Balancing nutrition for successful reproduction in ruminants

Proper nutrition is essential for sustaining optimal reproductive function in both humans and ruminants. Studies have shown that both undernutrition and overnutrition can negatively impact human reproductive processes. However, the Mediterranean diet, with its balanced and nutrient‐rich composition, is recognised for its positive effects on human reproduction. In the context of ruminants, a well‐balanced and nutrient‐rich diet is equally crucial for achieving peak reproductive performance. Grazing, which allows ruminants to feed on natural pasture in open environments, contrasts with indoor feeding, which is conducted in more controlled settings. A balanced dietary approach is vital for the successful reproduction of ruminant animals, and a combination of grazing and indoor feeding has been identified as an effective strategy to enhance their reproductive success. This review emphasises the impact of nutrition on reproductive outcomes in both human and ruminant populations, advocating for balanced dietary practices to improve reproductive success across species. A thorough understanding of the complex relationship between nutrition and reproductive health can offer valuable insights into enhancing fertility and reproductive efficiency in various contexts.


INTRODUCTION
Sexual reproduction involves the fusion of an egg and sperm, resulting in the creation of a new and genetically unique embryo. 1 Successful gametogenesis is essential to generate healthy and genetically accurate gametes.In males, spermatogenesis comprises three major biological phases: (1) the proliferation and differentiation of spermatogonia; (2) meiosis, which leads to the formation of haploid cells; and (3) spermiogenesis, culminating in the production of spermatozoa.In females, the process begins with a primary oocyte initiating the first meiotic division during embryonic development.This process then arrests before birth, resumes meiosis until ovulation, and finally completes meiosis after fertilisation.
Approximately one in six couples experienced subfertility around the world. 2 Nutrition plays a pivotal role in maintaining optimal reproductive function, but undernutrition and overnutrition pose significant threats to humans and ruminants globally.Recent reports indicate that the number of people affected by starvation reached 828 million in 2021, an increase of 150 million since the outbreak of COVID-19 pandemic.Concurrently, as per the World Health Organization, cases of obesity have surged nearly threefold worldwide since 1975.Both undernutrition and overnutrition are detrimental to overall health including reproductive health. 3,4In response to the challenges posed by both undernutrition and overnutrition, people have undertaken a diverse range of dietary strategies.Some nutritional and balanced dietary options, such as the Mediterranean diet, Dietary Approaches to Stop Hypertension (DASH) diet, Flexitarian Diets have emerged as prominently endorsed choices for healthy diets. 5For example, the Mediterranean diet emphasises a well-balanced eating pattern that highlights the integration of unprocessed plant foods, olive oil, and fish, while also promoting the reduction of red meat, animal fats, sugars, and salt.Reproductive health specialists commonly endorse the Mediterranean diet for individuals seeking to enhance their chances of successful conception and healthy pregnancies.
In terms of ruminant industry, grazing and indoor feeding stand are the two predominant feeding practices.These approaches play pivotal roles in shaping the nutritional regimes that govern the management of ruminant populations for various purposes, encompassing both meat and dairy production.Grazing allows animals to forage and consume natural vegetation in open pastures and indoor feeding entails ruminants to controlled environments, often within barns or housing facilities.Both approaches offer distinct advantages and challenges.Grazing embraces the natural inclinations of ruminants and leverages the potential benefits of diverse forage sources, and indoor feeding offers precision and control over nutrition, thereby enhancing productivity and addressing specific nutritional deficiencies.However, grazing may face limitations in terms of seasonal availability and quality of forage, while indoor feeding requires comprehensive management to mitigate issues related to animal health, welfare, and the environmental impact of intensive confinement.
In this review, we will explore the impact of undernutrition, overnutrition, and the balanced diet (taking Mediterranean diet as an example) on reproductive performance in humans to gain more understanding about the influence of nutritional status on mammal reproduction.With more focus on ruminant production, we will critically analyse the implications of two feeding approaches, namely the grazing system and indoor feeding, on ruminant reproduction.Our goal in comparing humans to ruminant animals is to use human physiology as a connecting point, facilitating a deeper examination of the intricate connection between nutrition and reproduction in ruminants.By exploring the effects of these practices on nutritional status and reproductive performance in ruminants, we intend to shed light on the intricate interplay between nutrition and reproductive success and management strategies.

