The first research work in lighting management for commercial poultry was conducted by Morris in the 1960´s. This work described the lighting management for egg-type layers, a tool that had not been used before. Since then, egg-type layers have lost their seasonal response to daylength (Morris et al., 1995) and do not respond physiologically to light stimuli as other birds do. This change in their physiological behaviour is thought to have been brought about the high selection pressure for high egg numbers per hen (Morris et al., 1995; Lewis et al., 1998). Lighting management in egg-type layers uses increments in daylength to synchronise sexual maturity more than to stimulate the onset of it. Exposure to long daylengths during rearing does not delay the onset of sexual maturity, and a period of exposure to short days is not a requisite to prepare the bird for photostimulation (Morris et al., 1995; Lewis et al., 1998). For decades, it was assumed that broiler breeders would show a similar response to that of an egg-type layer. However, it is well known that rearing meat-type layers in open-sided houses during the off-season, does result in delayed sexual maturity and a lower overall production of setting eggs. This reproductive behaviour provided an indication that the intensive selection for growth-related traits could have had a negative impact on the reproductive physiology of this type of bird. As a consequence, a large body of research has been produced since the late 1990´s investigating the physiological response of broiler breeders to different lighting patters and how this compares to that of egg-type layers. This paper will review the most significant findings of this research and how would these affect the manner in which meat-type breeders should be managed.
Reproductive physiology in broiler breeders
In the past 40 years, broiler breeders have been subjected to great selection pressure for growth and feed conversion traits, which has resulted in a decrease in the age at slaughter of its offspring by one day per year. With some broilers reaching a slaughter weight of 1.8 kg in as short as 33 days, this process has been very successful, but achieved at a cost. Meat-type breeders showed a lower reproductive performance than their egg-type counterparts, which is due to the negative correlation observed between growth and reproductive traits. It is only in the last decade that reproductive traits have been more prominent in genetic selection programmes, especially in female lines. However, the low heritability of quantitative traits gives paramount importance to the environmental management of these flocks to improve performance. Broiler breeder manuals indicate the use of changes in daylength to stimulate and synchronize a rapid onset of egg production and optimise reproductive performance. The lighting management of broiler breeder flocks depends, in many cases, on the facilities available and the time of the year, and location of the rearing farm. Different combinations of lighting treatments are recommended due to different types of facilities being used in the rearing and production periods. These could be controlled environment rearing and laying, open house rearing and breeding, and controlled environment/blackout rearing and open house laying. Flocks commercially reared in light-tight facilities are kept for the first days of life on daylengths of 23 or 24 h. From approximately 2 d of age, daylength is reduced to approximately 8 h and is kept constant until the age at photostimulation, which is normally applied when birds are transferred into the laying houses (Aviagen, 2006; Cobb, 2008). The success of the lighting management in this case depends mostly on the light proofing of the house. When rearing occurs in open houses, and flocks are placed from September to February in the Southern hemisphere (when natural daylengths are long), birds will come into lay later and tend to have a lower peak and less predictable performance throughout lay than the so-called in-season flocks (birds placed from March to August, i.e. winter months). Some management guides provided by breeding companies suggest various changes to the rearing protocols for out-of-season flocks. Some go as far as advising against rearing broiler breeding stock in open houses. This is unhelpful, since many rearing facilities in developing countries are open-sided houses. Therefore, understanding the reasons behind this lack of productivity could assist producers in overcoming this problem.
