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Citation of this paper

Effects of breed and management system on egg quality traits of two improved dual-purposes chicken breeds

F S Guni, S H Mbaga1, A M Katule1 and E H Goromela2

Tanzania Livestock Research Institute - Uyole, P O Box 6191 Mbeya, Tanzania
fadhili.guni@yahoo.com
1 Department of Animal, Aquaculture and Range Sciences, Sokoine University of Agriculture, P O Box 3004, Morogoro, Tanzania
2 Tanzania Livestock Research Institute - Naliendele, P O Box 1425, Mtwara, Tanzania

Abstract

This study was conducted to evaluate the egg quality traits of Sasso and Kuroiler chickens under semi-scavenging (on-farm) and deep litter (on-station) management conditions, as well as the phenotypic correlations between the traits. A total of 666 fresh eggs (246 from on-farm and 420 from on-station) were used to evaluate the external and internal egg quality traits. The eggs were collected from 240 hens raised on-station and 320 hens raised on-farm. The external egg quality traits evaluated were egg weight, length, width, shape index, shell weight, shell thickness and shell ratio while the yolk weight, albumen weight, yolk ratio, albumen ratio, albumen height and Haugh unit were the internal egg quality traits evaluated. The recorded data were analyzed using the General Linear Models (GLM) procedure of SAS software (SAS 2009). The results show that the mean values of all egg quality traits studied were higher for on-station than on-farm except shell, yolk and albumen ratios which did not differ between the two management conditions. With regards to breed effects, Kuroiler chickens had higher values for egg weight, egg length, yolk weight, albumen height and Haugh unit than Sasso chickens. Significant interaction effect of management system and breed was observed on egg weight and eggshell ratio. The Pearson’s correlation coefficients showed that egg weight was positively correlated with all external and internal egg quality traits of both Sasso and Kuroiler chickens, except with shape index and yolk ratio for external and internal egg quality respectively.

Keywords: egg quality, Kuroiler, on-farm, on-station, Sasso


Introduction

Poultry eggs have been traditionally considered as an important source of nutrients for humans, and nutritionally are a complete food that is used throughout the world regardless of religion and ethnic group (Stadelman and Cotterill 2001; Kraus and Zita 2019). Nowadays, it is widely recognized that eggs are more than a source of nutrients, but also play important roles in most pharmaceutical, food-processing and cosmetic industries (Mine and Kovacs-Nolan 2004; Abeyrathne et al 2013). In the egg processing industries, the shell, albumen and the yolk that form the egg, as well as their proportions, affect the amount and price of the product (Altan et al 1998).

Egg quality is the general term that refers to general standards which define both internal and external quality. It has also been defined by Stadelman (1977) as the characteristics of eggs that affect its acceptability to the consumers and is a more important price contributing factor in table and hatching eggs. External egg quality traits include size and shell qualities, while the internal egg quality traits include yolk and albumen qualities. The eggshell is an economically important trait as it determines the ability of eggs to withstand transportation shocks from producers to consumers (Mertens et al 2006). It has been reported by Bobbo et al (2013) that approximately 7-8% of the total amount of eggs get broken through the transfer from the production to the consumer leading to serious economic losses both to the producers, dealers and consumers (Hamilton 1982). The eggshell is also necessary to impede pathogenic challenges from the external environment of eggs and so to reduce food poisoning risks (Coutts et al 2006; Mertens et al 2006). On the other hand, albumen and the yolk in eggs have several proteins with functional properties such as nutrition, health, and antimicrobial effects (Kovacs-Nolan et al 2005; Ko and Ahn 2008; Abeyrathne et al 2013). There are some propositions that eggs with the heaviest yolks and the largest yolk to albumen (Y: A) ratios may contain the highest amounts of cholesterol (Hussein et al 1993; Campo 1995). Given that possibility, eggs containing a small proportion of yolk and a large proportion of albumen would appear to be suitable for consumers of table eggs, whereas eggs containing a large proportion of yolk should be more appropriate for processed foods such as mayonnaise, baking goods, creams and omelette, which use the yolk as a major ingredient. So, the knowledge of these traits and influencing factors is important. Previous studies have shown that the egg quality traits are influenced by several factors; among which are genotype and management system (Matt et al 2009; Tang et al 2015).