UNDERNUTRITION AND HUMAN REPRODUCTION Prenatal/maternal undernutrition and reproduction
Prenatal undernutrition has been associated with a number of postnatal disorders.This notion came from "the early or foetal origins of adult disease hypothesis", which describes a hypothesis that emphasised the effect of environmental factors in early life, particularly nutrition, on adult diseases and premature death. 6,7mproved maternal nutrition is considered as a global health target because maternal malnutrition is harmful to pregnancy and birth outcomes.It is relatively difficult to evaluate the foetal nutrition in humans; therefore, birth weight is commonly considered as a criterion of nutritional condition during foetal life.Clinical studies have reported that low birth weight was associated with male subfertility. 8,9For example, men born small for gestational age have higher gonadotropin and lower testosterone, which are the indicators of reduced fertility. 4It appears that maternal undernutrition during pregnancy and lactation impairs the balance between germ cell proliferation and cell death in rats, and this equilibrium is critical for spermatogenesis. 102][13] These findings clearly demonstrated the negative impact of maternal undernutrition on male reproduction.5][16][17][18] These studies have included reduced total diet, calories or protein and imposed various degrees of nutritional restrictions, and all revealed that maternal undernourishment is deleterious to the fertility of female offspring.It is thought that protein restriction during pregnancy has a more profound impact on foetal development since deficiency of amino acids demands more changes in maternal metabolism. 19

Postnatal/pubertal undernutrition and reproduction
While there is a wealth of research exploring the impacts of prenatal undernutrition on reproduction, the literature concerning the effects of postnatal or pubertal undernutrition on human fertility remains relatively limited.In order to address this gap, we will compile and synthesise existing relevant studies in rodents to exemplify this aspect.In rats, nutritional protein restriction during puberty increased the oestrus phase arrest without changing the length of the oestrus cycle in adult females. 20In addition, the low-protein diet reduced the level of androgen receptor and elevated the oestrus receptor β in ovaries of rats. 20Another study in rats showed that postnatal undernutrition delayed puberty onset. 21Rats that were fed a proteinrestricted diet after weaning displayed reduced testis weight, spermatogonia, spermatocytes, spermatids, Leydig cells and the diameter of the seminiferous 2 of 10 -MODERN AGRICULTURE tubules. 22Therefore, while the studies are sparse, it is very likely that postnatal/pubertal nutritional level is positively correlated with reproductive abilities of both male and female rats.More studies in other species are needed to further confirm this conclusion.

Adulthood undernutrition and reproduction
The occurrence of acute malnutrition induced by famine is closely linked to a substantial reduction in fertility, primarily attributed to the prevalence of amenorrhoea and anovulation. 23A 10%-15% decrease in normal body weight often leads to the cessation of menstrual periods.This phenomenon is thought to involve altered regulation of gonadotropin-releasing hormone secretion and changes in the dopaminergic and opioid systems. 24In developing countries, amenorrhoea and delayed menarche related to body weight are largely the result of nutritional deprivation and the demands of lactation on women with borderline body weight.However, western countries exhibit a different pattern.Anorexia nervosa serves as a prominent example of weight reduction leading to infertility.In the western world, self-imposed weight loss stands as the most common single cause of secondary amenorrhoea.While diagnosing severe anorexia is seldom overlooked, the more subtle degrees of weight loss and their impact on the menstrual cycle are often disregarded.Notably, simple weight loss of more than 30% of body fat can lead to menstrual dysfunction and ultimately amenorrhoea. 25

OVERNUTRITION AND HUMAN REPRODUCTION
The prevalence of overweight, defined as having a BMI of 25.0-29.9,and obesity, defined as a BMI of ≥30 kg/ m 2 , is on the rise worldwide. 26The World Health Organization (WHO) has declared obesity is an epidemic, affecting both developed and developing countries, impacting children, adults, and the elderly, and affecting both men and women. 27Overnutrition and obesity are detrimental to normal reproductive function.For women, studies have found a strong link between obesity and female infertility.Excessive weight and adipose tissue can interfere with the secretion and balance of female hormones, negatively affecting ovarian function and the ovulation process, thereby reducing the chance of conception. 28,291][32][33] In addition, obese men have an increased risk of low testicular function and abnormal sex hormone levels. 34,35Notably, the type of fat is the crucial factor when examining the connection between dietary fat and male fertility.The ratio of unsaturated to saturated fat can significantly influence semen quality. 33Taken together, overnutrition is detrimental to normal reproductive function in both females and males.

