Processing , Products , and Food Safety 258 Temperature effects on slow and fast

The negative effects of high environmental temperatures (T) on growth and muscle accretion in chickens are often confounded by a reduction in feed consumption. The growth performance of broiler chickens of slow (Red Ranger) and fast (Ross 708) growth strains were assessed when reared under neutral (NT), warm (WT), and pair-fed N (PNT; birds in NT fed the amount of feed consumed by those in WT) conditions to separate the direct (T) and indirect (reduced appetite) effects of WT. A total of 1,008 male chicks were reared in 12 environmentally controlled rooms (4 rooms/ T; 2 pens/ room) to 55 d. Tissue and blood samples were collected from 2 birds/ pen for muscle histology and plasma creatine kinase (CK) levels at 55 d. Six birds per pen (144 total) were processed to determine carcass yields, breast meat quality (color, pH, drip loss), and the incidence and severity of white striping (WS), woody breasts (WB), and necrotic fillets (NF).Slaughter BW differed significantly (P < 0.001) between the slow and fast growth strains and among NT (2,392 vs. 4,642 g), WT (2,148 vs. 3,354 g) and PNT (2,065 vs. 3,669 g), respectively. Carcass yields differed by strain (P < 0.001), with fast growth strain showing significantly higher average myofiber diameter. The proportion of breast fillet and tenders were reduced (P < 0.001) with WT, as compared with NT and PNT. Breast fillet were lighter in color in fast (L* = 56) compared with slow growth (L* = 51.8) strain. The proportion of breast fillets with severe WS, WB and NF was affected (P < 0.001) by strain (fast > slow), as well as rearing T (NT > WT > NPT). Incidence of severe WB (37.5 vs. 12.5%) and NF (33.3 vs. 7.3%) were significantly reduced in PNT as compared with WT trt. Plasma CK levels were highest for fast growth strain and increased from 49 to 55 d of age in NT due to high rate of muscle growth and protein turnover. Negative effects of T on meat yields, quality and breast myopathies were more acute with the fast growth strain in this study.