QUALITY AND CHEMICAL COMPOSITION OF EGGS AFFECTED BY REARING SYSTEM AND HEN'S AGE

: The aim of this study was to evaluate the effect of rearing system and hen's age on quality and chemical composition of eggs. The tested rearing systems were: cage, floor and organic (30 birds Isa Brown hybrid per group). Fifteen eggs per group were collected for analyses in each of three phases of the productive cycle (32, 48 and 72 weeks hen's age). In these samples were investigated external (egg weight and egg shape index), internal (albumen height, Haugh unit and yolk colour) and chemical composition of eggs (dry matter, minerals, protein and lipids content). The general conclusion is that the egg weight and Roche values were increased, while egg shape index, albumen height and Haugh unit were decreased with hen's age. Organic eggs had higher albumen height and more Haugh units in the 48 and 64 weeks compared to the floor and cage eggs. Yolk colour of the floor and cage hens most dependent on the hen's age, on the other hand - yolk colour of organic hens most dependent on the grass availability at the outlet. Eggs from the organic rearing system had a lower dry matter, proteins and lipid content as compared to the cage system.


Introduction
Although producers around the world are increasingly converting their facilities to cage-free, data from the International Egg Commission (2016) show that the majority of laying hens (89.7%) are housed in a cage system. The main reason why the cage system of laying hens is the most widespread in the world is that it is the most economical -it provides that the price of eggs is low and that product be accessible to all segments of the population. Egg consumption in Hungary, USA

Material and methods
The study was carried out in three rearing systems: cage, floor and organic.
In each system, the same genotype of laying hens (Isa Brown hybrid) was housed -30 birds per group. Each bird in battery system had at its disposal 750 cm² floor area. In floor and organic system the stocking density was 2.5 birds/m 2 . The organic layers also had about 5 m 2 per birds available outdoor area which was covered with grass and bushes and these hens were able to supplement their diets using vegetation and small creatures living in an outlet. Cage and the floor laying hens were fed with the same standard commercial diets whose average chemical composition is shown in Table 1. In organic system, except in the facility, the feeders and drinkers were located in the outlet. It is important to note that the diet from organic hens was complete without additions of synthetic amino acids, vitamins and minerals, with the use of more 80% organically grown components. Its chemical composition is also shown in Table 1. In all of three experimental groups feed and water were available ad libitum. Fifteen eggs per group were collected for analyses in each of three phases of the productive cycle (32, 48 and 72 weeks hen's age). In these samples (eggs were one day old) were investigated external, internal and chemical composition of eggs.
-Egg weight was measured on an electronic scale with accuracy of 10 -2 g.
-Egg shape index, or short-axis to long-axis length ratio (%), was determined using a special device (