MEDITERRANEAN DIET ON REPRODUCTION
To address the consequences of undernutrition and overnutrition, people have explored a wide array of dietary approaches, and among them, the Mediterranean diet has emerged as a highly recommended option.The Mediterranean diet promotes the consumption of unprocessed plant foods, olive oil and fish, while minimising the intake of red meat, animal fats, sugars and salt. 5This eating pattern offers a balanced and wholesome approach to nutrition, making it suitable for various health needs and goals.
An essential characteristic of the Mediterranean diet is its abundant presence of unsaturated fatty acids, a wide range of antioxidants and minimal presence of saturated fatty acids. 5,36,372][43] In contrast, saturated fatty acids may negatively impact female reproductive health, contributing to ovulatory disorders, reduced fecundability and endometriosis. 44,45Therefore, the Mediterranean diet is likely to be a favourable diet for optimised reproduction.Indeed, studies have demonstrated that the Mediterranean diet is positively associated with improved semen quality parameters. 46,47In addition, adhering more closely to the Mediterranean diet could potentially enhance the likelihood of a successful pregnancy and the birth of a healthy baby for women undergoing in vitro fertilization (IVF) treatment. 48verall, the Mediterranean diet is a balanced diet, which overcomes the consequences of undernutrition and overnutrition and is beneficial to cardiovascular health and reproductive performance.It is essential to remember that individual health needs, dietary preferences, and medical conditions may vary.Nonetheless, the Mediterranean diet stands as a highly regarded and health-promoting eating pattern that supports both heart health and reproductive well-being.

THE PREVALENCE OF GRAZING AND INDOOR FEEDING IN RUMINANT INDUSTRY
Grazing and indoor feeding are the two primary feeding practices for the ruminant industry, each with its unique set of advantages and disadvantages.These methods are heavily influenced by the local climate, available resources, and the specific goals of the livestock MODERN AGRICULTURE operation in a given region.Grazing, often referred to as pasture-based systems, involves allowing ruminant animals to feed on natural pastures and grass in open outdoor environments.This method is particularly wellsuited to regions with temperate climates, such as Ireland and New Zealand. 49Grazing is a cost-effective primary feed source that leverages the natural foraging instincts of ruminants.It is valued for several reasons, for instance, dairy products originating from pasturebased systems are preferred by consumers for their perceived naturalness.The animals have access to a diverse range of forage, and this is associated with enhanced animal welfare and environmental preservation. 50Such systems also create a more natural environment for ruminants, allowing them to exhibit normal behaviours. 51This not only benefits animal welfare but also has positive implications for sustainability and environmental conservation.
On the other hand, indoor feeding systems are more prevalent in regions such as the United States, China and Europe. 52,53In these systems, ruminant animals are confirmed to be in controlled environments and provided with a formulated diet comprising grass, maize/corn silage, carbohydrates, and concentrates to optimise milk and meat production.The rationale behind indoor feeding systems lies in their ability to provide protection from extreme environmental conditions such as excessive heat, cold, and wetness, which can negatively impact animal health and productivity. 51owever, indoor feeding systems have raised concerns related to animal welfare, including issues like increased lameness, reduced comfort, and a higher incidence of diseases, such as mastitis, which can ultimately impact animal productivity and well-being. 54hen it comes to reproductive performance in ruminants, both grazing and indoor feeding systems present their respective strengths and weaknesses, which we will explore in detail by discussing the advantages and disadvantages of each approach.

