Effect of dietary fat and environmental temperature on growth performance in broiler chickens : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Animal Science at Massey University, Manawatu, New Zealand

Abstract

This study investigated the effects of dietary fat level and environmental temperature on growth performance, nutrient utilisation, and survival in Ross 308 broiler chickens. Modern broilers have high growth potential, but their performance can be influenced by nutritional composition and environmental temperature. Dietary fat is commonly used as an energy-dense ingredient and may reduce metabolic heat production because of its lower heat increment (HI) compared with protein and carbohydrate. Therefore, this experiment evaluated whether a high-fat diet could improve broiler performance under an elevated temperature regimen. A 2 × 2 factorial design was used, with two dietary treatments, low-fat and high-fat, and two environmental temperature regimens, normal and elevated. Birds were fed experimental pelleted diets from day 10 to day 34. Both diets were formulated to contain similar targets of apparent metabolisable energy (AME) and similar crude protein levels; however, the measured AME content was numerically different between the low-fat and high-fat diets. During the experimental period, the normal-temperature regimen declined from approximately 25°C on day 10 to 21°C on day 34, whereas the elevated-temperature regimen declined from approximately 30°C on day 10 to 23°C on day 34. Thus, the elevated temperature group remained higher than the normal-temperature group throughout the experimental period, including a 2°C difference at day 34, with an additional cyclic daytime increase during weekdays. Growth performance was assessed through body weight, average daily gain (ADG), daily feed intake, and feed conversion ratio. Apparent metabolisable energy, ileal nutrient digestibility, and survival were also measured. Overall, elevated temperature significantly increased ADG and daily feed intake, but did not affect feed conversion ratio. Dietary fat level did not significantly influence overall ADG, daily feed intake, or feed conversion ratio, although the high-fat diet improved feed conversion ratio during Week 4. A significant Diet × Temperature interaction was observed for overall daily feed intake, but not for ADG or feed conversion ratio. Digestibility results showed that diet had a stronger effect than temperature. The low-fat diet produced a higher AME coefficient and starch digestibility, while the high-fat diet increased protein digestibility and digestible fat content. Survival ranged from 72.22% to 91.67% across the four treatment combinations, with no statistically detectable effects of diet, temperature. The results suggest that the elevated temperature regimen used in this study was moderate and should not be treated as an experimentally-based severe heat stress model. High dietary fat did not improve overall growth performance, although it may support feed efficiency during later growth stages.