The late maturation and poor production observed in out-of-season flocks could be related to some vestige of seasonal reproductive behaviour due to photorefractoriness. This latter concept has been observed in mammals, as well as birds (for a review, Nicholls et al., 1988) and has been reviewed by Sharp (1993) in chickens and other birds. Briefly, when birds that are seasonal breeders are reared on long days, they develop a condition that has evolved to prevent such birds from breeding in the year in which they are hatched, and this condition is known as juvenile photorefractoriness (Nicholls et al., 1988; Sharp, 1993). If such birds are reared on short days, juvenile photorefractoriness will not be dissipated, and consequently, sexual maturation will occur earlier than in birds reared on long days. When birds are subjected to short daylengths for long periods of time, juvenile photorefractoriness is completely dissipated; hence egg laying can be stimulated, whereas on long days, some species will take extremely long time to start to lay (Woodard et al., 1980). In seasonal birds, the breeding season is terminated by the development of adult photorefractoriness, which prevents birds from reproducing when the environmental conditions are not propitious for the survival of parents and offspring. The onset of adult photorefractoriness is thought to be programmed at the time of photostimulation or shortly thereafter (Proudman & Siopes, 2002). The development of a refractory state while exposed to long days is indicated by rapid gonadal regression, decreased LH and gonadal steroid secretion that can terminate reproduction (Sharp, 1993; Proudman & Siopes, 2002). Adult photorefractoriness can be dissipated, and birds can recover their reproductive function after transfer from long days to short days at the end of the breeding season (Nicholls et al., 1988). Turkeys are seasonal birds, and their reproduction is controlled by photoperiod, and this is a balance between two physiological states, photosensitivity and photorefractoriness (Proudman & Siopes, 2002). Like broiler breeders, turkey hens have not been subjected to the same rigorous selection for egg production that has been applied to egg-type hybrids, which appear not to exhibit photorefractoriness (Sharp et al., 1992). Sexual maturity of modern egg-type hybrids is not delayed when exposed to constant long-days (Lewis et al., 1998) and the age-related decline in rate of lay (at least to 72 weeks of age) is not reduced by exposure to lighting programmes designed to minimise the effects of photorefractoriness (Morris et al., 1995). Dunn and Sharp (1990) defined the critical and saturation daylength for Dwarf broiler breeders and commercial layers. The critical daylength was defined by as the minimal daylength required to stimulate gonadotrophin secretion in chickens reared on short days; whilst the saturation daylength, was the minimum daylength required to stimulate the maximum release of the same hormones. The results of this study showed that feed restriction (to conventional levels) did not depress photo-induced LH release, meaning that commercial feed restriction programmes did not affect the photoperiodic response of birds of either strain. The critical daylength for Dwarf broiler breeders was 10.5 h, while for commercial layers was between 10.5 and 12.75 h. The saturation daylength was between 10.5 and 12.75 h for dwarf broiler breeders and between 12.75 and 15.25 h for commercial layers. These results suggest that the most effective increases should be between photoperiods of 10.25 and 12.75 h for dwarf broiler breeders. These findings are similar to those described by Sharp et al. (1992), who indicated that an 11 h photoperiod seemed to be sufficiently stimulatory to enhance reproductive function, but not to drive the hens towards the development of adult photorefractoriness as rapidly as when exposed to 20 h. In light of these results, and the fact that genetic progress in meat-type birds virtually changes the available genotypes every 3 to 4 years, several experiments were conducted to determine the applicability of these in modern broiler breeder strains. In addition, the research reviewed in the following sections also addressed a number of research questions not resolved in the past.
To lay or not to lay - that is the question...
Lewis et al. (2003) demonstrated that meat-type layers do exhibit both, juvenile and adult photorefractoriness, which means that incorrect lighting management during the rearing period, will delay sexual maturity and result in a poorer overall performance. Even though there has been some selection for improved reproductive performance in broiler breeder stock, the sexual maturity and egg production data recorded in these trials, and the integrated model for constant photoperiods, strongly indicate that meat-type hybrids still exhibit photorefractoriness. The delayed age at first egg (AFE) for birds reared on constant long days, their subsequent increased egg weight and the minimal effect of an increment in photoperiod below 10 weeks of age suggest that maintaining spring-hatched birds on long photoperiods (to avoid increasing daylengths from infiltrating natural light) in poorly light-proofed housing might not be the right approach, especially as the marked delay in AFE will result in above target body weights at sexual maturity. Whilst evidence for turkeys, partridges and sparrows indicate that low light intensity may be used to eliminate photorefractoriness in birds maintained on long days, the necessary <1 lux illuminance to successfully achieve this would dictate the use of controlled environment housing and, in such circumstances, short days would be the preferred approach. The significant differences in sexual maturity and egg production between the various constant 11- and 16 h groups suggest that further research is needed to investigate the optimum daylength among the intermediate photoperiods. Additionally, the bi-modal distributions for AFE observed in the groups photostimulated at 67 or 124 d, and the evidence reported earlier for the effect of illuminance on the dissipation of photorefractoriness in turkeys, indicates that age at photostimulation and light intensity during rearing are also areas worthy of further investigation.
