A model to determine methionine + cystine requirements of rearing layers

Amino acid nutrition in poultry has evolved throughout the time. This development has been catalyzed by the use of models. The design and validation of biologic models in its different forms (static, dynamic, empiric, mechanistic, deterministic and stochastic models) have contributed to the integration of new concepts to ease the interpretation of relationships among biologic phenomena. Likewise, new research topics have been suggested.

Mechanistic models are best to explain animal responses. In amino acid nutrition, these models lead us to understand the relationship between maintenance, growth (protein deposition), and amino acid utilization efficiency to estimate amino acid requirements. This way, the theory proposed by Emmans (1981; 1997) becomes accepted by understanding that requirements are determined as a function of the genetic potential of rearing birds. Once the daily amino acid deposition in both the feather-free body and the feathers can be determined by the first derivation of Gompertz growing equation, related with deposition efficiency, the liquid requirement to achieve maximum protein deposition (PD_max) or maximum animal performance, can be estimated. As far as amino acid nutrition is concerned, only protein metabolism must be taken into account to estimate the maintenance requirement which, by rule, removes all the effects of different bodily tissues. This is due to the fact that each genotype has its own maximum protein weight and its own PD_max, and these parameters are not influenced by increased energy intakes, which is not valid for the adipose tissue (Emmans, 1981; Emmans and Kyriazakis, 1997). Based on the above-mentioned information, and due to the scarcity of model-related information, the purpose of this study was to estimate the parameters of a model to predict methionine + cystine requirements based on the protein metabolism of layer replacement pullets, during the rearing phase.

This research was performed in São Paulo State University (UNESP), Jaboticabal Campus, Brazil, to estimate the parameters of a model to predict methionine + cystine requirements, based on protein metabolism in layer pullets, from 1 to 18 weeks of age. The model adopted follows the structure proposed by Martin et al. (1994):

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

EAA_mg/ave/dia = [a . dPC/dt + b . dPP/dt]/k + [_MCORPO_met+cis + _MPENAS_met+cis];

(T.N.: assumed to be in English as follows):

EAA_mg/bird/day = [a . BPd/dt + b . FPd/dt]/k + [_MBODY_met+cys + _MFEATHERS_met+cys]

where: EAA = methionine + cystine requirement (mg/bird/day); a = methionine + cystine content in the feather-free body protein; b = methionine + cystine content in the feather protein; BPd/dt = bodily protein deposition obtained by Gompertz equation first derivative; FPd/dt = feather protein deposition obtained by Gompertz equation first derivative; k = efficiency in methionine + cysteine utilization; _MBODY_met+cys = methionine + cystine requirement for body protein maintenance; _MFEATHERS_met+cys = methionine + cystine requirement for feather protein maintenance. Parameters a and b were obtained in accordance with Emmans (1989).

The rearing parameters of 3 layer genotypes were obtained in previous assays performed in UNESP-Jaboticabal, based on the following Gompertz equation:

(T.N.: as is in the original document in Portuguese):

(T.N.: assumed to be in English as follows):

for the growth of feather-free body and for the feathers. The first derivative of this equation provides a gain (i.e. deposition) where BPd/dt (body) and FPD/dt (feathers) were obtained by BPd/dt and FPd/dt = PPt.B_PP.ln(1/(PP_m/PPt)), were PW_t = protein weight at time t, applied to both feather-free body and feathers; PW_m = protein weight at maturity applied to both feather-free body and feathers; PW₀ = protein weight at time zero or at hatch, applied to both feather-free body and feathers; B_PP = protein maturation rate or relative growth at inflection point, per day, applied to both feather-free body and feathers.

The efficiency of methionine + cystine utilization (k) was obtained in 3 assays using Dekalb White replacement pullets from 1 to 18 weeks of age (starting, early rearing, late rearing). Considering protein gain (body + feathers) and methionine + cystine composition in the body protein (body + feathers) of the birds in all 3 developmental stages, methionine + cystine depositions were obtained. Using a linear regression between deposition (Y) and intake (X) for deposition (methionine + cystine intake - methionine + cystine for maintenance) the amino acid utilization efficiency was estimated (linear coefficient).

Maintenance coefficient had been estimated in previous studies with adult roosters in UNESP, Jaboticabal, where: _MCORPO_met+cis (_MBODY_met+cys) = 87.26 mg methionine + cystine/unit of PP_m^0.73/dia (PW_m^0.73/day). Requirements are expressed based on the concept proposed by Emmans (1987):

 (T.N.: as is in the original document in Portuguese):

_MCORPO_met+cis = Met+Cis_m.PP_m^0.73. PPt/PP_m.

(T.N.: assumed to be in English as follows):

_MBODY_met+cys = Met+Cys_m.PW_m^0.73. PWt/PW_m.

