**2. Materials and methods**

*Animal Models in Medicine and Biology*

resulting from the specifics of the poultry reproduction.

another few days after the hatching [1].

studied on a much smaller scale in this context.

of the cases, regardless of the chicken sex.

76.09% (male chicks) and 76.41% (female chicks).

Keeping, breeding, and management of the parent flock during the eggs for fertilization production period, as well as the incubation of eggs for hatching, are very specific, specialized, and complex stages of the production process. Apart from the genotype (breed, hybrid), some nongenetic factors (age of the parent flock, keeping system and diet, morphometric egg traits, egg storage period, incubation conditions) play important role in the management of the one-day chicken production of the heavy hybrid (meat) and the light type hybrid (eggs for consumption),

The main difference between the reproduction of the birds—poultry—and the mammals is the fact that birds do not give birth to live offspring as mammals do. Instead, the new organism develops outside of the female's uterus, in the egg. The poultry breeds by means of eggs which must be fertilized in order to produce the young—offspring. Well fertilized egg for hatching is an embryo "package" with all the necessary nutrients which facilitate its development until it's hatched and for

Effects of the nongenetic factors, primarily effects of age on egg incubation values, but also effects of the egg mass, length of the storage period, mechanical and structural egg characteristics on the effective production of the one-day chicken, were extensively studied, particularly in the case of the broiler parents (heavy hen hybrids). Pure breeds and parent flocks of various light line hybrid hens were

Studies of Skewea et al. [1–14] were conducted, among other reasons, to demonstrate effects of age of different broiler parent flocks of various hybrids on incubation values of eggs (fertilization rate, hatchability, and the quality of one-day old chicken), as well as their phenotype correlation. The general observations would be that the egg mass and the newly hatched chicken mass increases with the age of the broiler parents; the fertilization and hatchability rate reaches its peak in the middle of the production cycle; relative share of the chick in the egg mass increases slightly with the age; and while the egg shape index in various stages of the production cycle demonstrated substantial variability. The absolute phenotype correlation between the egg mass before incubation and the one-day old chicks was determined in most

Narushin and Romanov [5, 15] point out that the egg shape index has significant effects on incubation indicators, and thus that eggs of abnormal shapes should not be used for incubation because they prevent the normal embryo development which

This observation was confirmed by Mitrović et al. [16] who studied mechanical (physical) and incubation values of eggs of two different hen breeds (Naked Neck hen and Sombor kaporka) kept in semi-intensive keeping systems. In their study, the egg shape index was 71.01 and 72.04%, respectively, and the relative share of the chicken in the egg mas was 73.61 and 76.68%, respectively. Abanikannda and Leigh [17] report higher egg shape index in eggs which hatched male chicks (Anak and Marshall hybrids) than in eggs which hatched female chicks (75.25 and 74.53%; 76.27 and 76.00%). In case of the Ross hybrid, the egg shape index measured was

In general, the literature survey shows that most of the authors studied effects of the nongenetic factors on egg incubation values and the production of one-day chicken of both sexes coming from broiler parents. This is understandable since both sexes of the heavy hybrids are used for the meat production. However, in case of light hybrids egg incubation, male chicks are generally destroyed, while female

results in increased embryo mortality during the incubation period.

chicks are used for the breeding and the commercial egg production.

**1. Introduction**

**220**

The experimental part of this study—incubation of the light line ISA Brown hybrid parent flock eggs—was conducted in the incubation station of the private poultry farm "Jugokoka," in Belgrade, Serbia.

Eggs originated from the flock bred on the parent farm in an installation which housed 5000 laying hens and 620 roosters during the production phase. With the purpose to determine morphometric egg traits and incubation egg values as well as the sex of the hatched chicks, eggs were collected from both the younger flock (YF) aged 33 weeks (YF33) and the older flock (OF) when the flock was 49 weeks old (OF49).

In both cases, incubated eggs were 4 or 5 days old. A total of 252 eggs were randomly collected for the first incubation (YF), and 225 for the second incubation (OF)—a total of 477 eggs. All eggs were laid in incubators of equal capacities and produced by the same manufacturer. The number and percentage of fertilized eggs were recorded at each round of laying of eggs into the incubator, as well as the number and percentage of hatched chicks out of the number of incubated and fertilized eggs, the number and percentage of eggs with dead embryos, and the progression of the mass loss until day 18 of the incubation period. This means that morphometric indicators of all eggs were individually measured before these were laid into the incubator. Egg mass, length and width of each egg were measured, marked on the shell with pencil, and each egg was disinfected in formaldehyde vapors. Upon completion of the incubation period, individual mass of the newly hatched chicks, both female and male, was measured (different sexes of this breed have different feather color).

Before the eggs were laid into the incubator, following measurements were made: egg shape index and egg volume, absolute and relative egg mass loss until day 18 of incubation, and the percentage of the chicken in the egg mass (relative share of the chicken in the egg mass).

Egg shape index (ESI) was calculated according to the following formula: ESI = EW (egg width)/EL (egg length) × 100. Particular attention was given to those eggs which hatched live and healthy chicken.

Egg volume was calculated according to the formula [34]: *V* = (π/6) × *L* × *W*<sup>2</sup> where *V* = egg volume; *W* = egg width, *L* = egg length; π = constant, 3.1416. When moved to the incubator hatching tray (day 18), eggs were individually placed

in separate compartments in order to be sure which chicken was hatched from which egg. Based on the egg mass and the newly hatched chicken mass, relative share of the chicken in the egg mass was calculated, that is, chick percentage (CP) in the egg mass, according to the following formula: −CP = [(chick mass − CM)/(egg mass − EM) × 100].

Information obtained in this way was entered into a database designed by the statistical software IBM SPSS statistics Version 22 (2013). Basic data processing was conducted by applying the standard variation statistical methods (descriptive statistics): arithmetic mean (*x*¯), […] arithmetic mean error (S¯x), standard deviation (*S*), and variation coefficient (VC).

Difference in significance between observed morphometric traits was tested by application of the corresponding variance analysis (two-level factorial experiment—2 flock ages × 2 sexes) with uneven numbers of repetition per treatment—classes, including interaction.

Following is the mathematical model of the variance analysis:

Yijk = μ + FAi + ESC + (FA × ESC)ij + eijk,

where:

Yijk—observed morphometric value in *i-*flock age, of the *ј*-egg/chick sex, and *k*-repetition;

μ—general mean;

FAi—effect of *i*-flock age (YF33 and OF49);

ESCj—effect *ј*-egg sex-chick during incubation period;

(FA × ESC)ij—effect of interaction between *i*-flock age and *ј*-chick sex; eijk—accidental error.

Based on the variance analysis and the results of the *F*exp. values, all significant and very significant differences were graded by Tukey test. Additionally, phenotype correlation coefficients (rp) between observed morphometric traits, primarily between the egg mass and egg shape index and other morphometric indicators, with stress on the chick's sex, were calculated according to the relevant formula.
