Description of the potential for nitrogen retention in pullets by different methodologies: maximum deposition and estimates of intake of methionine+cystine/Descricao do potencial de retencao de nitrogenio em frangas de postura por diferentes metodologias: maxima deposicao e estimativas da ingestao de metionina+cistina

This study aimed to describe the maximum potential of nitrogen deposition and to estimate the intake of methionine+cystine by nitrogen balance and comparative slaughter. Assays were performed in the periodsof 14 to 28, 56 to 70 and 98 to 112 days of age, using 168 Dekalb White pullets, distributed in seven treatments and eight replications. Treatments consisted of protein levels in the diets ranging from 75 to 435 g [kg.sup-1] dry matter in which methionine+cystine was the first limiting amino acid. The variables collected by comparative slaughter were nitrogen intake and deposition and, in nitrogen balance trials were collected nitrogen intake and excretion. With the exponential relationship between nitrogen intake and deposition was determined the maximum nitrogen deposition. The techniques were compared by the test of maximum likelihood ratio. The techniques described differently the maximum potential for deposition by the bird, but were similar in the estimation of methionine+cystine. Based on 60% of the maximum potential the intakes of digestible methionine+cystine were estimated at 163, 243 and 343mg [day.sup.-1] for the period of 14 to 28, 56 to70 and 98 to 112 days of age, respectively. Key words: methionine+cystine, nitrogen balance, comparative slaughter. Objetivou-se descrever o maximo potencial de deposicao de nitrogenio e estimar a ingestao de metionina+cistina pela tecnica do balanco de nitrogenio e abate comparativo. Foram realizados ensaios no periodo de 14 a 28, 56 a 70 e 98 a H2 dias de idade, utilizando 168 frangas Dekalb White, distribuidas em sete tratamentos e oito repeticoes. Os tratamentos consistiram de niveis de proteina na dieta, variando de 75 a 435 g [kg.sup-1] de materia seca, em que a metionina+cistina foi o primeiro aminoacido limitante. As variaveis coletadas pelo abate comparativo foram nitrogenio ingerido e depositado e, nos ensaios de balanco de nitrogenio, coletaram-se ingestao e excrecao de nitrogenio. Por meio da relacao exponencial entre ingestao e deposicao de nitrogenio, determinou-se a maxima deposicao de nitrogenio. As tecnicas foram comparadas pelo teste da razao de maxima verossimilhanca. As tecnicas descrevem de forma diferente o maximo potencial de deposicao pela ave, mas sao similares na estimativa da exigencia de metionina+cistina. Com base em 60% do maximo potencial, as ingestoes de metionina+cistina digestivel foram estimadas em 163, 243 e 343 mg [dia.sup.-1] para os periodo de 14 a 28, 56 a 70 e 98 a 112