Effets de la ractopamine sur le métabolisme de l'énergie du corps et du systéme splanchnique chez les bouvillons Holstein

This study was designed to examine the influence of ractopamine (RAC) on whole-body and splanchnic energy balance. Six growing Holstein steers [body weight (BW) = 402±39.5 kg] surgically fitted with an arterial and portal, hepatic, and mesenteric venous indwelling catheters were used in a repeated measures study. Treatments were a basal diet of alfalf1a cubes fed at approximately 1.5 × maintenance energy requirements (days 1-21) and basal plus RAC (430 mg head d^-1; days 22-42). On day 14 of each period, splanchnic and portal-drained viscera (PDV) energy balances were determined as the product of arterio-venous O₂ difference and blood flow. Blood flow was determined using down-stream dilution of p-aminohippuric acid. Whole-body energy balance was determined on days 15-21 of each period, which included 7 d total excreta collection and 3 d of respiratory gas exchange measurements. Body weight and DM intake were greater (P <0.05) for steers receiving RAC compared with those receiving the control diet; however, no difference was observed in either BW or DMI when expressed on a BW^0.75 basis. Similarly, as a function of BW^0.75, whole-body heat production (691 kJ kg BW ^0.75d^-1; P=0.96) and retained N (0.85 g kg BW ^0.75d^-1; P=0.34) and energy (298 kJ kg BW^0.75d^-1; P=0.71) were unaffcted by RAC. In contrast, RAC tended to decrease (P=0.09) energy use by splanchnic tissue0.s75(191vs. 156 kJ kg BW ^0.75d^-1), largely due to a reduction (P=0.12) in energy use by the PDV (100 vs. 86 kJ kg BW ^0.75d^-1). These data indicate that although whole-body energy use is not affected by RAC, energy use by splanchnic tissues is decreased, thereby increasingenergy use by peripheral tissues.