Environmental Physiology of Livestock (Collier/Environmental Physiology of Livestock) || Effects of Photoperiod on Domestic Animals

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BLBS093-c13 Collier October 29, 2011 10:17 Trim: 244mm×172mm

Chapter 13

Effects of Photoperiodon Domestic AnimalsGeoffrey E. Dahl and Izabella M. Thompson

Introduction

Photoperiod is defined as the alternating exposure to light and dark on a daily basis. Lightedperiods are termed the photophase, whereas darkness is the scotophase. Physiologically, thepattern of light exposure, particularly the relative duration of light and dark, affects a numberof endocrine pathways to culminate in daily, monthly, and annual shifts in functions relatedto growth, reproduction, pelage, and immune function. From an evolutionary perspective,adoption of photoperiod as an ultimate driver for annual changes in physiological function ispredictable because it is the most consistent environmental signal over extended periods oftime (Gwinner, 1986). But photoperiod can also be harnessed to improve production in allfarmed domestic species, especially in today’s intensive systems.

Light

One of the most frequent questions with regard to photoperiod concerns the relative differencebetween light and dark. Put another way, how dark is “dark” and how much light registers asa signal? Light intensity is measured and reported in footcandles (FC; English) or lux (Lx;metric), with a conversion ratio between the two of about 10 lux to 1 footcandle. Althoughthere are limited data on domestic species in regard to the minimal intensity of light that isperceived as “light,” an illumination as low as 5 footcandles registers as light. This does notmean, however, that an intensity of less than 5 FC is “dark,” as animals may acclimate to lowerintensities over time. In fact chickens can respond to intensities as low as 1 FC. The AmericanSociety of Agricultural and Biological Engineers (ASABE) recommends that all housing fordomestic animals achieve an intensity of 15 footcandles or more. The maximum intensity

Environmental Physiology of Livestock, First Edition. Edited by R. J. Collier and J. L Collier.C© 2012 John Wiley & Sons, Inc. Published 2012 by John Wiley & Sons, Inc.

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230 Environmental Physiology of Livestock

measured under natural conditions exceeds 1,000 FC on a sunny day and even a cloudy daywill routinely provide an intensity of 200 to 300 FC. Thus it is clear that animals have a wide,dynamic range of light perception, and the only true measure of darkness appears to be theabsence of light.

One exception to the foregoing discussion is that of low intensity red lamps. Although FCintensity is not measured per se in this single wavelength lamp source, the use of 7 to 15 Wlamps during the scotophase does not register as light. This observation is consistent for cattle,sheep, chickens, and swine, and the use of low intensity red lamps is a management tool forobservation and manipulation of animals during periods of darkness.

Type of light provided is also of practical interest in the production setting. The spectralqualities of incandescent, fluorescent, metal halide, and sodium vapor lamps vary particularlyin the color-rendering index, which is a measure of the “white” quality of the light transmitted.However, responses appear to be consistent in farmed species for all those lamps. Becausethe lamps also vary in the relative efficiency, it is to the producer’s advantage to use the mostefficient lamp type appropriate to each setting.

Endogenous Rhythms and Light Signal Reception

Photoperiod signals modify the endogenous rhythms present in animals that occur on a dailyor annual basis. The most obvious of these rhythms are the circadian (i.e., around a day)and circannual (i.e., around a year) rhythms that drive physiological systems that underlieacute and long-term functions. An example of a circadian event is the daily fluctuation inbody temperature between an apex in the afternoon and a nadir in the early morning hoursof a subjective day. The most widely adapted circannual rhythm is that which times seasonalreproductive events to optimize the chances for neonatal survival. These rhythms are innate tothe animal and oscillate in the absence of any external cues or “zeitgebers” (e.g., time givers),although the period or time between each physiological event will drift from a consistent24 hour or 365 day interval. Photoperiodic signals then serve to synchronize the endogenousrhythms to the day or year (Gwinner, 1986).

