Photoperiod Management in Farm Animal Husbandry: A Review

The production performance of farm animals is influenced by various factors, including diet, breed, and environment. Among these, lighting is a crucial component of the rearing environment, affecting animal behavior and physiology [1,2,3]. The light environment consists of three elements: photoperiod, light intensity, and light wavelength [4,5]. Climate and geographic location, particularly latitude, significantly impact the photoperiod experience of animals. In regions closer to the equator, animals are exposed to relatively consistent light durations throughout the year, whereas those in higher latitudes experience more pronounced seasonal variations in daylight. These seasonal fluctuations in photoperiod can significantly affect animals' biological rhythms, including their reproductive cycles and growth patterns. With the advancement of intensive farming, lighting measures are increasingly receiving attention, especially as they are influenced by the geographic and seasonal factors that vary across the globe. The photoperiod regulates the biological rhythms of animals and affects the secretion of hormones within the animal's body, particularly melatonin (MEL). MEL regulates the secretion and release of several other hormones through various pathways, such as growth hormone (GH), prolactin (PRL), and gonadotropins. For light environments in intensive farming, changing the light intensity or wavelength may require changing the light source, whereas modifying the photoperiod is relatively easy to achieve.

In the retinas of animal eyes, there are two types of photoreceptor cells: cone and rod (Figure 1). The former is sensitive to bright light, while the latter is sensitive to low light. The variation in these photoreceptor cells is considerable among different animals, hence, each species possesses its unique monochromatic or polychromatic vision. The visual systems of cattle, poultry, and pigs have distinct characteristics. Cattle have a wide field of vision and can perceive blue and yellow but have limited ability to distinguish red and green [6,7]. Poultry possess a highly developed visual system, enabling them to detect ultraviolet light and a broad spectrum of colors, including red, green, and blue [8,9]. In contrast, pigs have relatively weak vision, primarily distinguishing blue and green while exhibiting poor discrimination of red and yellow [10,11]. When the animal retina perceives light, the photoreceptor cells convert light signals into biological signals, influencing the organism's neuroendocrine system, particularly the hypothalamic–pituitary–gonadal (HPG) axis, thereby affecting circadian rhythms and other physiological activities [12].

Light signals are converted into chemical signals, influencing the HPG axis and regulating physiological functions. Effective light management plays a crucial role in optimizing reproduction and metabolism in livestock. In broilers, extended light exposure accelerates sexual maturity, as seen in Ross broilers [13]. Similarly, in swine, prolonged lighting (23L:1D) enhances metabolic capacity and increases average daily gain (ADG) [14]. However, photoperiod effects vary by species. In short-day breeders like dairy goats, reduced light exposure triggers earlier estrus, promoting mating and pregnancy [15].

This review aims to investigate the effects of photoperiod on farm animals and how optimizing light exposure can enhance their production performance. To conduct this review, we systematically searched multiple academic databases, including Web of Science, PubMed, and Google Scholar, using relevant keywords related to photoperiod, livestock production, and physiological responses. We included peer-reviewed studies from the past three decades, with some classical studies extending beyond this timeframe, that examined the impact of light duration on growth, production performance, and reproduction. Overall, research on the effects of the photoperiod has been more extensive for dairy cows, broilers, laying hens, ducks, geese, turkeys, pigs, rabbits, goats, and horses. By synthesizing these findings, we explored the mechanisms through which the photoperiod influences animal physiology. We hypothesize that optimizing photoperiod management can improve production efficiency while maintaining animal welfare. This study provides a scientific foundation and practical guidance for enhancing lighting strategies in intensive livestock farming.

The season is a complex factor affecting milk production in cows, including temperature, humidity, light, vegetation, etc. Photoperiod is one of the important factors in light that cannot be ignored. Winter light hours are the shortest, therefore cows' milk production will be at a low level without artificially extended light hours in winter [16]. Milk yield increases by 1 to 3 L/d when light hours are extended in winter [17]. In dairy farming, 16 to 18 h of light in a day is generally referred to as the long daily photoperiod (LDPP), while 8 h of light or less is generally referred to as the short daily photoperiod (SDPP). Studies on the effect of photoperiod on milk yield were first reported by Peters et al. [18], who reported that the milk yield of cows increased when light hours were extended to 16 to 18 h (16L:8D to 18L:6D) [19,20], but due to limited technology at that time, the mechanism was not clearly explained.

