Effect of ractopamine on whole body and splanchnic energy metabolism in Holstein steers

Abstract

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 alfalfa cubes fed at approximately 1.5&times 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 BW0.75 basis. Similarly, as a function of BW0.75, whole-body heat production (691 kJ kg BW0.75 d-1; P = 0.96) and retained N (0.85 g kg BW0.75 d-1; P = 0.34) and energy (298 kJ kg BW0.75 d-1; P = 0.71) were unaffected by RAC. In contrast, RAC tended to decrease (P = 0.09) energy use by splanchnic tissues (191 vs. 156 kJ kg BW0.75 d-1), largely due to a reduction (P = 0.12) in energy use by the PDV (100 vs. 86 kJ kg BW0.75 d-1). These data indicate that although whole-body energy use is not affected by RAC, energy use by splanchnic tissues is decreased, thereby increasing energy use by peripheral tissues.Key words: Bovine, ractopamine, beta-agonist, energy balance, metabolism