Light energy in the form of photons causes a conformational shift in photoreceptors foundin photoreceptor cells of the retina that in turn open sodium channels to alter the membranepotential of those cells. In the absence of light (i.e., dark), stimulation of the photoreceptorsis absent. Thus, photoreception is a transduction of the physical energy of light waves tothe chemical energy of neuronal stimulation. Subsequent to photoreceptor action, signals arepropagated through a series of interneurons to a collection of neurons within the hypothalamusknown as the suprachiasmatic nucleus (SCN; see Fig. 13.1). The SCN is termed the masterpacemaker for circadian events through expression of a group of transcription factors knownas “clock” genes.

Within the SCN neurons, four genes form a transcription-translation feedback loop thatinfluences neuronal signaling. On one side of the loop are CLOCK and BMAL1, whichencode transcription factors that heterodimerize and initiate the transcription of the Period(PER) and Cryptochrome (CRY) genes that form the other side of the loop. As PER:CRY geneproducts accumulate, they form heterodimers that translocate to the nucleus and repress theexpression of CLOCK and BMAL1 (Ko and Takahashi, 2006). A complete cycle of the loopoccurs at roughly 24 hour intervals, a duration that is consistent with circadian oscillation.Expression of the clock genes, however, is not limited to the SCN and numerous other tissues

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Effects of Photoperiod on Domestic Animals 231

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Figure 13.1. A model of photoperiodic input to the endogenous timing mechanism in animals. Lightimpacting the eye is perceived and transduced as a nervous signal along the retino-hypothalamic tractto the suprachiasmatic nucleus (SCN) and directly to the pineal gland. Within the SCN, the light signalimpacts the transcription-translation feedback loop of the “clock” genes BMal1, Clock, Period, andCryptochrome, which ultimately determine the timing of daily and annual events via outputs to thehypothalamus and pituitary. Additional input from the daily pattern of melatonin release from the pinealinfluences the relative timing of the clock gene expression such that the timing is synchronized with thegeophysical day under natural conditions.

express these genes as well, including the liver, mammary gland, and intestinal tissue. Whereasclock gene expression is constitutive in the SCN, they are not necessarily in other tissues andthus may be under the control of the master circadian oscillator.

Photoperiodic signals integrated at the SCN then modulate the expression of clock genes.Long days induce PER expression patterns quite different than exposure to short days inrodents (Naito et al., 2008), and it is likely that this is a conserved response across mammalianspecies. The rapid adjustment to altered photoperiodic input, and the eventual development ofrefractoriness to those inputs (Buchanan et al., 1992; Tournier et al., 2009), provides furtherevidence that photoperiodic cues modulate the endogenous rhythm of the SCN. The masterclock then affects other physiological systems, in particular endocrine systems related toreproduction, growth, immune responses, and lactation.

Endocrine Effects of Photoperiod

Melatonin is the primary indoleamine secreted by the pineal glands of birds and mammals(reviewed in Rieter, 1991; Rieter et al., 2010). Photic input from the eye is transduced in asimilar manner to that previously described to a sympathetic pathway innervating the pinealgland. Pinealocytes absorb tryptophan from the circulation and convert it to serotonin, which

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is then converted to N-acetylserotonin in a reaction catalyzed by N-acetyltransferase (NAT).This action of NAT is the rate-limiting step in melatonin synthesis, and NAT activity is the onlyprocess that exhibits a circadian rhythm in the production of melatonin (Illnerova and Sumova,1997). The enzyme hydroxyindole-O-methyltransferase then converts the N-acetylserotonininto melatonin. Melatonin diffuses freely across cell membranes and thus the circulatingconcentrations increase dramatically as synthesis accelerates upon exposure to dark conditions.

Pinealectomy of domestic mammals and birds eliminates the daily pattern of melatoninand the ability to track photoperiodic changes in mammals in particular. In the absence ofphotic input, animals continue to display endogenous rhythms of activity on a daily and annualbasis, but those rhythms become uncoupled and drift from the daily and yearly patterns ofexpression, a process termed “free-running” (Gwinner, 1986). In fact, free-running activitiesare hallmarks for endogenous rhythms and emphasize the importance of external cues – inthis case photoperiod – in aligning endogenous rhythms with the daily and seasonal patternsof physiological events.

