Broiler strains available in the poultry industry present different requirements for dietary lysine due to their different growth potentials as a result of their genetic makeup. This study aimed to determine the model parameters for maximum nitrogen retention (NR max T), the nitrogen maintenance requirement (NMR) and the efficiency of lysine utilization (bc , and NMR values were combined in a model to estimate Lys intake by simulating different percentages of the NR max T. The Lys intake estimates were 581, 1,538, and 2,171 mg day -1 for males and 512, 1,340, and 1674 mg day -1 for females during periods I, II, and III, respectively. Due to the flexibility of the model, it is possible to calculate the Lys intake for percentages of NR in the range of practical performance data.

-The objective of this study was to measure the response of Dekalb White laying hens to different intakes of digestible methionine + cystine (met+cys) to optimise their performance. Two hundred eighty-eight Dekalb White laying hens, ranging in age from 33 to 48 weeks, were used in the study. The birds were randomly allocated into eight treatment (levels of met+cys and the control treatment) groups with six replicates of six hens per unit. The experimental diets consisted of seven increasing levels of met+cys (1.37, 2.75, 4.14, 5.51, 6.89, 7.92 and 8.95 g kg −1 ) and were prepared using a dilution technique. A control treatment was used to confirm that the limiting response was due to met+cys intake. Egg production, feed intake, egg weight, egg mass and feed conversion per mass were measured. The data were analysed with repeated measures and regression analyses using Broken Line and Quadratic models, as well as using the combination of both models. The different met+cys intakes influenced the studied variables; all the variables except feed conversion per mass were significantly different between the periods and levels. The digestible met+cys intakes based on the association of the Broken Line and Quadratic models to optimise the birds' response to egg mass are 671 mg/bird d for 33 to 36 weeks, 728 mg/bird d for 37 to 40 weeks, 743 mg/bird d for 41 to 44 weeks, and 770 mg/bird d for 45 to 48 weeks.

The objective of the present study was to describe the growth of reproductive organs and, on the basis of this information, predict feed intake during the pre-laying phase of laying-type pullets and to evaluate the results of the models. Ninety-six ISA-Brown pullets from 15 to 28 weeks of age were used in the first experiment. The weights of the birds with and without feathers, ovaries and oviducts were measured, and samples were taken to analyse dry matter, gross energy and crude protein. Seventy-six ISA-Brown and 76 Hy-Line pullets from 15 to 24 weeks of age were used in the second experiment. Feed intake was measured daily for each hen until the first egg was laid. The energy for maintenance (EEM) was calculated on the basis of the actual protein content and protein weight at maturity. The effective energy (EE) requirement was calculated as EER = EEM + 50 deposition of protein (DP) + 56 deposition of lipids (DL). Feed intake was calculated by dividing the EE requirement by the EE content in the feed. The simulation of feed intake overestimated values of 0.41 g/day (P > 0.05) and 2.65 g/day (P < 0.001) for Hy-Line and ISA-Brown respectively. A significant linear bias was observed for Hy-Line (P < 0.001) but not for ISA-Brown (P > 0.05). The assessment of the results indicated that the models for predicting feed intake were more accurate and less precise for Hy-Line than for ISA-Brown. Thus, there was an agreement between the calculated and measured values for feed intake, which shows that the models provide a true estimation of feed intake during the pre-laying phase.

The objective of this research was to describe the effect of dietary protein content on the uniformity of egg production in ISA-Brown and Hy-Line laying strains. Six dietary protein levels (120–220 g protein/kg feed) were each fed to 16 individually caged hens, per treatment and strain, during the first 6 weeks of the trial from 28 weeks of age. During the second phase, from 35 weeks, only one feed was offered, this containing 175 g protein/kg. Egg production, feed intake, egg weight, egg output and changes in bodyweight were measured. Some birds were sampled before the trial began, after 6- and again after 10-weeks for carcass analysis. Maximum egg output differed between strains but the marginal response to dietary protein was the same in both strains, the coefficients of response being 220 mg protein/g egg output and 9.0 g per kg bodyweight. The coefficient of variation in egg output was low in both strains fed the highest protein feed but increased as the dietary protein level dropped, with the biggest increase occurring in outputs between birds fed 140 and 120 g protein/kg. These increases were particularly marked in the ISA strain, being almost twice as high as those of the Hy-Line strain. Similarly the lowest coefficients of variation in daily food intake were on the highest protein feeds, with a 2- to 3-fold increase on the lowest dietary protein levels, but with both strains in this case showing similar degrees of uniformity. Variation in body lipid content was higher in the ISA strain between dietary treatments. Uniformity in egg output is increased at the highest intakes of dietary protein because the amino acid requirements of an increasing proportion of the population are met by these higher protein contents. As the protein supply becomes marginal and then deficient uniformity is decreased not only because the most demanding individuals cannot consume sufficient to achieve their potential, but also because birds differ in their ability to deposit excess energy as body lipid when attempting to consume sufficient of a feed limiting in protein. This ability to fatten differs not only between individuals within a population but between strains, as shown in the differences between the two strains used in this trial.