|Some seasonality. Recent research shows that broiler breeders still exhibit a form of seasonal breeding, and this affects the amount of light breeders should be exposed to at various stages of production.
Prior to 2000 there had been few studies of the broiler breeder’s response to lighting, and recommendations for their lighting were mainly based on practical experience and our knowledge of how to light laying hens.
However, the research at the University of KwaZulu-Natal since that time has shown that broiler breeders cannot simply be treated as if they were merely large laying hens. Indeed, in many ways, they should be treated more like small turkeys. This is because broiler breeders, unlike modern egg-laying strains, still exhibit photorefractoriness, and this demands that lighting recommendations are specifically designed for them. A striking example of the difference in the response to lighting between broiler breeders and egg-laying hens is the divergent effect that age at photostimulation has on the timing of sexual maturity
|Figure 1: Effects of age at transfer from 8 to 16-hour photoperiods on age at 50% egg production for broiler breeders and egg-laying hens.
Originally, all domestic fowl were seasonal breeders and exhibited a condition called photorefractoriness; a natural phenomenon that, in its extreme, prevents an animal from breeding in its first year or when environmental conditions are not conducive to the successful rearing of its offspring. The condition exists in both males and females. Seasonal breeding birds are photorefractory when they hatch (juvenile photorefractoriness); that is they are unable to respond sexually to an otherwise stimulatory photoperiod (synonymous with long day) until they have experienced a period of neutral photoperiods (synonymous with short days), such as those that occur naturally in winter. Sexual development will be markedly retarded and egg production will be significantly reduced if they are reared on long days or are transferred to long days before they have dissipated photorefractoriness. This will be the outcome in precipitately photostimulated birds, even if growth is accelerated to achieve a body weight similar to that normally recommended for photostimulation. In wild birds and full-fed turkeys, photosensitivity generally occurs after about two months of short days. However, because broiler breeders are not fed ad libitum, they require almost five months of short days before they become photosensitive.
After prolonged exposure to a stimulatory daylength, an adult form of photorefractoriness develops and the reproductive system shuts down (adult photorefractoriness). It is thought that the mechanism responsible for terminating breeding is, like that for the initiation of sexual maturation, triggered by the initial transfer to long days. Furthermore, there is evidence to suggest that the longer it takes to dissipate juvenile photorefractoriness the sooner adult photorefractoriness will develop. In a species that exhibits the extreme or absolute form of photorefractoriness, such as partridge or pheasant, the onset of adult photorefractoriness occurs within three to five months of sexual maturation and breeding ceases until the following spring. When a species shows only a relative form of the condition, such as that exhibited by broiler breeders or turkeys, the breeding season is substantially extended, though still markedly shorter than, and inferior to, that of egg-laying hens. There is also quite a wide variation in reproductive state between individuals within a flock, especially in the latter half of the laying cycle; some females have a transitory pause in production and some males temporarily become infertile whilst the remainder of the flock continues to be productive. In contrast, modern egg-laying hybrids, because they no longer exhibit photorefractoriness, show few signs of ceasing production after 12 months in lay, with rates commonly exceeding 80 per cent.
|Figure 2: Egg production of broiler breeders maintained on 16 hours, reared on 8 hours and transferred to 16 hours At 10 weeks whilst still photorefractory, or reared on 8 hours and transferred to 16 hours at 20 weeks after achieving photosensitivity.
Research has shown that broiler breeders reared in lightproof housing on a photoperiod of between six and 10 hours and at a light intensity of about 25 lux (2.5 fc) mature at similar ages when photostimulated at about 20 weeks of age. This is because the slower sexual development expected for birds reared on a shorter photoperiod is countered by the larger, more stimulatory increase in daylength experienced when they are transferred to long days; the converse scenario applies to birds reared on a nine or 10-hour photoperiod.
However, these findings do not mean that daylength during the rearing period is unimportant, but that daylengths of 10 hours or less exert comparable influences because they are neutral photoperiods and, as a consequence, dissipate juvenile photorefractoriness at similar rates. The situation is very different when broiler breeders are reared on daylengths longer than 10 hours: juvenile photorefractoriness takes longer to dissipate, the birds experience a smaller, less stimulatory increase in daylength when photostimulated, sexual development is noticeably delayed, and overall egg production is significantly reduced.