It is important to understand that the photic control of melatonin secretion is separate fromthat of inputs to the SCN, yet the two systems interact to provide an output that is synchronizedto the external environment under natural conditions (Lincoln et al., 2003). For example, thedecreasing day length observed in the autumn means that melatonin is present during phasesof the circadian fluctuation of the clock genes when it would not be present during the longdays of summer. Relative presence or absence of melatonin then impacts the level of clockgene expression and ultimately affects the observed output of the SCN to peripheral tissues.

After melatonin, the impact of photoperiod on prolactin (PRL) secretion is the most con-sistent endocrine response across birds and mammals. Long days (i.e., a short duration ofelevated melatonin) drive an increase in PRL, whereas short days (i.e., extended duration ofelevated melatonin) are associated with decreases in PRL in chickens, cattle, sheep, and horses(Sreekumar and Sharp, 1998; Peters et al., 1981; Mikolayunas et al., 2007; Johnson, 1987).In cattle, an inverse relationship between the circulating PRL concentration and PRL-receptor(PRL-R) expression is observed in a number of tissues, including the mammary gland, liver,and leukocytes (Auchtung et al., 2003), such that high concentrations of PRL associatedwith long days reduce PRL-R mRNA expression, and this is associated with a reduction inresponsiveness to PRL (Auchtung and Dahl, 2004).

Altered day length induces shifts in circulating insulin-like growth factor I (IGF-1) in cattle(Dahl et al., 1997). Similar effects of long days are observed in farmed red deer, which suggestsa common response among ruminants to extended light (Suttie et al., 1992). The increase inIGF-1, however, is not due to an increase in growth hormone (GH), as there is no evidencethat photoperiod alters GH release or clearance in cattle or other domestic animals (Dahlet al., 2000). In addition, photoperiod does not affect expression of hepatic GH receptors(Kendall et al., 2003). One possibility is that long days increase circulating concentrationsof IGF binding proteins (IGFBP) that then alter clearance of IGF-1. Evidence to support thehypothesis that photoperiod affects IGFBP is indirect in that elevated PRL decreases IGFBP-5in bovine mammary cultures, an observation consistent with greater effective IGF-1 action(Accorsi et al., 2002). Further, summer season increased IGF-1 in both bST and non-bSTtreated cattle relative to winter conditions (Collier et al., 2008).

Reproductive Responses

Sheep have been used extensively as a model for study of circannual rhythms and haveprovided evidence that photoperiod modifies the expression of those rhythms via the pattern of

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melatonin release from the pineal gland (Karsch et al., 1984). The seasonal waxing and waningof reproductive competence observed in many sheep breeds results from a rhythm of alteredsensitivity to estradiol at the level of hypothalamic gonadotropin-releasing hormone (GnRH)secretion (Karsch et al., 1993). During the breeding season, GnRH release is stimulated bythe increasing estradiol secretion from developing follicles on the ovary, and that stimulationultimately causes the preovulatory surge of GnRH and, in turn, luteinizing hormone (LH) andrelease of the ova from the follicle. In contrast, ewes in anestrus are exquisitely sensitive tothe negative feedback effects of estradiol, such that as small follicles develop on the ovary andsteroid secretion increases, there is a profound suppression of GnRH release and no surge ofLH develops – therefore ovulation is absent (see Fig. 13.2).

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Figure 13.2. A summary model of the impact of photoperiod on reproduction in the ewe, a seasonalbreeder. The melatonin signal resulting from photoperiodic inputs synchronize the expression of theendogenous circannual rhythm of responsiveness to the negative feedback effects of circulating estradiol.During anestrus, naturally longer days emphasize the profound negative feedback response to estradiol,observed in the GnRH neurosecretory system, to limit GnRH and LH release to a low frequency pulsepattern characterized by an inability to mount a preovulatory LH surge. In the breeding season, thisinhibitory impact of estradiol is blunted such that rising concentrations of estradiol from the wavesof follicular development eventually induce a preovulatory surge of GnRH, LH, and ovulation. Theinhibitory inputs to the GnRH neurosecretory system are actively induced and retract on a seasonal basis,as the induction is a thyroxine-dependent process, through extension of an inhibitory dopaminergic inputfrom the A15 region of the hypothalamus. Adapted from Karsch et al. (1984).