Results and discussion
Egg quality parameters are presented in Table 2 shows that the egg weight was significantly influenced by hen's age (p≤0.05) while the effect of the rearing system was not significant (p≥0.05). Generally, egg weight was increased with hen's age, which is consistent with the results published by Zita et al. (2009) andRakonjac et al. (2017). It is important to note that the egg weight between the 32 and 48 weeks old hens differed significantly (p≤0.05), while in the later period (48-72 weeks) there were no differences (p≥0.05). That indicated that in an early period of laying there was increasing in egg weight but in later phases of egg production egg weight remain static (Padhi et al., 2013). On the other hand, the effect of rearing system on egg weight was not significant, similar to the results reported by the Mugnai et al. (2009) andRakonjac et al. (2017). Contrary to this, Lolli et al. (2013) and Kralik et al. (2013) found higher egg weight in free range than the cage system, while Ferrante et al. Lewko andGornowicz (2011) found the opposite -a higher egg weight in the cage than in the rearing systems with the outlet.
Egg shape index decreased with hen's age, and eggs from 32 week age hens had higher values of the index form of eggs from the 48 and 72 week age hens. As well as the weight of the eggs, the rearing system did not affect differences in this parameter (p≥0.05). Škrbić et al. (2011) also found that the egg shape index value decreased with hen's age (r = -0.15). These results Nikolova and Kocevski (2006) were explained by the fact that the higher shape index of eggs from younger hens showed a presence of more eggs of rounded shape, while lower shape index of eggs from older layers showed more percentage of eggs of an elongated shape which is typical for hens in deep age. Similar results were published and by Zita et al. (2009). The rearing system did not influence differences in egg shape index, which is in agreement with the findings by Đukić-Stojčić et al. (2009), Lewko and Gornowicz (2011), Ahhamed et al. (2014 and Dikmen et al. (2017). Haugh unit was decreased throughout the experimental period, due to the decreasing albumen height with hen's age. A significant effect of the age of hens on albumen height determined also Ojedapo (2013). Our results that Haugh unit decreases with hen's age (p≤0.05) are consistent with the findings by Škrbić et al. Padhi et al. (2013). Effect of rearing systems on Haugh units was not significant in the 32 weeks (p≥0.05), but eggs from organic rearing system had more Haugh units in the 48 and 64 weeks compared to the floor and cage (p≤0.05). There are a numerous reasons why the organic eggs have a more Haugh units compared to eggs from the floor and the cage system: less stress in the oviduct tract Quality and chemical composition of eggs… 339 (Castellini et al., 2006), effect of the higher concentration of ammonia in cage and floor systems that enhances the albumen pH affecting thus its consistency (Minelli et al., 2007), the high vitamin C content in albumen influenced intake fresh grass from outlet (Mugnai et al., 2009).
Both studied factors and their interactions had a significant effect on the yolk colour (p≤0.05). Generally, Roche values increased in floor and cage system with the hen's age. On the other hand, the yolk colour of organic eggs was relatively constant throughout the entire experimental period. Increasing Roche value with the hen's age in our experiment is in accordance to research Rizzi andChiericato (2005) andŠkrbić et al. (2011). Both groups -cage and floor consumed constant amount synthetic carotenoids in feed throughout the entire production cycle, and increasing intensity of yolk colour was a result of their greater synthesis in the body with hen's age. On the other hand, in organic laying hens, the intensity of yolks colour was much more dependent on season and availability of grass at the outlet (no synthetic colours in their feed).
Organic hens had available a significant amount of grass rich in carotenoids in an outlet in week 32, which caused a higher Roche values compared with a floor and cage laying hens (p≤0.05). At week 48, grass amount at the outlet was reduced, which caused that there were no significant differences between Roche values in all three investigated rearing systems (p≥0.05). Finally, at week 72, all the amount of the grass at the outlet was spent, and the floor and the cage produced eggs had a significant higher Roche values then organic (p≤0.05)organic hens no longer had grass available as an additional source of carotenoids, while the floor and the cage hens received synthetic carotenoids through feed.  Table 3 shows that the eggs from the organic rearing system had a lower dry matter content as compared to the other two systems (P≤0.05). The results similar to these are given by Matt et al. (2009), which found the higher content of dry matter in cage eggs compared to organic.
Ash content was significantly influenced by studied factors and their interactions (P≤0.05). These results are in agreement with the results published by Zhu et al. (2015), which found a significant difference in the content of a large number of minerals in eggs between the cage and the free-range layers.
Cage produced eggs had a higher content of protein and lipids compared to the organic eggs (P≤0.05). A significant effect of the rearing system on the content of protein and lipids in eggs was determined by numerous authors. Mat et al. (2009) obtained the similar results in their research, the cage eggs had higher protein and lipids content compared to free range eggs. Also, Pavlovski et al. (2011) determined higher protein content, andRadu-Rusu et al. (2014) higher lipids content in cage eggs compared to free range.

Conclusion
Based on the results of these investigations it can be concluded that the rearing system and hen's age had a significant effect on egg shape index, albumen height, Haugh unit and yolk colour, while egg weight was influenced only by the hen's age. The interaction of the observed factors significantly influenced yolk colour. On the other hand, the rearing system had a significant effect in all four parameters of the chemical composition of eggs, while the significant effect of the hen's age on these parameters was established for mineral and lipids content.