Sasso and Kuroiler chickens are dual-purpose breeds that have been introduced in Tanzania as a way of improving the productivity of the poultry industry and improve people’s livelihoods. The productive performance in terms of growth, egg production and survivability of these breeds has been recently evaluated under different management systems and environments, and the breeds have shown promising performance in Tanzania (Sanka et al 2020; Guni et al 2021a; Guni et al 2021b), Ethiopia (Kidie 2019; Biazen et al 2021) and Nigeria (Bamidele et al 2019). However, little research has been done on the egg quality traits of these breeds under different management systems in Tanzania. Therefore, this study intended to evaluate the egg quality traits of Sasso and Kuroiler chickens under semi-scavenging (on-farm) and deep litter (on-station) management conditions to establish if there are significant breed and management system effects on egg quality characteristics. Understanding the phenotypic correlations that exist between egg quality traits is also important for breed selection and trait improvement.


Materials and methods

Location of the study area

The study was conducted for a period of 52 weeks from December 2018 to December 2019 using Sasso and Kuroiler chickens under on-station and on-farm conditions. The on-station study was conducted at Sokoine University of Agriculture (SUA). The University is located at the foothills of the Uluguru Mountains in Morogoro, Eastern Tanzania, about 550 m above sea level. The on-farm study was conducted in two villages i.e. Wami-Sokoine and Wami-Luhindo about 45 Km from the University.

Management of chicks during brooding

A total of 1200 (600 Kuroiler and 600 Sasso) day-old chicks were purchased from AKM Glitters in Dar es Salaam and Silverlands in Iringa regions respectively to be used in this study. Brooding was done for six weeks at the Poultry farm of the Sokoine University of Agriculture. On arrival these chicks were weighed, wing tagged and thereafter placed into the deep litter brooding pens, where a 5 feet diameter circle was made using a brooder guard with a capacity of 300 chicks for each brooder. Commercial feeds purchased from the Silverland Company were used for both breeds throughout the brooding period and the on-station experiment. During the brooding period, chicks were fed a starter diet in form of crumbles containing 2941 Kcal ME/kg and 21.2% CP from day old up to the end of 2nd week and chick mash containing 3049 Kcal ME/kg and 20.3% CP from the 3rd up to end of the 6th week. Clean water was provided in ad-libitum. Chicks were vaccinated against Newcastle, Gumboro, and Fowlpox following veterinary vaccination schedules. Chick sexing was done at the end of the brooding period.

On-farm management of the experimental birds

At the end of brooding, three hundred and twenty (320) growing pullets were distributed to selected farmers in the two villages. The selection of villages and households participating in the study was done in collaboration with District and Ward livestock officers. The selection of villages was done purposely so that both on-farm and on-station evaluation be done in a similar environment in terms of altitude, rainfall pattern, temperature etc. On the other hand, the selection of households was done randomly from a list of farmers who had been keeping chickens for at least five years. In each village, 16 farmers/households were involved in the study. Each farmer was allocated with 10 pullets of a single breed. The allocation of the breed was done randomly by writing breed names in separate 16 pieces of paper (8 for Kuroiler and 8 for Sasso) and each farmer was required to select only one piece of the unfolded paper. Lastly, half of the households in each village received Sasso pullets and the remaining eight households received Kuroiler pullets. The pullets were reared under a semi-scavenging system of management. Experimental birds were provided with shelter/house where a simple enclosure was made around the house which allowed restriction of non-experimental birds and predators from entering the house. Cereals, crop by-products and kitchen leftovers were the main feeds supplemented to birds.

Photo 1. Kuroiler chickens under semi-scavenging management (on-farm) Photo 2. Sasso chickens under semi-scavenging management (on-farm)
On-station management of the experimental birds

Under the on-station experiment, a total of two hundred and forty (240) pullets were randomly allocated to 6 deep litter pens (3 for Kuroiler and 3 for Sasso) of 40 birds each and reared under total confinement. They were provided with a commercial grower ration containing 15.5% CP and 2762 Kcal ME/kg, from the 7th to the end of the 19th week of the age. Thereafter, layer rations containing 18.5% CP and 2965 Kcal ME/kg were provided from the 20th week of age up to the rest of the study period.

