N AND P UTILIZATION IN BROILERS FED AD-LIBITUM DIETS REDUCED IN PROTEIN AND GRADUALLY DIFFERING IN PROTEIN - ENERGY RATIOS

In the present study, 20 male Euribrid broiler chicks were fed from da y 10 to day 34 four diets differing in protein and energy content at p rotein:energy ratios (g/MJ MEn) of 18.3; 17.2, 14.4, and 13.1. Feed co nsumption and weight gain were repeatedly recorded. Body retention and excretion of nitrogen (N) as well as phosphorus (P) were determined b y tno different techniques. Applying the comparative carcass technique , at the 10th day of fattening 8 broilers and at the 35th day 20 broil ers were killed by anaesthesia and prepared for analysis with blood bu t without gut contents. The balance technique based on complete excret a collection was carried out from day 27 with day 34. Homogenized samp les of feed, carcasses and excreta were analyzed for their contents of N, P, proximate contents (only feed) and uric acid (only excreta). Co ncerning the whole experimental period, no significant differences in growth performance and feed efficiency ratio between the treatment gro ups were observed. Consequently, in the high-protein diets N intake wa s similarly higher by about 27% as compared with the low protein group s. N retention as determined by comparison of carcass contents average d 1.29 g/d and was similar between the treatment groups. In contrast, the calculated retention from intake and excretion was apparently foun d to be higher by 11% in the high-protein groups. There were several i ndications that this might have been mostly a result of a considerable and different N volatilization before the excreta were frozen. Nevert heless, N contents of the excreta still were sig nificantly higher in the high-protein groups by about 17%, and uric acid-N percentage of to tal-N was higher by about 21%. The effect of the dietary N content wid ely exceeded the effect of the protein:energy ratio since the broilers did not respond to the lower energy concentration by a higher feed co nsumption. Intake, excretion and retention of P were similar in all tr eatment groups with the mean dietary P utilization for body retention accounting for 48% using analysis and for 40% applying the balance tec hnique in the birds of higher average age.