The typical female broiler breeder management practice in closed houses is a short day length rearing phase, for instance 8L:16D, followed by an increase in day length, such as 14L:10D at about 20 weeks of age, which broiler breeders reaches an acceptable body weight and frame size (Aviagen, 2016; Cobb, 2016). This photo-stimulation starts a series of physiological alterations, which leads to sexual maturation and reproductive ability. Sharp et al. (1992) found that the decrease in egg production in aged hens was to a degree because of the progress of photo-refractoriness. The photo-refractoriness means lack of reaction to the long-day length, the situation similar to the end of the breeding season in nature (Tyler and Gous, 2011). Photo-refractoriness reduce the hypothalamic gonadotropin-releasing hormone (GnRH) concentration in commercial layer hens (Ubuka et al. 2013). On the other hand, it has also been recommended that in laying phase, the photoperiods should be to increase just enough to balance the progress of photo-refractoriness (Ubuka et al. 2013). Lewis and Gous (2006a) reported that the start of adult photo-refractoriness in broiler breeders exposed to more than 14 h daily light, occurred earlier than in broiler breeders reared under 11-12 h daily light. However, evidences that broiler breeders demonstrate photo-refractoriness (Lewis et al. 2003) and the reports that laying performance in broiler breeder reared on 8-h day length and exposed to either 11 or 12 h of light in laying period was better to that of birds kept under 16 h day length (Robinson et al. 1998; Ciacciariello and Gous, 2005; Lewis, 2006; Lewis and Gous, 2006b; Lewis et al. 2007) have questioned the long photoperiods practice for broiler breeders. Lewis and Gous (2006a) observed a poorer laying performance in broiler breeders reared on 16 h daily light and attributed it to accelerate adult photo-refractoriness because of the stimulatory effect of the photoperiods. The objective of this trial was to find the best photoperiods for photo-stimulated female Ross 308 broiler breeders.
MATERIALS AND METHODS
Birds kept on the floor, on 8-h photoperiods, with illumination from a single 60 W incandescent lamp in each room located 1.8 m above the ﬂoor and producing a mean illuminance of 5 lux for female and 7 lux for male broiler breeders. The stocking densities in rearing period were 7 and 3.5 birds per m2 for females and males, respectively, and 5.5 mixed sex birds per m2 in production phase. Table 1 shows the chemical composition of the diets. The pullets were photo-stimulated at 21 weeks of age by increasing the photoperiods from 8L:16D to 11L:13D. Then the birds were given an additional 60 min of light per week cumulatively, such that in different experimental groups in separated poultry houses the 13L:11D, 14L:10D, 15L:9D and 16L:8D photoperiods were achieved in 23. 24, 25 and 26 weeks of age, respectively. The photoperiods remained unchanged until the experiment was finished at 54 weeks of age. At 23 weeks of age, 600 male and 6000 female birds were allocated to each 16 experimental poultry house (4 replicates for each experimental group) with 5.5 birds per m2. The average light intensity at the egg production period was 70 lux for all the experimental groups. Eggs were collected from 26 to 50 weeks of age six times daily and stored in a cooler until set. Egg production was recorded daily and average weekly egg production percent was calculated for each replicate house. The number of settable eggs, double-yolked eggs, abnormal and large eggs, unsettable cracked eggs, were recorded throughout the laying period and was expressed as a percentage of the total number of producing eggs. The hatchability was determined as the number of chicks hatched per 100 eggs set. The total individual egg production was calculated using the average daily egg production yield in each house multiplied by 175 days egg production period as describe by Lewis et al. (2010). The average weight of 180 eggs per house was used to calculate individual egg weight, weekly and whole experimental period egg mass production. The mortality rate was recorded daily and expressed as the mean mortality percentage per week. The feed consumption per produced egg was calculated from the following formula:
Feed consumption= total feed intake between 26 to 50 weeks of age (g) / total egg produced between 26 to 50 weeks of age
Twenty five birds per pen were randomly selected and weighted at 22, 26, 30, 35, 40, 45 and 50 weeks of age. The body weight gain during egg production period was calculated as the difference in average body weights at 22 and 50 weeks of age. All experimental birds consumed a same diet formulated to meet Ross 308 broiler breeder requirements (Aviagen, 2016) through 50 weeks of age (Table 1). Data were subjected to analysis of variance by GLM procedures of SAS 9.2 (SAS, 1994) using a completely randomized design with 4 photoperiod treatments (13, 14, 15 or 16 h) and 4 replicates poultry house. Significant differences (P<0.05) between means were determined using the Duncan multi range test.
