| The poultry breeding industry has put forward the “Extended Layer Breeding Program”to reduce the production costs and carbon emissions recently,which will extend the elimination age from 72 weeks to 80-100 weeks to achieve the goal of “Laying 500 eggs at100 weeks of age”.However,in the intensive breeding process,most of the aged laying hens suffer from severe metabolic disorders after the high-intensity metabolism during the laying peak period,which leads to the accumulation of fat in the liver and abdomen,and therefore compromising the function of antioxidant defense system and reproductive system of hens.The metabolism disorders in aged hens will do harm to their health status,laying performance and the physicochemical quality of meat and egg,which severely restricts the development of poultry industry.Therefore,how to improve the laying performance and the physicochemical quality and oxidative stability of eggs and meat of aged hens has become an important issue for the current poultry industry.The idea that improving the laying performance and the physicochemical quality and oxidative stability of eggs and meat of aged hens has also become a new economic growth point in the implementation of “transition to new from old economic engines” in poultry industry.In addition,consumers are paying more and more attention to the relationship between health and diet supplement,and their demands for high value-added functional foods are increasing by years.The nutritional value of eggs and meat can be improved through diet formulas,which will further enhance the economic benefits of the laying hen industry chain.The main purpose of this study is to reveal the key pathways of improving the laying performance and the quality of animal products of aged laying hens through investigating the differences in physiological functions,the physicochemical quality and oxidative stability of meat and egg from hens with different ages.Based on the above two key pathways,three experiments were conducted to explore the effects and mechanism of Se-enriched Yeast(Se Y)and DHA-enriched microalgae(MA)on the laying performance and meat and egg quality of aged laying hens.In addition,the selenium and DHA species enriched in meat and eggs were also identified by using high performance liquid chromatography-inductively coupled plasma mass spectrometry(HPLC-ICP-MS)and ultraperformance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry(UPLC-Q-TOF-MS).The main findings are shown as follows:Firstly,the differences in the physiological function and the quality of meat and eggs from different week-aged laying hens and their correlations were investigated.Combined with previous studies,we found that the hepatic lipid metabolism and antioxidant capacity of aged laying hens were significantly weakened(mainly downregulate the glutathione metabolism pathway)compared with hens in their early and peak laying periods.The weakened hepatic lipid metabolism and antioxidant capacity had negative impacts on the laying performance and the physicochemical quality and oxidative stability of meat and eggs.The above results suggested that the laying performance and the quality of meat and eggs from aged laying hens can be improved through the regulation of the antioxidant function and lipid metabolism.Subsequent studies were based on the above two key pathways and the biological activity of selenium and DHA to explore the effects and mechanism of Se-enriched Yeast(Se Y)and DHA-enriched microalgae on the laying performance and the physicochemical quality and oxidative stability of meat and eggs from aged laying hensSecondly,the effects of dietary selenium supplementation with different sources on the meat and egg quality of aged hens were investigated based on the antioxidant activity of selenium.The biological efficiency of Se in Se Y forms was significantly higher than that of the same dose of sodium selenite(Se S)in increasing the deposited amount of selenium in animal products and improving the body's antioxidant function and the physicochemical quality and oxidative stability of meat of aged laying hens.Furthermore,the deposited forms of selenium in meat and eggs were analyzed by using the HPLC-ICP-MS.In whole eggs,selenomethionine(Se Met),methylselenocysteine(Me Se Cys),selenocysteine(Se Cys2),and selenite Se(IV)were detected,while Se Met,Se Cys2,Me Se Cys,and Selenourea(Se Ur)were identified in the breast muscle.Se Met was the main deposited form of selenium in meat and eggs.Dietary Se Y supplementation significantly improved the physicochemical quality and oxidative stability of aged hen meat through increasing the activity of glutathione peroxidase(GPX)and total superoxide dismutase(T-SOD)in the breast muscle.The increased antioxidant activity in breast muscle inhibited the peroxidation of phospholipid membrane and muscle protein as well as maintained the integrity of muscle cell membrane and the structure and function of muscle