Using Nonlinear Growth Models to Fit the Egg Production Curve in Khazak Hen

Introduction: The egg production curve is defined graphically as the relationship between the number of eggs and laying time, which indicates the biological efficiency of a hen and can be effective in the selection and nutritional management of laying hens. Egg production is an essential section of the poultry industry. Appropriate mathematical models accurately represent the production phases of the hen and provide a valuable tool for biological comparisons and interpretations. Also, egg production curves help predict egg production, determine the appropriate age for poultry culling, and economic decisions. Sigmoid growth models are often used to describe size over time in plants, animals, and humans. In laying hens, the shape of the cumulative egg production curve is similar to the growth curve. Therefore, different growth models may be used to model the cumulative egg production curve. Khazak hen is one of the native birds of the Sistan region (Iran), and natural selection has adapted this bird to the conditions of Sistan over the years. The body of this chicken is small, and has low growth and is mainly kept for egg production. Since laying patterns is different in populations. Thus, the use of an appropriate model to describe the specific laying pattern of each population is necessary. Therefore, this study was conducted to investigate growth models to describe the cumulative egg production and weight of eggs and select the best model for the Khazak hen.Materials and Methods: The present study was conducted in the Research Center of Domestic Animals (RCDA), the Research institute of Zabol, Zabol (Iran). Khazak pullets are identified using foot-banded numbers before they start laying. During the experiment, all birds had access to water and feed ad libitum. The egg production was recorded daily for each hen separately. Based on daily records, the weekly egg production of each bird was calculated and then used the calculation of the cumulative egg production. A total of 365 pellet egg production records were used to analyze the production curve from the first to the fortieth week of laying. Five growth models (Gompertz, Logistics, Richards, Lopez, and Weibull) were fitted on cumulative egg production and weight records. The goodness of fit criteria, including Akaike information criterion (AIC), mean square error (MSE), Bayesian information criterion (BIC), and adjusted coefficient of determination ( ), were used to compare the growth models and to select the best model. All models were fitted on egg production records using the nlme package in R software, and the parameters of each model were estimated. After fitting the models, the cumulative production values for different ages were predicted by the models and were compared with the actual values over 40 weeks.Results and Discussion: Based on the goodness of fit criteria, the Lopez mod had the highest  value and lowest values of AIC, BIC, and MSE for cumulative egg production. While the Weibull model was the best model than other models to describe cumulative egg weight in terms of the goodness of fit criteria. The Gompertz and Logistic models overestimated initial production and underestimated the final production compared with other models. Estimates of time and production at the inflection points using Lopez and Weibull models were close to actual values of cumulative egg production and weight, respectively. Also, prediction of cumulative egg production and egg weight in different weeks using Lopez and Weibull models was accurately, respectively. In literature, various models were reported as the best model to describe the egg production curve, which indicates that the appropriate model specific to each breed should be used to evaluate its curve. The overestimation and underestimation of initial and final production using Logistic models were reported in other research that was similar to our findings. The important application of egg production models in poultry is to estimate the economic and genetic value by predicting total egg production from some records, which can be a suitable tool for biological comparisons and interpretations.Conclusion: The results of the present study, showed that the Lopez and Weibull models were the best models to describe the cumulative egg production and egg weight based on four good fit criteria, respectively. Therefore, these models can be used to describe the cumulative egg production and egg weight in Khazak hens. The application of these growth models can be useful to nutritional management and breeding programs to improve and change cumulative egg production and egg weight.