Broiler performance and avian uncoupling protein mRNA expression during acute heat stress as influenced by thermal manipulation and dietary fat source

Abstract

Acute heat stress (AHS) in broilers results in reduced performance, increased mortality, and economic loss for producers. The effects of AHS are intensified as birds approach market age, as susceptivity to hyperthermia increases with age and body weight. Embryonic acclimation to high temperatures through thermal manipulation (TM) has been proposed to improve bird thermal tolerance post-hatch and may enhance breast muscle yield. However, high incubation temperatures can be detrimental to embryonic development and subsequent post-hatch growth and it is unclear if the cyclic application of TM is sufficient to reduce these negative effects. Dietary supplementation with unsaturated fatty acids (USFA) has also been demonstrated to improve bird performance and reduced fat deposition under standard rearing conditions, although it is unclear if these benefits remain during AHS. Both TM and USFA may also contributed to a reduction in oxidative stress, possibly through the upregulation avian uncoupling protein (avUCP). Ross 708 hatching eggs were exposed to a TM protocol of 39.5°C for 12 h daily from incubation days 7 to 16. Standard commercial diets were offered during until 28 d, and evaluation of dietary fat occurred during the finisher period. Dietary treatments consisted of fat sources varying in fatty acid composition and were formulated to include either soya oil, poultry fat, or olive oil at 4.5 % each. At 42 d, AHS was imposed, and a sample of male birds were selected for processing at 49 d. Increased eggshell temperatures were observed in the TM group, resulting in an increase in embryo mortality and decrease in hatchability. Although chick weight and yolk-free body mass were similar at hatch, TM birds exhibited decreased feed intake and body weight for most of the rearing period. Livability was approximately double for TM birds following AHS, although it is unclear if this was due to TM adaptation or smaller BW. Carcass and portion weights were decreased for TM birds, as was the occurrence of breast muscle myopathies; however, as this advantage may be related to the reduced growth demonstrated by the TM birds. Dietary fat source was not shown to significantly alter broiler performance or carcass characteristics. An interaction between the treatments was noted following AHS and may suggest modifications to lipid metabolism arising from TM. Neither TM nor dietary fat source influenced avUCP mRNA expression or oxidative stress as measured by concentration of thiobarbituric acid reactive substances (TBARS). Exposure to AHS resulted in the downregulation of avUCP concurrent with increased TBARS, which may support the role of avUCP in free radical regulation. Males demonstrated a higher level of avUCP expression as well as increased TBARS, which may be associated with their increased growth rate and susceptibility to oxidative stress as compared to females. This is the first report of sex differences in avUCP mRNA expression in broilers and future work into heat interventions may benefit from increased focus on male birds.