A model to estimate lysine requirements of laying hens

Lysine is the second limiting amino acid in poultry rations. In the ideal protein concept, lysine is the reference amino acid (Pack, 1995; Silva et al., 2005). Research methods to determine amino acid requirements or their ideal profile include the dose-response method and the factorial method (Sakomura and Rostagno, 2007). The dose-response method determines the requirements based on the performance response of animals fed diets containing growing levels of the test nutrient. The factorial method is based on the principle that the animal requires nutrients to maintain its vital processes, activities, growth, and/or production (Sakomura and Rostagno, 2007). A vast majority of the studies to define amino acid requirements are based on the dose-response method. Nevertheless, this method does not consider differences among genotypes, genders, and production conditions.

The factorial method represents a tool to better understand the energy/protein metabolism of animals. This is important for model-based studies, which search the definition of an adequate system for production purposes. The early factorial models to predict amino acid requirements of layers were developed by Hurwitz et al. (1973). Total amino acid requirements were obtained by adding maintenance, growth, and egg production requirements, and the coefficients expressing each of these fractions were determined independently. The authors concluded that the models yielded estimated values lower than those recommended in the literature, and attributed this to the fact that models consider the inefficiency of birds to transform dietary amino acids into egg protein.

From the economic stand point, and under field production conditions, it is interesting to determine a level leading to optimize the response of a poultry population. In this context, Fisher et al. (1973) applied a model (Reading´s model), that allows for estimating a lysine level based on the response of a population. This model is based on the hypothesis of linear rations between amino acid intake, egg production traits, and maintenance for each bird. In view of the lack of papers and considering the high relevance of models to determine amino acid requirements, the purpose of this study was to develop a model to determine lysine requirements of layers, based on Reading's model.

The experiment was carried out in the Poultry Sector, São Paulo State University (Setor de Avicultura da Universidade Estadual Paulista, UNESP), Jaboticabal Campus, Brazil. Three hundred and eighty four (384) 32-week-old Dekalb White birds were used under a completely-at-random design with 8 treatments (8 lysine levels), and 6 repetitions. The experimental unit included 8 birds. Treatments consisted of seven different lysine levels (0.273; 0.364; 0.455; 0.546; 0.637; 0.818, and 0.909%), plus one additional treatment, to confirm weather the experimental responses were a function of limiting the test amino acid, obtained through the supplementation of 0.091 g/kg, at the level of 0.273%.

Diets were formulated using the dilution technique. A high crude protein (CP) diet was formulated containing 0.909% digestible lysine. This diet was then sequentially diluted with another iso-energetic, protein-free diet, in order to obtain increasing lysine levels.

The experiment lasted for 10 weeks: 6 weeks for bird adaptation and 4 weeks for data collection. Egg production (EP) was quantified daily, and eggs were weighed on 3 days per week. Body weight (BW) of birds was determined on weeks 1, 6, and 10. For this purpose, all birds in two of the repetitions in each treatment were weighed.

The variables used to estimate Reading´s model parameters included lysine intake, egg mass (EM), and BW of birds, and their coefficients of variation (EMCV and BWCV). The coefficient of maintenance was estimated in accordance with Siqueira (2009). Data was analyzed using the EFG software in the amino acids optimize module, as per the following mathematical model: Lysine (mg/day) = a. EM + b. BW, where a = production requirement, and b = maintenance requirement. Dietary lysine utilization efficiency was obtained by the regression of lysine deposition in the eggs from the dietary lysine for egg production, represented by the straight line regression coefficient.

Using the parameters in the model proposed, a simulation was performed to estimate the requirement of individuals within a 1,500 bird population, generated at random. The variation parameters for live weight and egg mass used in the simulation were those normally found under practical conditions. The mean lysine requirement for the population was determined.

Parameters in the model proposed to estimate lysine requirements for layers are shown in Table 1. The maintenance parameter (32.3 mg/kg BW), differs from that proposed by McDonald and Morris (1985) who reported 73 mg/kg BW, and it also differs from that found by Rostagno et al. (2005) i.e., 84.2 mg/kg BW. The production parameter (9.46 mg/g EM) is similar to that reported by Pilbrow and Morris (1974) which is 9.5 mg/g EM, and it is also similar to that reported by McDonald and Morris (1985), who found a value of 9.99 mg/kg BW.

The efficiency of digestible lysine utilization obtained by the model (73.47%) is similar to that reported by Rostagno et al. (2005) which was 72.17% (8.3 mg lys/g egg/11.5 mg de lys/g EM).

Table 1. Maintenance, production, weight gain, and lysine efficiency parameters determined for layers

From the parameters shown in Table 1 the following model was developed to estimate lysine requirements:

(Translator's notice: as is in the original document in Portuguese language):

Lis = 32.3.PV + 9.46.MO + 27.GP

Literally translated as follows:

Lys = 32.3.BW + 9.46.EM + 27.WG

Where Lys = digestible lysine requirement (mg/bird/day; BW = mean body weight; EM = mean egg mass, and WG = body weight gain. Based on the model developed, a simulation was performed using BW, EM, BWCV, and EMCV values to estimate digestible lysine requirements for layers. Table 2 shows the mean results of the requirements in the population of 1,500 birds. In this simulation, the model obtained was compared with other models found in the literature. When the goal is optimizing a population response, the mean value and its standard deviation (SD) must be used. This way, the level estimated to optimize the population response was 0.599% digestible lysine. The model proposed was most similar to that reported by Pilbrow and Morris (1974) (0.676%), while Gous et al. (1987) reported a value of 0.686%. The digestible lysine requirement obtained by the simulation using the model proposed herein was lower than that recommended in the Brazilian Tables for Poultry and Swine (Tabelas Brasileiras de Aves e Suínos, Rostagno et al., 2005) which recommend a value of 0.782%.

Table 2. Digestible lysine requirements for layers, estimated based on the model proposed

The model has the ability to estimate the requirements considering the variability within a layer population. The dietary level estimated to optimize population response was 0.599% digestible lysine.

Gratitude is expressed to FAPESP, for financing this research and for the scholarship granted.

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