Evaluating the exterior quality of hatching eggs

Egg quality has a significant impact on hatchability and chick quality. Since a hatchery manager’s main aim is to produce the highest possible number of first-quality chicks, the evaluation of hatching egg quality should be an essential part of the standardized procedures in modern hatcheries. Quality factors for eggs can be divided into two general groups: exterior and interior. This article presents an overview of the main exterior quality factors. 

Exterior quality factors for hatching eggs

Good egg quality is a prerequisite for the production of healthy day-old chicks. It is therefore important to have a quality control procedure in place to evaluate the quality of incoming hatching eggs. Carefully checking the exterior qualifications of the eggs prior to incubation is strongly recommended. It can be helpful to break down the quality control process into steps and focus on the following factors: egg shape and size, eggshell quality, eggshell cleanliness and egg colour. 

1. Egg shape and size

The first thing to look at when evaluating incoming eggs is their shape and size. A good-quality hatching egg has an oval shape with a large end (air cell end) and a clearly recognizable sharp end. The egg receives its particular shape because it is pushed through the oviduct. It is also free of deformities. The shape index of a hatching egg is defined as the ratio of the width to the length of the egg. As an example, an egg with a width of 4.2 cm and a length of 5.7 cm has a shape index of 74, which is considered to be within the optimal shape index range of 72-76. 

2. Eggshell quality

The eggshell has an essential function in providing a sealed, protected environment for the embryo to grow in while allowing the transfer of gases and water, which are vital to the embryo’s development. Therefore, it is important that the eggshell is strong and intact. Eggs with the following deficiencies should never be incubated: 

• Thin shell
• Shell with calcium deposit (white, irregularly shaped spots deposited on the shell)
• Wrinkled shell
• Mottled shell (translucent areas) 
• Cracked shell (hairline cracks or any other type of cracks)

Abnormal shells may result from improper nutrition (lack of calcium, phosphorus, vitamin D), disease or the poor physical condition of the hen. Shell thickness is also influenced by flock age: As a hen ages, her eggs become larger, which means the proportion of calcium per egg lowers, causing the eggshell to be thinner. 

The most common test for shell thickness is a specific gravity test that uses the salt bath method. 
Several salt baths of varying specific gravity – salt concentrations ranging from 1.060 to 1.090 in increments of 0.005 – are prepared. The specific gravity of the solution in which an egg floats is the specific gravity of the egg. The eggs are first placed in the tub with the lowest salt concentration. The specific gravity estimate is recorded for those eggs that float. The eggs that sink are removed and placed into the next tub until all eggs have been recorded to float. 

Past trials have revealed that eggs with specific gravity values between 1.070 and 1.085 indicate a good shell quality and generally give better hatchability results. During those trials, specific gravity also proved to be a more determining factor in reaching high hatchability rates than egg weight loss during incubation. In other words, good eggshell quality is critical for successful incubation and hatching. 

3. Eggshell cleanliness

It is important to keep only clean eggs for hatching. A dirty egg can be a sign of contamination and should never be incubated: 

• A shell that has dirt or foreign material adhering to its surface 
• A blood-stained shell 

Moreover, an egg that appears ’clean‘ can still have a certain count of bacteria covering its shell. Using ultraviolet light will reveal the true condition of the eggshell.