Influence of Shape Index, Specific Gravity, Area-Volume Ratio, and Egg Weight Loss During Incubation on the Hatchability of Eggs from Layer Hen Grandparents

ABSTRACT

The physical parameters of eggs significantly impact embryo development and hatching success. This study aimed to examine the influence of the egg shape index (SI), specific gravity (SG), weight loss (WL), and area-volume ratio (AV) on hatching eggs from brown-layer hen grandparents. A total of 7,500 eggs were randomly selected from two batches of Lohmann Brown grandparents at various ages (39, 44, 50, 58, and 66 weeks). SG was determined by flotation (1,070 to 1,090 g/L), while egg length, width, and weight were measured digitally. Each egg was tracked to determine hatching success, with unhatched eggs collected for embryonic diagnosis. Physical parameters were categorized into minor, intermediate, and major groups, based on viability frequency; and embryonic mortality was analyzed using the chi-square test and odds ratio (OR). The study found a lower probability of embryo death in eggs with intermediate SI (75.1% to 79%) and AV (1.01 cm²/cm³ to 1.04 cm²/cm³). No significant differences were observed between SG and WL groups. SI proved to be a reliable parameter for assessing egg quality and predicting hatchability.

Keywords:
Eggs’ characteristics; Embryo; Hatchery; Layer hens

INTRODUCTION

As part of its sustainability efforts, the poultry industry has constantly been aiming to improve performance indicators such as hatchability. Considering that to achieve optimal hatching some variables within the process must be continuously controlled, physical characteristics such as the egg shape index (SI) and area-volume ratio (AV) are not often monitored.

Egg quality parameters, hatchability, and embryonic development are highly influenced by the physical characteristics of hatching eggs (Narushin & Romanov, 2002). One of these parameters, the egg shape index (SI), is essential for preventing incorrect placement of hatching eggs on setter trays, and maintaining a desirable egg shape for final customers (Icken et al., 2006). Previous studies have discussed the role of the SI on hatchability (Scholtyssek, 1994; Narushin & Romanov, 2002; Cavero & Schmutz, 2009; Sahin et al., 2009; Blanco et al., 2014), but disagree on whether it can be a good parameter to enhance hatching rates.

Over many years, eggshell strength has been incorporated by commercial breeding companies in their selection programs, since the direct selection on the percentage of cracked and broken eggs is not so effective. This way, several techniques and instruments have been developed to measure eggshell strength. Egg-specific gravity (SG) is the most widely used estimator for shell strength, as it is fast, practical, and cheap (Hamilton, 1982).

Weight losses (WL) during egg storage happens due to the shell’s water diffusion (Tona et al., 2001), and depend on the environment temperature and relative humidity, where the eggs are stored, and the length of storage period (Khan et al., 2014). Weight losse from hatching eggs are related to egg characteristics such as shell and membrane structures, and initial egg weight, as well as to important incubation conditions, such as temperature, relative humidity, and airspeed (Christensen & McCorkle, 1982).

Other important measurement in poultry studies are the egg volume and surface area, which must meet certain requirements from the early stages of incubation up to hatching (Narushin & Romanov, 2002). Moreover, most studies have focused on how egg shape can be mathematically described, with only a few trying to define the various shapes’ functions (Deeming, 2018).

Still concerning egg characteristics, most of those studies have been performed in broiler breeder hatching eggs, and little is known about the effect of physical egg parameters on layer breeder hatching eggs. Therefore, the aim of the current study was to investigate the influence of egg characteristics such as SI, SG, WL, and AV on hatchability, and their correlation with the fertility of brown eggs from laying hen grandparents.