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The seasonal change in sensitivity to estradiol results from a shift in neuronal inputs to theGnRH neurosecretory system. A dopaminergic pathway, projecting from the A15 area of thehypothalamus, extends and retracts from synaptic association with GnRH neurons dependingon the season. The neuronal synapses from the A15 neurons inhibit the secretory activity ofGnRH neurons and thus releases GnRH to a level that achieves a preovulatory surge (Goodmanet al., 2000). It is of interest that thyroid hormones are required for the initiation of anestrus,as they act in a permissive manner in the neuronal extension of the inhibitory inputs to theGnRH neurosecretory system (Karsch et al., 1995). Photoperiod is not required for the shiftin responsiveness to estradiol, as seasonal changes occur in ewes kept on a constant long orshort day (Woodfill et al., 1991). That is, an endogenous circannual rhythm of reproductiveactivity drives the cycle of breeding season and anestrus. However, the natural shifts thatoccur in day length do synchronize the timing of the rhythm such that parturition occurs atthe optimal time for neonatal survival. This is characterized by increasing temperatures andgreater food availability for the dam. Chickens, turkeys and ducks are all photoresponsive andphotoperiod management was first exploited to improve productivity in these species (Payneand Simmons, 1934). As with mammals, light exposure affects the onset of reproductiveactivity, i.e., seasonality, and the daily event of ovulation in females. Ovulation is driven bya circadian oscillation in the secretion of FSH and LH, likely driven by a surge of GnRH,which causes ovulation and oviposition to occur (Johnson, 2000). Long days of 12 to 14 hrsof light are used to extend the productive period of egg laying under typical managementconditions.

Of interest the photoperiodic effect in galliforms does not appear to be mediated by pinealsecretion of melatonin. Pinealectomy eliminates circulating melatonin in chickens (Pelham,1975). But, neither pinealectomy nor orbital enucleation of turkeys alter photoperiodic in-fluences on gonadotropin and prolactin secretion (Siopes and El Halawani, 1986). There isevidence that photoreceptors that perceive light signals are deep within the brain rather than theeye or pineal, and those receptors then influence GnRH secretory neurons in the hypothalamus(Kuenzel, 1993).

Light management of chickens varies based on production outcome, be it growth or egglaying (North and Bell, 1990). Because egg size is directly related to the age of the hen, pulletswill often be managed to reach sexual maturity at a later age and larger size to avoid smallegg size in the initial production cycle. Light exposure is reduced as the pullets are growingto delay sexual maturity (North and Bell, 1990). This does not affect overall productivity ofthe hen, but small eggs are more difficult to market, although they are no different from anutritional perspective.

Consistent with other farm species, the most noticeable impact of light on equine perfor-mance is linked to reproduction, and there are effects on puberty and the breeding season. Theprimary effect on puberty appears to be a negative impact of long days to ensure that foals donot reach reproductive competence to foal at a time of year less compatible with survival ofthe foal (Wesson and Ginther, 1982). Because horses have a gestation length of 11 months, thetiming of reproductive events dictates a long-day breeding linkage to ensure that foaling occursat the time most advantageous to survival. Light manipulation for mares involves an increasein day length during the autumn to induce early reproductive activity, and this treatment resultsin a breeding season of extended duration, as horses begin to ovulate sooner under a long-dayphotoperiod in autumn than under natural day length (Sharp et al., 1975). Subsequent studiesindicated that the earlier resumption of reproductive activity did not reduce the length of thebreeding season, providing evidence that long days drive the onset of ovulatory cycles andshort days result in anestrus (Kooistra and Ginther, 1975).

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While an effect of season on reproductive performance is apparent in swine, and pho-toperiodic mechanisms are likely at play, the impact on overall reproduction is less than thatof other domestic species (Peltoniemi and Virolainen, 2006). The effects of photoperiod onpuberty attainment vary with some studies indicating a positive effect of short days (Dufourand Bernard, 1968), whereas others report an advantage of long days (Ntunde et al., 1979). Itis likely that other environmental factors such as the season in which the study commenced,nutritional regime, and the effects of other management tactics may affect the response tophotoperiod in animals to a greater extent than photoperiod.

