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METABOLISM AND NUTRITION |
Department of Animal Sciences, Oregon State University, Corvallis 97331
1 Corresponding author: Gita.Cherian{at}oregonstate.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Key Words: hatching egg • docosahexaenoic acid • arachidonic acid • breeder age • hatchability
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Manipulation of yolk n-3 FA has been widely documented in layer birds raised for table egg production. In addition, the roles of other factors such as hen age and strain in affecting yolk n-3 FA content and egg quality characteristics are well documented in table eggs (Cherian et al., 1995; Scheideler et al., 1996; Gonzalez and Leeson, 2001). Although the relationship between hen age and strain and egg characteristics has been reported in broiler breeders raised for hatching eggs (Lapao et al., 1999; Wolanski et al., 2007), very little information is available on dietary n-3 FA modulation and egg quality aspects of hatching eggs from broiler breeders. Assessing egg quality characteristics and the effect of n-3 PUFA and selenium in breeder hens during a 4-wk (27–31) production period, Pappas et al. (2006) reported reduced egg weight with addition of fish oil (n-3 FA source) in the hen diet. In addition, Peebles et al., (2000) reported that inclusion of corn oil (n-6 FA source) led to reduction in shell weight in older hens.
The success of producing hatching eggs rich in n-3 FA depends on maintaining egg quality aspects, n-3 FA concentration, fertility, and hatchability during the production cycle. The objectives of this study were to determine the effects of hen age and dietary n-3 oils on hatching egg quality, yolk FA composition, fertility, and hatchability. It was hypothesized that 1) addition of n-3 oils rich in n-3 FA will lead to an increase the content of yolk DHA and 2) eggs from younger hens will be of higher quality, will incorporate higher levels of DHA, and will have increased fertility and hatchability than eggs from older hens. Long-chain PUFA such as AA and DHA are given special emphasis in the current study because in species that produce precocious hatchlings, such as chickens, the role of yolk long-chain PUFA is likely to be crucial during hatching and in the early posthatch period due to rapid cell proliferation and intense accretion of these FA in the tissues (Noble and Cocchi, 1990; Cherian and Sim, 1992).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Eggs high or low in n-3 PUFA were obtained from Cobb hens fed corn-soy diets containing 1.75% fish oil + 1.75% yellow grease (low n-3) or 3.50% fish oil (high n-3). The experimental diets were isonitrogenous (16.2% CP) and isocaloric (2,863 kcal of ME) and were fed to 48 hens, 3 replicates of 8 hens per treatment, from 26 to 62 wk. During the experimental period, the hens were fed a weighed amount of feed as per Cobb management guidelines. The eggs were collected in a 7-d period every 4 wk from 26 to 62 wk (end of the trial). Egg quality characteristics (egg weight, yolk weight, shell weight, shell thickness, yolk color, albumen weight, Haugh unit) were done every 4 wk from 26 to 62 wk. Egg total lipid and fatty acid composition were determined on wk 26 and at every 8-wk interval up to 62 wk of age. Hatchability and fertility were measured every 8 wk at 30, 38, 46, 54, and 62 wk of age. For each diet and week, a total of 110 eggs were collected and were held in a cold room at 65°F and 70% relative humidity. In addition, a total of 560 commercial eggs (Cobb) were purchased from a local hatchery at 5 different months and were stored under the same conditions as that of the experimental eggs. Commercial eggs were collected at 5 different months to simulate any variation that may have arisen due to hen age or diet supply. The commercial eggs were used as an external control to compare the lipid and FA composition of low and high n-3 eggs.
Egg Quality Characteristics
The eggs (low n-3, high n-3, and commercial) were taken from the cold room and were warmed to room temperature before setting in the incubator. Six eggs, 2 from each replicate, were taken randomly from each diet and week for measurement of egg quality characteristics, total lipids, and fatty acid assay. Egg quality measurements were carried out by the same person during the entire trial. The eggs were weighed, and yolks were separated using an egg separator and weighed. Albumen weight was calculated by subtracting yolk and shell weight from total egg weight. Albumen height was documented, and Haugh unit was calculated. Yolk color was determined by comparing yolk color to the Roche color fan. Shell thickness was measured using an electronic micrometer. Any remaining eggs that were not incubated were discarded as per Oregon State University poultry farm standard operating procedures. From the commercial eggs collected each period, 12 eggs were randomly selected at each collection period and were weighed. The yolk was separated and weighed. Two yolks were pooled to obtain a sample size of 6 yolks per collection for lipid and FA analyses.
