Digestibility and nitrogen balance in Cinta Senese growing pigs fed different protein levels

The aim of this study was to evaluate the protein digestibility and the N retention of four diets, containing 12, 14, 16 and 18 % of crude protein (CP) in Cinta Senese growing pigs. Bentonite as internal indicator was added to each diet (2% as fed). Eight castrated males, weighing 55 kg of lw on average, were cyclically fed with the four diets according a Latin-square design. The animals were regularly weighed before each experimental cycle to adjust the daily amount of feed, according to their metabolic weight (90 g DM/kg MW). Every cycle consisted in 10 days of diet adaptation in box, two days of adaptation in metabolic cage and three days of trial, when feces and urines were collected. The whole trial lasted a total of 8 weeks. Sampling took place at fixed hours, once a day for urines and twice a day for feces. On feed and feces, the following analysis were carried out: moisture, protein, ether extract, ash, NDF, ADF, ADL and acid insoluble ash (AIA). On urines, nitrogen content was determined. Total tract apparent digestibility (TTAD), balance and efficiency of nitrogen utilization and energy partition of the experimental diets were calculated. Data were analyzed by SAS using the GLM model and considering sampling day and animal as fixed effects and diet and metabolic weight as continuous effects. TTAD of dry matter, organic matter and protein showed a parabolic trend with maximum at 14.5-15.0 % of CP. Concerning the N-related parameters, results showed that the N intake, N adsorbed and total N excreted increased with the dietary CP content. Contrariwise, the biological value (BV) of the diet and the N retained/intake ratio linearly decreased as the CP increased. Indeed, the lowest BV was observed for the 18% CP diet (45.28%), while the highest was showed by 12% CP diet (59.82%). Energy digestibility followed a parabolic trend, while metabolizable energy decreased linearly from 12%CP to 18%CP diets. In conclusion the protein requirement for Cinta Senese growing pig (from 50 to 80 kg lw) can be fulfilled by the 12% CP diet.