One of the reasons for a bird having superior egg production to its peers is that it has a longer laying cycle (breeding season) and a shorter interval between cycles, and so the intense selection for egg numbers in egg-laying fowl over the past 50 years has virtually eliminated photorefractoriness from modern genotypes of laying hen. In contrast, the less stringent selection for egg numbers in meat-strains of fowl (and turkeys) means that they continue to exhibit photorefractoriness, albeit in a relative form, and to have inferior rates of egg production.
|Figure 3: Egg production of broiler breeders reared on 8 hours and photostimulated at an average body weight of 2.1 kg (4.6 lbs) at either 15 weeks or 20 weeks.
A flock of broiler breeders will not become fully responsive to photostimulation until about 20 weeks of age when reared on eight hour photoperiods in lightproof housing at a light intensity of between 10 and 100 lux (one and 10 fc) and to a growth curve similar to that recommended by Aviagen. If they are transferred to long days much earlier than 20 weeks, before all birds in the flock have dissipated photorefractoriness, the photoresponsive birds will have their sexual development accelerated, whilst those that are not photoresponsive (still photorefractory) will have their sexual development retarded. The result will be a flock that is sexually uneven, has a suboptimal peak rate of lay, and is nutritionally very difficult to manage. Data in Figure 1 for broiler breeders grown on a typical growth curve demonstrate the disastrous effect of photostimulating too soon. Whereas birds transferred to long days at 21 weeks had a five-week advance in maturity, a five week spread in individual ages at first egg, and all birds maturing before un-photostimulated controls held on eight hours, photostimulation at 13 weeks resulted in only a two-week advance in maturity, but, more seriously, a very wide spread of ages at first egg, and a proportion of the flock retarded. When birds were transferred to long days at 10 weeks, less than five per cent of the flock were photoresponsive, individual maturities were even more widely spread, and more than half of the birds had their sexual maturation delayed.
A postponement of photostimulation to 23 weeks or later will produce a flock that, despite having a narrow spread of individual ages at first egg and no delays in sexual development, has an increasingly later sexual maturity (Figure 1) and, as a consequence, a reduction in total egg production and an increase in egg weight. This is because there are close relationships between age at sexual maturity, total eggs, settable eggs and average egg weight to 60 weeks: egg numbers are reduced by two to three eggs (Figure 2) and average egg weight is increased by 0.7 g (Figure 3) for each one-week older age at 50 per cent lay. When sexual maturation is retarded by photostimulating either too soon or too late, more feed is required to get a flock to point-of-lay, which, together with the reduction in the number of eggs, results in a less efficient conversion of feed into egg.
The research at the University of KwaZulu-Natal show that birds should be photostimulated at between 20 and 22 weeks of age.
|Figure 4: Typical egg production profiles for laying hens, broiler breeders, turkeys, and partridge.
The rate at which photorefractoriness is dissipated when birds are reared in lightproof housing at between 10 and 100 lux (one and 10 fc) is proportional to growth and so the minimum age at which a flock of broiler breeders can be successfully photostimulated will depend on its body weight; faster growth permitting earlier stimulation and slower growth necessitating later photostimulation. However, no flock should be transferred to long days before all birds in the flock have become photoresponsive, and that, with typical broiler breeder body-weight targets, is unlikely to be that much earlier than 20 weeks. When a flock’s growth differs significantly from the Aviagen target, the minimum age for photostimulation should be delayed by about four days for each 100 g (3.5 oz) that body weight is below target at 20 weeks, but may be advanced by four days for each 100 g (3.5 oz) that it is above target. Although broiler breeders may respond to photostimulation before 20 weeks, the findings from the KwaZulu-Natal studies have shown that there is no economic advantage from growing birds more quickly than that recommended by Aviagen just to facilitate earlier photostimulation. Although faster growth allows earlier photostimulation, advances sexual development and increases total egg production to a given age, more feed is required to get the flock to point of lay, more abnormally large eggs are produced and more eggs are laid on the floor, so fewer settable eggs are produced and the efficiency with which feed is converted into eggs is reduced. The research work in South Africa has also shown that Aviagen’s recommendation to grow birds to reach average body weight of between 2.0 and 2.2 kg (4.4 and 4.8 lbs) at 20 weeks is optimal, though sexual maturity will be delayed by 2 days for each 100 g (3.5 oz) that a flock is below and advanced by two days for each 100 g (3.5 oz) that it is above target body weight at 20 weeks. The only possible justification for an acceleration of growth beyond that recommended by Aviagen is the need to expedite the dissipation of juvenile photorefractoriness when birds are reared on long days or when spring-hatched birds are exposed to increasing or decreasing natural lighting conditions. The decision is, however, an economic one of higher feed costs versus delayed maturity and fewer eggs.