Feed restriction and body weight control are effective tools to counteract the rious effect of fast growth on egg production. However, these result in a delay in sexual maturity. The need for the birds to achieve a certain body weight and/or body composition in order to start laying, raised the question of whether the manipulation of the body weight profile and lighting programme could overcome the delay in sexual maturity in meat-type breeders. Ciacciariello & Gous (2005) investigated the extent to which the onset of lay could be manipulated and whether this manipulation could be applied to commercial production systems, with the ultimate objective of increasing the number of hatchable eggs per hen. The results of this study showed that sexual maturity can be successfully advanced, but this earlier onset of lay did not translate in a financially viable strategy. The potential for increasing the number of settable eggs by altering the growth curve and lighting programme was ultimately constrained by the dissipation and development of juvenile and adult photorefractoriness respectively. An important result of this study was the lack of significant differences in terms of sexual maturity when birds were photostimulated to 12 or 16 h. Birds on the 12 h treatment produced more eggs, indicating that both photoperiods were equally stimulatory to trigger the onset of lay. In addition, using a maximum photoperiod of 12 h could have reduced the development of adult photorefractoriness and would certainly represent a saving on the electricity costs throughout the breeding cycle.
Due to the nature of these experimental treatments, many trials were needed to generate the necessary information to understand the physiological requirements of these birds and scrutinise the suitability of the current management practices. Lewis & Gous (2006a) investigated whether further increases after the initial increase in photoperiod would not improve egg production by further stimulating gonadotrophin secretion, as suggested by Sharp (1993). Birds were transferred from 8 to either 11 or 16 h at 20 weeks of age, and some were subsequently subjected to further different increments in photoperiod up to 16 h. The results of this study showed that 11 h were less stimulatory than 16h, and that further increments in photoperiod during the laying cycle did not improve egg production. These results refuted the hypothesis formulated by Sharp (1993).
Following from the previous studies, Lewis & Gous (2006b) designed an experiment to investigate which final photoperiod would produce the best performance. Birds were transferred from 8 to 10, 11, 12, 14, 16 and 18 h at 20 weeks. The best results were observed with a final photoperiod of 14 h. However, this was a short trial which concluded at 39 weeks of age. The authors concluded that the optimal final photoperiod should be around the 11 to 12 h for a full breeding cycle. This hypothesis was tested in the report by Lewis et al. (2010). In this experiment birds were transferred from 8 to 11, 12, 13 or 14 h. The results indicated that the best final photoperiod for a 60 week cycle was obtained when birds were transferred from 8 to 13 h.
Are the recommendations of the primary breeders in sync with the bird´sphysiology?
