Livestock Research for Rural Development 22 (4) 2010 Notes to Authors LRRD Newsletter

Citation of this paper

Effect of long-term heat stress on egg quality traits of Ethiopian naked neck chickens and their F1 crosses with Lohmann White and New Hampshire chicken breeds

Aberra Melesse*, S Maak** and G von Lengerken***

* Department of Animal and Range Sciences, Hawassa University, P.O.Box 1798, Awassa, Ethiopia
** Research Unit Muscle Biology and Growth, Leibniz Institute for Farm Animal Biology (FBN), Wilhelm-Stahl-Allee 2, D-18196 Dummerstorf, Germany,
*** Institute of Animal and Nutritional Sciences, Martin-Luther University Halle-Wittenberg, Theodor-Lieser-Str. 11, D-06120 Halle (Saale), Germany   ;


An experiment was conducted on forty eight female chicks from each of the following genotypes: Naked neck (Na, from Ethiopia), New Hampshire (NH), Lohman White (LW), and F1 crosses of Na x NH and Na x LW. The different genotypes were tested to assess the effect of long-term heat stress (30-32 °C) on performance traits. The female chicks were randomly divided into an experimental group (24 each) and a control group (24 each, ambient temperature18‑20 °C). All hens were kept in individual cages up to 68 weeks of age. Two eggs from each hen were collected at 27, 43, 55 and 68 weeks of age for egg quality assessments.


Eggshell quality traits of F1 crosses exposed to long-term heat stress did not significantly differ from control group. On the contrary, long-term heat stress significantly reduced eggshell quality traits of commercial chicken breeds (LW and NH). Under heat stress conditions, shell thickness, breaking strength, shell percentage and deformation for F1 crosses were much better compared to the average of heat stressed LW and NH breeds. The NaxLW cross kept at heat stressed environment exhibited better shell thickness and yolk quality traits associated with lower mortality compared with NaxNH cross. Both F1 crosses showed evidence of heat stress resistance as indicated by improved eggshell quality and low mortality traits.

Key words: Angete-Melata chickens, commercial chicken breeds, crossbreeds, eggshell quality, warm environment


Stress due to high environmental temperature is widely recognised as one of the primary problems in poultry production, especially in tropical and subtropical areas (Cahaner and Leenstra 1992; Deep and Cahaner 2001; Deeb et al 2002; Maak et al 2003; Aberra et al 2005). The effects of hot environment has been found to vary in magnitude among different chicken breeds or stocks (Deeb and Cahaner, 2001a,b; 2002). Due to unsuitability of local chickens for commercial production, commercial chicken breeds developed in temperate climates are now being widely imported and used in regions with hot climates. Nevertheless, various works have shown that the use of such chicken stock in hot climates resulted in large economic losses. This is because genotypes selected in temperate climates are highly susceptible to high ambient temperatures in hot regions resulting in depressed productivity (Yalcin et al 1997). Replacing the local chickens with improved commercial chicken breeds, on the other hand, would be very expensive and not a viable option for developing countries. Although the performance of tropical local chickens in terms of egg and meat production is very low, they still possess some desirable genetic qualities (tolerance to heat stress, resistance against some diseases, good egg and meat flavour, hard eggshells, high fertility and hatchability as well as high dressing percentages) which are relevant for the further improvement of their productivity (Horst 1988; Horst and Mathur 1992; Gueye, 1998; Shariatmadari 2000; Fassill et al 2009).


Among the local chicken eco-types found in Ethiopia, the naked neck, Na, chicken (locally known as Angete-Melata) is well known for its better performance as compared to other native chicken ecotypes (Teketel 1986; Mebratu 1997). Although the native chickens performed better under higher levels of management than under village conditions, they still do not perform competitively under commercial conditions (Alemu 1995; Tadelle et al 2003a). Thus, most workers expressed the opinion that the adaptive traits of indigenous breeds can best use in synthetic or composite strains and in crossbreeding programmes, as most indigenous stocks lack the productive capacity of the highly improved commercial stocks (Barlow 1981; Cunningham 1982). Crossing the native chickens with commercial chicken stocks may especially result in improved productivity combined with better thermo-tolerance, which can fit to the regions with hot climates. The general objective of the present study was thus to assess the effect of long‑term heat stress on egg quality parameters of native naked neck chickens and their F1 crosses with commercial chickens. 


Materials and Methods 

Experimental animals


Two entirely different experiments were conducted at Martin-Luther University, Halle Wittenberg, Germany. In the firs phase experiment, a total of 240 hens from five different commercial chicken breeds were used to test their adaptability potential to long-term heat stress environments. Finally, based on their performance and adaptability to heat stress, the LW and NH breeds among the five commercial chicken breeds were identified and eventually crossed with Ethiopian naked neck breeds to produce the F1 crossbreed population. Artificial insemination was used for all populations. Female chicks of each genotype were then identified and randomly divided in to control and experimental groups and raised on the floor pen up to 20th week of age under normal (18-20 °C) and high (30-32 °C) ambient temperatures, respectively. Thereafter, 120 pullets from each group were moved to three-tiered individual cages in temperature regulated houses with the respective ambient temperatures as indicated in Table 1.

Table 1.  Genetic structure and size of experimental flocks

Ambient temperature

F1 crosses

Native Na and commercial breeds






Normal (Control group, 18-20 °C)






High (Experimental group, 30-32 °C)






Total (n)


The thermo-neutral environments in „control group“ were established at 31-32 °C during the 1st week; 28-29 °C during the 2nd week; 25-27 °C during 3rd  week; 22-24 °C during the 4th week and 18‑20 °C thereafter.


