Interaction of dietary energy and protein on growth performance, carcass characteristics and digestibility in finishing barrows when fed at a constant digestible lysine to metabolizable energy ratio

Abstract The objective of this study was to determine the performance response to changes in dietary fat, protein, and fiber content in finishing barrows while maintaining a constant apparent ileal digestible (AID) Lys to metabolizable energy (ME) ratio (AID Lys:ME). The experiment was conducted in 2 trials, each containing 25 individually penned pigs. In both trials, barrows (initial wt=85.3 kg) were blocked by weight and assigned to one of five experimental diets (0.40, 0.44, 0.48, 0.52, and 0.56% digestible Lys) with a constant AID Lys:ME (1.44 g/MCal). Lysine content was changed by altering the proportions of corn and soybean meal and energy was altered by the addition or removal of cellulose and fat. Diets contained 0.2% titanium dioxide as a digestibility marker. Pigs were fed experimental diets for 28 d, and body weights, feed intakes, and ultrasound measurements of tenth rib backfat (BF) and loin eye area (LEA) were recorded on d 14 and 28. Blood samples were taken on d 28 to determine serum urea nitrogen (SUN), insulin, T 3 , and T 4 . A quadratic effect of increasing dietary Lys was seen on ADG on d 28 showing that the addition of Lys increased gain ( P P 3 increased with increasing Lys ( P 4, or insulin. Ultrasound estimates of BF and LEA tended to increase linearly and quadratically, respectively, as dietary Lys increased ( P P