The Effect of Turning Eggs on Embryonic Structures

What is turning, and why should it be investigated?

Incubation is a surprisingly complex topic with relevance to several financial industries. The production of eggs and chicken is vital to western consumerism, and the sale of day-old chicks is a smaller but still profitable niche. To optimize hatch rates, one must have optimized embryonic development. Several factors outside of temperature and humidity influence this, of which the first, and most important, is turning.

When used within the poultry industry, the term ‘turning’ refers to rotating eggs back and forth by forty-five degrees a minimum of three times per day, either by hand or with a machine. This mimics the action of a broody hen moving back and forth on a nest and is commonly reputed to increase hatch rates. However, this seemingly well-grounded idea is often thrown around without citing any research, with the result that some will argue against the practice based on personal experience alone. To best clarify the effects of turning, one must look at what turning impacts, namely, the development of embryonic structures. The rate of albumen absorption, membrane adherence, and the development of the chorioallantoic membrane (a combination of the chorion and allantois, two structures within the egg that grow together to form a vital part of the embryo’s temporary structure) comprise three major areas of investigation to find potential links and impacts of turning practices. Other factors such as porosity, water loss, and hot spots can all contribute to structural development and must be controlled to properly investigate the consequences of turning.

Extant research contains a significant amount of evidence by which to form conclusions. It is an important question to poultry producers who want to squeeze every percent out of hatch rates and therefore has been thoroughly documented. Though this extensive documentation is valuable, little effort has been made to boil it down into a form comprehensible to the layman while retaining scientific logic and reasoning. The small-scale poultry industry would benefit from research into the existing body of evidence using controlled studies to form solid conclusions about turning efficacy. Despite the research all but proving that turning is both beneficial and necessary for proper development, there are those that claim either that turning is not necessary or that it can be stopped earlier than day eighteen of incubation. The intent of this article is to investigate those claims, and if permissible, to refute them through a structured analysis of the available data. If research suggests some basis to these claims, then that basis will be explored and consolidated with opposing data to come to a conclusion.

Overall, what does the current literature say about turning?

Current literature strongly suggests that turning has a sizeable impact on the development of the chorioallantois, the absorption of albumen, and rates of adherence of the embryo to wall membranes. The period of incubation from days three to seven seems to be the most important, as eggs that are turned only outside of this window present with significantly reduced hatch rates [1]. Also of interest is the evidence that suggests that turning after day fifteen does not contribute to hatchability, despite common practices involving turning eggs until day eighteen. Other literature, however, noted marked drops in hatch rates when turning was stopped prior to day eighteen [2]. This discrepancy should be investigated, and potentially charted to best clarify which is the most relevant result and which may have been influenced by other uncontrolled factors. The objective of this article is to not only look at the results of various studies, but to quantify them in a manner appropriate for the statistics. Individually, one can pull sources that back up either claim. However, when put against each other, it seems likely that one will emerge as better statistically supported.

What sources will be used when discussing turning?

The chorioallantoic membrane has been studied in medical journals due to its contributions to human medicine, which has had the less intentional side effect of improving knowledge of the CAM as it relates back to poultry production. Several poultry science journals and textbooks have information on embryonic development as well. Location of the applicable research will involve searching through databases such as library resources, open texts on sites such as PubMed, and reference to personal collections of textbooks on avian physiology. These sources contain peer-reviewed articles that in some cases have even been duplicated and therefore have a higher degree of reliability. Some original data will be included from previous trials set up specifically to gauge the impact of specific turning methods on chorioallantoic membrane development. It is impossible to statistically validate this data given the extremely small sample size and the fact it is not verified to be reproducable, but it provides a personal proof-of-concept for the larger, statistically validated studies done by other researchers.

Why is this investigation relevant?

This article intends to begin to explore the nuances of the impact of turning on embryonic structures with the aim of improving practices particularly in the small-scale poultry industry. There is somewhat of paradigm existing around turning practices without much originality or research into different strategies that may be more effective. Understanding how turning influences development, then, is hoped to fuel a greater understanding and curiosity in turning and how the development of these structures may be improved.


[1] Deeming, D. Charles. “The Role of Egg Turning during Incubation.” Avian Biology Research, vol. 2, no. 1-2, Apr. 2009, pp. 67–71, 10.3184/175815509x431849.

[2] Tona, K., et al. “Effects of Strain, Hen Age and Transferring Eggs from Turning to Stationary Trays after 15 to 18 Days of Incubation.” British Poultry Science, vol. 42, no. 5, Dec. 2001, pp. 663–667, 10.1080/00071660120088614.


Introduction

Within the world of poultry production, much emphasis is put on optimizing the process of incubating eggs to create the next generation of birds. Researchers have put significant time into investigating various ways to improve hatch rates, and by association, embryonic development. Among the three most influential categories of contributing factors (temperature, humidity, and turning) the one that has the most sway over various internal structures is turning. Internal structures such as the chorioallantoic membrane, or CAM, has been confirmed to be heavily influenced by turning procedures. Albumen absorption rates and the adherence of embryos to shell membranes can also be impacted. The research used in this article presumably generalizes to most bird species, though for the sake of attention to detail, the species studied will be specified. Individual experiences may lead one to conclude turning is not necessary for proper development of embryonic structures, but evidence strongly shows that turning eggs until at least the fifteenth day of incubation is essential for optimal development and hatch rates.


Claims and Evidence

When reviewing the current literature, one cannot help but note that most of it indicates that turning, and turning correctly, is of utmost importance. The period between days three to seven is particularly influential, as when eggs are turned outside of that window but not inside it, hatch rates are still significantly reduced. (Deeming 67). According to Boleli et al., “It [turning] is critical particularly during the first week of incubation, due to the long distance between the embryo and the shell, and to the high albumen density.” Two sources, then, verify this claim that turning during the period around three to seven days of incubation is imperative to optimize hatch rates and development.

