CHANGES IN PLASMA UREA CONCENTRATION CAN BE USED TO DETERMINE PROTEIN-REQUIREMENTS OF 2 POPULATIONS OF PIGS WITH DIFFERENT PROTEIN ACCRETION RATES

The experiment had two objectives: 1) to determine the protein require ments of two strains of growing-finishing pigs based on growth perform ance, carcass characteristics, tissue accretion rates, and organ weigh ts and 2) to evaluate whether protein requirements can be determined f rom changes in plasma urea concentration. Forty-six Gene Pool (GP) and 46 Hampshire (H) gilts with an initial BW of 28.5 kg were used. Pigs were allotted to two trials of a randomized complete block experiment with a 2 x 6 factorial arrangement of treatments. Five pigs from each strain were randomly selected and slaughtered at the beginning of each of the two trials. The remaining 72 pigs were individually penned and allotted to one of six dietary treatments (10, 13, 16, 19, 22, or 25% CP). Pigs remained on the experiment until the mean weight of a treat ment group within each strain reached 115 kg (16 wk for GP and 14 wk f or H), at which time all pigs of that strain were slaughtered. The onl y strain x protein level interactions that were detected were for carc ass protein and water accretion rates. Gene Pool pigs grew less rapidl y and utilized feed less efficiently than H pigs (P < .001). Average d aily gain (quadratic, P < .05) and ADG/ADFI (quadratic, P < .05) were increased as protein level increased until a plateau was reached. Back fat depths were decreased (linear, P < .001) and longissimus muscle ar eas were increased (linear, P < .001) as protein level increased. Prot ein accretion rate was lower (P < .01) and fat accretion was higher (P < .01) in GP pigs than in H pigs. Protein accretion increased (quadra tic, P < .001) and fat accretion decreased (linear, P < .001) with inc reasing dietary protein level. Examination of the response of plasma u rea concentration over time suggested that GP pigs required 13% CP fro m 30 to 80 kg and 10% CP thereafter, whereas H pigs required 19% CP fr om 30 to 45 kg, 16% CP from 45 to 100 kg, and 13% CP thereafter.