Photo 3. Kuroiler chickens under deep litter management (on-station) Photo 4. Sasso chickens under deep litter management (on-station)
Measurement of the external egg quality traits

Samples of eggs were collected at four weeks intervals beginning at 28th to 52nd week of age. A total of 666 fresh eggs (246 eggs from on-farm and 420 eggs from on-station) were used to evaluate the external egg quality traits. External egg quality traits such as egg weight, length, width, shape index, shell weight, shell thickness and shell ratio were determined. Egg weights were obtained by weighing individual eggs using a digital weigh balance whereas the length and width of the eggs were measured using a digital vernier calliper. The egg shape index (%) was calculated as the ratio of egg width to egg length times 100. The eggshells with their membranes were dried on open-air and weighed using a digital weighing balance. The shell weight was divided by egg weight to get the shell ratio. The thickness of shells was measured using a digital vernier calliper.

Measurement of the internal egg quality traits

The eggs used for external egg quality measurements were also used to measure the internal egg quality traits. The internal egg quality traits that were evaluated include yolk weight, albumen weight, yolk ratio, albumen ratio, albumen height and Haugh unit. The internal egg quality measurements were obtained by carefully breaking the egg followed by separation of the albumen and the yolk contents. The weight of albumen was obtained by taking total internal egg weight (i.e. yolk weight + albumen weight) minus yolk weight. The albumen weight and yolk weight were determined using a digital weighing balance. Albumen and yolk ratios were calculated by taking their weights as the percentage of total egg weight. Haugh Unit (HU) was calculated according to Haugh (1937) by fitting the average albumen height and egg weight into the following equation: HU=100 log (H + 7.57 – 1.7W0.37), where H = Albumen height and W = Egg weight.

Statistical data analysis

The General Linear Models (GLM) procedure of SAS software (SAS 2009) was used to analyze all traits measured with the MANOVA option for calculating partial correlation coefficients among the egg quality variables. Management system and the breed were considered as fixed effects while individual farmer or pen effect within a management system was taken as a random effect.

The following statistical model was used to analyze the external and internal egg quality traits observed on a pen or household basis (i.e. the pen or household was the observation unit):

Yijklm = μ + Mi + Bj + (MB)ijk + FP(MB)ijkl + Eijklm (1)

Where:

Yijk = observation (Egg quality traits) from the kth farmer/pen within the jth breed and ith management system;

μ = General mean common to all observations in the study;

Mi = Effect of the ith management system (i= on-station, on-farm);

Bj = Effect of the jth breed (j= Kuroiler, Sasso);

(MB)ijk = Effect associated with the interaction between management system and breed;

FP(MB)ijkl = Random effect of the kth farmer/pen within the jth breed and ith management system;

Eijklm = Random effects peculiar to each bird;

Note: Effects of the management system and breed for egg quality variables were tested using the farmer/pen variation within the management system and breed (i.e. FP(MB)ijk) as the error term.


Results and discussions

Effects of management system and breed on external egg quality traits

The least-square means for the effects of management system and breed on external egg quality traits of chickens are presented in Table 1. Management system significantly (P<0.05) affected all external egg quality traits except shell ratio, while breed of chicken significantly (P<0.05) affected egg weight and egg length.

Table 1. Least square means (se) for the effects of management system and breed on external egg quality traits of chickens

Variable

Management

p-value

Breed

p-value

On-farm

On-station

Kuroiler

Sasso

Egg weight (g)

53.200.37b

59.730.24a

<.0001

57.130.33a

55.800.29b

0.0365

Egg length (mm)

55.940.19b

57.050.12a

<.0001

56.890.17a

56.100.15b

0.0060

Egg-width (mm)

41.280.11b

43.000.07a

<.0001

42.280.09

41.990.08

0.1069

Egg Shape index (%)

73.920.27b

75.480.18a

<.0001

74.730.24

74.970.22

0.1062

Shell weight (g)

6.080.06b

6.940.04a

<.0001

6.580.05

6.440.05

0.1131

Shell ratio (%)

