Assuming that part of Methionine (Met) is converted into Cystine (Cys), but ignoring the rates with which such phenomenon occurs may lead to an excessive supply of Met in poultry diets. Such inconvenient could be easily avoided with the knowledge of the ideal Met:Cys/Total sulfur amino acids (TSAA) ratio and the rates of Met conversion into Cys. Met sources did not affect performance. Met:Cys/TSAA ideal ratio was determined using curvilinear-plateau regression model. Both optimum body weight gain and feed conversion ratio were estimated in 1007 g/day and 1.49, respectively, at 52% Met/TSAA ratio. Feed intake was not affected by Met:Cys/TSAA ratios. In the labelled amino acid assay, the rates with which Met was converted into Cys ranged from 27 to 43% in response to changes in Met:Cys/TSAA ratios, being higher at 56:44. Based on performance outcomes, the minimum concentration of Met relative to Cys in diets for broilers from 14 to 28 d of age based on a TSAA basis, is 52% (52:48 Met:Cys/TSAA). The outcomes from labelled amino acid assay indicate that highest the Met supply in diets, the highest is its conversion into Cys.

Context Genetic improvements in modern strains have led to continuous increments in broiler growth rates, which, as a consequence, have resulted in higher economic returns for broiler producers over the last decades. Aim The present study was conducted to characterise the potential growth of the body and feathers of Cobb 500, Hubbard Flex and Ross 308 male and female broilers, as well as to assess the changes in chemical composition that occur up to 16 weeks of age. Methods Birds were fed isoenergetic diets divided in four phases and formulated to marginally exceed the nutritional requirements of the strains throughout the growing period. They were maintained in a controlled environment so as not to limit growth. A dual energy X-ray absorptiometry (DXA) scanner was used to follow the in vivo body composition of 12 broilers of each strain and sex (total of 72 broilers), and the feather weight and composition was determined in four birds of each strain and sex selected at intervals during the growing period (total of 288 broilers) through comparative slaughter with later chemical analysis. Key results Parameters of Gompertz growth curve to describe the strains were estimated for body and feather weight as well as for the growth of their chemical components. Conclusion Differences in the growth rates between strains were evident, indicating the possible differences in selection methods used by geneticists in the different breeding companies. These genetic parameters would explain part of the variation on broiler´s performance which impacts on the way they should be fed and housed during growth. Implications The accurate description of genetic growth potential is useful information to be associated with factorial models that predict nutritional and feed intake requirements of birds. The main advantage of DXA technology is to decrease the variation of body deposition on the Gompertz model, resulting from the use of the same bird throughout its life. Despite the speed of obtaining chemical values of the body, the method is unsuitable for measuring the growth of feathers, which is also important data to be collected and related to the broiler strains.

Nutritionists have been discussing whether the dietary supplementation of cyst(e)ine is required as a part of the dietary methionine (Met) in the total sulfur amino acid (TSAA) requirement to achieve optimum performance in broilers. Part of Met is converted to cysteine (Cys) to meet the Cys requirement, especially for feather growth. The TSAA requirement has been determined by using graded levels of free Met in the diet, without supplementation of free cyst(e)ine. It has also been argued that the Met to Cys ratio (Met : Cys) changes with age and even with different Met sources. The objective of this study was to evaluate the two sources of Met, while determining the proportion of Met and Cys in total dietary TSAA that optimize the performance of broilers. A performance assay was carried out in a factorial arrangement (5 × 2) using 1080 broilers from 42 to 56 days of age fed diets having different dietary proportions of Met and Cys (44 : 56, 46 : 54, 48 : 52, 50 : 50 or 52 : 48) while maintaining the same dietary TSAA in the diets. Two synthetic Met sources (dl-Met or l-Met) were used for each of the diets with different dietary Met : Cys ratios. Twenty-one broilers of the same age were fed the diets 44 : 56, 48 : 52 and 52 : 48 by supplementing the diet with L-(15N) Met or L-(15N2) Cystine to study the metabolism of TSAA. No differences were observed between Met sources for feed intake, BW gain and feed conversion ratio (FCR; P > 0.05); however, FCR was numerically improved at 50 : 50 Met : Cys. Regarding TSAA utilization, the conversion of Met to Cys increased with increase in Met : Cys ratios, but the concentration of Met intermediates decreased. Broiler chickens responded to different dietary proportions of sulfur amino acids by altering their sulfur amino acid metabolism, and diets containing 50 : 50 Met : Cys is recommended for broilers of age 42 to 56 days.

Stable isotope methods have been used to study protein metabolism in humans; however, there application in dogs has not been frequently explored. The present study compared the methods of precursor (13C-Leucine), end-products (15N-Glycine), and amino acid oxidation (13C-Phenylalanine) to determine the whole-body protein turnover rate in senior dogs. Six dogs (12.7 ± 2.6 years age, 13.6 ± 0.6 kg bodyweight) received a dry food diet for maintenance and were subjected to all the above-mentioned methods in succession. To establish 13C and 15N kinetics, according to different methodologies blood plasma, urine, and expired air were collected using a specifically designed mask. The volume of CO2 was determined using respirometry. The study included four methods viz. 13C-Leucine, 13C-Phenylalanine evaluated with expired air, 13C-Phenylalanine evaluated with urine, and 15N-Glycine, with six dogs (repetitions) per method. Data was subjected to variance analysis and means were compared using the Tukey test (P0.05). However, only 13C-Leucine and 13C-Phenylalanine (expired air) presented an elevated Pearson correlation and concordance. This suggested that caution should be applied while comparing the results with the other methodologies.