Egg Incubation
Low and high n-3 eggs (n = 104 for each diet and week) and commercial eggs (n = 100) were incubated at a dry bulb and wet bulb temperature of 37.5 and 29.4°C, respectively. Egg incubation conditions for each week were identical and eggs were incubated in the same setter. At 18 d of incubation, the eggs were candled, and infertile eggs were removed and counted. The eggs were transferred to hatch baskets, and the hatch was pulled at 21.5 d. Hatched chicks from all treatments were counted, and those eggs that did not hatch were removed from the hatcher and were also counted. All protocols were approved by Oregon State University’s Animal Care and Use Committee to ensure adherence to Animal Care Guidelines.
Total Lipid and Fatty Acid Analysis
Total lipids were extracted from egg yolk by the method of Folch et al. (1957). The mass of total lipid content was determined gravimetrically. The FA methyl esters were prepared as reported earlier (Cherian et al., 1997). The FA analysis was performed with an HP 6890 gas chromatograph equipped with an autosampler, flame ionization detector, and SP-2330 fused silica capillary column (30 mm x 0.25 mm i.d). Samples (1 µL) were injected with helium as a carrier gas onto the column programmed for ramped oven temperatures (initial temperature was 110°C, held for 1 min, then ramped at 15°C/min to 190°C and held for 55 min, then ramped at 5°C/min to 230°C and held for 5 min). Inlet and detector temperatures were both 220°C. The FA methyl esters were identified by comparison with retention times of authentic standards (Nuchek Prep, Elysian, MN). Peak areas and percentages were calculated using Hewlett Packard ChemStation software (Agilent Technologies Inc., Wilmington, DE). The FA values are reported as grams per egg or as percentage. An internal standard (C19:0) was used for FA quantification.
Statistical Analysis
Repeated measure ANOVA was used to compare different egg quality aspects, egg lipid, and FA composition and hatchability. Significant differences among treatment means were analyzed by the Student-Newman-Keuls multiple range test, and significance was set at P < 0.05. Means of interaction were analyzed by comparing the feeding period (weeks) separately for each diet by Student-Newman-Keuls. Percentage data underwent angular transformation (arc sine square root percentage transformation) before analysis. Computations were done using the GLM procedure of SAS 9.1 (2002).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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A significant decrease in egg weight, yolk weight, shell weight, and yolk color was observed for high n-3 when compared with low n-3 eggs. No difference due to diet was observed for albumen weight, height, or Haugh unit (Table 1
). Duration of feeding time (weeks) was significantly different for all the egg characteristics assessed except albumen weight (Table 1). As the hens aged, egg weight increased significantly for high and low n-3 eggs. The eggs laid by 62-wk-old hens were over 17% heavier than those laid by hens from 26-wk-old hens for high and low n-3 (P < 0.05). However, as the hens aged, shell weight and shell thickness deteriorated for high and low n-3 eggs (P < 0.05). Shell weight (% of egg weight) was lowest at 62 wk of age (9.6%) when compared with 11.7 and 11.8% at 42 and 46 wk of age, respectively, for low and high n-3 eggs (P < 0.05). Shell thickness (mm) was lowest at 62 wk of age (0.57%) when compared with 0.86% at 30 wk of age, respectively, for low and high n-3 eggs (P < 0.05). When expressed as percentage of egg weight, the yolk weight was affected by diet and age. The yolk weight constituted 28.9 and 30.0% for high and low n-3 eggs (P < 0.05). As the hens aged, yolks from 26-wk-old hens were smaller (26.0% of egg weight) than those from 62-wk-old hens (32.3%; P < 0.05). Albumen weight when expressed as percent of egg weight was 61.8 and 59.2% (P > 0.05), for high and low n-3 eggs, respectively. Yolk color, Haugh unit, and albumen height deteriorated after 46 wk in low and high n-3 eggs (data not shown; P < 0.05).
