This review discusses the benefits seen of heat treatment during storage in order to reduce embryonic mortality caused by prolonged storage in avian species.
Heat treatment during storage
The longer eggs are stored, the higher the losses in hatchability. Stored eggs have a higher rate of embryonic mortality between days 2 and 3 of incubation, and need more time to complete incubation. This causes some live chicks to be rejected at take-off because they hatch too late. Several studies have investigated the possibility to limit the hatchability loss after long storage by applying short periods of heat treatment during storage, with differing results. Heat treatment for stored eggs has been around since the 1950’s, most commonly known as SPIDES but also known as PRESI in Canada and IDEAS in Turkey. In recent years, more and more successful attempts have been reported applying heat treatment during storage, even for large-scale trials. Nicholson (2012) and Aviagen (2014) have shown a consistent improvement of the hatchability of long stored eggs (Ross 308 & Ross 708 broiler eggs, as well as various GP & GGP lines) by applying one or more heat treatments in 34 small to large-scale trials.
Effect of storage according to developmental stage
Development of the avian embryo begins immediately after fertilization in the infundibulum and continues as albumen and shell are deposited over the next 24-26 hours. The embryonic developmental stage at the moment of oviposition (egg laying) is variable for different genetic lines as well as parental ages. This may be genetically determined or linked to variations in oviductal transit time and/or body temperature. The effect of long storage times on embryonic development highly depends on the developmental stage of the embryos at oviposition. Although there are variations of staging at point of lay, according to breed type, etc. the stage should always be well away from the “point of no return”. However collection timing, farm storage, transport, etc. can all affect the embryo stage if the eggs are poorly managed and it must be remembered that to fill a commercial incubator will usually take more than one days egg collection or eggs from more than one farm. It has been reported that embryos at the pre-gastrula stage at oviposition are less able to withstand prolonged storage compared to embryos at the gastrula stage. For these embryos, incubation during storage may improve hatchability, since it can advance them to the developmental stage in which hypoblast formation is complete. In contrast, if development is already well advanced and embryos have started to form the primitive streak, incubation during storage may be detrimental since it brings the embryo in a more advanced stage of primitive streak formation (to around stage 3/4 H&H, 1951, period of active cellular migration and differentiation). Storage during such a period could impede critical embryonic processes. So there is some sort of “point of no return”- once this is reached, the embryonic development cannot be stopped anymore via returning the eggs to cold storage.
How does heat treatment during storage help?
In the egg holding room, eggs are kept at or under a so-called threshold temperature or physiological zero for development. However, some partial, but not a global or proportionate development can take place at these subthreshold temperatures. Different cells or tissues in these early embryos may have different threshold temperatures for development, resulting in uneven or disproportionate development. When this disproportionate development progresses too far it may interfere with embryonic viability and, hence also hatchability. Total heat treatment aims to be around 12 hours, but this will depend on egg size, cold store temperatures and egg temperature at removal. Using current technology adapted to work in a specialised way, has allowed the process to be refined to give accurate and consistent results irrespective of egg age, breed, size, egg weight or storage time of the loaded eggs. This consistency would the answer the question as to why previous heat treatments have been shown to give a benefit but with varying levels of success.
Storing eggs is an inevitable practice when incubating eggs on commercial scale. It is impossible to synchronize breeder egg production with final product production, which often generates high levels of losses. Heat treatment during egg storage allows a significant reduction in these logistical discrepancies. There is undoubtedly a huge potential in restoring the hatchability of stored eggs, and even improving post-hatch performance. The initial series of commercial trials demonstrated that the operational parameters required in order to achieve a degree of benefits was relatively broad; however achieving the optimal gains considerably narrowed the parameter limitations. Add to this the logistical need to minimize space usage within the hatchery, it was clear there is a need for dedicated, accurate equipment that has a practical capacity. It is crucial to accurately and consistently control the key incubation parameters, as inadequate application of the technique will result in suboptimal results or might even lead to major losses. The precise measurement and control of the egg shell temperature in the incubator, as well as a controlled and uniform warm-up and cool-down phases of the eggs, are key to achieve consistent, optimal gains. The trials currently being undertaken under precise controlled conditions indicate gains can be achieved on eggs with relatively short storage. Taking an immature embryo to a more robust developmental stage either prior to or during initial storage has a major effect on embryonic viability as this gain is seen in the improved hatch of set figures.
Presented at the 9th Turkey Science and Production Conference