The impact of stress and selected environmental factors on cows’ reproduction

ABSTRACT
 The selection of cattle in terms of the increase in milk yield has been resulted in a negative energy balance of cows. A negative energy balance inhibits the growth of dominant follicles and the secretion of estradiol. This results in a decrease in the activity of ovaries. Furthermore, in high-yielding cows there is often a decrease in fertility. In addition, there are many factors influencing the cows’ fertility. Unsuitable nutrition, litter in a barn, as well as the stress experienced by cows can reduce the reproductive rate of cattle. Oxidative stress can cause damage, the genetic material of oocytes, and endometrium damage. Metabolic stress can lead to a longer period of infertility. Furthermore, heat stress impairs the maturation of the dominant follicle and may cause placental retention. Chronic stress, e.g. in lameness or during transport, may cause changes in the secretion of reproductive hormones due to the high concentration of cortisol in the blood. Moreover, mycotoxins can cause miscarriage or inflammation of the uterus. Pesticides cause hormonal dysfunction in the hypothalamic-pituitary-gonadal axis. Dairy cattle should be provided with suitable conditions to obtain satisfactory production and reproductive rate.


The impact of stress on the efficiency of cow reproduction
The suitable housing systems of cattle are necessary to achieve specific production and reproductive parameters of cows, as well as to achieve animal welfare (Krueger et al. 2020). Welfare is a state when animals, e.g. do not feel a mental or physical discomfort. Animals then are not sick, injured, or lame and they do not experience stress or pain (Thomsen and Houe 2018).
Disorders of welfare often result from improper conditions of breeding cattle and often are associated with stress. These disorders have a negative impact on production and reproductive traits in cattle (Polsky and von Keyserlingk 2017). Locomotion diseases in cows, such as lameness, have a negative influence on the fertilization rate and delaying of conception (Blackie and Maclaurin 2019). Animal health problems or reduce milk yield of cows as a result of stress contribute to earlier culling of animals from herds (Gussmann et al. 2019).
The stress experienced by animals may have crucial negative results in economical and health aspects (e.g. reducing fertility) (Lucy 2019). The main effects are the decrease of milk yield and quality of milk or the reduction of reproduction efficiency of cows. There are, among others, oxidative stress, metabolic, heat, and chronic stress (Strzałkowska et al. 2014).
The oxidative stress occurs in the perinatal period and the first weeks after the calving. Then cow's energy demand is much higher than the energy provided in a ration of forage. Then the inflammation of the udder or the uterus in cows is frequent. Because there are the following dysfunctions related to the response of inflammation due to the adaptation to lactation (Mavangira and Sordillo 2018). Due to the increased biochemical processes in the organism, energy demand and oxygen demand increase. The side effect of these changes is the production of reactive oxygen species (ROS), which are usually neutralized by antioxidants. However, when there is an imbalance, the production of ROS increases (Mavangira and Sordillo 2018). The reactive oxygen species express their adverse effects on disorders within the reproductive system through, among others, damage of genetic material of oocytes or endometrium damage. The results are the decrease in fertility and conception rate in cattle (Boudjellaba et al. 2018). Furthermore, ROS destroys proteins and lipids in cells and provides for their apoptosis (Mavangira and Sordillo 2018).
The nutrition of cattle has a direct impact on metabolism. All nutritional deficiencies, mostly in lactation cows affect metabolic changes (Coroian et al. 2017). Metabolic stress provides changes in metabolic pathways related to the immune functions of the cow during the early stage of lactation. This affects the susceptibility to the occurrence of infectious diseases, including inflammation, e.g. mastitis (El-Tarabany et al. 2016). In high-yielding cows, fertility is often reduced or even absent (Bragança and Zangirolamo 2018). The infertility period may be extended during the postpartum period when following a more energy demand than supply (El-Tarabany et al. 2016).
