The research objective was to determine the effect of ad libitum and restricted diets including mixture concentrates in which the content of crude proteins and exogenous amino acids had been reduced by 10% on nitrogen parameters in animals’ blood and urine. The study involved 36 growers: (PL × PLW) × Duroc crossbreds assigned into three groups. All the fatteners were fed standard mixture concentrates: grower type (25-70 kg) and finisher (71-110 kg). Group I (control) received standard mixtures with crude protein and exogenous amino acid levels recommended by the Polish Standards [1993]. The content of crude protein and exogenous amino acids in the diets for groups II and III was reduced by 10% throughout both fattening periods. The animals from groups I and II were fed a restricted diet, whereas those from group III had free access to feedstuffs (ad libitum). Blood was taken twice, at the fatteners’ body weight of 55 and 95 kg, and a 6-day urine collection was performed at 55-58 kg and 95-100 kg BW. The nitrogen parameters established in urine and blood were analyzed statistically. The concentration of uric acid was lower by 16.7% and that of urea nitrogen (BUN) by 8.5% in the blood of fatteners from group II, fed mixtures with a lower protein content in the restricted feeding system. The amounts of urea nitrogen (UUN) and ammonia nitrogen excreted in urine decreased by 13.5% and 13.2% respectively. As for the animals receiving a diet with a reduced protein level ad libitum, the rate of urine nitrogen was lower by 8%. A 10% reduction of protein and exogenous amino acids in relation to the Polish Standards (2) in both restricted and ad libitum feeding systems, has indicated the potential for the reduction of urinary nitrogen release to the environment by 13.5 and 8% respectively.
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