There are many reasons why extended light increases milk production in cows, with light affecting hormone production in cows being the main reason. Several hormones are involved in lactation in cows, such as PRL, estrogen, and GH. The role of PRL in dairy cows is to promote somatic cell proliferation and regulate mammary gland development and lactation initiation [21,22]. On the one hand, PRL levels in cows increase when they are exposed to LDPP (18L:6D) [23], and thus milk yield increases. On the other hand, when stimulated by light, light inhibits MEL secretion from the pineal gland, causing MEL levels to be low during the day and high at night [23,24], and it has been shown that MEL signaling can inhibit PRL secretion [25]. Figure 2 shows the inhibitory effect of MEL on PRL.

However, SDPP (8L:16D) increased the milk production of dry cows in their next lactation by 10% relative to dry cows in an LDPP (18L:6D) environment [23,26,27,28]. The reason can be three-fold. Firstly, the shortening of the light period during the dry period was found by some studies to increase the dry matter intake (DMI) [27,28]. Contrastingly in a study by Crawford et al. [29], there was no difference in DMI between SDPP (8L:16D)- and LDPP (18L:6D)-treated cows during the dry period. Therefore, the changes in DMI may not be the only reason for the increased milk yield during the next lactation. Secondly, cows were exposed to an SDPP (8L:16D) during the dry period, which promoted mammary gland development [30,31]. SDPP (8L:16D) also increased the expression of the PRL receptor gene [27]. Finally, the mRNA expression of insulin-like growth factor-2 was increased in cows exposed to SDPP (8L:16D) [31]. It has been reported that insulin-like growth factor-1 could improve mammary gland development and lactation by promoting cell proliferation and inhibiting apoptosis [32], hence the growth of IGF-2 mRNA expression in SDPP (8L:16D) promoted the mammary gland development in cows and contributed to increasing milk yield in the next lactation [31].

Milk composition indicators mainly include milk fat rate, milk protein rate, somatic cell score, and urea nitrogen, which are directly related to milk quality. Milk composition is influenced by a variety of factors, and the main ones currently recognized include breed, parity, lactation stage, calving period, season, temperature, management practices, etc. [33]. In 1999, Miller et al. [34] suggested that photoperiodic management does not affect milk composition. However, a lot of evidence has suggested that the composition of milk produced by cows is closely related to the duration of light exposure [35,36,37]. For instance, an early study found that LDPP increased milk fat yield in dairy cows by 0.3% [38]. Similarly, Lim et al. [39] found that the milk fat yield of cows increased with an increase in lactation under LDPP. However, there were also different findings that dairy cow milk fat yield decreases under LDPP [17,40].

In addition, milk compositions show circadian rhythmicity. For example, higher MEL levels have been reported in milk produced at night than during the day [35]. Therefore, milk produced at night can improve sleep quality [41]. When the concentration of circulating MEL in cows increases, the lactose content of milk decreases, but the fat, protein, and casein content increases, so the milk produced at night has a better milk composition [36]. However, milk production has decreased. Similarly, if we look at the annual rhythm, the long light hours in summer and the short winter months, result in higher fat and protein content in milk during the winter months [42]. This is also the reason for the difference in milk composition between summer and winter. There is also a circadian rhythm in the absorption of fat in animals [43], which is higher at night [44]. Lacto-fatty acids are also derived from rumen microbial synthesis. Ruminal microbes and their metabolites have also been reported to exhibit circadian rhythms [45]. Using lipidomics and metabolomics studies, Teng et al. [37] found that milk from cows at night contained higher levels of various fatty acids such as stearic acid, eicosapentaenoic acid, myristic acid, and cis-9-palmitoleic acid compared to milk during the day.

Calf growth is influenced by a number of factors, such as feed intake, individual differences, health status, etc. Photoperiod can also affect feed intake [46]. When light duration was extended from 10 to 18 h, the ADG of calves increased, and LDPP (18L:6D) also increased the colostrum intake in calves [47]. For the growth and development of calves, LDPP accelerated the onset of puberty in calves [47].

We have summarized the positive effects of different photoperiods on dairy cows at various physiological stages in Table 1. This section highlights the significant impact of photoperiod on dairy cow production, including milk yield, milk composition, and calf growth. Extended light exposure (16L:8D to 18L:6D) during lactation has been shown to increase milk yield by 1–3 L/day, primarily by regulating PRL secretion. However, a SDPP (8L:16D) during the dry period enhances mammary gland development and improves milk production in the subsequent lactation. While photoperiod effects on milk composition remain inconsistent, evidence suggests that nighttime milk contains higher MEL and beneficial fatty acids, improving milk quality. Additionally, photoperiod influences calf growth, with extended light exposure accelerating feed intake, ADG, and puberty onset. Despite these findings, inconsistencies exist across studies, and further research is needed to determine optimal photoperiod management strategies for different physiological stages of dairy cows.

