Ways to lessen the impact of prolonged hatching egg storage due to Covid-19
Written by Gerd de Lange, Senior Poultry Specialist
The current Covid-19 pandemic is causing widespread market disruptions. While some countries are much worse affected than others, global demand for poultry meat has dropped as restaurants and fast-food chains have been closed. Some governments have banned imports of poultry meat to protect their country's internal market and processing plants are calling for a reduction in chick placements of 15–25 % in an attempt to stabilise the market. In addition, the closure of many traditional wet markets is having a dramatic effect on the poultry sector in less developed countries.
Although it is hard to predict the situation ten weeks from now (the time it takes between a hatching egg being laid and broiler meat reaching the market), in anticipation of the situation continuing, several hatcheries have reduced the number of eggs set. To reduce the supply of hatching eggs to the hatchery, older flocks can be culled earlier and, for younger flocks, forced moulting might be an option. Needless to say, this is not the time to send floor eggs to the hatchery or to be sloppy with egg grading.
When hatching eggs are supplied continuously, each day that they are kept in the storage room adds to the eggs' age. This has a negative impact on hatchability and chick quality once the eggs are set. This article offers hatchery managers some practical measures for reducing the negative effects of prolonged egg storage.
A closer look at a new-laid egg
A fertile egg laid by a breeder hen contains an embryo consisting of 40,000–60,000 already differentiated cells. At the moment of oviposition, it is in developmental stage IX or X (Eyal-Giladi and Kochav, 1976). As the egg cools down from the same temperature as the hen's body, cell division and development continue as long as the internal egg temperature remains above 25 °C/77 °F. If cooling down goes too quickly the embryo might not reach stage XII–XIII, its storage resistant stage. Ideally the egg should cool down gradually to below 25 °C/77 °F over a period of approximately 6 hours. When performing a fresh egg break-out, a well-developed embryo can be recognised by its typical ‘doughnut form’: a white circle of 3.5–5 mm diameter with a translucent centre on top of the yolk. When the egg is kept below this temperature, the embryo goes into a state of dormancy. The technical term for this is ‘diapause’ (Fasenko, 2007). See more information here. Embryonic development will only start up again when the temperature is increased at the onset of incubation, or if the egg holding conditions are poor. If eggs cool down too slowly after oviposition, the embryo will develop too far and gastrulation will start; these embryos will not survive storage, not even for a few days under good conditions.
A side effect of the eggs cooling down is the formation of the air cell. The shell is rigid and hardly shrinks at all, but the egg content does. This causes negative pressure in the egg and as a result, air enters through the pores to counteract this. Air takes the path of least resistance, so most enters at the blunt end, where more pores are located than at the equator or sharp end of the egg.
During cooling down, and also during first few days of storage, CO2 diffuses through the pores in the egg shell into the environment, simply because the CO2-concentration outside the egg is about 100 times lower than inside the egg. This creates a rise in the pH of the albumen from 7.6 initially to 9.2, while the pH of the yolk stays constant at about 6.5. The embryo, which is situated between the yolk and the albumen, is exposed to this pH gradient, which is essential for proper embryonic development. The elimination of CO2 from the egg also plays a role in making the thick, gelatinous albumen more liquid, and this is believed to improve the embryo's gas exchange, and thus supply of oxygen to the embryo, during early incubation. Both these changes are part of the process of preparing the fertilised egg for incubation, known as the adaptation period. This is why very fresh eggs are not suitable for incubation; for best results, 1 or 2 days of storage are needed, and eggs from some layer breeders might require even longer.
The consequences of prolonged egg storage
- Reduced internal egg quality : watery albumen and weaker yolk vitelline membrane
- Egg weight loss due to evaporation of water
- Reduced viability of embryo and thus lower hatchability
- Reduced chick quality (especially navel) and strong indications of reduced farm performance
- Longer incubation time
Storage reduces the quality of hatching eggs. As the ratio between thick albumen and thin albumen decreases, ultimately the albumen will become very watery. Water will diffuse from the albumen to the yolk, which can lead to yolk mottling, and the yolk vitelline membrane becomes weaker. Water evaporates out of the eggs, resulting in egg weight loss which can be visualised with the aid of a candling lamp in a darkened room. The air cell will gradually increase in size.
Storage also has a negative impact on the embryo itself. As storage progresses, blastoderm cells will degenerate and die, reducing embryo viability. The direct effect of this will become evident when the eggs are incubated; both early and late embryo mortality will be higher and hatchability lower. Moreover, the longer the eggs are stored the faster hatchability declines. The data displayed in Figure 1 were collected over a 3-year period from 3 different hatcheries in the Netherlands and represent a total of 511 breeder cycles (or 24,234 batches). Contrary to what is commonly believed, the graph shows that hatchability of young breeders is affected more by prolonged egg storage (8–14 days) than that of old breeders (0.8 vs 0.4 % per day of storage for age groups 25–30 and 51–60 weeks, respectively). Nevertheless, the daily loss of hatchability increases for all flock ages with storage duration. Note that losses of 1–1.5% hatchability per day after 7 days of storage are not uncommon in practice. Looked at from this perspective, Figure 1 is on the optimistic side.
