The effect of varying dietary nutrient densities on performance: experimental investigations on the response of broiler chickens to different energy and lysine levels.

Masters Degree. University of KwaZulu-Natal, Pietermaritzburg.

The overall objective of the study was to determine the influence of varying dietary apparent metabolizable energy (AMEn) and digestible lysine (dLys) inclusion levels on the overall growth performance of Cobb 500 broilers. Two experiments were conducted, and in each experiment a total of 1440 unsexed day-old Cobb 500 broiler chicks were randomly allocated to one of 48 pens (i.e. 30 chicks per pen) in a completely randomized block design. In experiment 1, the effect of incremental changes in dietary AMEn while maintaining a constant dLys: AMEn ratio was assessed. Experimental diets were formulated to contain eight different dietary AMEn concentrations ranging from 11.30 to 13.05 MJ/kg (+0.25MJ/kg) respectively, while adjusting dLys levels tomaintain a constant dLys: AMEnratio of 0.97. The broilers received a common broiler started diet for a period of 14 days, after which the experimental diets were introduced (Day 14-35 of the trial). Feed intake decreased linearly with incremental changes in dietary AMEn. Dietary AMEn intake and energy efficiency ratio (EER) differed significantly between treatments. Dietary AMEn increased linearly with incremental changes in AMEn, while EER exhibited a quadratic response. No significant differences in body weight gain (BWG) were observed for the entire treatment period (Day 14-35). According to the response curve equation (𝑌 = −58.952𝑥2 + 1473.9 − 6937.9), BWG was optimised at 12.55 MJ/kg. Increasing AMEn while maintaining the dLys: AMEn ratio improved feed conversion ratio (FCR), and significant differences occurred between the lowest and the highest nutrient densities. In experiment 2, the influence of varying dietary dLys levels in isoenergetic diets was evaluated. Experimental diets were formulated to contain eight different dietary dLys levels ranging from 0.95 to 1.30% (+0.05%). Dietary AMEn concentration was kept constant at 12.1 MJ/kg giving dLys: AMEn ratios 0.78, 0.83, 0.87, 0.91, 0.95, 0.99, 1.03 and 1.07. The broilers were fed a common broiler started diet for the first 14days, whereafter the experimental diets were fed from Day 14-35 of Experiment 2. Feed intake and AMEn intake were not affected by the incremental changes in dietary dLys. Energy efficiency ratio, dLys intake and digestible lysine ratio (dLysER) differed significantly between dietary treatments. Digestible Lys intake increased, while dLysER decreased with an increase in dLys inclusion level. Overall, BWG differed significantly between treatment groups, with an increasing quadratic response (𝑌 = −1119𝑥2 + 2871.7𝑥 + 301.11, 𝑃 < 0.05). Feed conversion ratio decreased (improved) linearly with increasing dLys levels. It can be concluded that broilers can sustain optimal growth performance in a range of dietary AMEn concentrations, provided that an ideal balance between dietary AMEn and nutrients is maintained. Furthermore, adjusting the dLys: AMEn ratio when feeding one dietary AMEn concentration for growing and finishing will optimise broiler performance. Keywords: broiler, digestible lysine, dietary energy, apparent metabolizable energy, energy concentration; digestible lysine: energy ratio; performance