There is a variety of lighting programmes found in management manuals regarding the optimum daylengths during rearing and laying. The current recommendations from two of the largest primary breeders are, for light-proofed houses, 8 h up to 20 weeks of age, and small increases from 11 to 15 h per day at 27 weeks of age (Aviagen, 2006). Cobb (2008) recommends a similar programme, differing only in the maximum daylength being achieved at 23 weeks of age. The literature reviewed so far, clearly indicate that these recommendations might not only be inaccurate, but they could also be detrimental to the performance of the flock. If light-proof houses are available for rearing and breeding, the lighting management of broiler breeders should allow for a long period on 8 h during the rearing period in order to dissipate juvenile photorefractoriness, followed by a single increase in photoperiod from 8 to 13 h at 20 weeks of age. In order to provide more suitable management programmes when light-tight facilities not be available, especially when rearing off-season flocks, the following trials were conducted. Lewis et al. (2005), tested whether the relaxation of growth control could be used to counteract the delay in sexual maturity observed in breeders reared on long days. This paper describes two trials that were conducted to assess the effects of growth rate when rearing broiler breeders on constant 14-h photoperiods, and to quantify the risks of rearing on naturally changing daylengths. In the first trial, two opposing hypotheses were tested, (1) that accelerated growth would counter the delaying effect of rearing on constant long days, and (2) that slower growth would ensure that the body weight at a given rate of egg production would match that recommended for normally reared broiler breeders. In the second trial, birds were reared on accelerated and standard growth curves, and, within each group, were given a lighting regimen that simulated natural daylengths for birds hatched on the shortest day or six weeks after the shortest day, simulated natural daylengths for birds hatched on the longest day of the year, or maintained on 14 h to 20 weeks. Data from both trials confirmed the earlier findings of Gous & Cherry (2004), Payne (1975) and Lewis et al. (2003; 2004) that rearing on constant long photoperiods to 20 weeks delays age at sexual maturity (ASM) compared with conventional broiler breeder lighting. However, it is clear that the extent to which ASM is retarded depends very much on growth rate during the rearing period. The similar ASM, egg numbers, mean egg weights, and feed conversion efficiencies for the increasing- and the decreasing-daylength regimes grown conventionally in Trial 2, but reduced egg numbers and inferior feed utilisation for the birds maintained on 14 h to 20 weeks, indicate that the practice of rearing broiler breeders on constant long days to avoid the effects of precocity in open-sided or non-lightproof housing is both unnecessary and incorrect. Indeed, application of the gross index margin formula showed that maintaining birds on 14 h would result in at least a 6-unit reduction compared with any of the simulated natural-lighting regimens. However, the policy of rearing non-controlled environment broiler breeders on a constant daylength might still be well-founded for birds kept at low latitudes, where the longest anticipated natural daylength is only mildly stimulatory, or when other seasonally changing environmental factors, such as light intensity and temperature, adversely affect sexual maturation. These results were further confirmed in another two trials reported by Lewis & Gous (2007). The conclusion from these two experiments is that, irrespective of breed, season of hatch, or light-tightness of the facilities, broiler breeders should be given a period of short days during the rearing period to expedite the acquisition of photosensitivity, and that the economic consequences of not doing so are such that serious consideration should be given to light-proofing facilities rather than to the use of long days.
Do male breeders show a similar physiological response to lighting stimuli?
Recent research has been conducted to investigate the response of male meat-type breeders to lighting programmes. Although male and female lines of grand-parent and great-grandparent stock have different selection objectives, preliminary work in females indicated that, in spite of the expected poorer performance in the male lines, both are equally affected by the development of photorefractoriness (Ciacciariello & Gous, 2005). Although the males in a broiler breeder flock represent only a small proportion of the population, their contribution to the success of the production cycle is essential. Even though it could be expected that they show a similar response to that observed in females, confirmation of this hypothesis would be ideal, in order to provide the best possible management and ensure maximum performance. Tyler & Gous (2008) subjected meat-type males to constant photoperiods. Day-old males were given 48 h constant illumination and later subjected to 8, 10, 12, 14, 16 or 18 h constant photoperiods. Birds demonstrated juvenile and adult photorefractoriness in this trial, and the results suggested that males and females respond similarly to constant photoperiods. A follow up study was conducted to evaluate the response of male broiler breeders to different ages at photostimulation, through testes development and asymmetry as well as age at first semen production and serum testosterone concentration, and to determine whether flock fertility may be maximised if males are reared on the same lighting programme as females. Where male and female broiler breeders are reared on the same lighting programme, males are likely to attain sexual maturity before females, which is necessary to maximise fertility. All broiler breeder males should respond to photostimulation at 98 d or later, while some respond to earlier photostimulation. Such responders could be selected and utilised in a breeding programme to eliminate seasonality in broiler breeders. Such a genotype would be useful in production systems using open-sided houses, where short daylengths cannot be applied during the rearing period, resulting, at present, in differences in fertility between in-season and out-of-season flocks.
Whilst the physiological response of female meat-type breeders has been studied in detail, there is still a great deal of work to be done with regards to the response of male breeders. The research reviewed in this paper provides novel information on the response of this kind of stock to a powerful management tool. These recommendations need to be tested in commercial conditions to evaluate whether they could be applied to less controlled and varied production situations.
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