11.470.10

11.670.06

0.1499

11.560.09

11.590.08

0.9683

Shell thickness (mm)

0.530.00b

0.560.00a

0.0008

0.550.00

0.540.00

0.9921

a-b Means with different superscripts within a row and effect differed significantly (p<0.05)

Egg weight differed (P<0.05) between the two management systems with on-station birds laying heavier eggs (59.730.24 g) than on-farm (53.200.37 g). The observed difference between the two management systems on egg weight might be due to insufficient feeding prevailing under on-farm that does not support the birds with adequate levels of nutrition needed to exploit their genetic potential. Similarly, Champati et al (2020) reported heavier eggs for intensively reared chickens than for semi-intensive while Dong et al (2017) and Kkyılmaz et al (2012) also observed variation in egg weight for different rearing systems. In contrast to the present findings, Patel et al (2018) and Sokołowicz et al (2018) did not find significant differences in egg weight between deep litter and other rearing systems. Conflicting reports from these authors are likely due to the effect of a variety of factors, such as genotype used, nutrition, and environment (Rakonjac et al 2014).

The shape index is the ratio between the width and length of the egg, which is a good indicator of uniformity in the size of the eggs. In the present study, the egg shape index was higher for on-station (75.480.18) than for on-farm (73.920.27) which could be explained by the size and weight of an egg. Normally egg length and width are the determinants of the shape of an egg, which were also higher for on-station eggs (57.050.12 and 43.000.07 mm for egg length and width, respectively) than on-farm (55.940.19 and 41.280.11 mm for egg length and width, respectively). Sokołowicz et al (2018) had a comparable observation where the egg shape index was found to be higher for birds under deep litter than those from free-range and organic systems. Similarly, using Red Island Red (RIR) and Fayoumi chicken breeds, Bekele et al (2009), found a higher egg shape index for eggs from the on-station than from on-farm. On the contrary, Sekeroglu et al (2010), Oke et al (2014), and Champati et al (2020) reported the effect of rearing system on egg shape indices not to be significant. The shape index in the present study varied from 73.920.27 – 75.480.18 %. This value falls within the range of 72-76% reported by Altuntas and Sekeroglu (2008) as the standard/normal shape. Therefore, both Sasso and Kuroiler chickens had eggs of standard size that fit properly in normal egg trays. It has been suggested that the eggs with a shape index below 72% are sharp and those above 76% are roundish (Altuntas and Sekeroglu 2008) which increase the possibility of breakages during transportation.

Eggshell quality is also associated with levels of resistance to breakages during transportation. In this study, the management system significantly (P<0.05) affected shell weight, and shell thickness in favour of on-station. The lower values for on-farm eggs for shell quality traits are most likely to be associated with poor feeding and inadequate Calcium (Ca) and other trace minerals intake. It has been reported by Roberts (2010) that Calcium supplementation is a key for eggshell quality, each eggshell contains up to 3 g of Ca, and so the diet of hens must contain an adequate amount of Ca in utilizable form. Since the on-station birds were provided with a commercial diet, it is anticipated that they had well-balanced minerals required for eggshell formation. Nevertheless, several authors have reported varying results on the effect of the management system on shell weight and shell thickness. For example, Ogunshola et al (2018) reported heavier eggshells in the deep litter system than in the cage system but observed no significant difference in shell thickness between these systems. On the other hand, Dahloum et al (2018) did not find differences in shell weight of eggs from different rearing systems. Khn et al (2014) also did not find differences in shell weight and thickness of eggs from the litter-floor and free-range systems. Likewise, Patel et al (2018) observed no differences in shell thickness of eggs from deep litter, semi-scavenging and backyard management. Inconsistent results might be associated with the interaction of the management system with several factors affecting these traits including genotype used, age, oviposition time, and nutrition (Ketta and Tumova 2016).

