THE USE OF THE EXCRETION OF NITROGEN-COMPOUNDS AS AN INDIRECT INDEX OF THE ADEQUACY OF DIETARY-PROTEIN IN CHICKENS

An experiment was carried out to study the effect of changes in either the quality or the quantity of dietary protein intake on the excretio n of nitrogen (N) compounds in the chicken. Thirty-two White Rock male broilers (1 day old) were raised in batteries and fed a commercial st arter diet for 9 days. Then they were randomly divided into eight grou ps each of four birds, of similar body weight (mean live weight: 178 ( s.e. 1.9) g), and individually housed in metabolism cages. Following a paired-feeding design based on metabolic body weight (kg M(0.75)), ea ch group of birds was given, for an experimental period of 20 days, ea ch of four levels of protein (60, 120, 180 oi 240 g/kg : 5 days each) in two groups of isoenergetic (14.5 kJ metabolizable energy per g dry matter) and semisynthetic diets based on soya-bean meal, known to be f irst limiting in methionine, either unsupplemented (diets S) or supple mented with 2 g/kg DL-methionine (diets SM). Excreta were collected ev ery 3 days for 48 h, frozen and stored at -20 degrees C. The excreta s amples were subjected to chemical analysis for uric acid by a rapid hi gh-performance liquid chromatographic method, for urea and ammonia by a colorimetric method, and for total N by the Kjeldahl procedure. In g eneral, the excretion of major N compounds was markedly affected by ei ther the quality or the content of dietary protein. Overall, the excre tion of total N, uric acid-N, ammonia-N and urea-N significantly (P < 0.05) decreased with improvement in dietary protein quality and signif icantly (P < 0.05) increased with increase in protein intake. Regressi on equations were obtained relating the excretion of uric acid, urea, ammonia and total N on protein supply. For the partition of N compound s output, the ratios of uric acid-N and ammonia-N to total N significa ntly (P < 0.05) decreased on improving dietary protein quality and inc reased or remained unchanged, respectively, with the increase in dieta ry protein content. The use of the ratio of ammonia-N to total N is re commended as a rapid, easy and accurate indicator of dietary protein a dequacy, as an alternative to measures based on total N balance, witho ut the need for separation of urine and faeces.