The avian maternal environment: Its influence on the physiological mechanisms contributing to progeny production efficiency in chicken meat birds

Abstract

Alterations to the maternal and subsequent in-ovo environment during embryonic development, can lead to permanent phenotypic alterations, a phenomenon termed developmental programming. Developmental programming has enormous potential in the chicken meat industry due to meat chickens now spend ~40% of their life in-ovo, the stressors encountered by breeder hens throughout production that can affect the in-ovo environment during egg formation, and the pedigree breeding structure of the industry. Despite evidence suggesting maternal/in-ovo stress can influence progeny body weight (bwt), the physiological mechanisms contributing to performance variation in commercial poultry remain relatively unknown. This thesis therefore aimed to further ascertain performance variations in commercial chicken meat birds in response to alterations to the maternal/in-ovo environment, and to identify the contributing physiological mechanisms influencing these phenotypes. Furthermore, nutritional additives to both breeder and progeny diets were investigated as a potential method to alleviate performance variation in response to altered maternal/in-ovo environments. As reductions in offspring bwt have been repeatedly identified in response to both maternal and in-ovo stress, the first study aimed to investigate contributing physiological mechanisms. Great grandparent lineage meat chicken embryos were exposed to a corticosterone (CORT) or control (CON) solution at embryonic day (ED) 11, hatched and then fed either a control or arginine supplemented diet until 35 days of age. Exposure to in-ovo CORT failed to influence male and female bwt at any stage, whilst dietary arginine supplementation had no effect on bwt or body composition traits. However, in-ovo CORT exposure did affect body composition and led to a decrease in total lean mass, and subsequent increase in total fat mass in exposed female birds. In response to the findings generated in the first study, a second experiment was designed to examine the effects of the maternal environment on offspring body composition traits across both sexes, and whether phenotypes were consistent between genetic lines of chicken meat birds. Breeder hens from two genetic lines, a high performing line, and an underperforming line, were fed either a control or saccharomyces cerevisiae (SC) supplemented diet from 23 weeks of age. Eggs were collected from 32 week old birds, with the subsequent progeny hatched and reared under identical conditions until 42 days of age. The results indicated breed differences in growth traits, as well as sex by breed interactions for body composition traits. The provision of SC appeared to increase plasma CORT concentrations in breeder hens, and subsequently reduced yolk testosterone concentrations, however SC failed to influence any performance measure in both breeder hens and progeny. A third study was developed to identify whether in-ovo exposure to CORT influenced muscle fibre development at hatch through the metabolic actions of the sex steroid hormones within the yolk. This study utilised the in-ovo method from Chapter 3 to expose commercial chicken meat birds to CORT at ED 11. Exposure to In-ovo CORT did not influence steroid hormone abundance levels in the yolk, except for the conjugate etiocholanolone glucuronide, nor did it affect muscle fibre number and muscle fibre cross sectional area at hatch. However, muscle fascicle area occupied by muscle fibres was reduced in CORT treated birds at hatch, potentially indicating increased intramuscular fat content in CORT treated birds. Furthermore the relative mRNA expression of the CCAAT/enhancer-binding protein beta (CEBP/β) was increased in CORT treated birds. Additionally, total yolk lipid content was decreased in CORT treated birds at hatch. In summary, this thesis examined the effects of the maternal environment and exposure to in-ovo CORT on progeny performance traits, and explored potential physiological mechanisms contributing to noted differences in body composition measures in chicken meat birds. The results indicate that in-ovo exposure to CORT appears to alter body composition, including higher fat mass (% bwt), with both sex and line differences evident in adult birds while no influence on muscle fibre development was detected in embryonic birds. Furthermore, our results suggest progeny phenotypes in response to alterations to the maternal/in-ovo environment are a result of a complex interaction between breeder age, bird strain/line and the subsequent in-ovo environment created. However, further analysis of the metabolic fate of yolk hormones and the mechanisms in which progeny are exposed to these compounds is required to gain a better understanding as to how the maternal environment can influence performance traits in chicken meat birds. We however do believe that targeting the maternal/in-ovo environment provides a novel and innovative method to improve performance outputs as well as flock uniformity in chicken meat birds.