The objective of this study was to estimate genetic and phenotypic correlations of body weight at 6 weeks of age (BW6), as well as final carcass yield, and moisture, protein, fat and ash contents, using data from 3,422 F2 chickens originated from reciprocal cross between a broiler and a layer line. Variance components were estimated by the REML method, using animal models for evaluating random additive genetic and fixed contemporary group (sex, hatch and genetic group) effects. The heritability estimates (h²) for BW6, carcass yield and percentage of carcass moisture were 0.31 ± 0.07, 0.20 ± 0.05 and 0.33 ± 0.07, respectively. The h² for the percentages of protein, fat and ash on a dry matter basis were 0.48 ± 0.09, 0.55 ± 0.10 and 0.36 ± 0.08, respectively. BW6 had a positive genetic correlation with fat percentage in the carcass, but a negative one with protein and ash contents. Carcass yield, thus, appears to have only low genetic association with carcass composition traits. The genetic correlations observed between traits, measured on a dry matter basis, indicated that selection for carcass protein content may favor higher ash content and a lower percentage of carcass fat.

A digestibilidade in vitro da proteína de fontes protéicas tem sido correlacionada com a digestibilidade in vivo. Entretanto, fatores como a origem protéica, enzima usada e sua concentração, pH e processamento têm sido relacionados com a significância da correlação entre as estimativas. Para atuar somente no fator da concentração enzimática, este trabalho teve por objetivo testar a pepsina nas concentrações de 0,2, 0,02, 0,002 e 0,0002%, utilizando o procedimento padrão do AOAC (1995). Duas farinhas de carne e ossos com baixo ou alto teor de proteína (PB) foram usadas para determinar o coeficiente de solubilidade da PB em pepsina e HCl (CSCPPEPH). Centrifugação foi usada para obter o nitrogênio (N) na fase solúvel, em vez da filtração e análise do N no resíduo não filtrado. Foi conduzida uma análise de variância e o modelo não linear assintótico foi ajustado aos dados. A variável CSCPPEPH em diferentes concentrações de pepsina, para as duas farinhas de carne e ossos, mostrou maior discriminação da solubilidade com baixa concentração de pepsina. O nível de 0,0002% de pepsina é melhor preditor da PB solúvel nas farinhas de carne e ossos. Os resultados obtidos permitem assumir que a concentração de 0,2%, definida pelo AOAC, não é correta para o propósito de determinar o intervalo de solubilidade em farinhas de carne e ossos de alta e baixa PB.

In vitro protein digestibility of protein sources has been correlated with in vivo digestibility values. However, factors like protein origin, enzyme used and its concentration, pH and processing have been related with the significance of the correlation between the estimates. To address only the enzyme concentration factor, this paper had the objective of testing pepsin at 0.2, 0.02, 0.002 and 0.0002% using the standard AOAC (1995) procedure. Two meat and bone meals (MBM) with low and high crude protein (CP) content were used to determine the coefficient of solubility of CP in pepsin and HCl (CSCPPEPH). Centrifugation was used to establish the nitrogen (N) in the soluble phase, instead of filtration and analysis of N in the residue. The variance analysis and a non-linear asymptotic model were adjusted. The CSCPPEPH under different pepsin concentrations for the two MBM showed higher solubility discrimination with low pepsin concentration. The level of 0.0002% pepsin is better to predict the CP soluble in MBM. This finding implies the assumption that 0.2% pepsin found in the AOAC is not correct for the purpose of determining the range of solubility in high and low CP content in MBM.