α, and NF-κB) in intestinal porcine epithelial cell line J2 (IPEC-J2).

Results

The SCFA tested showed significant effects on IPEC-J2, which proved to be dependent on the type and specific concentration of the fatty acid. Acetate stimulated cell viability and NO production in a dose-dependent manner (P < 0.05), and specifically, 5 mM acetate activated the barrier response through claudin-4, and immunity through β-defensin 1 (P < 0.05). The same effect on these parameters was shown by propionate supplementation, especially at 1 mM (P < 0.05). Contrarily, lactate and butyrate showed different effects compared to acetate and propionate, as they did not stimulate an increase of cell viability and regulated barrier integrity through zonula occludens-1 and occludin, especially at 30 mM and 0.5 mM, respectively (P < 0.05). Upon supplementation with SCFA, the increase of NO release at low levels proved not to have detrimental effects on IPEC-J2 proliferation/survival, and in the case of acetate and propionate, such levels were associated with beneficial effects. Furthermore, the results showed that SCFA supplementation induced β-defensin 1 (P < 0.05) that, in turn, may have been involved in the inhibition of TNF-α and NF-κB gene expression (P < 0.05).

Conclusions

The present study demonstrates that the supplementation with specific SCFA in IPEC-J2 can significantly modulate the process of barrier protection, and that particularly acetate and propionate sustain cell viability, low oxidative stress activity and intestinal barrier function.

Heat stress during critical windows of the estrous cycle and risk of pregnancy establishment in embryo-recipient dairy heifers

This study aimed to determine effects of exposure of recipient dairy heifers to heat stress (THI≥ 73) during the estrous cycle coinciding with embryo transfer (ET) on the risk of pregnancy establishment after transfer of in vivo produced embryos. Recipients exposed to THI values ≥ 73 during days zero (recipient estrus), 7 (day of ET), 14 (seven days after ET), 15 and 16 (maternal recognition of pregnancy) of the ET cycle were considered as heat stressed heifers (n=254), while heifers in the control group (n=470) were not exposed to THI≥ 73 at any of the previous days. Results revealed no significant effects of any of the investigated factors on the risk of pregnancy following ET. However, the mean THI above 77 was associated with a drastic numerical decrease in PR/ET (36.63%), when compared to a mean THI 72 (78.78%). In addition, PR/ET after transfer of second and third grade embryos were numerically lower in heat-stressed recipients, compared to first grade embryos (41.17 vs 56.36%, respectively). Our findings confirmed that transfer of blastocysts were associated with numerically higher PR/ET in heat stressed and control recipients, as compared to morula stage. Interestingly, PR/ET tended to be higher when sexed embryos were transferred to the control recipients compared to heat-stressed ones. In conclusion, PR/ET in dairy heifers was not significantly affected by heat stress during critical windows of their estrous cycle coinciding with ET, whereas transfer of sexed embryos gives lower results under conditions of heat stress.