Intepretive Summary: Ractopamine and age alter oxygen use and nitrogen metabolism in tissues of beef steers

By: Joan H Eisemann, John A Nienaber, Gerald B Huntington

Ractopamine (RAC) is a feed additive that stimulates rate of gain, feed efficiency, and carcass leanness in finishing cattle. The objective of the present study was to quantify the effects of age and RAC on whole body metabolism and nutrient use by tissues of the portal-drained viscera, liver, and hindquarters of steers. Whole body oxygen consumption and oxygen uptake in hindquarters increased in response to RAC in older but not younger steers. Retained nitrogen was greater for younger than older steers and increased with RAC in both ages of steers. Uptake of α-amino nitrogen by liver decreased with RAC. For hindquarters tissues, uptake of α-amino nitrogen was lesser in older than younger steers and increased in both ages in response to RAC. Ractopamine increased protein synthesis in hindquarters of older steers but not in younger steers. In contrast, in response to RAC, protein breakdown decreased in younger steers and was less in younger than older steers. Ractopamine enhanced lean tissue growth by altering protein metabolism in hindquarters and increasing supply of α-amino nitrogen to peripheral tissues. The response to RAC was greater in older steers.