EFFECTS OF UREA AS A SOURCE OF EXTRA DIETARY NITROGEN ON GROWTH-PERFORMANCE AND CARCASS TRAITS OF FINISHER PIGS

Forty-two crossbred pigs weighing 54.2 +/- 5.5 kg were used to evaluate the value of urea as a source of extra nitrogen (N) to enhance carcass leanness of finisher pigs. Three soybean meal (SBM) diets were formulated to contain 0.60 and 13.2% (Low), 0.73 and 15.5% (Med) or 0.85 and 16.7% (High) lysine and crude protein (CP), respectively. Two urea diets were formulated to be iso-nitrogenous (Iso-N) to the Med and High SBM diets. Two additional urea diets were formulated to be iso-lysinic (Iso-Lys) to the Med and High SBM diets by incorporating crystalline amino acids. The indispensable amino acid contents were at least 20.8 and 41.7% above the current NRC requirements for the Med and High Iso-Lys diets, respectively. To ensure an adequate supply of CP from common feed ingredients for pigs, corn and SBM contributed 13.2% CP to four urea diets, and urea supplied additional CP equivalent. Pigs were housed individually, and allowed ad libitum access to feed. All pigs were slaughtered at an average weight of 103.2 +/- 4.5 kg. Pigs fed the Low SBM diet tended to grow faster (P = 0.09) than those fed other diets (1.09 vs 1.02 kg/day). However, the average backfat (BF; 36.4 vs 32.1 mm; P < 0.05) and 10th rib BF (33.1 vs 28.9 mm; P = 0.10) were higher, and carcass specific gravity (1.046 vs 1.041; P < 0.05) and proportion of lean (47.6 vs 44.7%; P = 0.08) were lower in pigs fed the Low diet than those fed the Med and High diets, indicating that a higher weight gain was achieved by the increased rate of fat accretion. The results indicated that the carcass quality was improved by increasing lysine and(or) N levels from low to medium regardless of the source [SBM or urea and(or) amino acids]. In addition, simply incorporating urea to increase dietary N (Iso-N) from low to medium and amino acid supplementation of urea diets (Iso-Lys) were equally effective in enhancing the carcass quality of pigs, indicating that overall utilization of diets was improved by the inclusion of urea. There was, however, no improvement in the carcass quality of finisher pigs with further increases in dietary lysine and(or) N levels from medium to high.