protein,and therefore improving the water holding capacity(WHC)and color stability of the breast meat.High temperature stress is a major challenge for the global animal husbandry industry,which will induce oxidative stress in body and therefore compromises the quality of meat and eggs.As the poultry production system gradually shifts from "cage breeding" to “free-range breeding”,the natural high temperature stress may become more and more widespread.Therefore,a chronic heat stress(HS)model by continuous natural high temperature treatment was established in this study to further explore the molecular mechanism of dietary Se Y in alleviating oxidative stress.The laying performance as well as the physicochemical quality and oxidative stability of meat form aged hens exposed to continuous heat stress were significantly compromised.Long-term heat stress exposure increased the reactive oxygen free radicals(ROS)in tissues and cells of aged hens and then induced severe oxidative stress by weakening of the antioxidant enzyme activity,damaging the macromolecules such as lipids,proteins,and DNA,destroying mitochondrial structure and function,and inducing the apoptosis of muscle cells.All these responses ultimately did harms to the physicochemical quality and oxidative stability of the breast meat.Dietary Se Y supplementation could increase the antioxidant capacity of mitochondria,enhance the ability to eliminate superfluous ROS,improve the morphological structure and redox balance,inhibit cell apoptosis,and therefore improve muscle quality and oxidative stability.Under the protection of Se Y,dietary microalgal DHA supplementation on lipid metabolism,meat and egg quality,nutritional value and oxidative stability from aged laying hens were investigated.Dietary microalgal DHA supplementation promoted the ?-oxidation of liver fatty acids and reduced the content of free amino acids and lipids deposited in the liver tissues of aged laying hens.As a result,the incidence of fatty liver in aged laying hens significantly reduced.The deposition of DHA in liver,muscle and egg yolk increased in a dose-dependent manner,as a result,the n-6/n-3 PUFA ratio was significantly reduced,and the lipid health index was significantly improved.The DHA contents in egg yolk,breast muscle,and thigh muscle reached to 11.6,1.2,and 1.3 mg/g,and the ratio of n-6/n-3 PUFA ratio decreased to 2.49:1,1.89:1,and 2.27:1 from hens fed 2.0% microalgal DHA.Furthermore,the lipid molecule compositions of ordinary egg yolk and DHA-rich egg yolk were analyzed by lipidomics.A total of 1069 lipid molecules covering 8 categories and 30 sub-categories were identified and quantified.Among them,a total of 71 lipid molecules contained DHA,mainly in the form of glycerophospholipids,of which more than 85% of the total DHA were combined with phosphatidylcholine and phosphatidylethanolamine.Dietary microalgal DHA supplementation at high doses(? 1.5%)significantly reduced the oxidative stability of meat and eggs,suggesting that additional antioxidants should be added in addition to Se Y when adding more than 1.5% of microalgal DHA in hens' diets.Since DHA is mainly deposited in the lipid fractions of egg yolk and muscle,it is speculated that dietary fat-soluble natural antioxidants supplementation might effectively improve the oxidative stability of DHA-enriched meat and eggs.Therefore,in this section,the effects of dietary microalgal astaxanthin(ASTA;Haematococcus Pluvialis)supplementation(0-40 mg/kg)on the quality and oxidative stability of meat and eggs from aged hens fed diets containing 2.0% microalgal DHA were investigated.Dietary supplementation with ASTA significantly increased the content of ASTA and carotenoids enriched in meat and eggs.The deposited ASTA and carotenoids in meat and eggs fulfilled strong antioxidant properties through blocking the propagation of the oxidation chain,inhibiting the lipid oxidation of meat and eggs,and therefore improving the quality of meat and eggs and prolonging their shelf life.In addition,dietary ASTA supplementation also enhanced the antioxidant enzyme activity in liver by regulating the Nrf2/HO-1 signaling pathway,and therefore increasing the antioxidant level of meat and eggs indirectly.Based on the above results,it was recommended that the optimum level of astaxanthin was 20-30 mg/kg to aged hens' diets containing high levels of n-3 PUFA.In summary,dietary selenium-enriched yeast and microalgal DHA supplementation could improve the laying performance and the added nutritional value of the meat and eggs without compromising their quality and oxidative stability,which will further enhance the economic benefits of the laying hen industry chain.Our results could provide scientific guidance to produce nutritional,safe and healthy functional meat and egg products,and provide new ideas for the comprehensive development and utilization of the aged laying hens. |
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