RESULTS AND DISCUSSION
Table 2 shows the effects of photoperiod in laying phase on the performance parameters of broiler breeder hens. Photo schedule had a significant effect on egg production parameters. The egg production and egg mass in birds maintained on 16L:8D photoperiods were less than the 13L:11D, 14L:10D or 15L:9D h photoperiod groups (P<0.05). Ubuka et al. (2013) suggested that a longer photoperiod increase GnRH secretion by stimulating the hypothalamus pathway. This process would result in more gonadotropin serration from the anterior pituitary and consequently an increase in concentration of LH circulation and ovulation. However, the production of more than 2 eggs for 26-50 week production period of the 13, 14 and 15-h birds, compared with the 16-h hens, agreed with the previous report on broiler breeders transferred directly to 12-h at 20 weeks over hens transferred to 16 h (Ciacciariello and Gous, 2005). This also is agreed with the findings of Lewis et al. (2008) who reported that broiler breeders pullets reared on 8 hours and transferred to 13 h stimulator photoperiods, showed an improved sexual maturation for broiler breeder pullets reared on 8 h. The cumulative number of eggs produced in 50 week also supports the report of Lewis and Gous (2006b) who found that broiler breeders reared on 8-h photoperiods do not require more than 14- h in the laying phase to optimize sexual maturity and egg production.
Table 1 The chemical composition of the Ross 308 broiler breeders diets
Ingridients of mineral and vitamin premix in starter and growth (period): vitamin A: 11000 IU; vitamin D: 3500 IU; vitamin E: 100 IU; vitamin B1: 3 mg; vitamin B2: 6 mg; Nicotinic acid: 30 mg; Pantonic acid: 13 mg; vitamin B6: 4 mg; Biotin: 0.2 mg; Folic acid: 1.5 mg; vitamin B12: 0.02 mg; Colin: 1300 mg; K: 3 mg; Cu: 16 mg; Zn: 110 mg; Mn: 120 mg; Se: 0.3 mg and I: 1.25 mg.
Ingridients of mineral and vitamin premix in production (period): vitamin A: 11000 IU; vitamin D: 3500 IU; vitamin E: 100 IU; vitamin B1: 3 mg; vitamin B2: 12 mg; Nicotinic acid: 55 mg; Pantonic acid: 15 mg; vitamin B6: 4 mg; Biotin: 0.25 mg; Folic acid: 2 mg; vitamin B12: 0.03 mg; Colin: 1050 mg; K: 5 mg; Cu: 10 mg; Zn: 110 mg; Mn: 120 mg; Se: 0.3 mg and I: 2 mg.
Table 2 Effects of photoperiod on the performance of broiler Ross-308 breeder
TEP: total egg production for 175 d experimental period; MEP: mean egg weight (g) and TEMP: total egg mass production (kg).
The means within the same column with at least one common letter, do not have significant difference (P>0.05).
SEM: standard error of the means.
The more earlier decrease in egg production by the hens exposed to more than 13 h daily light is indicative of a more rapid start of adult photo-refractoriness, as explained by Lewis et al. (2003) and Lewis (2006). Confirmation of the photo-refractoriness occurrence in broiler breeders has also been reported by Robinson et al. (1998) and Lewis et al. (2007) for hens transferred to 15-h and 16-h day lengths, respectively. Dawson (2001) suggested that it is the preliminary increase in photoperiod that activates the sexual maturation and also the adult photo-refractoriness and the faster transfer to stimulatory photoperiod, the more rapid sexual maturation and adult photo-refractoriness. In this regard, Lewis et al. (2010) found a positive relationship for the hinged regression between the egg numbers and photoperiod. The total hatching egg percentage was significantly lower for 15L:9D and 16L:8D h than for 13L:11D and 14L:10D h birds (P<0.05). The 13L:11D and 14L:10D h hens had signiﬁcantly less double-yolked and broken or abnormal eggs, followed by the 15L:9D h birds, and the 16L:8D h birds (P<0.05). This was a surprising observation as previous study has revealed that the number of eggs laid out-side the nest box and abnormal eggs, reduced with increased day lengths (Lewis and Gous, 2006a; Lewis et al. 2007). The effect of more rapid sexual maturation and body weight on double-yolked egg production was formerly reported in egg type hens by Lewis et al. (1997). However, Lewis and Backhouse (2004) suggested that these unflavored effects are a result of the advance in average oviposition time and higher number of eggs laid before lights-on when hens are exposed to day lengths shorter than 12.25 h, which is out of the photoperiod treatments of current study. In another study, Ciacciariello and Gous (2005) found that each 100 g higher body weight in hens resulted in about two more double-yolked egg production. The relationship between birds body weight and abnormal and double-yolked eggs is also clear in the present study. The mean egg weight 14-L hens were significantly more than for 16-L h birds (P<0.05), with 13 h and 14 h birds were intermediated (P<0.05). The lower average egg weight of broiler breeders shifted to 16 h photoperiods was probably a consequence of their earlier age of sexual maturity and lower body weight at maturity. The negative effect of daily light duration on average egg weight has been reported by other authors who shifted broiler breeders from short to extended day lengths at about 20 weeks (Ciacciariello and Gous, 2005; Lewis et al. 2005; Lewis and Gous, 2006a; Lewis and Gous, 2006b; Lewis et al. 2010) and suggest the positive age effect of sexual maturity on egg weight (Lewis et al. 1994). The percentage of hatchability was higher in 13L:11D h hens followed by the 15L:9D birds, and the 16Lh birds (P<0.05), with 14L:10D h hens intermediated between 13L:11D h and 15L:9D h. The mortality rate for 15L:9D and 16L h birds was significantly more than for 13L:11D h birds (P<0.05), with 14 h intermediate between 13 h and 15 h (P<0.05) (Table 3).