Poultry are highly photoreceptive animals with a well-developed visual system, making them particularly sensitive to lighting conditions. In modern intensive farming, they are predominantly raised in controlled indoor environments without access to natural light, relying entirely on artificial illumination [48]. The effects of photoperiod on poultry have been extensively studied, particularly in broilers, where lighting schedules significantly influence growth rate, feed efficiency, and welfare. However, photoperiod management also plays a crucial role in the productivity and health of laying hens, turkeys, ducks, and geese. The following sections will explore the species-specific effects of photoperiod on different types of poultry, with a particular emphasis on broilers.

Pigs are diurnal animals, which means they are active during the day and rest at night [109]. The European Union mandates that the lighting period in pig farming environments must be at least 8 h per day [110]. Artificial manipulation of the photoperiod, by extending the duration of light exposure, can alter pigs' production performance and immune function, among other factors.

Weaning is a challenging process for piglets, often leading to stress that can cause intestinal diseases, reduced growth performance, and in severe cases, death [111,112]. Research has confirmed that after weaning, artificial manipulation of light to extend the illumination period to 23 h increases the feed intake and energy metabolism rate of weaned piglets [14]. Extending the lighting period from 8 to 16 h has been shown to improve the daily weight gain and immune performance of weaned piglets [113]. Similarly, Martelli et al. [110] found that compared to an 8L:16D photoperiod, a 14L:10D photoperiod increased the average daily weight gain of pigs and reduced abnormal behaviors such as standing inactive, sitting inactive, and over-exploring/over-sniffing of the floor. The reason is that extending the lighting period reduces the secretion of MEL and other neurochemicals associated with temperament, increases feeding activity, extends feeding time, and enhances digestive absorption capacity [14]. For growing and fattening pigs, extending the lighting period has a positive impact on production performance without adversely affecting carcass traits or meat quality [114].

Extending the duration of light exposure has a positive effect not only on the production performance of pigs but also on the reproduction performance of replacement gilts. Studies have found that for every additional hour of light, the age at first mating (AFM) of gilts decreases by 1.13 days [115]. Tummaruk [116] discovered that when the lighting period is extended from 11.5 h to 12.5 h, the time to first estrus in L × Y gilts is shortened by 3.04 days.

Extending the lighting period from 8 to 20 h has a promotive effect on the growth of suckling piglets and positively influences their activity without affecting the sows' activity and nursing time [117]. However, too long lighting periods may have negative effects. Compared to a 16 h long light exposure, an 8 h light exposure during gestation is beneficial for sows to have a greater number of live piglets; however, the piglets' birth weight is lower [113].

In boar farming, the assessment of ejaculate parameters holds significant economic importance [118]. The lighting environment has a crucial impact on the semen quality of boars, with the mechanism being the influence of light exposure duration on the body's MEL levels. It has been found that MEL promotes testosterone production in the testes and slows down testicular aging [119]. In addition, MEL maintained normal dimethylation of spermatogonial stem cells and meiotic cells in the testes [120], which affected sperm production [121]. Knecht et al. [122] found that during short light exposure periods (8L:16D), the average semen volume of three boar breeds (PLW, P × L, D × P) was significantly higher than during long light exposure, with the most pronounced effect observed in the D × P breed. Under extreme lighting conditions (0 h of light and 24 h of light), negative impacts on boar semen concentration, semen volume, and sperm acrosome integrity were noted [123].

Lighting management in pig production, including gestating sows, gilts, boars, and growing pigs, is complex, as different lighting cycles have varying effects depending on production objectives. Studies have shown that extending light exposure (e.g., 16L:8D) for sows and piglets improves reproductive performance and growth rates, while prolonged lighting also enhances feed efficiency in growing pigs. Conversely, for boars, reducing light exposure to less than 8 h per day improves semen quality, as excessive light may negatively affect sperm production and fertility. According to the studies cited above, adopting lighting cycles tailored to the physiological stages and growth needs of pigs, such as 16L:8D for lactating sows and 8L:16D for boars, helps meet their physiological requirements, aligns with their biological rhythms, ensures overall health, and optimizes both productivity and reproductive efficiency.

We have summarized the positive effects of different photoperiods on pigs at various physiological stages in Table 4.