Heat stress is related to the external environment. The temperature optimum for cattle ranges from 4°C to 16°C with an air humidity of 60-80%. Fundamental factors in the reception of temperature conditions are air movement, relative humidity, as well as sunlight. Therefore, the THI index (Temperature Humidity Index) was developed. According to the THI index, the thermal comfort zone for cattle is between 24°C and 26°C, and the relative humidity reaches 60-70%. The physiological and metabolic disorders are the results of stress. This type of stress can have a negative influence on the success of cow fertilization, pregnancy during the peri-implantation period, as well as a negative influence on hormonal disorders and embryo mortality (Bragança and Zangirolamo 2018;Schüller et al. 2014;Liu et al. 2017). In the warm seasons, the efficiency of artificial insemination of dairy cows is lower than in winter. The impact of the season on the reduction of reproductive performance in cattle is associated with the occurrence of hormone secretion disordersmainly luteinizing hormone (LH). Moreover, heat stress impairs the maturation process of dominant follicles and may also affect placental retention or postpartum uterine inflammation (Wakayo et al. 2015).
The milk yield of cows during heat stress also decreases (Liu et al. 2018). Heat stress reduces the conception rate in dairy cattle. The high-yield cows are the most susceptible to heat stress due to the fact the heat tolerance decreases with increasing milk yield (Tao et al. 2020). This type of stress also reduces the immune value of cow colostrum (Mellado et al. 2014). Moreover, heat stress also affects production traits. There is a reduction in the fat and protein content in the milk of cows exposed to heat stress (Liu et al. 2017).
Chronic stress has a significant impact on the functioning of a cow's ovaries by inhibiting the secretion of hormones on the pituitary-thyroid-ovary axis. In cows with lameness occurs for a longer period from the beginning of oestrus to ovulation. The time from calving to the beginning of the oestrus cycle is also longer (Strzałkowska et al. 2014). Cows with lameness as a result of experienced stress and/or pain have a higher level of cortisol in the blood, which inhibits the secretion of reproductive hormones. This has a negative influence on the production of oocytes and on the expression of behaviour during oestrus (Ratanapob et al. 2020).
Transport is an important factor that influences the experience of stress in cows. The perception of stress during animal transport predisposes to the development of respiratory disease in calvespneumonia caused by BHV-1 virus, as well as exposes them to diarrhoea (Damtew et al. 2018;Van Engen and Coetzee 2018). Dehydration and reluctance to feed intake may also occur during transport (Van Engen and Coetzee 2018). Furthermore, stress during transport can reduce fertility by affecting the secretion of hormones regulating the function of the gonads. The effect of stress on cows is more frequent loss of embryos, delayed maturation of ovarian follicles, as well as ovulation disorders (Damtew et al. 2018).
Miscarriages in cows are crucial aspects influencing the economics of cattle breeding in animal production. Miscarriages can be caused by the twin pregnancy of cows, as well as the presence of an extra corpus luteum (García-Ispierto et al. 2006). Twin pregnancies in cows are usually shorter and are often associated with difficult calving (Mellado et al. 2014). Early foetal loss may be caused by the heat stress experienced by the cow, affecting the deterioration of the uterine blood supply (García-Ispierto et al. 2006). Calf mortality may also have a negative impact on the production rates of the herd due to, e.g. genetic selection limitations (García-Ispierto et al. 2006). High environmental temperature contributes to the increase in calf mortality rates, mainly in hot regions where the risk of heat stress is high. Moreover, calves of cows that had been exposed to heat stress during the dry period showed weakened immunity, lower birth weight, and lower weight gain (Mellado et al. 2014). Moreover, heat stress causes a significant increase in corticosteroids in the blood serum of calves. They inhibit the absorption of colostrum immunoglobulins by the intestines and contribute to the death of young animals (Liu et al. 2018). The heat stress experienced by the cow reduces the weight of the placenta as well as reduces the blood flow through the uterus. This results in a lower birth weight of the calf (Liu et al. 2018). Moreover, higher calf mortality is observed more often among male than female calves. This may be due to their greater birth weight, which may cause misalignment of the cow foetus increasing the risk of difficult calving (Mellado et al. 2014). Perinatal mortality in calves is associated with the occurrence of difficult calving and a reduction of milk yield. It also increases the number of retained placenta and inflammatory diseases of the uterus. The cause of stillbirths (of calves) in difficult calving may be too late intervention by a veterinarian. As a result, calving takes longer and metabolic acidosis occurs which the calves prone to die during calving (Nogalski and Mordas 2009).