Effects on Growth

Despite the fact that cattle are not typically considered to be seasonal breeders, the effects ofphotoperiod on other physiological endpoints are profound. Growth, especially in prepubertalcalves, is stimulated by exposure to long days and the composition of that increase in massfavors leaner components and height relative to short days (Osborne et al., 2007; Petiticlercet al., 1983; Rius et al., 2005). This growth improvement persists until calving even when pho-toperiod conditions are not controlled after puberty, with calves that developed during longdays calving in at heavier bodyweights and taller stature. Long days also increase mammaryparenchymal growth (Petitclerc et al., 1984; 1985), which appears to improve the lactationperformance in the first lactation (Rius and Dahl, 2006). Exposure to long days also hastensthe onset of puberty relative to shorter days (Hansen et al., 1983; Petiticlerc et al., 1983), butthe effects on growth appear to be independent of the earlier achievement of reproductivecompetency. In general, exposure to long days is beneficial to calf growth and long-term pro-ductivity. Similar effects of photoperiod on lean body growth are observed in other ruminantsincluding lambs and kids.

Photoperiod manipulation can accelerate growth in poultry, and this is exploited to improvethe efficiency of gain in birds destined for slaughter. In contrast to mammals, however, a strictlong day is not associated with the optimal growth pattern in chickens (Classen et al., 1991).Broiler chicks reared under an increasing photoperiod from 6 to 23 hours between day 4 and35 of life had improved performance relative to controls raised under a constant 23 hours oflight. This observation is consistent with earlier work that lean body growth was depressed atan almost constant photoperiod of 23L:1D (Robbins et al., 1984). Turkey growth efficiencyshows a similar advantage of intermittent lighting versus a near continuous photoperiod (Nollet al., 1991).

Like reproduction, the effect of photoperiod on swine growth is variable and less robust thanin other farmed animals. Young piglets respond positively to a 23L:1D after weaning relativeto an 8L:16D regimen, and, in particular, feed intake and average daily gain benefit from theextended lighting (Bruininx et al., 2002). The impact of the light:dark cycle in older pigs,however, is absent or subtly related to the season of observation (Ntunde et al., 1979; Diekmanand Hoagland, 1983), and thus of less consequence to the overall management of pig growth.

Impact on Lactation

Photoperiod manipulation of mature cows has substantial impact on lactational performance,both during established lactation and during the dry period. During an established lactation,

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Figure 13.3. Summary figure of the effects of long day exposure on milk yield in cows. White barsrepresent cows under 16 to 18 hrs of light/day, whereas black bars represent yields of cows undercommon management of approximately 12 hrs of light/day. The studies represent a range of initialproduction levels, days in milk, and management conditions, as well as a broad geographic base wherethe experiments were conducted.

cows receiving 16 to 18 hours of light each day produce 2 to 3 kg more milk than those on a12 light:12 dark photoperiod regardless of stage of lactation or production level (Dahl et al.,2000; see Fig. 13.3).

The greater yield during long days does not affect milk composition, as protein, lactoseand fat content are all unchanged. During long days, cows will increase dry matter intake toaccommodate the increase in yields, although the increase in milk output precedes the milkresponse. Similar increases in milk yield have been observed in sheep and goats exposedto long days when lactating. Evidence points to the aforementioned increase in IGF-1 as anendocrine mechanism underlying this response (Dahl et al., 1997).

The effect of melatonin has variable impact with regard to lactation in cattle. Feeding ofmelatonin to mimic a short day did not affect milk yield in early or mid-lactation cows (Dahlet al., 2000). However, melatonin implants did accelerate the decline in milk yield in latelactation cows on pasture (Auldist et al., 2007). The discrepancy in responses may be relatedto the relative difference in production and the nutrition of the cows in the two studies. Butresponse variability does suggest that melatonin may explain only a portion of the observedresponses to photoperiod during lactation.

