Manipulation of porcine carcass composition by ractopamine.

The effect of dietary ractopamine and protein level on growth performance, individual muscle weight and carcass composition of finishing pigs were evaluated in two experiments. Twelve barrows and 12 gilts (Exp. 1) and 32 barrows (Exp. 2) with an average initial weight of 64 kg were penned individually and offered ractopamine at 0 or 20 ppm in diets containing 13 or 17% CP in 2 x 2 factorial experiments for 28 d. In both experiments, dietary ractopamine improved daily gain (P less than .1) and gain-to-feed ratio (P less than .05) at 17% dietary protein level but depressed these response criteria at 13% protein level. Leaf fat was reduced (P less than .05) and longissimus muscle depth was increased (P less than .1) by feeding ractopamine regardless of dietary CP concentration. Longissimus, psoas major, semitendinosus, biceps and quadriceps femoris (P less than .05) and tensor facia latae (P less than .1) muscles were 8 to 22% heavier with ractopamine feeding at 17% dietary CP level. Results from both trials suggest that ractopamine improves growth rate and carcass leanness at the higher dietary protein level but improves only carcass leanness at the lower protein level.