EFFECTS OF GRAZING ON RUMINANT REPRODUCTION
Due to the complex stomach structure, ruminants can digest and absorb grasses that humans cannot and convert them into meat and milk that are consumable by humans.Eisler et al. highlighted the vital role of grazing ruminant industries in tackling challenges arising from population growth, demand for animal protein in developing nations, and production limitations due to urbanisation and climate change.Access to high-quality forage during grazing positively influences the reproductive performance of ruminants.Adequate nutrition from grazing supports the development of healthy ovaries and uterine function, leading to improved conception rates and increased ovulation rates. 55Besides, grazing on diverse plant species provides a wide array of essential nutrients and micronutrients, which are vital for proper foetal development and successful pregnancies.
However, in many environments, the quantity and quality of pasture are often inadequate during the breeding season, which causes the loss of testis mass of ruminants. 56,57Furthermore, overgrazing can lead to the depletion of available pasture, leaving ruminants undernourished.A series of reports have assessed the long-term influence of maternal undernourishment on the fertility of female offspring.Compromised foetal ovarian development may potentially reduce ovarian reserve and adulthood fertility.By using Scottish Blackface ewes as an example, maternal protein restriction reduced the weight of the ovaries. 58Similarly, another study on Scottish Blackface ewes suggested that prenatal undernutrition reduced the ovulation rate in female progeny. 59Mossa et al. worked on bovines, which have a gestational length and birth rate similar to humans, and found that maternal restricted nutrition during the first trimester resulted in subfertility of the female progeny, evidenced by reduction of antral follicles, decreased concentrations of anti-Müllerian hormone, and increased concentrations of folliclestimulating hormone. 60any studies, including ours, have endeavoured to answer the question of whether underfeeding affects reproduction in sexually mature male animals.The mature male sheep has a long history as a model to assess its response to nutrition.The first robust study was conducted by Mori et al., and they found that semen quality of severely underfed rams was improved by refeeding. 61Subsequent reports in the next 6 decades further supported the previous conclusion by evaluating the testis weight, sperm production and spermatogenic efficiency. 62,63However, not all studies agree, 64 and this might be because the traditional measurements (sperm concentration, viability, morphology, and subjective motility) are not accurate and the results are hard to reproduce.To overcome this difficulty, we simulated weight reduction in sheep under grazing conditions in a laboratory environment, utilised computer-assisted semen analysis and sperm chromatin structure assay, and we found that undernutrition reduced sperm velocity and caused more DNA damage in sperm. 56mall non-coding RNAs are potent regulators of gene expression and possess diverse biological functions. 65,66Small RNAs are composed of three major groups: small interfering RNA (siRNA), microRNA (miRNA), piwi-interacting RNA (piRNA).In this review, we will only focus on miRNAs and piRNAs since they are closely related to our sheep model.9][70] In our nutritionally treated sheep model, we identified 44 miRNAs, which were differentially expressed in sheep fed with high and low diets.The predicted functions of these miRNAs were involved in sperm number and sperm quality. 68piRNAs are non-coding RNAs with a preferred signature of a 5 0 uridine nucleotide. 71piRNAs are associated with piwi proteins (MIWI, MILI, MIWI2), and are required for repression of retrotransposons, whose mobilisation is 3][74] We identified 35 putative piRNAs, oar-piR-12568 for instance, that were affected by nutritional treatment in sexually mature male sheep. 68These findings led us to conclude that the reductions in sperm production and sperm quality induced by undernutrition are mediated by small RNAs.To our knowledge, the changes in small RNAs related to nutritional treatment in adults, particularly in females, were not reported in other studies, and this is worth further investigation.

EFFECTS OF INDOOR FEEDING ON RUMINANT REPRODUCTION
Compared with the grazing system, the effect of indoor feeding on reproduction in ruminant animals is an area of limited study.Nevertheless, some observations and research findings shed light on the intricate relationship between indoor feeding and reproductive performance.One notable advantage of indoor feeding is the provision of controlled and consistent nutrition.It has been reported that lambs raised in indoor feeding systems tend to gain more body weight compared with those solely relying on grass for their nutrition. 75To authenticate this comparison, we extensively compiled literature on the feeding habits of ruminant animals.We systematically gathered data illustrating changes in body weight before and after indoor feeding and grazing and presented the results visually (Table 1).Our analysis suggests a more pronounced rise in body weight in animals subjected to indoor feeding in contrast to those engaged in grazing (Figure 1).This weight gain can be seen as an indicator of improved reproductive ability in animals that are fed indoors. 56,68mproved nutrition can indeed have a positive impact on reproductive success by ensuring that animals are in optimal body condition for breeding.However, there are complexities in the effects of indoor feeding on reproductive performance.Animals kept indoors often have less opportunity for natural behaviours.They may spend less time eating, drinking, and standing, and allocate more time to resting. 75While this reduced physical activity may contribute to the weight gain mentioned earlier, it can also lead to excessive weight gain, which can be detrimental.Studies have shown that over-nutrition or excessive obesity resulting from indoor feeding may negatively affect the reproductive efficiency, as in rams. 76Excessive postnatal feeding causes anovulation of sheep during their first breeding season. 77The number of total embryos was higher for cows in normal weight groups compared with cows in the obese group. 78In addition to the direct impact on reproductive ability, the lack of physical activity associated with indoor feeding can have broader implications for animal well-being.Animals with limited exercise may experience higher stress levels and compromised overall health.These factors, in turn, can further influence reproductive performance.Inadequate exercise may lead to issues such as muscle weakness, decreased bone density, and compromised immune function, all of which can have a cascading effect on reproductive ability.
In summary, indoor feeding offers the advantage of superior and controlled nutrition along with protection from harsh environmental conditions.However, it also comes with the risk of obesity, which can compromise overall health and adversely impact reproductive ability in ruminant animals.The effects of indoor feeding on reproduction are complex and multifaceted, highlighting the need for further research to better understand and optimise the reproductive performance of animals in indoor feeding systems.