Photoperiod in lay
The age at which a flock of broiler breeders reaches 50 per cent egg production, when it has been reared on a neutral photoperiod (10 hours or less) and transferred to a longer photoperiod, is highly dependent upon both the photoperiod to which is transferred and the age at which this occurs. The influence of the age at photostimulation has been discussed in an earlier section, where it was concluded that the appropriate time is between 20 and 22 weeks. Assuming that all birds in the flock have become photosensitive by these ages, the rate at which a final photoperiod induces sexual development may be described by a graph called a photoperiodic response curve (Figure 4). It can be seen that photoperiods can be divided into two distinct classes according to their ability to induce sexual development: neutral photoperiods, also called short days, which have minimal influence over sexual maturation, and stimulatory photoperiods, which significantly accelerate it. The shortest photoperiod that is able to significantly initiate sexual development is termed the critical daylength, and that which maximizes it, the saturation daylength. In practical terms, these are generally accepted as being 11 hours and 13 hours respectively for birds reared on eight hours and photostimulated at about 20 weeks. Photoperiods longer than the critical, but shorter than the saturation, daylength accelerate sexual development, but not maximally, and are considered to be only mildly stimulatory. The data in Figure 5 show that in addition to the age at 50 per cent egg production being influenced, the photoperiod to which the birds are transferred also determines the rate at which daily egg production increases to peak; more-stimulatory photoperiods induce steeper increases because sexual development within the group is more uniform.
In the section on photorefractoriness we noted that prolonged exposure to long days results in the development of an adult form of the refractory condition. However, if broiler breeders are transferred to a photoperiod that is shorter than the saturation daylength, that is mildly stimulatory, there will be a delay in the onset of the condition and a consequential improvement in the persistency of lay; and so broiler breeders transferred to 11 or 12 hours at 20 weeks generally have superior rates of lay to others transferred to 16 hours, resulting in the production of five to 10 more eggs to 60 weeks. It has been suggested that there may be merit in making the initial transfer to a mildly stimulatory photoperiod and delaying further increases until later in the laying cycle to counteract the reduction in photoperiodic drive. Unfortunately, not only does this not work, irrespective off when the increases are given or their size and frequency, but it has the opposite effect in that it accelerates the decline in rate of lay, most likely by hastening the onset of adult photorefractoriness. Although the research in South Africa supports the Aviagen recommendation to transfer from 8 hours to 11 or 12 hours at 20 or 21 weeks, depending on a flock’s body-weight uniformity, it neither supports the advice to continue increases to a 15-hour maximum nor the suggestion that further increases to 16 hours may be beneficial when rates of lay are not increasing satisfactorily. This lighting strategy results only in a more rapid decline in egg production because birds on longer daylengths become photorefractory more quickly and use more energy for daily maintenance (each extra hour of daylength uses one per cent more energy and reduces the amount of energy available for production). Longer daylengths are also associated with poorer shell quality, and this might conceivably lead to inferior hatchability.
Broiler breeders maintained on 11 or 12 hours during the laying period have better egg production than birds given a 16-hour day, however, on such short photoperiods, egg-laying will generally begin before the lights come on, and this will increase the likelihood of eggs being laid on the floor.
Notwithstanding that 11 or 12 hours may be suitable for birds kept in cages, 13 or 14 hours may be better recommendations for birds kept on the floor. In production units fitted with automatic
nests the installation of dim lights in the nest boxes set to come on two hours before the main house lights may be a practical solution where early egg-laying is a problem.
Current research at the University of KwaZulu-Natal is looking at the responses to various photoperiods between 11 and 14 hours to identify the most appropriate for use during the laying period.