MATERIALS AND METHODS

Hatching eggs were collected from two grandparent flocks from a Lohmann Brown female line (around 4,000 females per flock), with a male/female ratio of 10%, placed on a closed, negative pressurized house, with a full plastic slatted floor and automatic nest system, fed in compliance with the standard requirements according to the genetic line manual. In the hatchery, 10 trays, each of 150 eggs from the same production day, were randomly selected, and these samples were taken from the same flocks at 39, 44, 50, 58, and 66 weeks of age. The SG was determined by the flotation method using saline solutions from 1070 to 1090 g/L, in 5g/l intervals as described by (Freitas et al., 2004), and hatching eggs were grouped into trays by SG. Egg length and width were measured with a digital caliper (Starfer®, 0.01 mm). Based on length and width measurements, the area-volume ratio (AV) was calculated as the area over volume, according to Wang et al. (2021). Moreover, SI was calculated as width over length, according to (Wang et al., 2021). Finally, egg weight was individually measured with a digital scale (Tanita Corporation®, 0.01 g), and every egg was identified with a sequential number to be tracked until hatching. A total of 7,500 hatching eggs were included in the study.

After 7 days in storage (room temperature kept at 17°C and 70% relative humidity), eggs were set in a single-stage incubator (Chick Master^® Avida Genesis IV). For all ages, the 10 trays were set in the middle of the setter trolley, and the trolley was set next to the incubator door. At 18.5 days of incubation, egg weight was individually measured again to calculate the weight loss until transferring (WL) as the percentage 1 - (egg weight at transfer/initial egg weight). Hatching eggs were transferred to hatcher baskets, and no egg was removed during the process. At hatching, all eggs that were not found were considered as hatched. After chick take off, the remaining eggs were collected for embryo diagnosis reporting. The fertility and hatchability of fertile eggs (HOF) were calculated based on the results.

Spearman’s correlation was used for the statistical analysis comparing SI, AG, SG, or WL, and fertility or hatchability. Using Microsoft Excel, a histogram of the dates was built for each independent variable. To choose the best model for date analyses (logistic regression or chi-square) we built a viability distribution (%) for each variable. To calculate the categories of SI and AV according to viability, the number of classes was the square root of the number of categories in the SI (considering only integers), AV, or WL. In the case of SG, we considered the categories available in the database (1070, 1075, 1080, 1085, 1090, and 1095). Afterwards, the data were divided by the number of classes to find the approximate cut-off point of the categories.

Thereafter, the SI, AV, SG, and WL results were divided into three categories (according to the frequency of viability of that data), and embryonic mortality association was analyzed by chi-square test followed by calculation of the odds ratio (OR), considering a contingency table among the minor, intermediate and major categories. The programs GraphPad Prism 9.2 and R Studio were used, considering 95% confidence.

The Committee on Ethics in the Use of Animals (CEUA) of the Federal University of Uberlandia was informed of advice number A001/21.

RESULTS

Hatchability was correlated with SI, SG, WL, and AV (p<0.05). Fertility was correlated with SI, SG, and AV (p<0.05), but not with WL (p>0.05) (Table 1). A graph was prepared to illustrate the distribution of chick hatching across different values of SI, AV, SG, and WL (Figure 1). Intermediate values of SI and AV were associated with higher hatchability. Therefore, we divided the data into three categories - minor, intermediate, and major - to study their association with hatchability (Figure 1). In the case of SG and WL, it was impossible to verify the effects of extreme values. Thus, the categories were divided considering 50% of the intermediate data and 25% of the major and minor extremes.

Table 1
Correlation among SI, AV, SG, or WL and fertility or hatchability.

Variables	p-value	R-value
SI and hatchability	<0,001	0,05652
SI and Fertility	0,0452	0,02706
AV and hatchability	<0,0001	0,1122
AV and Fertility	0,0004	0,0571
SG and hatchability	<0,0001	0,06328
SG and Fertility	0,0058	0,03728
Wl and hatchability	<0,0001	0,145
WL and Fertility	0,7262	-

• SI: Shape index. SG: specific gravity WL: weight loss WL. AV: area-volume ratio.

Figure 1
Distribution of Hatchability (%) among the different categories of SI, AV, SG, and WL.

The distribution of hatchability was similar across ages (Figure 2), although the sample size was smaller in some ages and categories (Figure 3 and Figure 4).

Figure 2
Distribution of HOF by flock age among the variables.

Figure 3
Frequency distribution of hatching eggs by flock age among the variables.

Figure 4
Frequency distribution of hatching eggs for each variable.