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No difference was noted in egg total fat content due to dietary treatments (Table 2). The total lipids constituted 5.58 and 5.38 g for low and high n-3 eggs, respectively (P > 0.05). When egg total fat content was expressed as percent of egg weight, a significant difference due to hen age on yolk fat content was observed (Figure 1). Eggs from 42-wk-old hens contained higher fat than those at other ages (P < 0.05). The effect of dietary oils and breeder hen age on egg yolk fatty acid content (g per egg, percent fatty acids) is shown in Table 2. No effect of diet on total saturated fats (16:0+18:0) was observed. However, when reported as percent fatty acids, a significant increase in saturated fatty acids due to feeding high n-3 was observed (Table 2). A trend for lower incorporation of total monounsaturated fats (16:1+18:1) was noted for high n-3 eggs (P = 0.086). No effect of diet on yolk PUFA (n-6+n-3) or polyunsaturated:saturated ratio was observed among the 2 treatments. Total n-3 FA, DHA, and the DHA:AA ratio were higher in high n-3 eggs when compared with low n-3 eggs (P < 0.05). Concomitantly, AA, total n-6, and the n-6:n-3 ratio were higher in low n-3 eggs when compared with high n-3 eggs (P < 0.05) (Table 2).
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). The incorporation of DHA was lowest at wk 26 for low and high n-3 eggs (Figure 2). The peak incorporation of DHA was at wk 38 for low and high n-3 eggs. The dietary n-3 oils (1.75%) did not sustain egg yolk DHA and a reduction in yolk DHA was observed after 38 wk of age in low n-3 eggs (Figure 2). However, AA incorporation increased steadily as the hens aged (Figure 2). The low n-3 and high n-3 eggs at 62 wk of age had the highest level of AA when compared with eggs less than 62 wk of age. A positive correlation between hen age and egg yolk AA content was observed. The r2 values for AA in low n-3 and high n-3 were 0.91 and 0.90, respectively (P < 0.05). Hen age also resulted in the significant increase in n-6:n-3 ratio in eggs from hens fed the low n-3 diet. Peak incorporation of FA was observed at 38 wk of age for saturated, monounsaturated, and PUFA (data not shown).
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The average egg and yolk weights of commercial eggs were 61.5 and 18.9 g. Total lipids constituted 29.7%. The DHA content was negligible and constituted 0.5 to 1%. The AA was the major long-chain PUFA in the commercial egg. In addition, other long-chain n-6 FA such as 22:4 n-6 and 22:5 n-6 were present in these eggs. The total content of long-chain n-6 PUFA (AA+ 22:4 n-6+22:5 n-6) constituted over 0.3 g per commercial egg as compared with 0.09 and 0.07 in low and high n-3 eggs (Figure 3).
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Overall fertility was 98.6 and 97.4%, and hatchability of fertile eggs was 80 and 83.8% for low and high n-3 eggs, respectively. Dietary treatments did not affect fertility or hatchability. Fertility was 96%, and hatchability of fertile eggs was 80% for commercial eggs.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The resistance to change in total fat content through dietary n-3 FA is in agreement with our previously reported research (Cherian and Sim, 1991; Cherian et al., 2007). However, accretion of lipids in yolk was affected by age of the hen as depicted in Figure 1. Eggs from 26-wk-old hens had less fat deposited than those eggs from 42 wk of age. The increase in egg or yolk weight associated with age did not lead to any increase in yolk lipid deposition. Yolk lipid formation depends upon a hen’s capacity to initiate and sustain the assembly of VLDLy, a small VLDL particle that is rich in triglycerides (Walzem, 1996). Hen age appears to affect the ability to assemble VLDLy correctly and, as a result, may lead to reduction in yolk lipid accretion (Walzem et al., 1999). This impairment in yolk lipid formation is reflected in the high concentration of LDL, HDL, and cholesterol in the blood of chicks hatched from eggs laid at 26- vs. 48-wk-old hens (Latour et al., 1996).
