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ENVIRONMENT, WELL-BEING, AND BEHAVIOR |
Department of Poultry Science, Auburn University, AL 36849
1 Corresponding author: lienrog{at}auburn.edu
| ABSTRACT |
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Key Words: broiler light intensity photoperiod breast meat yield stress
| INTRODUCTION |
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Awareness of lighting programs has recently increased as governmental (Commission of the European Communities, 2005), poultry industry (National Chicken Council, 2005), and industry customer groups (Food Marketing Institute and National Council of Chain Restaurants, 2003) have established guidelines that define minimum light intensities and amounts and durations of darkness that must be provided to broilers daily. These guidelines have increased interest in the efficacy of restricted photoperiods (16 to 20 h) of different intensities [0.1 to 2 footcandles (FC); 1 FC = 10.76 lx], because all of them restrict the use of continuous or near-continuous photoperiods and the Commission of the European Communities (2005) restricts the use of low intensities (<2.0 FC), which are still commonly used in the United States.
Results of the few studies on the influence of either photoperiod or light intensity on physiological stress responses of chickens are inconsistent, although there is a widely held assumption that long photoperiods and very high or low light intensities compromise their welfare (Gordon, 1994; Buyse et al., 1996b; Bessei, 2005). Heterophil:lymphocyte (H:L) ratios of broilers were observed to be unaffected by photoperiod (Blair et al., 1993). Similarly, H:L ratios of layers were unaffected by photoperiod; however, exposure to 23L:1D increased their duration of tonic immobility (a measure of psychological stress) relative to 14L:10D (Campo and Davila, 2002). Plasma corticosterone levels of broilers were elevated by exposure to continuous as opposed to intermittent photoperiods or higher light intensity in 1 of 2 trials testing each parameter but unaffected in the other (Buckland et al., 1976). Renden et al. (1994a) also observed no effect of photoperiod on broiler corticosterone levels.
Only a small percentage of reports focusing on the influence of photoperiod and light intensity on broiler production continue data collection past slaughter. Renden et al. (1992b, 1993b, 1994b, 1996) observed that relative to those of broilers exposed to 23L:1D, restricted, intermittent, and increasing photoperiod programs can influence the yields of different parts and cuts even though transient BW reductions were overcome by the time of slaughter. Recently, it has also been observed that breast meat yield increased linearly as rearing photoperiod treatments were increased in 3-h increments from 14L:10D to 23L:1D (Schwean-Lardner et al., 2006) and that reducing light intensity from 2 to 0.25 FC resulted in a slightly greater amount of wing at the expense of breast (Downs et al., 2006). The purpose of the present study was to compare effects of relatively low light intensities (1.0 vs. 0.1 FC) and long photoperiods (18L:6D vs. 23L:1D), both within ranges commonly used in commercial production and mentioned in recent guidelines, on the live performance, a physiological stress response, and carcass and breast meat yields of broilers.
| MATERIALS AND METHODS |
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Experimental Treatments
Photoperiod and light intensity were the 2 factors varied by the experimental treatments. All rooms were provided 23L:1D with an intensity of 3 FC until d 8. Three replicate rooms were then subjected to the following photoperiod and intensity treatments in a 2 x 2 factorial arrangement: either 1 FC (1FC) or 0.1 FC (0.1FC) from 8 to 49 d and either 23L:1D from 8 to 49 d (23L) or 18L:6D from 8 to 42 d followed by 23L:1D from 42 to 49 d (18L). It should be noted that the 18L treatment was returned to 23L for the last 6 d before slaughter, so the 18L and 23L treatments only differed in duration from 8 to 43 d. This was done, because it is common industry practice to maximize photo-period for 3 to 7 d before slaughter, and it is provided for by recent guidelines (Food Marketing Institute and National Council of Chain Restaurants, 2003; Commission of the European Communities, 2005; National Chicken Council, 2005). Light intensity treatments differed from 8 to 49 d.
Measurements
Individual BW were determined, and BW uniformity (% within ± 10% of BW mean) was calculated at 8, 15, and 49 d. Pen group BW was determined at 29 and 43 d. Feed consumption by each room was determined at 8, 15, 22, 29, 36, 43, and 49 d. Feed conversion by each room was calculated at 8, 15, 29, 43, and 49 d. Mortalities were recorded daily, necropsied, and the cause of death was classified as either metabolic (ascites, skeletal disorders, or sudden death syndrome) or nonmetabolic (other causes) based on position and posture when found as well as internal and external abnormalities.
At 40 d, a blood sample was drawn from each of 3 birds per pen without regard to sex by venipuncture with heparinized syringes. Immediately, 2 thin smears were prepared from each blood sample on clean microscope slides and allowed to air-dry. Smears were stained with a modified Wrights stain, then cover-slipped, and a total of 100 heterophils and lymphocytes were identified and counted under 100x oil immersion. Other types of leukocytes were not counted or used in calculations. The H:L ratios were calculated by dividing the number of heterophils by the number of lymphocytes.