Figure 1: Daily decrease in hatchability in relation to duration of egg storage at the hatchery, excluding 1 to 4 days storage at breeder farm (Yassin et al., 2008)
Storage also has an impact on chick quality. Typically, more black navels are seen in chicks hatched from stored eggs. There are also strong indications that broilers' 7-day body weight is reduced, which could be a predictor of reduced slaughter weight.
Finally, storage slows down embryonic development during incubation resulting in a longer incubation time. In practice, the rule of thumb '1 hour extra incubation time for each day of storage beyond 4 days' works well. When eggs that have been stored for a long time are set together, the setter should be started earlier. But if eggs that have been stored for different lengths of time (e.g. 4–14 days) are set together, this will definitely result in a wider hatch window.
To summarise, best practice is to set hatching eggs 2–3 days after laying. Avoid storing eggs for longer than 7 days. If they need to be stored for longer than 10 days, there are some practical measures available for reducing the negative effects of prolonged egg storage.
Practical measures to minimise the impact of prolonged hatching egg storage
Where prolonged egg storage (beyond 10 days) is the only option, the main aim is to maintain the highest hatch potential. To do this you need to preserve the quality of the albumen and the yolk and most importantly the quality and vitality of the embryo, as far as is possible.
- Create optimal storage conditions:
- 12–14 °C/ 53.6–57.2 °F
- 80–85 %RH
- Aim for uniformity by allowing some circulation of air and keeping trolleys 10 cm from wall
- Avoid direct air flow from cooler or humidifier over eggs
- Be aware of ‘sweating’ when eggs are moved to setter room for incubation or for heat treatment
- Bear in mind that reducing the temperature of an egg storage room on a breeder farm might not be a good idea!
- Store eggs sharp-end-up if possible; alternatively turn them 2 – 4x/day as is done during incubation
- Apply 1 or more heat treatments during storage; that means heating up the eggs uniformly to at least 32 °C/90 °F for some hours
Optimal climate conditions in egg storage room
If longer storage than normal is foreseen, it is essential to reduce the temperature in the egg storage room (see Table 1 for recommendations) at the hatchery to a lower level than would be needed to simply arrest further development of the embryo. (See also here). A lower temperature not only reduces the rate of thinning of the albumen, thus preserving its internal quality; it also slows down other cellular metabolic processes within the embryo, which is crucial for preserving the vitality of the embryo during diapause. Make sure that the sensors used to control egg storage-room temperature and the thermometers used to check the climate are reliable and recently calibrated.
Ensure that the temperature in the egg storage room is kept uniform by allowing minimal recirculation and not placing trolleys directly against the wall; keep a gap of approximately 10 cm. Also avoid placing eggs directly in the flow of air from the cooler or humidifier. Of course, this may be difficult to do in an egg storage room that is overloaded beyond its normal capacity. An option could be to utilise ‘free rooms’ as makeshift egg storage rooms by placing mobile coolers in them, or consider renting a refrigerated trailer.
A potential risk of lower storage temperatures is ‘sweating’ (condensation of water on the egg shell surface) if cold eggs are moved to the much warmer setter room, where the temperature is about 25 °C/77 °F. Water can act as a transport medium via which bacteria on the shell can enter the egg through the pores, and this may result in exploders later on during incubation. You can help to prevent this by warming up the eggs gradually before putting them in the setter room (if you have a room available at an intermediate temperature) and by keeping relative humidity low in the setter room. For more information see here. If this is impossible under the current circumstances, consider spraying the eggs with a liquid disinfectant – better wet with disinfectant than condensation water!
The above does not automatically mean that the temperature of the egg storage room at the breeder farm should be lowered as well. It is necessary to prevent hatching eggs from cooling down too fast after oviposition, as this can cause embryonic development to be arrested before the storage-resistant stage is reached. Moreover, during truck loading there will be an increased risk of sweating if the breeder farm egg store is too cold. When eggs are not stored at the breeder farm for longer than 3–4 days, a temperature of 18–21 °C/64.4–69.8 °F is fine. When prolonged egg storage at the hatchery is foreseen, 18 °C/64.4 °F is likely to be a better storage temperature. If that is not possible, consider increasing the frequency of transporting hatchings eggs from the breeder farm to the hatchery. More information here .
To prevent the eggs from dehydrating, it is advisable to keep relative humidity in the egg storage room above 75 %. To prevent the growth of moulds, however, relative humidity should not exceed 85 %. Take care that the eggs placed close to the humidifiers do not become wet! While the use of buggy bags to trap humidity and CO2 has been promoted in the past to minimise...
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