In addition to the time frame, the angle also appears to have significance to embryonic development. Eggs turned at an angle shallower than forty degrees presented with significantly higher rates of malpositioning (Elibol and Brake 1433). The standard angle of turning of forty-five degrees is out of the danger zone for high rates of malpositioning, indicating that changing the paradigm by turning eggs shallowly is not a beneficial step and may easily reduce hatch rates via malpositioned embryos dying prior to hatch. When turning geese eggs at a seventy-degree angle, growth and hatch rates were significantly higher. (Guo et. al). This may give some room for the standard procedure of turning to be interrupted. However, further research needs to be done to verify this holds true for other species of birds.

Also of interest is the albumen quantity. Greater quantities of albumen remailing at the end of incubation indicate unutilized resources. This can be seen to a greater effect in eggs that have not been turned (Tona K. et. al). For optimal embryonic development, it is desirable for the embryo to use everything at its disposal to grow into a vigorous, profitable chick. Turning, then, increases the absorption rates of the albumen, contributing to chick health and the proper development of an embryo.

Original Research

Though not large enough to be statistically validated, a small experiment by the author will be used to provide a personal proof of concept for the existing data, namely, that embryonic structures can be influenced by the frequency and method of turning. The subject consists of eight shipped Japanese quail eggs, which all survived until the lockdown period past day eighteen, four in each group. Group A was put in semi-automatic turners, going back and forth forty-five degrees, and turned three times a day. They were also spun a quarter turn every time they were turned in a standard fashion. This was intended to explore the theory that shifting the gravitational pressure from the egg contents would encourage the development of the CAM to fill the bottom of the egg rather than stopping partway down the sides, which can influence hatch rates. Group B was turned normally in the same type of turners.



Based on the evidence from a sample set (n = 8), eggs turned without being spun presented with a fifty percent higher rate of total closure than the eggs turned normally. However, fifty percent of the air cells in group A were saddled (dipped at the side,) and none of the air cells in group B were. This indicates that despite assisting the CAM development, modifying one’s turning method may damage the other membranes present in a shipped egg.

Opposing Claims

Tona et. al have done research that suggests that not only must eggs be turned until day 15, but there are negative effects present when stopping prior to day 18. This conflicts with much of the other available evidence. They then go on to say that it is most important for eggs from older flocks, leaving some questions about what procedure is optimal for eggs from younger flocks.

Anecdotal evidence from people or groups sometimes suggests that turning is not necessary at all. These claims are not formatted as a scientific study, but rather as individual observations over a period.

In another article, Elibol and Brake claim that the most important window for turning is days 0-2. This runs counter to and is outnumbered by the claims by Deeming and Boleli, namely that days 3-7 comprise the most important days out of the incubation period for turning.

Investigation of Opposing Claims

In the study by Tona et. al, one important factor was not adjusted for: the age of the flock. The authors concede that it may be especially important when the flock’s average age is higher compared to a young flock. This evidence, then, can not be directly compared to other evidence that suggests no link between early dismissal of turning procedures and lowered hatch rates.

Most of the anecdotal arguments that are made claiming that things such as turning timeframe have no effect are based on extremely small sample numbers (n < 50). The studies done on larger batches (n > 1,000) are much more statistically reliable. Additionally, when results are kept from memory and not logged, there is a greater opportunity for misconception of the results; a slight decrease in hatch rates would likely not be noticeable.

Conclusion

When the available evidence is weighed, it becomes clear that there are concrete benefits to retaining conventional turning practices, but there may also be a benefit to escaping that paradigm and twisting the eggs when turning or turning at a wider angle.


Boleli, IC, et al. “Poultry Egg Incubation: Integrating and Optimizing Production Efficiency.” Revista Brasileira de Ciência Avícola, vol. 18, no. spe2, Dec. 2016, pp. 1–16, 10.1590/1806-9061-2016-0292.

Elibol, Okan, and John Brake. "Turning frequency during incubation." Poultry World, no. 4, 2017, pp. 52-53. ProQuest, https://www.proquest.com/trade-journals/turning-frequency-during-incubation/docview/1915356198/

Elibol, Okan, and John Brake. “Effect of Egg Turning Angle and Frequency during Incubation on Hatchability and Incidence of Unhatched Broiler Embryos with Head in the Small End of the Egg.” Poultry Science, vol. 85, no. 8, 1 Aug. 2006, pp. 1433–1437, pubmed.ncbi.nlm.nih.gov/16903474/, 10.1093/ps/85.8.1433.

Deeming, D. Charles. “The Role of Egg Turning during Incubation.” Avian Biology Research, vol. 2, no. 1-2, Apr. 2009, pp. 67–71, 10.3184/175815509x431849.

Guo, Binbin, et al. “Wider Angle Egg Turning during Incubation Enhances Yolk Utilization and Promotes Goose Embryo Development.” Animals: An Open Access Journal from MDPI, vol. 11, no. 9, 24 Aug. 2021, p. 2485, pubmed.ncbi.nlm.nih.gov/34573451/, 10.3390/ani11092485.

Tona, K., et al. “Effects of Strain, Hen Age and Transferring Eggs from Turning to Stationary Trays after 15 to 18 Days of Incubation.” British Poultry Science, vol. 42, no. 5, Dec. 2001, pp. 663–667, 10.1080/00071660120088614.

Tona, K., et al. “Effects of Turning Duration during Incubation on Embryo Growth, Utilization of Albumen, and Stress Regulation.” Poultry Science, vol. 84, no. 2, 1 Feb. 2005, pp. 315–320, 10.1093/ps/84.2.315.