Increases in DHA and the DHA:AA ratio and a reduction in the n-6:n-3 ratio in eggs by feeding n-3 oils to hens corroborate with previously reported research on laying and breeder hens (Cherian and Sim, 1991; Ajuyah et al., 2003a,b). The enzyme 
6-desaturase is the rate-limiting step in the synthesis of AA and DHA from their 18-carbon precursors (Brenner, 1971). There is a competition between n-6 and n-3 FA in which n-3 FA are used as the preferred substrate in the desaturation elongation pathway leading to an increase in the DHA:AA ratio in the egg. However, it is interesting to note that as hen age progressed, the pattern of AA and DHA accretion differed in low n-3 and high n-3 eggs. The presence of 1.75% n-3 oils in the diet did not sustain DHA concentration in eggs after 38 wk of age for low n-3. Similarly, DHA was lowest at wk 26 for low and high n-3 eggs. Reductions in DHA at wk 26 and 62 for high n-3 eggs suggest that hen age may affect yolk incorporation of long-chain n-3 FA. Nielsen (1998) reported a 20% increase in yolk DHA in egg yolks from young vs. old layer hens. The decrease in DHA observed in the present study and those of Nielsen (1998) suggests that hen age reduces the ability to accrue DHA in yolk lipids or diminishes the desaturation and elongation of n-3 FA as reported in mammals (Ulmann et al., 1991). A positive correlation has been reported with egg weight and hatched chick weight (Shanawany, 1987). Therefore, chicks hatched from large eggs laid toward the end of laying or eggs laid before 30 wk may have lower levels of DHA for tissue accretion compared with those from hens at 38 wk of age. Whether the low level of DHA is in any way related to chick health pre- and posthatch is not known. Nevertheless, it has been reported that eggs laid by hens during the middle of their production cycle perform significantly better throughout incubation than eggs from younger or older hens (Christensen et al., 1996; Fairchild et al., 2002). Similarly, first-week mortality of chicks was reported to be higher in broilers hatched from eggs laid by younger hens than by older hens. The DHA is the major PUFA of the central nervous system in avians (Noble and Cocchi, 1990; Cherian and Sim, 1992). During avian embryogenesis, DHA is preferentially taken up from yolk sac lipids and is incorporated into cell membrane phospholipids of the developing embryo (Cherian and Sim, 1992; Cherian et al., 1997). In addition, it has been reported that cardiac, brain, immune, and hepatic tissue status of long-chain n-3 PUFA and DHA during growth was higher in broiler chicks hatched from high n-3 eggs (Ajuyah et al., 2003a,b; Wang et al., 2004; Cherian, 2007).
Low levels of DHA in commercial hatching eggs may reflect the dietary source of lipids fed to breeder hens. In the United States, supplemental dietary fat is typically provided as an animal-vegetable blend with lipids from rendering sources (tallow) or the food industry (restaurant grease, hydrogenated oil). Fats from these sources are rich in saturated, trans, n-6 FA and are poor in n-3 FA (Cherian, 2007). Though only few commercial eggs were assayed for FA content, the high content of long-chain n-6 FA along with negligible levels of n-3 FA in commercial hatching eggs suggests that current feeding practices of breeder stock are not placing enough emphasis on the PUFA quality of dietary lipids. Recently it was reported that chicks hatched from n-6 FA-enriched eggs incorporated higher levels of AA in the thrombocytes and produced more proinflammatory leukotriene B4 eicosanoid during growth than chicks hatched from high n-3 eggs (Hall et al., 2007). Therefore, it may be argued that chicks hatched from eggs laid by older hens or commercial eggs lacking in long-chain n-3 FA may be more prone to inflammatory disorders during growth due to the high maternal supply of AA.
Poor hatchability, economic loss due to culls, and chick mortality during the first 2 wk of life remain a problem for the broiler industry. In addition, inflammatory (e.g., joint abnormalities, leg weakness), metabolic and cardiovascular disorders are major causes of morbidity and mortality in broiler chickens during growth. In view of the important roles long-chain n-3 PUFA play in cellular signaling mechanisms (Salem et al., 2001) and lipid and eicosanoid metabolism in progeny (Cherian, 2007; Hall et al., 2007), availability of maternal (yolk) DHA is likely to be crucial during the early pre- and posthatch period in broiler chickens producing precocial hatchlings that are selected for rapid growth and muscle yield. Further research is needed in exploring avenues that will increase n-3 FA while maintaining quality-related traits in hatching eggs.
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ACKNOWLEDGMENTS |
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Received for publication August 10, 2007. Accepted for publication February 11, 2008.
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