[Methodical investigations on the metabolism oriented determination of methionine requirement of broiler chickens. 3. Degradation of 14C-(CH3)- and 35S-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 oriented estimation of the methionine requirement. Furthermore it was studied, if this method can give additional informations to weight gain and 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/d; 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 methionine requirement was measured after i.v. injection of 14C-(CH3)- and 35S-methionine by 14CO2-excretion with breath and 35S-excretion with excrements. Both parameters gave similar results and showed an increased methionine degradation at methionine contents exceeding 4.5 g methionine/kg DM. Because there was no increase in methionine degradation up to 4.5 g/kg DM, an increased reutilization of methionine within the range between 3 and 4.5 g methionine/kg DM was supposed, which may act as a spare-mechanism. For practical feeding applications, also in case of low feed intake, a minimal methionine content of 4.5 g/kg DM is recommended.