Virginiamycin and caloric density effects on live performance, blood serum metabolite concentration, and carcass composition of broilers reared in thermoneutral and cycling ambient temperatures

One experiment utilizing Cobb x Cobb male broilers was conducted to evaluate virginiamycin (VM; 0, 15, 20 ppm) and diet caloric density (CD; 2,945, 3,200 AMEn/kg) effects on broiler live performance, blood serum metabolites, and carcass composition. The starter period exposed birds to recommended brooding conditions, whereas from 3 to 7 wk birds were exposed to thermoneutral (TN, 24 C) or cycling temperature (CT, 24 to 35 C) environments (E). During the 21-d starter period, VM levels and high CD increased (P < 0.05) BW gain (G) and gain:feed (G:F) improved (P < 0.05) with 20 ppm VM and high CD. During 3 to 7 wk, CT reduced (P < 0.05) most live performance and carcass variables as well as heat production (HP) and energetic efficiency whereas energy content per gram of tissue increased. The main effect of VM and CD on blood serum constituents was not significant; however, CT decreased (P < 0.05) serum Na, K, Ca, Mg, Fe, albumin, and total protein. Within CT, G increased (P < 0.05) with high CD and with the 15 ppm VM combination, whereas within TN, G was unaffected by CD, but increased (P < 0.05) with the 20 ppm VM and low CD combination compared with the control. High CD increased (P < 0.05) BW, G, carcass weight, dressing percentage, carcass percentage fat, carcass dry matter, carcass energy content per bird, HP, fat, and protein gain but reduced (P < 0.05) carcass percentage protein and energetic efficiency. Carcass weight, breast yield, fat, and protein gains as well as dry matter carcass energy content increased (P < 0.05) with VM compared with controls. The reduced (P < 0.05) calorie intake and HP with concomitant increase (P < 0.05) in calorie gain with 20 ppm VM increased (P < 0.05) caloric efficiency. In summary, the results suggest that VM improves bird performance by reducing HP and that reduced HP during high CT improves body temperature homeostasis.