The basis of animal health monitoring is blood biochemical and haematological profile analyses. Blood tests of cows present the adaptability of cattle to changing environmental conditions. Moreover, any reproductive disorders also change the profiles of these analyses (Coroian et al. 2017).

The influence of the housing system on cow's reproduction
A crucial aspect that influences the proper oestrus activity of dairy cows is also the housing system and their stocking density. Cows in the tie-stall housing system are kept in separation from other cows. Then they have limited interaction with cows and no possibility to, for example, rubbing animals against each other or sniffing. In a result, it influences the expression of visible symptoms of estrus (Berry et al. 2016;Ritter et al. 2019). Moreover, cows kept in isolation have a lower concentration of estradiol regulating, e.g. the sexual cycle (Berry et al. 2016). In a tie-stall housing system, cattle are characterized by a higher milk yield with a simultaneous reduction in fertility rates compared to the free-stall housing system (Sawa and Bogucki 2011). Free-stall barns enable animals to interact and express natural behaviour. Therefore, in this housing system, cows are characterized by a higher fertility rate (Sawa and Bogucki 2011).
The type of bedding material in the barn is a crucial factor to ensure the health, longevity, and high milk yield of animals. A suitable litter guarantees comfort when animals are resting and can prevent the occurrence of lameness and mastitis (Singh et al. 2020). Lameness in cattle has a negative impact on animal welfare, because reduces the activity of cattle and oestrus expression (Blackie and Maclaurin 2019;Ratanapob et al. 2020). The bedding material should provide comfort and encourage cows to lie down, as well as protect them from possible injuries (Singh et al. 2020). The litter for cows can be made of organic materials, such as straw, wood shaving, sawdust, and inorganic materialsconcrete, sand, or rubber mats. The padding material should be dry to prevent the growth of microorganisms in the wet litter. Cattles were the most preferers soft bedding material, such as straw (Sinha et al. 2017). Furthermore, straw reduces the risk of lameness and improves reproductive performance. In turn, wood shavings due to their relatively large size and sharp tips, can cause injury and damage to the animal's skin. Infection of the udder and itching can cause sawdust because they can stick to the skin and udder of cattle. The sand bedding prevents from bacterial growth. Cows kept on sand have the lowest percentage of mastitis compared to other bedding materials (Singh et al. 2020). The sand provides a grip to cows (Sinha et al. 2017). According to Sinha et al. (2017) the time of feeding animals, lying and chewing on the sand is longer than on the concrete bedding. Cows keeping on concrete are characterized by a higher risk of occurrence of hoof disease and lameness. Moreover, the claws are being worn out faster. On the one hand, the claws are being worn out faster on concrete, and on the other hand, the claws are not being worn out on rubber mats, which lead to the overgrowth (Singh et al. 2020).
The selection of cows for reproductive traits has contributed to the achievement of better reproductive rates in herds (Eetvelde et al. 2017;Stangaferro et al. 2018). The health of the cows, proper nutrition of animals, metabolic status, as well as environmental conditions has an impact on the reproductive success and health of the calves (Eetvelde et al. 2017).
Assisted reproductive techniques (ART) have contributed to the genetic improvement of animals and the production of more bovine embryos. Nevertheless, oocyte collection from cows is associated with damage to the ovaries and affects the functioning of reproductive organs. This reduces the fertility of the animals and results in irregular oestrus. In dairy cattle also has been observed a negative effect of oocytes collection on follicle development (Figueiredo et al. 2020). Recent research presents that the use of assisted reproductive techniques may reduce the reproductive performance of dairy cattle (Figueiredo et al. 2020). It had been proved that IVP embryos (in vitro embryo production) are characterized by darker cytoplasm due to the higher content of lipids, slower growth, and more frequent occurrence of apoptosis compared to in vivo embryos (Eetvelde et al. 2017). Moreover, the use of these techniques exposes gametes and/or embryos to different environmental conditions than those in the female and male reproductive systems (Eetvelde et al. 2017).