Where Met+Cys_m = methionine + cystine requirement (mg/day); PW_m, PW_t and PW_m = as described above, and the PW_t/PW_m ratio is u or degree of protein maturity, a factor that corrects for the time-dependent changes. The _MPENAS_met+cis (_MFEATHERS_met+cys) requirement was considered equivalent to feather losses. Daily feather loss was suggested to be 0.01*PP_penast*b (0.01*PW_featherst*b)(Emmans, 1989): the equivalent to this amount must be replaced daily; where 0.01 = rate of feather losses from the body in g/day; PW_featherst = feather protein weight at time t; b, has been described above. Based on these parameters, a model was designed to estimate the digestible methionine + cysteine requirements for 3 different genotypes: Hy Line, Hisex and Dekalb in all 3 phases (starting, early rearing, late rearing.)

The model prepared in accordance with  Emmans´ theory (1981; 1997) to predict methionine + cystine requirements based on protein metabolism for layer pullets is shown below:

(T.N.: as is in the original document in Portuguese):

Met+Cis_mg/ave/día = [36 . dPC/dt + 76 . dPP/dt]/0.794 + [87.26 . PP_m^0.73. u + 0.01.PPpenast*76];

(T.N.: assumed to be in English as follows):

Met+Cys_mg/bird/day = [36 . BPd/dt + 76 . FPd/dt]/0.794 + [87.26 . PW_m^0.73. u + 0.01.PWfeatherst*76];

 

where Met+Cys = methionine + cystine requirement (mg/bird/day). Values 36 a 72 correspond to the methionine + cystine content in the feather-free body and in the feathers, respectively. The parameters BPd/dt and FPd/dt are the depositions of protein in the body and in the feathers obtained by first Gompertz equation derivative. The methionine + cystine utilization efficiency determined was 79.4%. The value of 87.26 is the methionine + cystine requirement for body protein maintenance, and 0.01 is birds daily feather loss rate.

The model consists of simple equations that include parameters with a biological meaning, which facilitates understanding birds' protein metabolism (Martin et al., 1994). To use this model it is only necessary to know the body composition of the birds of a given genotype, and the description of their growth.

Based on the parameters shown in Table 1, the growth of 3 layer genotypes was simulated, and the model was applied to estimate their methionine + cystine requirements. The results obtained by the model designed are shown in Table 2. The differences observed among the 3 genotypes correspond to differences in the parameters describing the growth or lean tissue in the birds. These differences may imply a higher or lower protein deposition (PW_m) or development precocity (B_pp). This means that these bird genotypes need to be re-evaluated in order to update these parameters, that are constantly modified due to intense genetic selection. Specifically for the Dekalb White genotype, the Hurwitz and Bornstein´s (1973) and Rostagno et al. (2005) growth models were simulated. Results are shown in Table 2.

The model proposed described the daily digestible methionine + cystine requirement, consistent with previous reports (Hurwitz and Bornstein, 1973; Rostagno et al., 2005).

Differences exist between the models published by Hurwitz and Bornstein (1973) and Rostagno et al. (2005), which can be attributed to the concepts used for the parametric structure of such models. The maintenance used by Hurwitz and Bornstein (1973) considers cell system amino acid demands as a linear function, but metabolism does not increase in a manner proportional to animal body weight. Bonato (2010) showed that the equivalence used by these authors was 107.1 mg/kg in a protein metabolic weight unit of 449 mg, PP_m^0.73.u /day, nearly 5.7 times more amino acid is destined to maintenance, as compared with the coefficient used in the model proposed herein (87.26 mg PP_m^0.73.u /day).

Table 1. Parameters in Gompertz equations for the feather-free body protein weight (BPW) and feather protein weight (FPW) in different layer strains^s

Table 2. Digestible methionine + cystine requirement estimates for rearing layer pullets

Similar to this study, Hurwitz and Bornstein (1973) used the rearing requirements based on the ideal protein concept or the amino acid composition in bird carcass protein. This way, as amino acid deposition increases in the carcass with the time, the requirement increases accordingly.

Using the metabolic weight to determine maintenance requirements is foreseen in the model proposed by Rostagno et al. (2005) and the parameter for growth requirements considers birds body weight gain, which is subjected to variations in its tissue composition (fat/protein ratio) as a function of energy intake (Emmans, 1981; Emmans and Kyriazakis, 1997). On the other hand, the model proposed herein expresses maintenance requirement on a per-requirement unit basis, while for growth it takes into consideration the protein depositions in both the body and the feathers.

Under the conditions used for this comparison, it was seen that up to 6 weeks of age, similar estimates are obtained with all the models, but from the sixth week on, conceptual differences exist among the models. The requirements foreseen by Rostagno et al. (2005) are higher up to week 13 of age, and afterwards the requirements decrease as weight gain also decreases. It is important to mention that, under field conditions, from week 13 on fat gain can play an important role in the requirement estimate, since the bird is in a physiological stage approaching PD_max.

The model proposed herein described in a consistent manner the requirements of digestible methionine + cystine, considering the physiological changes experienced by the bird as age increases. This knowledge contributes to better understand birds' protein metabolism and aids to cut corners in nutritional programs.

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