There was an association between SI or AV and embryonic mortality. The probability of embryo death is higher in the extreme SI or AV ranges than in the intermediate ones. When the extreme categories of SI and AV are compared, there is a greater probability of embryo death in the major categories of SI (table 2). Thus, the probability of embryo death is 2.61 times greater with an SI <75% or 3.61 times greater with an SI >79%, as compared to a SI between 75% and 79% (table 2). Additionally, the probability of embryo death with an AV >1.059 cm2/cm3 is 1.70 times greater than that of an AV between 1.00 cm2/cm3 and 1.059 cm2/cm3 (table 2). A similar trend occurred in SI at different ages, except that there was no association between SI <75 and SI >79% at 44, 58 and 64 weeks of age. In the case of AV, the probability of chicken embryo death is greater with AV >1.059 than with AV from 1.00 to 1.059 at all ages, except at 39 weeks. The probability of chicken embryo death was greater with AV <1 than with AV >1.059 at all ages, except at 64 weeks. At 64 weeks of age, the probability of embryo death was 1.9 times greater with AV <1 than with AV from 1.00 to 1.059 (table 2). SG and WL had no association with embryonic mortality (p>0.05) or age in general.

Table 2
Odds Ratio among each variable and embryonic mortality by flock age.

Comparison	p-value*	OR*	39W	44W	50W	58W	64W
SI (%)
< 75 and 75 - 79	<0.0001	2,61	2.30	2.63	2.28	2.65	3.34
>79 and 75 - 79	<0.0001	3,61	4.17	3.23	3.37	3.78	5.2
< 75 and > 79	<0.0001	0.70	0.55	--	0.67	--	--
AV
<1 and 1.00-1.059	0.071	1.13	--	--	--	--	1.90
>1.059 and 1,00-1.059	<0.0001	1.70	--	1.85	1.57	1.96	2.48
<1,00 and >1.059	>0.99	1	1.85	0.68	0.46	0.63	--
SG
<1080 and 1080-1085	0.886	1.03
>1085 and 1080-1085	0.755	0.970	--	--	--	--	--
<1080 and >1085	>0.999	1.01	--	--	--	--	--
WL
<8.58 and 8.58-10.67	0.197	0.90	--	--	--	--	--
>10.67 and 8.58-10.67	0.508	1.05	--	--	--	--	--
<8.58 and >10.67	0.480	0.94	--	--	--	--	--

• *Result for all flock ages. OR appears only at ages where p≤0.05 (the symbol - means p≥0.05) at different ages. A. Shape index (SI), B. Specific gravity (SG), C. Area-Volume (AV), D. weight loss (WL).

DISCUSSION

Fertility and hatchability correlation coefficients in our study (table 1) indicate a low correlation (Mukaka, 2012), which can be explained by the evaluated variables, the interaction of factors involved in fertility, and the physical properties of hatching eggs. Based on that, the data were stratified for a better understanding.

Eggs at the extreme ends of SI and AV seemed to have lower hatchability, so we stratified the data into three categories (Figure 1) to calculate the association of hatchability with the variables studied. Through the odds ratio, we confirmed that there is a higher likelihood of embryo death in the extreme groups of SI and AV compared to the intermediate group. When we compared data specifically by flock age, we found a similar behavior, although there were few observations (low number of eggs) for the extreme groups (Figure 2 and Figure 3). As the physical properties of hatching eggs change as age increases, different cut-off points can be defined to perform comparisons with different ages accurately, so this should be considered for further studies. Like Nikolova and Kocevski (2006), we saw that the SI decreases as flock age increases (Figure 3).

Several studies have shown that eggs’ quality parameters and hatchability are influenced by the SI. However, these studies do not agree on the influence of the SI on hatchability rates, as discussed below. On the other hand, other authors found that round eggs tend to hatch better (Alasahan & Copur, 2016; Narushin et al., 2016; Patil et al., 2020). There is a positive correlation between shell stability and SI for brown and white eggs (Cavero & Schmutz, 2009). This was also confirmed by Blanco et al. (2014), who found that dynamic stiffness and breaking strength were positively correlated with the SI, confirming that higher shell stability is provided by round eggs. In laying hens, the Haugh unit (p<0.01) increased as the egg shape changed from sharp (from 68 to 71.9) to standard (from 72.2 to 75.9) to round (from 76.1 to 82.3) (Duman et al., 2016). Moreover, a positive correlation was determined between the shape index and the Haugh unit (p<0.05).