The insemination of dairy cows ensures the use of a large proportion of normal sperm for fertilization (qualitative selection in the production of insemination doses) (Diskin 2018). This method shows similar conception rates to natural mating. In order for the treatment to be effective, the symptoms of oestrus must be detected earlier (Eetvelde et al. 2017).
Oestrus monitoring is a fundamental aspect in optimizing the insemination time of animals. The oestrus and progressive ovulation have to be properly identified in order for a cow to become pregnant (Crowe et al. 2018). A tool often used to detect oestrus is automates activity monitoring systems (AAM) (LeRoy et al. 2018). The activity monitors detect oestrus when the cow's activity increases, which usually occurs to several hours (usually up to 17 h) after increasing activity (LeRoy et al. 2018). This system identifies about 80% of animals during oestrus, which predisposes them to insemination. The ovulation occurrence with detected oestrus symptoms usually occurred within 20-30 h (LeRoy et al. 2018). Nevertheless, it is difficult to determine the optimal insemination time for each animal individually (Diskin 2018;LeRoy et al. 2018). According to Diskin (2018), the best time for insemination cows is 12 h after observing the oestrus. The heifers should be inseminated up to 8 h after AAM detects oestrus symptoms with the highest probability of fertilization (LeRoy et al. 2018). At the time of insemination, the quality of the embryos decreases since the onset of oestrus (Diskin 2018). Moreover, the viability of sperm in the reproductive tract of cows is also important in terms of reproductive indicators. They remain in the reproductive system for 12-24 h of which it takes 6-8 h to reach the isthmus of the fallopian tube (LeRoy et al. 2018).
Temporary artificial insemination (TAI) protocol has been developed to increase the reproductive performance of cattle. They are supposed to help synchronize oestrus and ovulation. Synchronization is usually based on the OvSynch protocol (OVS) with the use of stimulation, e.g. progesterone, prostaglandin, gonadotropin-releasing hormone (GnRH) (El-Tarabany et al. 2016). Another programme used is PreSynch. PreSynch uses prostaglandin injections to improve the fertilization rate of cows. According to El-Tarabany et al. (2016) the highest rate of conception and pregnancy following insemination was obtained with PreSynch than with OvSynch protocol.
To increase herd profitability, the management system can be combined with a voluntary waiting period (VWP). VWP is a post-calving time that improves the health of the animals as well as regeneration after calving. The optimization of the time to the next pregnancy during lactation is then progressing (Stangaferro et al. 2018). Dairy herds were set a voluntary waiting period of about 50 days. After this time we can begin further reproduction and increase the breeding (Bragança and Zangirolamo 2018). Shortening the calving interval in cows results in an increase in the detection sensitivity of oestrus (Bekara and Bareille 2019).
Cow reproductive rates are also influenced by, e.g. climate change. By increasing the average temperature in the world, the risk of heat stress contributing to, e.g. embryo mortality increases (Alam and Silpa 2020;Saif et al. 2020).

The influence of nutrition on cow reproduction
Changes in diet may have an impact on the fertility of animals, including they may affect the normal function of the ovaries (Singh et al. 2020). Cows that experience a negative energy balance (NEB) may experience longer anestrous periods. The anestrus period intensifies during a lengthening time of poor nutrition. To resume ovulation, there must be an adequate energy supply is needed to increase the concentration of insulin in the blood. Metabolism is related to reproduction and therefore metabolic disorders may affect fertility. According to Berry et al. (2016) changes in the concentration of insulin in the blood of cows may affect the oocyte. As a result, the oocyte may be exposed to metabolic changes. It has been proved that higher energy assimilating results in higher insulin levels, which accelerate the growth of the dominant ovarian follicle while reducing the quality of the oocyte (Laskowski et al. 2016). The low concentration of insulin in the blood of cows after calving negatively affects the fertility of animals, delaying ovulation and creating unfavourable conditions for the development and maintenance of pregnancy (Rodrigues 2014).