Rabbits are typically crepuscular animals and have been intensively farmed globally for wool, skin, and meat production. Observations of wild rabbits' behavior indicate minimal activity between 10:00 and 16:00, with activity peaks occurring at 20:00 and 08:00 [124]. With the intensification of farming, most commercially produced rabbits reside in hutches with artificial lighting, making the light environment particularly important. However, research on the photoperiod for rabbit farming is relatively early-stage and primarily focused on reproduction performance. Although rabbits are nocturnal, extending the duration of light exposure may positively affect their reproduction performance. Minj et al. [125] found that, compared to short light exposure (12 h), long light exposure (16 h) environments resulted in increased levels of FSH and LH hormones in female rabbits. Quintela et al. [126] suggested that longer light exposure improved the estrus response in female rabbits, with better receptivity to artificial insemination observed after extending the light period from 8 h to 12 h six days prior to insemination. Similarly, longer light exposure (14 or 16 h) also enhances the ejaculate quality of male rabbits and improves sexual activity, increasing the sexual receptivity of female rabbits [127].

We have summarized the positive effects of different photoperiods on rabbits in Table 5.

Goats are globally renowned for their wool, with cashmere historically used to produce high-end knit fabrics [128]. Cashmere goats possess primary hair follicles (PHF), which produce coarse hair, and secondary hair follicles (SHF), which produce cashmere [134,135]. Cashmere production is influenced by genetics, nutrition, and environmental factors, with photoperiod being a key regulator of hormonal secretion. Studies show that MEL, PRL, GH, IGF-1, and thyroid horIGF-1mones play vital roles in cashmere growth [128,136,137]. MEL stimulates PRL secretion, triggering the anagen phase of hair follicles [138]. A SDPP (8L:16D) induces earlier hair follicle growth by upregulating genes associated with follicle development, with miRNAs likely playing a regulatory role [128,139]. Li et al. [66] found that under 7L:17D conditions, SHF density increased while PHF density decreased, and SDPPs elevated serum MEL levels, promoting the expression of β-catenin, BMP2, FGF5, and PDGFA, all linked to follicle development. Similarly, prolonged darkness increases MEL, PRL, and GH levels, enhances antioxidant enzyme activities (T-SOD, CAT, T-AOC), and reduces MDA concentration, thereby improving immune function and antioxidant status in goats [129,130,140].

There are fewer studies on the effect of photoperiod on the lactation performance of dairy goats. Similar to dairy cows, SDPP (8L:16D) increased lactation in dry lactation goats in the next lactation period, which may also be due to the ability of SDPP to increase PRL levels in dry lactation goats [141]. Similarly, LDPP (16L:8D) increased lactation and decreased ovulatory activity in lactating dairy goats [142].

We have summarized the positive effects of different photoperiods on goats in Table 5.

Photoperiod plays a key role in regulating various physiological aspects in horses, including coat growth, reproductive function, and hormonal regulation. Extended light exposure inhibits MEL secretion, thereby activating the molting process. Many horse owners prefer not to have thick coats on their horses, as this can impact temperature regulation in racehorses and affect the visual aesthetics of performance horses. Studies have shown that an extended photoperiod (14.5L:9.5D) helps young horses and fillies shed their winter coats [131,132]. Additionally, when the daily light duration is extended to 14.5 h for young Thoroughbreds, a significant increase in fat-free mass is observed compared to foals under natural light conditions [133]. We have summarized the positive effects of extended photoperiods on horses in Table 5.

Light exposure plays a critical role in regulating hormone secretion in farm animals, influencing growth, reproduction, immune function, and product quality. This review highlights species-specific responses to photoperiod manipulation. In dairy cows, prolonged light exposure enhances milk yield, while short-day photoperiods during the dry period promote mammary gland recovery. In poultry, a balanced light-dark cycle (e.g., 16L:8D) optimizes egg production and broiler growth, whereas excessive light exposure increases stress and inflammation. For pigs, extended lighting benefits sows and piglets by improving reproductive and growth performance, but reduced lighting enhances semen quality in boars. In cashmere goats, SDPPs stimulate secondary hair follicle growth and improve wool production. However, excessive or inappropriate photoperiod adjustments may disrupt circadian rhythms, induce stress, and compromise animal welfare. While current research provides valuable insights into photoperiod management, the optimal lighting duration for different species and physiological stages requires further investigation. A more precise understanding of species-specific lighting needs will enable the development of tailored photoperiod strategies to enhance productivity while maintaining animal welfare.