By contrast, Scholtyssek (1994) found 74 to be the optimal SI, and the authors argued that eggs with a higher index (too round) might decrease hatchability performance. According to Cavero & Shmutz (2009), SI has a negative correlation with hatchability, whereby round eggs show lower hatchability; whereas Sahin et al. (2009) did not find any significant effects of SI on hatchability. Based on the data distribution in this study, intermediate SI between 75 and 79 showed found a lower probability of embryo death.

In our study, we found that an intermediate AV and SI were related to a better hatching result. We did not find an objective reason for the similar behavior in both intermediate groups of AV and SI (the first shows the proportion of surface area per unit of volume, and the latter is the proportion of width over length). A proper explanation is that the embryo changes its axial orientation in the egg during the later stages of embryonic development (Ragozina, 1962; Rolnik, 1968). Therefore, both narrow and more markedly round egg shapes are likely to prevent the rotation of the embryo inside the egg. The SI is a better tool than AV because the SI results were more consistent across almost all ages.

Concerning SG, most of the observations were concentrated around 1.090 mg/L in our study (figure 4). This finding may be related to the favorable environmental conditions and nutritional quality of the feed that a grandparent operation demands, combined with good liver and ovary health. Due to the high number of hatching eggs in the 1.090 mg/L group, it was impossible to find significant differences (p>0.05) among SG groups.

Jin et al. (2011) reported that temperature and storage time have a great influence on the WL, albumen pH, and Haugh units of Lohmann Light-Brown hen eggs. In our study, eggs were stored for seven days at 17°C in the egg room. When studying hatching eggs, Visschedijk et al. (1985) found a variation coefficient in eggshell conductance of 22%. Due to the eggshell conductivity variation, it is not possible to estimate an optimal percentage of WL, but there is an optimal range, whereby it seems that WL should be between 6.5% and 14.0% of the initial egg weight to guarantee an adequate air cell before internal piping (Molenaar et al. 2010). In our study, around 88% of the observations were inside this range (figure 4), which may explain why there were no significant differences (p>0.05) among WL groups.

Differences in egg weight losses have also been found in other studies, depending on the hens’ age and housing system. Concerning individual factors, heavier eggs have been obtained in enriched-cage environments as compared to free-range environments at 51 weeks old; and freshly laid eggs have been found to be heavier than those stored for 14 and 21 days. The significant interaction of evaluated factors affecting egg weight loss included the pH and the Haugh unit score (Vlčková et al. 2019). In our study, the housing system was a closed, negative pressurized house with a full plastic slatted floor.

CONCLUSIONS

Concerned to the egg characteristics such as SI, SG, WL, and AV the the intermediate value of SI (75.1% to 79%) and AV (1.01 cm²/cm³ to 1.04 cm²/cm³ presented lower probability of embryo death in brown grandparents. SI is a more suitable parameter to measure egg quality, as it presents more consistent results at all hen ages. It is possible that the effects of SG were not observed because our eggs were of high quality, with few showing extreme values.

ACKNOWLEDGMENTS

The authors thank the Layer Genetics Brazil GP hatchery (Nova Granada, Brazil) and the Novo Mundo hatchery (Uberlândia, Brasil).

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• Funding
  This research was funded by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES).
• Data availability statement
  The datasets used and analyzed during the current study are available from the corresponding author upon reasonable request.
• Disclaimer/Publisher’s Note
  The published papers’ statements, opinions, and data are those of the individual author(s) and contributor(s). The editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions, or products referred to in the content.

Edited by

• Section editor:
  Maria Fernanda Burbarelli

Data availability

The datasets used and analyzed during the current study are available from the corresponding author upon reasonable request.

Publication Dates

• Publication in this collection
  01 Nov 2024
• Date of issue
  2024

History

• Received
  04 July 2023
• Accepted
  30 Nov 2023