peer-reviewed articleTo investigate the effect of lysine level in low-protein diets (LPD) an experiment was conducted with 36 barrows fed 174, 140 and 118 g/kg of crude protein (CP), and three digestible lysine levels. Low-protein diets were supplemented with crystalline amino acids (AA) to the same concentration as in the standard CP diet. Growth performance, carcass characteristics and plasma urea nitrogen concentration were evaluated. In nursery pigs, the addition of lysine to LPD quadratically increased backfat thickness (BF; P < 0.01), longissimus muscle area (LMA; P < 0.001), and lean meat percentage (LMP; P < 0.05). In the growing phase, reducing dietary protein decreased average daily gain (ADG; P < 0.09), final body weight (BW; P < 0.09), fat free lean gain (FFLG; P < 0.09), and LMA (P < 0.038); and increased BF (P < 0.05). Addition of lysine to LPD quadratically increased ADG (P < 0.01), average daily feed intake (ADFI; P < 0.05), final BW (P < 0.006), and FFLG (P < 0.002). In the finishing period, pigs fed LPD had lower ADFI (P < 0.03) and feed:gain ratio (FGR; P < 0.003). Addition of lysine to LPD linearly increased BF (P < 0.04), and quadratically increased ADFI (P < 0.02), FGR (P < 0.009), BF (P < 0.08), and LMA (P < 0.08). The plasma urea nitrogen concentration was reduced (P < 0.05) in all phases in pigs fed LPD. Lysine requirement for fattening barrows fed LPD may be higher than the recommended concentration for standard CP diets
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