A comparison of the various equations published for the estimation of characteristics of hen’s eggs, the importance of reporting the compression rate for shell strength measurements, and distinction between egg specific gravity and density

SUMMARY

Prediction equations allow the estimation of dependent variable from the value obtained from the measurements of an independent variable. Comparisons of estimates obtained for of 85 equations that were published for the prediction of shell strength parameters were made. Egg weight, specific gravity, length, width and thickness were the independent factors use to estimate surface area (SA), egg volume (EV), shell weight, percent shell, sphericity, thickness, compression and impact fracture strength, and shape index. Values (n = 5–20) from published results were used to create a data set for the testing of these equations.

Comparisons, based on coefficient of variation (CV), among the calculated estimates obtained with the majority of the equations (72) showed the variability was small, especially those for SA and EV, However, the CV for other equations (7) showed their estimates varied over wide range; whereas, the estimates for the remainder (6) were outside the expected acceptable range. Ten equations, as published, required an ‘adjustment factor’, either multiplication or division, in order to produce an estimate that was within the expected range.

It is essential that the rate of compression used to measure compression fracture strength of egg shell be reported because, since the egg shell is a brittle material, the value obtained when fracture strength is measured by compression is dependent on the compression rate. Without knowing the compression rate, it is not possible to establish whether the difference among published shell strength measurements is actual or due to differences in compression rates. There is a need to clarify that the ‘saline flotation method’ measures the density of the egg, NOT specific gravity. In addition, the use of various abbreviations for the same shell strength variable causes confusion that could be clarified by the development of standardised abbreviations. Finally, more care is needed to ensure the original authors are cited when reporting the sources of prediction equations.

Abbreviations: EW: egg weight; SW: shell weight; EV: egg volume; SA: surface area; L: longitudinal length; W: axial width (diameter); SG: specific gravity; STk: shell thickness; sd: standard deviation; obs: observational; CFS: quasi-static compression fracture strength; DFm: non-destructive deformation; SW/SA: shell weight per unit surface area; D_g: geometric diameter; SI: shape index; μm: micro metre; N: Newton; n: number of observations; CV: coefficient of variation

KEYWORDS:

• Egg shell
• strength
• equation
• estimation
• variability
• egg density
• compression rate

Acknowledgements

No specific funding was received to write this review. The author is very gratefully to Dr. Wayne Bryden, University of Queensland, Gatton, Queensland, Australia for accessing references for the author, reviewing the draft, and for his constructive suggestions.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Notes on contributors

R. M. G. Hamilton

Dr. Hamilton was a research scientist with Agriculture and Agri-Food Canada initially at their Animal Research Centre in Ottawa and later at the Atlantic Food and Horticulture Centre first in Kentville, Nova Scotia and then at the Nova Scotia Agriculture College at Truro. His areas of research were: egg shell strength and factors that affected its measurement; nutritional and physiological factors that influence egg shell quality; effects of trichothecene mycotoxins on the productive performance of poultry; evaluation of naked oats (Cavena), low-trypsin soybeans and ‘sweet’ lupins as feed ingredients for poultry, and barley for laying hens; and effects of feed texture on poultry production. He has authored more than 150 articles of which 95 are papers published in national and international scientific journals. As a graduate student (McGill University and Western University) he was awarded an NRC and MRC Canada, and an Ontario Graduate fellowship. In 1970 he was recipient of the Honour Student Award from the American Oils Chemists’ Society and in 1981, the Young Scientist Award from the Poultry Science Association. By invitation, he was on the committees for graduate students, external examinator of thesis submitted to national and international universities by graduate student. Finally, he was the leader of the poultry nutrition program in Ottawa and Kentville for over 10 years. Dr. Hamilton was a member of the organising committee that planned the XXI World’s Poultry Congress that was held in 2000 in Montréal.