Abrupt or step-up transfers
Lighting program for broiler breeders invariably involve an initial increase to 11 or 12 hours followed by a series of 30-minute or one-hour increases to reach a 15 or 16-hour maximum. However, such a step-up regimen is more for the benefit of the producer than the bird because research with egg-laying hens, broiler breeders and turkeys have consistently failed to show any benefit from giving a gradual increase compared with a single abrupt increase to the desired daylength. Whereas a single abrupt increment tends to result in a slightly steeper increase in egg production and higher peak rate of lay (indicative of a more sexually uniform flock), and a step-up program usually has marginally better persistency, there will be no significant difference in egg production for the complete laying cycle.
Light intensity (luminance)
Research findings have shown that illuminating broiler breeders with a light intensity (luminance) of at least 15 lux (1.5 fc) at bird-head-height during the rearing period will result in a satisfactory level of performance in lay. However, to optimize the timing of sexual maturation and the rise to peak rate of lay, and to maximize overall egg production, luminance in the laying period does not need to be any brighter than 7 lux (0.7 fc); a threshold very similar to the 5-10 lux (0.5 - 1 fc) concluded for laying hens. Furthermore, as is the case with laying hens, the response to luminance in the laying period is independent of that experienced during the rearing period. This means that it is immaterial whether the light intensity is increased, decreased or held constant when broiler breeders are transferred from the rearing to the laying facilities, provided the laying house luminance is 7 lux (0.7 fc) or brighter. The findings also question the correctness of beliefs that light intensity and daylength for broiler breeders must be increased simultaneously and that it is the combination of increasing daylength and light intensity that stimulates their sexual maturation.
Although the research showed that there is no need for luminance in the laying period to be brighter than seven lux (0.7 fc) to maximize reproductive performance, a South African study has shown that birds illuminated at 25 lux (2.5 fc) lay significantly more eggs on the floor than others given light at 55 or 70 lux (5.5 or fc). This indicates that when broiler breeders are kept on the floor, which is generally the case, light intensity needs to be considerably brighter than the biological threshold of seven lux (0.7 fc) to minimize floor-egg-laying and maximize hatching egg production. Aviagen’s current recommendation to rear at an intensity of between 10 and 20 lux (1 and 2 fc) and to transfer to between 30 and 60 lux (3 and 6 fc) for the laying period therefore seems to be the correct advice for birds housed on the floor. However, the research refutes the suggestion that egg numbers may be improved by further increasing the laying-house luminance to between 100 and 150 lux (10 and 15 fc).
Floor-egg-laying is not a problem in cages and so electricity can be saved by illuminating caged broiler breeders in the laying period at an intensity lower than the 30-60 lux (3-6 fc) recommended for floor birds.
There will, however, still be a need to comply with prevailing welfare regulations. Welfare codes commonly stipulate that luminance should be such that it allows all birds to see one another and for them to be seen clearly, to investigate their surroundings visually, and to show normal levels of activity. It would be prudent, therefore, to adopt the 20 lux (2 fc) minimum luminance advocated for the laying period by some welfare organizations as the recommended figure for lighting broiler breeders in cages.
There are no specific reports of the response of broiler breeders to colored light (wavelength). Some claims made by manufacturers are not endorsed by scientific research. Responses to light from colored LED (light emitting diode) lamps, which have a very narrow spectral emission, have yet to be assessed in broiler breeders. Nevertheless, it must be remembered that white light includes all colors of light, so there is no need to replace white lamps with coloured lamps, particularly if they are replacing incandescent lamps which already emit more than 70 per cent of their light in the red band.
In contrast to us, birds are able to see in the UV-A part of the ultraviolet range of radiation (shorter wavelengths than light). Broiler breeder plumage, though appearing to be uniformly white to us, has varying UV-A reflective markings that allow birds to identify each other as individuals. Although it is known that broiler breeders use this facility in mate selection, its effect on fertility is only speculative. UV-A radiation penetrates very poorly to the part of the brain that controls sexual activity (hypothalamus) and so it is likely that UV acts principally on the retina of the eye to stimulate behavioral responses and is minimally involved in reproductive performance. Data from recently completed research conducted in South Africa supports this suggestion.
Light source (lamp type)
There is no scientific evidence that light source has any consistent effect on any aspect of reproductive performance in broiler breeders.
This research was sponsored by Aviagen Group For a full pdf of this article, click here