With regard to breed effects on external egg quality, results show that only egg weight and egg length differed (P<0.05) between the two breeds. Kuroiler chickens had heavier (57.130.33 g) and longer (56.890.17 mm) eggs than Sasso chickens (55.800.29 g and 56.100.15 mm for egg weight and length, respectively), which might be due to variations in genetic make-up between the breeds. However, using similar breeds, Sanka et al (2021) did not find significant differences in egg weight, which might be due to differences in the management of the birds, specifically on feeding practices. The overall egg weight for Sasso chickens in the present study is within the range of 45.7 – 59.9g reported by Sanka et al (2021) and Kidie (2019) for the same breed. Likewise, the egg weight for Kuroiler in this study is within the range 46.25 – 59.0 g reported by the same authors. In contrast, Bamidele et al (2019) reported that the overall egg weight for Kuroiler and Sasso was 54.0 and 54.9 g respectively, which was lower than the egg weight observed in the present study. This difference may be attributed to variation in feeding, hen’s age, and other environmental factors affecting egg weight in chickens.

Effects of management system and breed on internal egg quality traits

The least-square means for the effects of management system and breed on internal egg quality traits of chickens are presented in Table 2. Management system significantly (P<0.05) affected all internal egg quality traits except yolk ratio and albumen ratio. On the other hand, yolk weight, albumen height and Haugh unit were influenced (P<0.05) by the breed of chickens.

Table 2. Least square means (se) for the effects of management system and breed on internal egg quality traits of chickens

Variable

Management

p-value

Breed

p-value

On-farm

On-station

Kuroiler

Sasso

Yolk weight (g)

17.120.14b

19.470.09a

<.0001

18.610.13a

17.980.11b

0.0029

Yolk ratio (%)

32.300.25

32.660.16

0.3047

32.620.22

32.350.20

0.3481

Albumen weight(g)

29.890.29b

33.120.19a

<.0001

31.870.25

31.140.23

0.1456

Albumen ratio (%)

56.110.29

55.390.19

0.0892

55.760.26

55.740.23

0.9565

Albumen height (mm)

6.800.05b

7.580.03a

<.0001

7.290.45a

7.090.41b

0.0080

Haugh unit

84.120.27b

86.980.17a

<.0001

85.980.24a

85.120.21b

0.0243

a-b Means with different superscripts within a row and effect differed significantly (p<0.05)

It was observed that yolk weight and albumen weight differed (P<0.05) between the two management systems with on-station eggs showing higher values than on-farm. The higher mean values for yolk weight and albumen weight from on-station eggs in this study might be related to the size of an egg as these traits have a significant association with egg weight (Suk and Park 2001). It has to be noted that eggs from on-station were also heavier than those of on-farm, as described earlier in this study. This observation conforms to the arguments put forward by Zhang et al (2005) and Aygun and Yetisir (2010) that egg weight influences the weight of components of eggs especially albumen and yolk. In agreement with the results of the present study Sokolowicz et al (2018) and Dong et al (2017) also observed variation in rearing systems on yolk weight.

The Haugh unit (HU) is calculated from the height of the inner thick albumen and the weight of an egg, and it is considered to be a typical measure of albumen quality. It is generally accepted that the higher the Haugh unit value, the better the quality of the egg. In this study, the albumen height and Haugh unit were also affected by the management system with on-station eggs showing higher values (7.580.03 mm, albumen height and 86.980.17, Haugh unit) than those of on-farm (6.800.05 mm, albumen height and 84.10.27, Haugh unit). The higher score in albumen height and Haugh unit for eggs from on-station than on-farm could be associated with better management and nutrition of the birds; which have a significant influence on internal egg quality traits (Gerber 2012). This observation concurs with that of Bekele et al (2009) who also found higher values for eggs from on-station than on-farm. Sokolowicz et al (2018) also found a significant rearing system effect where eggs from the deep litter system outperformed free-range in Haugh unit value. However, the current finding disagreed with Dong et al (2017) who did not find any differences between rearing systems on those traits.

