Optimising energy density in broiler diets using a central composite design : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Animal Science at Massey University, Palmerston North, Manawatu, New Zealand. EMBARGOED until 2nd June 2028

Abstract

A study was conducted to optimise energy density in broiler diets. A 3 x 5 central composite design (CCD) was employed, having three factors (starter, grower, and finisher), each tested at five levels of nitrogen corrected apparent metabolizable energy (AMEn) (2550, 2650, 2800, 2950, and 3050 kcal/kg). AMEn had a significant linear effect on the day 0-14 feed intake (FI) (P < 0.01) and feed conversion ratio (FCR) (P < 0.01), with both parameters decreasing as dietary AMEn increased. Body weight (BW) at day 14 was not affected by AMEn (P > 0.05). Similarly, AMEn did not affect BW at day 28 but resulted in a significant effect on FI and FCR from day 0-28. FI from day 0-28 decreased linearly as AMEn in the grower phase increased (P < 0.01), while FCR linearly decreased as AMEn in both the starter (P < 0.01) and grower (P < 0.01) phases increased, with AMEn in the grower phase having a more pronounced effect. The final (day 35) BW showed a significant quadratic response (P < 0.05) to AMEn in the finisher phase. BW achieved the maximum response at AMEn levels of 2,936, 2,789, and 2,755 kcal/kg in the starter, grower, and finisher phases, respectively. Total FI was affected only by AMEn in the finisher phase, with FI decreasing linearly (P < 0.05) as energy increased. Interestingly, the overall FCR (day 0-35) responded linearly to AMEn in the starter phase (P < 0.05) but was not affected by AMEn in the subsequent phases. The breast and fat pad percentage were affected by AMEn (P < 0.05) while the leg and carcass percentage were unaffected (P > 0.05). Breast yield was maximised at AMEn levels of 3,050, 2,550, and 2,550 kcal/kg in the starter, grower, and finisher phases, respectively. Interestingly, higher AMEn in the starter phase supported better breast yield in this study. Furthermore, an interaction was observed between the starter and finisher phase AMEn on the breast yield (P < 0.05). High fibre diets (low AMEn) did not have any positive effects on GIT development as GIT characteristics were unaffected (P > 0.05). In general, the results provide important insights into the impact of dietary energy on several performance traits. The models allow for response optimisations, and the figures provide important insights into the growth trends and how they are impacted by dietary energy.