At 49 d, following a 10-h feed withdrawal period, 8 males and 8 females per room were randomly selected for processing at the Auburn University Poultry Processing Unit and placed in transport coops. Birds were weighed, hung on shackles, and electrically stunned (50 V, 20 mA, 400 Hz). Killing was by a single cut severing the right carotid artery and jugular vein. Following a 95-s bleed-out, birds were subscalded in a 2.44-m-long single pass steam-injected scalder at 56.6°C for 90 s, defeathered in a 1.22-m-long picker for 42 s, eviscerated using an automatic eviscerator, and chilled in a 2°C ice-slush of static tap water for 2 h. Carcasses were then drained, weighed, and deboned to obtain skinless, boneless breast fillet (pectoralis major muscle) and breast tender (pectoralis minor muscle) weights. Carcass yields are expressed as a percentage of live weight, and carcass part yields are expressed as a percentage of chilled carcass weights.
Statistical Analysis
A randomized complete block design with a 2 x 2 factorial arrangement of the light intensities and photoperiods tested was used for this study. Data for group BW, feed consumption, feed conversion, uniformity, mortality, and H:L ratio parameters were analyzed for intensity and photoperiod effects and their interaction in 1 statistical model using PROC GLM (2004 version, SAS Institute Inc., Cary, NC). Individual BW and carcass and parts weights and yields were analyzed for sex effects (nested within rooms) and all possible interactions among sex, intensity, and photoperiod treatments. However, because there were no significant interactions involving sex, data were combined across sex. All data were analyzed independently at each age with rooms serving as experimental units. Percentage data were arc sine-transformed before analysis. Statistical significance was reported at P
0.05.
| RESULTS AND DISCUSSION |
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Uniformity was reduced in the 0.1FC treatment at 15 d (Table 3
). This was due to the lack of an increase in uniformity from 8 to 15 d in the 0.1FC treatment as it did in the 1FC treatment and is normal in broilers of this age. This adverse effect is likely related to the concurrent transient decrease in feed consumption in the 0.1 FC treatment and due to the previously mentioned abrupt change in light intensity. Although uniformity is a serious concern of the industry, only 1 previous report has documented the influence of lighting programs on uniformity, and it also noted few significant effects (Downs et al., 2006).
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Physiological Stress
Percentages of heterophils and lymphocytes and the H:L ratio at 40 d were unaffected by treatments (Table 3
). Relative to ratios reported in response to specific stressors by Gross and Siegel (1983), the average H:L ratio of 0.44 observed in the present study indicates a low level of stress. The H:L ratios of 3 Spanish breeds of laying hens housed in floor pens averaged 0.61 at 40 wk and were unaffected by photoperiods ranging from 14L:10D to 23L:1D (Campo and Davila, 2002). Similarly, H:L ratios averaged 0.43 at 2 wk and 0.60 at 5 wk of age and did not differ in commercial broilers exposed to either 23L:1D or an increasing photoperiod program (Blair et al., 1993). These results are correlated with the observation of Renden et al. (1994a) that photoperiod does not affect plasma corticosterone levels of broilers. However, in 1 of 2 trials, broilers exposed to continuous light had significantly higher plasma corticosterone levels than those on intermittent lighting programs, and in 1 of 2 trials, corticosterone was greater in broilers exposed to a light intensity of 0.75 FC than in those provided 0.1 FC (Buckland et al., 1976). Additionally, Campo and Davila (2002) did observe that tonic immobility durations were elevated in hens exposed to 23L:1D instead of 14L:10D. Therefore, it appears that although H:L ratios in the present study indicated that longer photoperiods and brighter intensities did not induce physiological stress, there is evidence in the literature that different lighting programs may influence either physiological or psychological stress responses.
Processing Performance
The statistical relationship of the treatment liveweights (Table 4
) from the sample of birds processed was slightly different than that for the BW of all birds (Table 1
) in the present trial. Similar to the BW results, the sample liveweights of the 0.1FC-18L and 1FC-23L treatments were greatest. However, the sample liveweight of the 0.1FC-18L treatment was not greater than that of the 1FC-18L treatment, as was the case with BW. Also, the 1FC-18L sample liveweight was greater than that of the 0.1FC-23L treatment, whereas these treatments had similar BW. An interaction effect on chilled carcass weight (Table 4
) resulted in the 0.1FC-23L treatment carcass weight being reduced an average of 78 g relative to the other 3 treatments, which were similar. Chilled carcass yield averaged 0.5% greater in the 23L than in the 18L treatment. An interaction effect resulted in the 1FC-23L treatment having greater whole breast weights (average + 45 g), fillet weights (average + 40 g), and fillet yields (average + 1.1%) than the other 3 treatments, which were similar (Table 4
). The whole breast yield of 23L averaged 1.0% greater than that of the 18L treatment. Tender weights of the 1FC treatment averaged 5 g more than those of the 0.1FC treatment. There were no significant effects on tender yield.
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| ACKNOWLEDGMENTS |
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Received for publication October 12, 2006. Accepted for publication March 23, 2007.
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