Cows in the transition period (3 weeks before and 3 weeks after calving) are characterized by hormonal changes and metabolism that have an influence on the production, reproductive traits, and health of the animals. Cows during the transition period experience a negative energy balance. It occurs when the energy demand exceeds the energy supply feed. The cows then release free fatty acids into their bloodstream as reserves to cover their energy demands. In the result, free fatty acids are metabolically transformed in the liver and their excess in the blood may indicate metabolic problems or disorders (Barletta et al. 2017).
The excess of fat also is a cause of weakening the cow's immune system or reduces productivity. Therefore, the level of fat in the body should be monitored. Body Condition Scaling (BCS) is used to control the condition of the body and it is supporting the management of cattle nutrition programmes. Loss of condition affects the fertility of cows (Bezdíček et al. 2020). Poor body condition (obesity or emaciation) reduces the number of Graaf's follicles in the ovary (Bezdíček et al. 2020). A low condition during calving may delay ovulation (Sharma et al. 2018). Furthermore, the loss of condition after calving is caused by high postpartum concentrations of free fatty acids as well as metabolites (e.g. hydroxybutyric acid) and reduces the quality of oocytes which affects the quality of the embryo. NEB limits the growth of the dominant follicle and the release of estradiol and LH (Barletta et al. 2017). The loss of condition in the transitional period results in inhibition of ovarian function after calving and delayed ovulation (Sharma et al. 2018). Moreover, the lower condition of cows contributes to the susceptibility to mastitis (Bezdíček et al. 2020).
Better results of embryo transfer are obtained in cows in good condition than in emaciated or obese animals. A higher rate of pregnancy loss is associated with a reduction in body weight at about 50 days of pregnancy (Bezdíček et al. 2020). There are histopathological changes in the corpus luteum of thin and fat cows due to the lower concentration of progesterone. The poor condition of animals can lead to the formation of cysts on the ovary. The emaciated caused a reduction in the size of the ovary, as well as the ovarian follicles (Bezdíček et al. 2020).
Low intake of energy from the feed by the cow before calving may result in the risk of postpartum disease and a reduction in lactation efficiency (Bezdíček et al. 2020). During this period may occur metabolic diseases as well as infectious diseases e.g. mastitis or uterine inflammation. These diseases affect reproduction (Bezdíček et al. 2020). The negative energy balance of cows results in a delay in the activity of the ovary after calving by inhibiting the secretion of estradiol. It reduces the sensitivity of the follicle to LH and follicle-stimulating hormone (FSH) (Berry et al. 2016). According to Santos (2008), the relationship between metabolism and hormonal balance in animals is also visible during early lactation. Then large amounts of energy are needed to maintain lactation, and the reproductive functions of animals may be impaired. In the case of negative energy, balance, ovulation may atrophy due to an increase in the concentration of free fatty acids in the blood with a decrease in insulin-like growth factor-I and insulin. This may have an impact on ovarian dysfunction and cow fertility (Santos 2008). Certain fatty acids such as linoleic acid contained in food can improve ovarian function and have a positive effect on the conception rate of cattle (Santos 2008;Rodney et al. 2015). Mentioned linoleic acid reduces the length of the period between the calving interval and a pregnancy. Enrichment of the diet with the addition of fat has a positive effect on the reproduction of cattle. This type of supplementation increases the caloric content of the feed and modulates the ovarian functions and thus fertility (Rodney et al. 2015). Cattle genetic selection programmes in terms of milk yield increased the cows' predisposition to increase the negative energy balance occurring in early lactation (Berry et al. 2016;Bekuma and Galmessa 2019).
