This study investigated effects of light intensity and photoperiod on live and processing performance and physiological stress of broilers. One hundred broilers were housed in each of 12 rooms, provided 23L:1D with 3 footcandles (FC) of intensity to 8 d, and then subjected to the following 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 43 d followed by 23L:1D from 43 to 49 d (18L). At 40 d, blood samples were drawn and heterophil:lymphocyte ratios determined. At 49 d, 16 birds from each room were processed to determine weights and yields. There were interaction effects on BW from 29 to 49 d. At 29 d, BW was reduced by either 18L or 0.1FC treatments. At 43 d, BW was greatest in 1FC-23L, reduced in 1FC-18L and 0.1FC-23L, and intermediate in the 0.1FC-18L treatment. At 49 d, BW of 1FC-23L and 0.1FC-18L were similar and greater than those of 1FC-18L and 0.1FC-23L treatments. Feed consumption was reduced by 18L treatment from 15 to 29 d and the 0.1FC treatment at 15 d. Feed conversion and mortality were not affected by treatments. The 0.1FC treatment decreased uniformity at 15 d. Heterophil:lymphocyte ratios averaged about 0.45 and were not affected by treatments. Carcass yield and tender weight were reduced by the 0.1FC treatment, whereas whole breast yield was reduced by the 18L treatment. There were interaction effects on whole breast weight and fillet weight and yield, which were reduced by either the 18L or 0.1FC treatments. These results indicate that although the combination of 18L:6D and 0.1FC may result in broiler live performance comparable to that achieved with 23L:1D and 1FC, and no combination of the photoperiods and intensities tested caused physiological stress, breast meat is generally reduced by either 18L:6D or 0.1FC.

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Skeletal muscle development proceeds from early embryogenesis through marketing age in broiler chickens. While myofiber formation is essentially complete at hatching, myofiber hypertrophy can increase after hatch by assimilation of satellite cell nuclei into myofibers. As the diameter of the myofibers increases, capillary density peripheral to the myofiber is marginalized, limiting oxygen supply and subsequent diffusion into the myofiber, inducing microischemia. The superficial and deep pectoralis muscles constitute 25% of the total body weight in a market-age bird; thus compromise of those muscle groups can have profound economic impact on broiler production. We hypothesized that marginal capillary support relative to the hypertrophic myofibers increases the incidence of microischemia, especially in contemporary high-yield broilers under stressing conditions such as high environmental temperatures. We evaluated the following parameters in four different broiler strains at 39 and 53 days of age when reared under thermoneutral (20 to 25 C) versus hot (30 to 35 C) environmental conditions: capillary density, myofiber density and diameter, and degree of myodegeneration. Our data demonstrate that myofiber diameter significantly increased with age (P > or = 0.0001), while the absolute numbers of capillaries, blood vessels, and myofibers visible in five 400 x microscopic fields decreased (P > or = 0.0001). This is concomitant with marginalization of vascular support in rapidly growing myofibers. The myofiber diameter was significantly lower with hot environmental temperatures (P > or = 0.001); therefore, the absolute number of myofibers visible in five 400X microscopic fields was significantly higher. The incidence and subjective degree of myodegeneration characterized by loss of cross-striations, myocyte hyperrefractility, sarcoplasmic vacuolation, and nuclear pyknosis or loss also increased in hot conditions. Differences among strains were not observed.

This study investigated the effects of different light intensities provided via an increasing photoperiod program on broiler live performance and processing characteristics. A total of 1,080 male broilers were evenly distributed in 12 rooms. Six rooms were subjected to intensities of either 15 footcandles (FC) from 1 to 51 d (Bright), or 0.5 FC from 1 to 9 d and 0.1 FC from 9 to 51 d (Dim). Both intensity treatments were provided in an increasing photoperiod program (23L:1D, 1 to 9 d; 12L:12D, 9 to 16 d; 14L:10D, 16 to 23 d; 17L:7D, 23 to 30 d; 20L:4D, 30 to 37 d; and 23L:1D, 37 to 51 d). Feed consumption and BW were determined, and feed conversions were calculated approximately weekly. Mortalities were necropsied and recorded daily. At 51 d, 30 birds from each room were processed and cut up to determine weights and yields. Beginning at 23 and 30 d, respectively, BW and feed consumption were greater in the Dim treatment. At 51 d, Dim treatment BW was 4.7% greater and feed consumption was 3.9% greater. Feed conversion, metabolic and total mortality, and BW uniformity were not influenced by light intensity. Weights of lean carcass, total breast, fillets, tenders, and legs were from 4.9 to 6.2% greater in the Dim treatment, which was proportional to the BW difference and resulted in similar yields of these parts. However, wings were 9.9% heavier in the Dim treatment, which resulted in greater wing yield. Equal fat pad weights resulted in reduced fat pad yield in the Dim treatment. These results indicate that BW, feed consumption, and most parts weights were increased proportionally by providing 0.1 vs. 15 FC of light intensity via an increasing photoperiod program, and that only the yields of minor parts were affected by intensity.

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