With regard to breed effects on internal egg quality, it was observed that the albumen height and Haugh unit differed (P<0.05) between the two breeds. Kuroiler had higher mean values for albumen height (7.290.45 mm) and Haugh unit (85.980.24) than Sasso (7.090.41 mm, albumen height and 85.120.21, Haugh unit). Differences between breeds/strains for albumen height and Haugh unit have been reported by several authors (Bekele et al 2009; Kucukyılmaz et al 2012). In addition to albumen height and Haugh unit, the yolk weight was also heavier (18.61 g) for Kuroiler than for Sasso (17.98 g). Yolk weight and egg weight are positively correlated traits; probably this might be a reason for heavier yolk for Kuroiler, as the breed had also heavier eggs than Sasso. It was further observed in this study that, neither the management system nor the breed affected the yolk ratio and albumen ratio. This may imply that the share of these traits to the total egg weight of the two breeds is similar regardless of breed or management system. In agreement, Sanka et al (2021) also observed similarity in yolk and albumen ratio for Kuroiler and Sasso eggs under semi-scavenging management. Moreover, Patel et al (2018) reported similar observations on the yolk ratio but they reported contrasting results on the albumen ratio.

Effect of interaction between management system and breed on egg quality traits

Interaction effects between management system and breed on egg quality traits are presented in Table 3. The results show that there were significant interaction effects between management system and breed on egg weight and eggshell ratio. This may imply that with exception of egg weight and shell ratio, the response of the two breeds on other evaluated egg quality traits was similar when subjected to different management systems. It was observed that, while the two breeds had comparable egg weight and shell ratio on-farm, these traits differed on-station where Kuroiler outperformed Sasso on egg weight but had a lower shell ratio than Kuroiler. The probable reason for this variation might be due to differences in strength of correlation coefficients for egg weight and shell ratio between the two breeds; the correlation between egg weight and shell ratio was higher for Sasso than for Kuroiler (Table 4); therefore for every increase in egg weight, the per cent share of the shell became higher for Sasso than for Kuroiler. Similar to the present findings, Bekele et al (2009) and Kucukyılmaz et al (2012) also found significant interaction effects on egg weight when two breeds were compared under two different rearing systems, but in contrast, Sokolowicz et al (2018) did not find significant interactions between the rearing system and breed on egg weight.

Table 3. Least square means (se) for the interaction effect between management system and breed on egg quality traits of chickens

Variable

On-farm

On-station

p -value

Kuroiler

Sasso

Kuroiler

Sasso

Egg weight (g)

53.510.61c

53.050.50c

60.740.38a

58.550.38b

0.0458

Shell ratio (%)

11.600.15ab

11.320.12b

11.490.09b

11.890.09a

0.0046

a-c Means with different superscripts within a row differed significantly (p<0.05)

Phenotypic correlation coefficients for external egg quality traits

The phenotypic correlation coefficients for external egg quality traits are shown in Table 4. Significant and positive correlations were observed between egg weight and egg length, egg width, shell weight, shell ratio as well as shell thickness of both Sasso and Kuroiler chickens. The highest correlations were observed between egg weight and egg width for Sasso and Kuroiler chickens (0.80 and 0.66, respectively), while the lowest significant and positive correlation (0.15) was observed between egg weight and shell thickness for both Sasso and Kuroiler chickens.

Table 4. Phenotypic correlation coefficients for external egg quality traits of Sasso and Kuroiler chickens