Lack of ovulation in dairy cattle may indicate disorders in the functioning of the hypothalamic-pituitary-ovarian axis. There are mechanisms in the central nervous system that regulate the secretion of the LH necessary for ovulation. These mechanisms are based on the direct action of insulin, which affects the GnRH (gonadoliberin neuronal expression) neurons responsible for the release of follicle-stimulating and LHs (Berry et al. 2016;Laskowski et al. 2016). Then with insulin, the ovaries do not have the ability to respond to LH and FSH, and the growth of ovarian follicles is blocked and ovulation is inhibited (Laskowski et al. 2016;Berry et al. 2016). In the anovulatory period, cows have a high concentration of the follicle-stimulating hormone, which, like the LH, are released by the pituitary gland (Berry et al. 2016).
In cows with hypocalcemia, there is a relationship between postpartum disease and reproductive performance. Cows diagnosed with calcium deficiency are at risk of placental retention and endometritis (Rodney et al. 2015). There are a lower fertilization rate and a reduced number of calves with increased milk yield (Berry et al. 2016).

The influence of environmental pollution on reproduction in cattle
The fundamental aspects of influence on reproduction are pollutants that are common in nature. The main contaminants are chemical environmental pollutants, such as toxic metals (Alam and Silpa 2020). They can penetrate the body through the digestive system, respiratory system or the skin. Dysfunction within the reproductive system is mainly influenced by lead. Lead poisoning results in pregnancy loss. Moreover, other contaminants, such as mycotoxins in feed, also can affect reproduction. Due to mycotoxins, animals consuming contaminated feed may not show symptoms of oestrus (Alam and Silpa 2020). Mycotoxins, apart from their carcinogenic activity, present estrogenic activity, which induces hyperestrogenism in cows. This results in enlargement of the reproductive organs, as well as their inflammation or miscarriage and impaired maturation of the oocytes. Toxins are produced by mould fungi, exhibit a highly toxic effect, and often contribute to placental retention in cows (Rodrigues 2014).
Another factor affecting the reduction ratios in cattle breeding are pesticides and herbicides. These substances are ubiquitous in the environment. They contribute to hormonal dysfunction in the hypothalamic-pituitary-gonadal axis. Exposure of animals to pesticides causes the formation of ovarian cysts. It also contributes to the occurrence of polycystic ovary syndrome by modulating the endocrine system and reduces the concentration of estrogen and progesterone (Sayad et al. 2019;Saif et al. 2020). Lipophilic substances contained in pesticides accumulate, for example, in the ovaries. In the ovaries, these substances adversely affect the wall of the ovarian follicle, reducing the quality of the blastocyst, limiting the implantation of the embryo, or even preventing fertilization (Saif et al. 2020).
The farms aim to continuously increase their productivity, which is why the methods of assisted reproduction (ART) are implemented (Daly et al. 2020). There are methods of assisted reproducing such as multiple ovulation, embryo transfer, or in vitro fertilization. There is a genetic gain in the form of the increased number of calves. There is variation in the response of cow's ovaries to hormonal stimulation, which affects the pregnancy and calving rates. The results of assisted reproductive techniques are also influenced, among others by stressors such as climate (Daly et al. 2020).
It is necessary to provide the cows with suitable breeding conditions to obtain suitable production and reproductive indicators of dairy cattle. This is crucial for cows to achieve welfare. The animals should be housed in suitable systems and the barns should have an adequate still for the comfort and health of the cows. Animal nutrition is also essential factor. Many factors are influencing the fertility of dairy cows.

Conclusion
Many factors affect the efficiency and the reproductive rate of cattle. The main factors are nutrition, housing systems, and stress. Significant factors reducing cow fertility are also contamination of feed by mould, fungi, and environmental pollutione.g. pesticides and toxic metals. It is important to provide suitable conditions for cattle to obtain high productivity and fertility of animals.

Disclosure statement
No potential conflict of interest was reported by the author(s).