Ph. D.
Department of Agricultural, Food and Nutritional Science (AFNS)
University of Alberta
Edmonton, Alberta,
Canada
Introduction
What is egg storage?
In commercial turkey breeding operations fertile eggs produced by turkey hens are stored in on-farm coolers until the eggs can be shipped to the hatchery. After a fertile egg is laid (oviposited), temperature is the primary catalyst that influences embryonic development. Egg storage coolers are set below temperatures that could promote embryonic development. This temperature threshold is called "physiological zero" and is accepted to be about 21°C (69.8°F) (Edwards, 1902). Once the eggs reach the hatchery, they are again stored for various lengths of time at temperatures below physiological zero. A relative humidity of approximately 75% is maintained in most egg cooler rooms to prevent excessive moisture loss from the egg during storage.
Why are hatching eggs stored?
Hatching eggs are not incubated immediately after hatching for a variety of logistical reasons. Hatching eggs are stored on-farm as breeder farms are not located close enough to one another to make daily shipment of eggs to the hatchery economically efficient. Most eggs are shipped from the farms twice per week and are therefore not stored on-farm longer than 4 days.
Duration of storage at the hatchery is dependent upon the demand for poults, and the availability of enough incubators to hold the eggs, or enough eggs to fill incubators. During times when hatching eggs are plentiful, incubator space may be limited, therefore requiring storage of surplus eggs. In anticipation of a reduction in egg production rates, especially during the summer months in the southern United States, hatchery managers "stock pile" eggs in storage for extended periods. The main objective of egg storage is to stop all embryonic development until the eggs can be set at normal incubation temperatures (37.5°C; 99.5°F). A secondary objective of cool storage is to discourage bacterial growth.
Embryonic development related to storage
At the time of lay, the embryo has already been undergoing development while it travels through the reproductive tract of the hen. By the time the egg is laid, the embryo consists of approximately 60,000 cells. At this time the egg must be cooled down to prevent further embryonic development from occurring. Using a microscope to assess stages of turkey embryonic development, it is known that during prolonged egg storage development of the turkey embryo ceases (Bakst and Gupta, 1997). What is not known is if each individual cell in the embryo stops all development.
It is important to maintain egg cooler temperatures below physiological zero to inhibit all embryonic development. This maximum storage temperature at which to store eggs without initiating embryonic development has been reported at two levels; 20-21°C (Edwards, 1902; Proudfoot and Hamilton, 1990) and 27°C (Funk and Biellier, 1944). It should be noted that all of these authors were examining chicken not turkey eggs. Some researchers have suggested that there are different temperature thresholds for different species (Webb, 1987) or different cells of the embryo (Kauffman, 1948). For example, some cells may still develop at 25°C while other cells do not. If this differential growth within the embryo occurs, it could lead to abnormal development and/or increases in early embryonic death once proper incubation temperatures are provided. This may be the physiological reason behind why prolonged turkey egg storage increases turkey embryonic abnormalities (Kosin and Mun, 1965; Arora and Kosin, 1966a; Sittman et al., 1971a) and embryonic mortality (Sittman et al., 1971b).
What is the effect of prolonged egg storage?
Hatchery managers have long used the "rule of thumb" that eggs should not be stored for longer than 7 days. The resulting effects of egg storage for more than one week include a decrease in yolk membrane strength, a decrease in the number of embryonic cells, an increase in incubation length, an increase in the hatch window (the time span between when the first poult hatches to when the last poult hatches), an increase early and late embryonic mortality, and a reduction in hatchability and poult quality.
Yolk membrane strength
Previous researchers have noted that the membrane surrounding the yolk becomes weak as storage duration increases (Fromm, 1964; Britton, 1973). An electron microscopy experiment examined the structure of yolk membranes of fertile turkey eggs stored for 1 or 14 (Fasenko, 1996). This research showed that both the thickness of the various layers of the yolk membrane and the density of material making up these layers is reduced in eggs stored for 14 versus 1 day.
Why is this important? This may have implications with respect to early embryonic mortality. There is contact between the inner surface of the yolk membrane and the embryo of a freshly laid egg that is imperative for further development of the embryo (New, 1959). In eggs that have been stored for long periods of time the embryo is not adhered as well to the yolk membrane (Fasenko, unpublished observations). If the structure of the yolk membrane is altered due to storage such that this reduces the contact of the membrane with the embryo, this may be a contributing factor in early embryonic mortality.
Embryonic viability
Previous researchers studying the effects of prolonged egg storage on embryonic health discovered that cell death increases as storage duration is lengthened (Arora and Kosin, 1966b; Bakst, unpublished observations). This reduction in healthy embryonic cells may be the reason behind an increase in early embryonic mortality. For example, Fasenko, et al., (2001), documented a 1.8% increase in early mortality (1-7 days) when egg storage was increased from 4 to 14 days. There may be a minimum number of healthy embryonic cells that are required for normal embryonic development to occur. Late embryonic mortality at internal pipping (beak of embryo breaks into the air cell), and external pipping (beak breaks through the shell) was 1% and 3.5% higher, respectively, in eggs stored for 14 versus 4 days (Fasenko, et al., 2001).
The possible reason for the increase in late mortality is due to the extended incubation period required to hatch in stored eggs or possibly a difference in the metabolism of embryos from stored eggs (discussed below under embryonic metabolism paragraph).
Embryonic development
It is well documented that the required incubation length is extended when eggs are stored for extended periods. Researchers have put forth two possible reasons for this. The first is that the embryo of a long-term stored egg does not begin development once incubation temperatures have been provided (Arora and Kosin, 1966).
