Studies on Chicken Hatchability and Its Relation with Egg Yolk Metabolites

Hatchability is an important trait for the chicken industry because of its high impact on chicken output. The embryonic survival ability in fertile eggs is greatly influenced by the offspring genotype and the quality of the egg environment, especially egg yolk metabolites. This study is aimed at estimating genetic parameters of hatchability traits and determining the relationship between yolk nutrients and chicken embryonic survival ability in order to identify important metabolites for the embryonic health of chickens and other species. In chapter 2, the functions of egg yolk metabolites were further discussed. We summarized the literature from the past decades on finding metabolites in egg yolk and albumen that impact embryonic survival ability and on determining the function of different nutritional components during distinct stages of incubation. The function of fatty acids, carbohydrates, amino acids, vitamins and steroids were introduced in detail in this chapter. In general, fatty acids related pathways are most active during middle and late incubation stages and associated with chicken embryonic energy metabolism, neural system development, immune ability and lipid uptake. Carbohydrates are an important energy source when embryonic oxygen consumption is insufficient. Essential amino acids control protein synthesis and impact chicken embryonic growth rate. Vitamins with strong anti-oxidation ability could inhibit toxic effect of avidin in the egg. Steroids such as cholesterol are important hormone precursors. A good nutritional status of the parent birds is crucial for the transfer of an adequate, balanced supply of nutrients required to the egg for a normal development of the embryo. The consequences to the embryo may be lethal if the egg contains inadequate, excessive, or imbalanced levels of nutrients. In chapter 3, we estimated the genetic parameters of hatchability traits: Fertility (FER) hatchability of total eggs set (HTE) and fertilized eggs (HFE) and susceptibilities to early (EES), middle (MES), late (LES) embryonic survival ability, external pipping ability (EXP) and internal pipping ability (INP). 5,983 hatching egg records obtained from 100 unselected Brown layer hens (BL) and 99 White layer hens (WL) from Friedrich Loeffler Institute and 80,916 egg records selected from 2909 Brown layer hens (BLC) and 3319 white laying hens (WLC) from Lohmann company were included in this study. To determine the time of embryonic mortality, eggs not exhibiting a viable embryo at candling on days 7 and 18 of incubation and eggs that did not hatch were opened and examined macroscopically to assess their true fertility and estimate the time of embryonic death. Heritabilities were estimated using a generalized linear animal model and results suggest that heritability for fertility (FER), hatchability of total eggs (HTE), hatchability of fertilised eggs (HFE), early embryonic survival ability (EES), middle embryonic survival ability (MES), late embryonic survival ability (LES), external pipping ability (EXP) and internal pipping ability (INP) exhibit ranges of 0.10-0.13, 0.04-0.08, 0.05-0.06, 0.03-0.10, 0.01-0.18, 0.02-0.09, 0.001-0-083 and 0.001-0.11, respectively. The estimated genetic correlations between different reproductive traits for all breeding lines were positive ranging from 0.15 to 0.94. The ranges between minimum and maximum breeding values of HTE and HFE were large and on average approximately 30%. The low heritabilities found for these traits led to the conclusion that the potential selection response is expected to be limited. Taking into consideration the very high reproductive ability and the short generation interval in chicken, it appears still feasible to select animals for improved hatchability. Based on an estimation of the hatchability of all egg set (HTE) breeding values from the results in chapter 3, chicken from 4 lines were divided into high and low hatchability groups and used to determine the metabolite profile in chapter 4. Generalized linear mixed models were applied in association analysis between metabolites and hatchability traits (HTE and 5 other traits). We found, in total, 24 fatty acids, 4 vitamins, 10 steroids, and 7 amino acids to be significantly related to the hatching traits (P≤0.05). Fatty acids usually had the greatest impact on hatchability among these egg yolk nutrient groups. Four branched-chain fatty acids (hexadecanoic acid, 14-methyl; pentadecanoic acid, 14-methyl; tetradecanoic acid, 12-methyl; and tetradecanoic acid, 13-methyl) were associated with hatching traits in the brown lines. Two monounsaturated fatty acids (9-hexadecenoic acid and 9-octadecenoic acid) and 9, 12-octadecadienoic acid impeded embryonic survival in the Lohmann brown line but had the opposite effect in the other lines. Five polyunsaturated fatty acids (docosahexaenoic acid, docosapentaenoic acid, docosatetraenoic acid, eicosatetraenoic acid; and eicosatrienoic acid) promoted hatchability and hatchability traits in specific lines. In conclusion, genetic parameter estimation results suggest that heritabilities for FER, HTE, HFE, EES, MES, LES, EXP and INP are generally low. These results seem to lead to the expectation that the potential selection response will be limited. However, considering the high reproductive performance and low generation interval in chickens and the wide range of estimated breeding values for hatchability, successfully selecting animals for increased embryonic survival appears to be a feasible endeavour. Certain types of egg yolk nutrients among the amino acids, vitamins, steroids and fatty acids were related to embryonic development in chicken, with fatty acids being the most important ones. These identified metabolites may be useful for improving the embryonic health of chickens and other species.