METHODICAL INVESTIGATIONS ON THE METABOLI SM ORIENTED DETERMINATION OF METHIONINE REQUIREMENT OF BROILER-CHICKENS .3. DEGRADATION OF C-14-(CH3)- AND S-35 METHIONINE AFTER FEEDING OF SYNTHETIC DIETS

In a chicken experiment was investigated, whether the kind of applied isotope and labelling site influence the results of the metabolism ori ented estimation of the methionine requirement. Furthermore it was stu died, if this method can give additional informations to weight gain a nd N-retention data concerning methionine requirement. 48 male broiler chickens received synthetic diets with a complete amino acid mixture from day 7 posthatching. The methionine content of the 8 experimental diets varied from 2.25 to 7.5 g methionine/kg DM (1.17 to 3.87 g/16 g N). The low weight gain (13 to 22 g/d) and N-retention (0.55 to 0.87 g ld; from day 13 to 17 posthatching) was due to the inferior intake of the synthetic diets. Methionine supplementation increased weight gain and N-retention only up to 3 g methionine/kg DM. The metabolic methion ine requirement was measured after i.v. injection of C-14-(CH3)- and S -35-methionine by (CO2)-C-14-excretion with breath and S-35-excretion with excrements. Both parameters gave similar results and showed an in creased methionine degradation al methionine contents exceeding 4.5 g methionine/kg DM. Because there was no increase in methionine degradat ion up to 4.5 g/kg DM, an increased reutilisation of methionine within the range between 3 and 4.5 g methionine/kg DM was supposed, which ma y act as a spare-mechanismen. For practical feeding applications, also in case of low feed intake, a minimal methionine content of 4.5 g/kg DM is recommended.