Breed

Trait

Egg
weight

Egg
length

Egg
width

Shape
index

Shell
weight

Shell
ratio

Shell
thickness

Sasso

Egg
weight

1

0.68***

0.80***

-0.12*

0.40***

0.29***

0.15**

Kuroiler

1

0.59***

0.66***

-0.01ns

0.50***

0.18**

0.15*

Sasso

Egg
length

1

0.33***

-0.76***

0.32***

-0.14*

0.07ns

Kuroiler

1

0.33***

-0.66***

0.22***

-0.18**

0.06ns

Sasso

Egg
width

1

0.33***

0.26***

-0.29***

0.11*

Kuroiler

1

0.47***

0.33***

-0.11ns

0.33***

Sasso

Shape
index

1

-0.13*

-0.06ns

0.00ns

Kuroiler

1

0.05ns

0.08ns

0.09ns

Sasso

Shell
weight

1

0.74***

0.27***

Kuroiler

1

0.61***

0.15*

Sasso

Shell
ratio

1

0.17**

Kuroiler

1

0.06ns

Sasso

Shell
thickness

1

Kuroiler

1

*** (p < 0.0001); ** (p < 0.001); *(p < 0.05); ns(p > 0.05)

This observation indicates that egg weight has a direct relation with egg width, egg length, shell weight, and shell thickness, thus may suggest that it is possible to use egg weight in determining the egg width, egg length, shell weight, and shell thickness on both Sasso and Kuroiler chickens. This observation supports the suggestion put forward by Ozcelic (2002) that the egg weight values are more appropriate in determining the shell quality since shell weight and shell thickness are mainly measured after breaking the egg. In agreement with the present results, Oluwami and Ogunlade (2008) also observed a significant correlation between egg weight and other external egg quality, with the correlation between egg weight and egg width also being the highest (0.88), following the same trend as observed in this study. Positive correlations between egg weight and shell weight and shell thickness have been also reported by Farooq et al (2001). On the contrary, a significant but negative correlation (-0.12) was observed between egg weight and shape index for Sasso chickens, while for Kuroiler the relationship was non-significant but also negative (-0.01). In agreement with the present findings, Kul and Seker (2004) and Oluwami and Ogunlade (2008) also found negative correlation coefficients between egg weight and shape index.

Phenotypic correlation coefficients for external and internal egg quality traits

The phenotypic correlation coefficients for external and internal egg quality traits are shown in Table 5. Significant and positive correlations were observed between egg weight and yolk weight, albumen weight, albumen ratio and albumen height. In addition, the correlation between egg weight and Haugh unit was also significant and positive for Kuroiler while for Sasso chickens the relationship was not significant but positive. The highest correlations in both breeds were observed between egg weight and albumen weight (0.83 and 0.85, respectively) for Sasso and Kuroiler chickens. These results may imply that egg weight can be used to estimate internal egg contents (yolk and albumen weight) as well as the albumen ratio without breaking the egg.

Table 5. Phenotypic correlation coefficients for external and internal egg quality traits of Sasso and Kuroiler chickens

Breed

Trait

Yolk
weight

Yolk
ratio

Albumen
weight

Albumen
ratio

Albumen
height

Haugh
unit

Sasso

Egg
weight

0.39***

-0.42***

0.83***

0.23***

0.39***

0.02ns

Kuroiler

0.55***

-0.27***

0.85***

0.23***

0.55***

0.23***

Sasso

Egg
length

0.27***

-0.27***

0.57***

0.16**

0.28***

0.02ns

Kuroiler

0.32**

-0.15**

0.44***

0.00ns

0.33***

0.13*

Sasso

Egg
width

0.35***

-0.29**

0.70***

0.25***

0.36***

0.07ns

Kuroiler

0.42***

-0.11*

0.56***

0.13*

0.41***

0.21***

Sasso

Shape
index

-0.02ns

0.07ns

-0.08ns

0.00ns

-0.02ns

0.03ns

Kuroiler

0.02ns

0.05ns

0.04ns

0.11*

0.02ns

0.04ns

Sasso

Shell
weight

0.20***

-0.12*

0.27***

-0.00ns

0.19***

0.04ns

Kuroiler

0.28***

-0.13*

0.43***

0.12*

0.28***

0.12*

Sasso

Shell
ratio

-0.07ns

0.17**

-0.30***

-0.17**

-0.07ns

0.04ns

Kuroiler

-0.10ns

0.05ns

-0.15**

-0.04ns

-0.09ns

-0.03ns

Sasso

Shell
thickness

0.16**

0.03ns

0.06ns

-0.06ns

0.16**

0.11ns

Kuroiler

0.06ns

-0.07ns

0.12*

0.02ns

0.07ns

0.02ns

*** (p < 0.0001); ** (p < 0.001); * (p < 0.05); ns(p > 0.05)

This observation is supported by the report of Moula et al (2010) who also observed a strong and positive correlation between egg weight and albumen weight (0.972) and between egg weight and yolk weight (0.552). Several studies have shown that egg weight is genetically linked to the weight of all three of the major components of an egg i.e. shell, albumen, and yolk. Washburn (1990) showed that the link between egg weight and albumen weight is higher than those between egg weight and shell or yolk weight. On the other hand, the correlations between egg weight and yolk ratio for Sasso (-0.42) and Kuroiler (-0.27) were significant but negative, indicating that heavier eggs in the present study had a lower yolk ratio. This is in agreement with the report of Padhi et al (2013) on Vanaraja chickens.

