عنوان مقاله [English]
Introduction: This research was undertaken to evaluate the effects of different levels of dietary fiber and fat on the growth performance of broiler chicks using the central composite design and response surface methodology at 1-7 d and 7-14 d of age. The response surface methodology is a set of statistical and mathematical methods that help the researcher in design of experiment within the incomplete factorial designs. In this method, the obtained data is converted into a mathematical model and the obtained model is optimized to determine the values of the input variables in order to achieve the best output.
Materials and Methods: This study was carried out at the Research Farm, Faculty of Agriculture, Ferdowsi University of Mashhad, Iran. A total of 420 one-day-old Ross 308 male broiler chicks with average weights of 46.90 ± 1.03 g were randomly distributed into 60 battery brooder cages. According to the scheme produced by 3-levels, 3-factors central composite design (CCD), 60 cages of 7 birds each were assigned to 15 experimental diets containing 3 levels of sugar beet pulp (SBP; 0.00, 1.75 and 3.5%), tallow (T; 0.00, 0.50 and 1.00%), and soybean oil (SO; 0.00, 0.50 and 1.00%), from 1 to 7 d and 7 to 14 d of age. Diet samples were analyzed for neutral detergent fiber, acid detergent fiber and insoluble fiber. Soluble fiber was calculated from the difference of total crude fiber from its insoluble fraction. Fatty acids profiles of tallow and soybean oil were determined using gas chromatography. The average daily body weight gain (ADG) was calculated from the weight gain of birds in each cage. Feed conversion ratio (FCR) was corrected for mortality and represented as grams of feed consumed by all birds divided by grams of body weight gain. The experimental data (60 data lines) obtained by CCD were fitted to the second-order polynomial equation by Minitab 2017.
Results and Discussion: The polynomial equation from raw experimental data for ADG (R2 = 0.79; root MSE = 1.65) and FCR (R2 = 0.88; root MSE = 0.14) at 7d of age was generated as follows:
ADG (g/bird) = 27.54 – 1.07 × SBP – 5.57 × T – 1.99 × SO – 0.17 × SBP × SBP + 1.95 × T × T + 1.77 × SO × SO – 0.45 × SBP × T – 0.05 × SBP × SO – 0.71 × T × SO
FCR= 0.87 – 0.03 × SBP + 0.38 × T – 0.08 × SO + 0.05 × SBP × SBP – 0.29 × T × T + 0.13 × SO × SO + 0.15 × SBP × T + 0.04 × SBP × SO + 0.002 × T × SO
The estimated parameters for SBP and T terms in the ADG model, and SBP, T, SO, SBP×SBP and SBP×T terms in the FCR model were significant. In the ADG and FCR models the linear terms had higher contribution to explain existing variation in the response of the chicks. Maximum ADG was observed with diet containing 0.15% SBP, 0.00% T and 0.00% SO and minimum FCR was observed with diet containing 0.07% SBP, 0.00% T and 0.28% SO. The predicted ADG and FCR at the optimal points were 27.54 g/bird per day and 0.96, respectively. The coefficient estimates for ADG and FCR models and the corresponding absolute t-values showed that among the investigating nutrients and their interactions, the linear effect of dietary SBP the largest effect on ADG and FCR of chicks. Lack of fit for both ADG and FCR models was insignificant, showing that the observed data are in good agreement with the model. The polynomial equation from raw experimental data for ADG (R2 = 0.78; root MSE = 3.60) and FCR (R2 = 0.80; root MSE = 0.14) at 14d of age was generated as follows:
ADG (g/bird) = 52.50 – 7.81 × SBP – 26.01 × T + 14.37 × SO + 0.66 × SBP × SBP + 11.22 × T × T – 14.17 × SO × SO + 3.58 × SBP × T + 0.27 × SBP × SO – 3.46 × T × SO
FCR = 1.01 + 0.07 × SBP + 0.35 × T – 0.26 × SO + 0.02 × SBP × SBP – 0.10 × T × T + 0.26 × SO × SO + 0.03 × SBP × T + 0.03 × SBP × SO + 0.14 × T × SO
The estimated parameters for SBP, T, T×T, SO×SO and SBP×T terms in the ADG model, and SBP, T and SO terms in the FCR model were significant. In the ADG and FCR models the linear terms had higher contribution to explain existing variation in the response of the chicks. Maximum ADG and minimum FCR were observed with diet containing 0.30% SBP, 0.00% T and 0.50% SO. The predicted ADG and FCR at the optimal points were 56.65 g/bird per day and 0.95, respectively. The coefficient estimates for ADG and FCR models and the corresponding absolute t-values show that among the investigating nutrients and their interactions, the linear effect of dietary SBP the largest effect on ADG and FCR of chicks. Lack of fit for both ADG and FCR models was significant, showing that a more complicated modeling method or other testing with extra variables should be made.
Conclusion: Current results showed that with increasing age and evolution of the birds’ gastrointestinal tract, the negative effects of soluble fibers were decreased and the broilers will be able to digest and absorb fats more efficiently. Central composite design reduces the number of trials and costs. Response surface model can be used to describe the relationship of nutrients to reach the optimum point.