COMBINATION OF GRAZING AND INDOOR FEEDING ON RUMINANT REPRODUCTION
Given the unique advantages and drawbacks of the grazing systems versus indoor feeding, it is increasingly recognised that a combination of both outdoor grazing and indoor feeding may offer an optimal strategy to maximise ruminant reproduction and overall productivity.Several studies have highlighted the benefits of incorporating both pasture and concentrate supplementation into the feeding regimen.For example, in dairy cows, a feeding approach that combines pasture with concentrate supplementation has been found to result in increased milk production when compared to a feeding regimen of pasture only. 79,80Furthermore, research on Nalor sheep reared within the semiintensive breeding approach (a combination of grazing and indoor feeding) exhibited enhanced sperm motility and improved plasma membrane integrity in comparison to sheep raised under the extensive breeding system involving pure grazing. 81These studies highlight the potential synergistic effects of a mixed feeding strategy on reproductive performance.
In summary, enhancing ruminant reproductive performance requires a thoughtful evaluation of the intricate relationship between grazing systems and indoor feeding strategies.Rather than viewing these approaches as mutually exclusive, a more comprehensive approach that leverages the strengths of both methodologies can potentially lead to superior reproductive outcomes across diverse ruminant species.By carefully balancing outdoor grazing with controlled indoor feeding, to some extent mimicking some best diet options of humans, livestock managers can tailor their practices to the specific needs of their animals and production goals, ultimately promoting healthier and more reproductively successful ruminant populations.We therefore recommend the following strategies for ruminants: (1) Formulate a balanced concentrate mix with grains, protein sources (soybean meal, sunflower meal), and essential vitamins and minerals; (2) Utilise high-quality forages such as alfalfa and clover, and implement rotational grazing for optimal pasture utilisation; (3) Regularly monitor animal performance, body condition, and pasture conditions to assess the effectiveness of feeding strategies.

CONCLUSIONS AND FUTURE DIRECTIONS
This review article examines the impact of nutrition on reproduction in both humans and ruminants.Although the diets of humans and ruminant animals are primarily different due to their distinct digestive systems, nutritional requirements and dietary choices, there are some similarities for the nutritional considerations to meet the dietary requirements of humans during the reproduction process and for ruminant animals to optimise their reproduction performance.In the case of humans, it has been well-documented that both undernutrition and overnutrition can have adverse effects on reproductive processes.Some healthy diet options like the Mediterranean diet are becoming more and more popular because of their balanced and nutrient-rich composition that is beneficial for human reproduction and overall wellbeing.For ruminants, the management of nutrition becomes a critical factor in ensuring optimal reproductive performance.Conventional feeding methods utilised indoors typically result in a more substantial weight gain compared to those involved in grazing (Figure 1).Through a strategic balance of outdoor grazing and regulated indoor feeding, resembling some of the best dietary practices observed in humans, livestock managers can customise their methods to meet the distinct needs of their animals and production objectives.This, in turn, fosters healthier and more reproductively successful populations of ruminants.

AUTHOR CONTRIBUTIONS
Effects of feeding systems on the weight of animals.
T A B L E 1