Figure 1 Effect of photoperiod on total egg production and mean egg weight of Ross 308 broiler breeder hens
Table 3 Effects of photoperiod in laying phase on the mortality rate and body weight of broiler breeder hens
FIPPE: feed intake per produced egg (g).
The means within the same column with at least one common letter, do not have significant difference (P>0.05).
SEM: standard error of the means.
This appears contradictory to the lack of the photoperiodic effect on the incidence of mortality in laying period, which previously reported for broiler breeders (Ciacciariello and Gous, 2005; Lewis et al. 2005; Lewis and Gous, 2006a; Lewis et al. 2007; Lewis et al. 2010). In the current study, there were no signiﬁcant differences in the mean body weight of birds at 22 wk of age. However, At 26 weeks, birds given the ﬁnal photoperiod of 13-L and 14-L h had signiﬁcantly heavier mean body weight than groups transferred to 15L:9D or 16L h (P<0.05) (Table 3). At 30 weeks, groups given the ﬁnal photoperiod of 13L:11D h had a signiﬁcantly heavier mean body weight than groups transferred to 14L:10D or 15L:9D h (P<0.05); and 16L h birds were intermediate. At 35 and 40 weeks, the group given a ﬁnal photoperiod of 13L:11D h had a signiﬁcantly heavier mean body weight than groups transferred to 14L:10D or 16L h (P<0.05). The final photoperiod did not affect the weight gain of birds between 22 to 50 weeks of age. These results support the previous reports that hen’s body weight is an important factor of double-yolked egg production (Hocking, 1993). The heavier body weights for broiler breeders exposed to shorter photoperiod during laying phase is in agreement with previous reports of Lewis and Gous (2006a) and Lewis et al. (2010) and to a lesser extent the results of Joseph et al. (2002), who observed heavier body weights between 41 to 48 weeks for broiler breeders kept for 14 h day length than whose given an increased day length up to 18 h. This higher body weight can be explained, in part, by the MacLeod et al. (1988) theory that the maintenance energy requirement of hens is decreased by about 1% for each 1 h decrease in photoperiod, and consequently, with the same daily feed provision and almost comparable egg production, the saved maintenance energy at shorter photoperiods results in increasingly heavier body weight as birds age (Lewis et al. 2010). Figure 1 shows the reducing effect of long photoperiod on total egg production and mean egg weight, with a linear trend being followed over all treatments in 35 to 50 weeks of age. The 16L h birds had signiﬁcantly poorer feed conversion ratio (FCR) than any of the other groups (P<0.05) (Table 3). There is a probable explanation for this observation. All experimental birds were given the equal daily amount of feed; consequently, hens exposed to longer day lengths would be estimated to have a poorer FCR, because the extra daily illumination enhanced birds maintenance need and in the same way decreased the quantity of energy accessible for egg production (MacLeod et al. 1988). This finding does not support the result of Lewis et al. (2010), who found that the FCR was improved with longer photoperiods.
The findings of this study suggest that the optimal photoperiod for Ross 308 broiler breeders in the laying period up to 50 wk of age was about 13 h, and the birds reared in this pattern had consistently better egg production, hatchability and FCR, especially in comparison with birds reared on 16 h photoperiod.
We gratefully acknowledge the support and funding received from the University of Mohaghegh Ardabili.