The extended period of time the embryo spends at cool temperatures may somehow inhibit it from responding immediately. The second is that embryos from eggs stored for long periods have a lower metabolic rate at certain times during incubation or throughout the entire incubation period (Mather and Laughlin, 1977). Unpublished data in broiler embryos from my lab has shown it to be a combination of both reasons. Turkey embryos likely respond to storage in a similar manner, but this has yet to be confirmed.
Embryonic metabolism
During incubation the main source of energy for growth and development of the embryo comes from the fat (lipids) in the yolk. Metabolism of lipids requires oxygen that is provided to the embryo by crossing through the pores of the shell from the exterior egg environment. During the final stages of hatching, the energy demands of the embryo to pip out of the shell are large and the oxygen that is provided through the pores in the shell is not in sufficient amounts to continue to use only lipid as the energy source. During this time, the embryo must rely on stored carbohydrate (glycogen) in the liver. Research has shown that this time period in which oxygen is in short supply is extended by 7 hours in embryos from 14 versus 4 day stored turkey eggs (Fasenko, 1996).
Extended egg storage somehow changes the biological quality of a late incubation embryo such that its metabolism is not as efficient as an embryo from a short-term stored egg. In broiler breeders, Christensen et al. (2001) showed that a line of breeders that was resistant to extended egg storage are able to maintain larger glycogen reserves in the heart and liver than embryos from lines susceptible to the negative effects of egg storage.
In turkeys, embryos from eggs stored for 14 days rely more heavily upon the metabolic pathway of gluconeogenesis during pipping and hatching than do embryos from 4 day stored eggs (Fasenko, 1996). Gluconeogenesis is a metabolic pathway that converts non-sugar (carbohydrate) resources such as amino acids into carbohydrate. This is not an ideal situation as amino acids are the building blocks for protein
(i.e. muscle). Because the incubation time period in which oxygen is in short supply is extended due to long-term storage, the glycogen stores in the liver are likely depleted and therefore, the embryo must rely on alternative metabolic pathways (gluconeogenesis) that do not require oxygen to break down energy sources.
Hatchability and chick and poult quality
It is widely known that extending the storage duration reduces hatchability. In one study the hatchability of turkey eggs was reduced from 70.4% in 4 day stored eggs to 65.2% in eggs stored for 14 days (Fasenko, et al., 2001).
Most of the research on offspring quality as it relates to egg storage has been conducted in broilers. Unpublished research from my lab has shown that the percentage of chicks culled due to unhealed navels, deformities, and general signs of weakness was higher in hatched chicks from eggs stored for 14 versus 4 days. In this study the body weight of chicks from the long-term stored eggs was 1.4 g lighter. The negative effects on chick quality were significantly reduced by incubating the eggs for 6 hours prior to storage for 14 days.
Management options to reduce negative effects of storage
Egg Storage Conditions
Temperature. Wilson (1991) and Meijerhof (1992) have provided the most recent reviews of the literature pertaining to optimum egg storage conditions. Care should be taken in interpreting these results as most of the research has been conducted on chicken eggs. In general, as storage time increases, storage temperatures decrease. Eggs stored for less than 7 days should be held at 16-17°C (~61-63°F), while eggs stored longer than 7 days should be held at 10-12°C (~50-54°F).
Humidity. Humidity should be kept as high as possible during storage, but not so high as to promote moisture settling on the eggs or eggs "sweating" once they are removed from the cooler for transport. Both of these issues encourage bacterial growth. Most coolers can be maintained at a humidity of 75% without the above concerns taking place.
Pre-Storage Incubation
In both chickens and turkeys, research on incubation of freshly laid eggs prior to any storage has helped to reduce the negative effects of extended storage. Research has shown that incubating turkey eggs for 12 hours, at normal incubation temperatures, prior to storage for 14 days improved the hatchability of these eggs from 64.4% (no pre-storage incubation) to 70.9% (Fasenko, et al., 2001). This positive effect of pre-storage incubation on hatchability of long-term stored eggs has also been demonstrated in broiler breeders (Fasenko, et al. 2001b). Although broiler breeders require less pre-storage incubation time (6 hours), the same stage of embryonic development is reached as that of the turkey embryo from an egg incubated for 12 hours. These research studies indicate that the embryonic stage of development that is best able to withstand storage is the same in both turkeys and chickens.
As there are logistical problems with incubating eggs prior to any storage (no incubators are housed on farm), a similar research project in my lab was conducted on broiler eggs to determine if incubation after on-farm storage but before hatchery storage would provide the same beneficial effects on hatchability of stored eggs. Unfortunately, post-farm, pre-hatchery storage incubation was not as effective as incubating eggs prior to any storage. This indicates that there is something about a freshly laid egg that responds better to pre-storage incubation treatments.
Summary
Research over the past 50 years has clearly demonstrated that prolonged egg storage negatively affects the egg, the embryo, and the hatched poult or chick. Some more recent research has tried to determine the physiological reasons behind the negative impact of storage. By knowing the physiological mechanisms behind why egg storage is so detrimental, new management or breeding procedures may be altered in the future to reduce the negative effects of egg storage over 7 days.
Most of the research conducted to date on the detrimental effects of prolonged storage and the optimal conditions under which to store eggs to reduce these effects has been conducted using chicken eggs. Although the results of this research may be used as a guide in storing turkey eggs, one of the "take home messages" should be that turkeys are not simply "big" chickens. Future research on optimum storage conditions specifically for turkey eggs should be conducted to fill this information gap.
References are available on request
From Proceedings of the "Midwest Poultry Federation Convention", St. Paul, Minnesota, U.S.A.