Phenotypic correlation coefficients for internal egg quality traits

The phenotypic correlation coefficients for internal egg quality traits are shown in Table 6. Significant and positive correlations were observed between yolk weight and yolk ratio, albumen height and Haugh unit in both Sasso and Kuroiler eggs. The correlation between yolk weight and albumen weight was significant and positive for Kuroiler whereas for Sasso that relationship was not significant. The highest positive correlations of 0.91 and 0.93 for Sasso and Kuroiler respectively, were observed between albumen height and Haugh unit while the highest negative correlations of -0.61 were observed between yolk ratio and albumen ratio for Sasso and Kuroiler eggs.

Table 6. Phenotypic correlation coefficients for internal egg quality traits of Sasso and Kuroiler chickens

Breed

Trait

Yolk
weight

Yolk
ratio

Albumen
weight

Albumen
ratio

Albumen
height

Haugh
unit

Sasso

Yolk
weight

1

0.65***

0.02ns

-0.43***

0.60***

0.40***

Kuroiler

1

0.63***

0.21***

-0.34***

0.60***

0.43***

Sasso

Yolk
ratio

1

-0.60***

-0.61***

0.64***

0.88***

Kuroiler

1

-0.53***

-0.61***

0.64***

0.86***

Sasso

Albumen
weight

1

0.72***

0.03ns

-0.29***

Kuroiler

1

0.69***

0.21***

-0.09ns

Sasso

Albumen
ratio

1

-0.42***

-0.53***

Kuroiler

1

-0.34***

-0.48***

Sasso

Albumen
height

1

0.91***

Kuroiler

1

0.93***

Sasso

Haugh
unit

1

Kuroiler

1

*** (p < 0.0001); ** (p < 0.001); * (P < 0.05); ns (p > 0.05)

The significant positive correlations between albumen height and Haugh units obtained in the present study supports the findings of Oluwami and Ogunlade (2008), Kul and Sekar (2004), Akbas et al (1996) and Ozcelik (2002) who reported positive correlation values as 0.98, 0.95, 0.97 and 0.97 respectively. This may imply that as the albumen height is improved, also does the Haugh unit. Since the Haugh unit measures the freshness of an egg (Moula et al 2010) it is reasonable to use the albumen height to determine the Haugh unit. The highest negative correlations observed in this study may imply that when the yolk ratio is high, the albumen ratio is reduced at the same magnitude for both Sasso and Kuroiler chickens. Such information is important especially in pharmaceutical and food processing industries where the yolk and albumen ratio is necessary. For example, egg albumen contains many functionally important proteins among those are ovalbumin (54%), ovotransferrin (12%), ovomucoid (11%), ovomucin (3.5%), and lysozyme (3.5%) and have high potentials for industrial applications if separated (Abeyrathne et al 2013). Therefore, by understanding the variation that exists between breeds for internal egg quality traits (albumen vs. yolk), one could select certain breeds for their peculiarity in the intended qualities.


Conclusions

Based on the results of the present study, it is concluded that both the external and internal quality of eggs were influenced by the management condition. Eggs from on-station appeared to be better in quality than those of on-farm. Breed had significant effects on weight, egg length, yolk weight, albumen height and Haugh unit. Kuroiler laid heavier eggs with higher yolk weight, albumen height and Haugh unit score than Kuroiler. Egg weight as an important egg quality parameter has positive correlations with all traits except shape index for external and yolk ratio for internal egg quality traits.


Acknowledgements

The authors express their appreciation to the Department of Animal, Aquaculture and Range Sciences of the Sokoine University of Agriculture for providing necessary facilities for the on-station experiment. They are further grateful to both livestock officers and households for their cooperation during on-farm data collection.


Funding

This research was carried out under the financial support of the African Chicken Genetic Gains (ACGG) project in Tanzania sponsored by the Bill and Melinda Gates Foundation (Grant Agreement OPP1112198).


Conflict of interest

The authors declare no competing interests


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