During the dry period, exposure to short days improves immune status and milk yieldin the subsequent lactation relative to long days (Aharoni et el., 2000; Miller et al., 2000;Dahl and Petitclerc, 2003; Velasco et al., 2008). The endocrine basis for responses at themammary gland and circulating leukocytes is related to PRL secretion, wherein the elevatedPRL during long days reduces PRL-R expression in target tissues and depresses responses toPRL (Auchtung et al., 2004, 2005). At the mammary gland, long days during the dry period are

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associated with lower mammary cell uptake of tritiated thymidine, indicating lesser mammarycell division relative to cows on short days (Wall et al., 2005). These observations are furthersupported by examination of seasonal effects on milk yield and disease. Cows with calvingdates in the winter months, (characterized by short days and cooler temperatures), producemore milk relative to herd mates that calve in summer months (characterized by long days andhigher temperatures). Of interest, increases in ambient temperature have a direct relationshipto circulating PRL, similar to the effect of photoperiod. Ultimately the exposure of dry cows toshort days improves their capacity to produce milk and resist pathogens in the next lactation.Figure 13.4 presents a unified model of the effects of photoperiod during the life cycle of thedairy cow.

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Figure 13.4. Summary model of the physiological effects and management outcomes of photoperiodmanipulation of dairy cattle. Exposure to long days promotes lean body and mammary growth andaccelerates puberty in growing heifers relative to short days. These effects are associated with increasesin circulating IGF-1 and prolactin, which are both associated with altered melatonin profiles in cattle.During the latter stages of pregnancy in both pregnant heifers and dry cows, exposure to short days isrecommended to reduce circulating prolactin and increase expression of prolactin receptor at mammary,immune, and hepatic tissues. Cows and heifers exposed to short days during pregnancy subsequentlyproduce more milk than those to long days when dry. Lactating cows should be exposed to long days asthere is an increase in circulating IGF-1 and prolactin, and an increase in milk yield during establishedlactation.

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Other Effects

Hair, feather, and other pelage changes are some of the most readily observed effects ofchanges in photoperiod in birds and mammals. The short hair coat noted during summermonths in cattle and horses results from the increasing day length after the spring equinoxand not from higher temperatures associated with the summer (Kooistra and Ginther, 1975;Dahl et al., 2000). Moulting in birds, characterized by a loss of and subsequent regrowth offeathers is also driven by photoperiod. This is consistent with the adaptation of photoperiodsignaling as a proximate cue to predict the need to alter physiological processes and adapt toenvironmental conditions.

An emerging area of research on the effects of photoperiod has a focus on immune function.Whereas seasonal fluctuations in disease have long been noted, the potential relationship to hostimmune function is a more recent observation. Some of the first evidence that photoperiod mayalter immune status was generated in poultry (Skwarto-Sonta et al., 1983; Kirby and Froman,1991), and observations in seasonal mammals soon followed (Brainard et al., 1987).

The physiological mechanisms that underlie the effect of photoperiod on immune status arerelated in part to changes in PRL secretion rather than other hormones affected by light. Mela-tonin treatment limits mammary epithelial cell loss to neutrophil-induced damage observedin response to pathogen infiltration (Boulanger et al., 2002), but this results from the innateantioxidant action of melatonin rather than an endocrine effect. In contrast, long days induce adepression of immune status in cattle relative to short days (Auchtung et al., 2004), and thereis an inverse relationship between immune status and circulating PRL concentrations in cattle(Auchtung et al., 2003). Further, bovine lymphocytes express PRL-receptors and lymphocytesharvested from calves on short days are more responsive to PRL than those from calves ex-posed to long days (Auchtung and Dahl, 2004). Sows exposed to short days during pregnancyhave improved immune function relative to those pregnant during long days, and piglets fromshort day sows also benefit immunologically, relative to those from long-day sows (Niekampet al., 2006). Therefore, multiple lines of evidence suggest that seasonal cycles of disease maybe related to both pathogen and host physiology, and that photoperiod is an environmentalsignal to adjust immune status in domestic animals.

Summary

Photoperiod is the primary cue used by farm animals to time seasonal events to optimizesuccessful outcomes. Knowledge of the species-specific effects of light manipulation can beused to improve management efficiency, production, and health of farmed animals. This isparticularly apparent in intensive housing systems used for dairy and poultry production.

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Aharoni, Y, A Brosh, and E Ezra. 2000. Short Communication: Prepartum photoperiod effecton milk yield and composition in dairy cows. J Dairy Sci. 83:2779–2781.

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Environmental Physiology of Livestock (Collier/Environmental Physiology of Livestock) || Effects of Photoperiod on Domestic Animals