During the growing period, the birds kept on the floor pen had ad libitum access to feed and water. Standard starter (11.4 MJ/kg and 18 % CP) and grower rations (11.4 MJ/kg and 15 % CP) were provided to all growing chicks and pullets, respectively. The matured hens were fed on commercial laying feed with 11.4 MJ/kg energy and 17% crude protein contents. Pullets and matured hens kept in individual cage were fed in-group ad libitum (4 hens/feed pan) and supplied water with individual nipple drinkers. The dimension of each individual cage was 1000 cm2 (20 cm width x 50 cm length).


In heat stressed group, a Tinytalk® II Data Loggers device was employed to measure the pen temperature and relative humidity during the course of the experiment. The apparatus was loaded and unloaded at 72 days interval and was adjusted to record the pen temperature on two hours interval. The collected data were then converted to conventional data set with Tinytalk® software for further analysis.


Egg quality traits considered


Internal and external egg quality assessments were conducted at 27, 43, 55 and 68 weeks of hen’s age. Two eggs from each hen were collected at each age with a total of 1920 eggs. The following measurements were made: egg weight, shell breaking strength, egg specific gravity, egg deformation, egg length and breadth, shell weight fresh and dry, shell thickness, albumen and yolk heights, yolk diameter and colour. Eggs were weighed using triple beam balance. Egg length and width as well as yolk width were measured using electronic digital calliper. A tripod micrometer (Mitutoyo, 0.01 mm, Japan) was used to measure the height of the thick albumen midway between the yolk and the edge of the albumen. Yolk height was measured by tripod micrometer (Mitutoyo, 0.01 mm, Japan) and yolk diameter by electronic digital calliper. Yolk colour was measured by using the Roche Colour Fan. Egg shell thickness was measured according to Chowdhury (1987). Accuracy of shell thickness was ensured by measuring shell samples at the broad end, middle portion and narrow end of the shell and the average was then taken as a final measurement. Eggshell breaking strength was determined by compressing one egg at a time on an LR50 materials testing machine (Gutsch Nauendorf Germany) at a compression speed of 20mm/min. Egg deformation was measured at the equator of the egg with Gutsch apparatus (μm) (Gutsch Nauendorf Germany). Shell density, shell percentage (shell weight/egg weight), egg shape index (breadth/width) and yolk index (yolk height/diameter) were calculated. Individual Haugh unit (Haugh 1937) score was calculated using the egg weight and albumen height (Doyon et al 1986). The Haugh unit values were calculated for individual egg using the following formula:

HU= 100 log (AH + 7.57-1.7 EW0.37), where HU is Haugh unit, AH is thick albumen height in mm and EW is egg weight in gram.


Heat stress index or difference in performance in hot environment was calculated by using the following formula:

Statistical analysis


All statistical analysis was conducted with General Linear Models Procedure of SAS® (SAS Institute 1996) version 6.12. All egg quality parameters were analysed in a complete 2 x 5 factorial design (2 normal and high ambient temperatures; 5 genotypes). When significance differences in ANOVA were detected, mean comparisons were made by using Tukey's HSD test. All statements of statistical differences were based on P<0.05 unless noted otherwise.



The mean value of eggshell quality obtained at 27, 43, 55 and 68 weeks of age is presented in Table 2.

Table 2.  Mean (±s.d.) values of eggshell quality traits for Na, commercial breeds and their F1 crosses in normal and high ambient temperatures (n=240x4 measurements)

Shell quality traits





Na x LW

Na x NH

Egg weight, g













Deformation, µm













Breaking strength , N













Thickness, µm













Shell percentage













Specific gravity, g/cm3













A-EMeans between genotypes within a temperature with different superscripts are significantly different (p<0.05)
Means between temperatures within each genotype with different superscripts are  significantly different (p<0.05)
Temp.= Temperature N= Normal temperature H= High temperature

Eggs from heat stressed commercial layers were heavier than both native and Fcrosses. Significant differences between control and heat stressed groups in egg quality traits were observed. However, long-term heat stress did not affect shell thickness, breaking strength and percentage shell in Fcrosses. Moreover, no significant differences for these egg quality traits were noted between heat stressed and control Fcrosses at 27, 43, 55 and 68 weeks of age.


Significant differences in eggshell quality traits between heat stressed genotypes were also observed (Table 2). Accordingly, breaking strength, egg specific gravity and percentage shell were higher (p<0.05) for native Na chicken, followed by F1 crosses but lower (p<0.05) for NH and LW breeds. Shell thickness was considerably higher (p<0.05) for NaxLW, followed by NaxNH and Na chickens (Figure 1). The native Na hens produced eggs with less deformation being different (p<0.05) from other genotypes. The egg deformation for both NaxLW and NaxNH crosses was lower (p<0.05) than NH and LW breeds. As illustrated in Figure 1, the reduction in shell thickness due to heat stress was much higher (p<0.001) for both LW and NH chicken breeds, the average depression for both commercial breeds being 10.3%, which is very much larger than the average of F1 crosses (2.3%).

Figure 1.
 Magnitude of heat stress effect on shell thickness for Na, commercial chicken
genotypes and their F1 crosses [Each bar on the graph represents mean genotype
± SE]

As shown in Table 3, the F1 crosses kept in heat stressed environment were generally superior to commercial chicken breeds for most of eggshell quality traits.

Table 3.  Comparison of selected eggshell quality parameters between commercial breeds and their F1 crosses maintained under heat stress condition (Change to each commercial breed, %)

Egg shell quality parameters


Na x LW


Na x NH

Shell thickness, µm

340 b

371 (+9.1) a

337 b

359 (+6.5) a

Breaking strength, N

37.2 b

42.8 (+15.1) a

33.9 b

43.6 (+28.6) a

Shell percentage, %

8.7 b

9.7 (+11.5) a

8.8 b

9.6 (+9.1) a

Egg deformation, µm

75.2 a

61.5 (-18.2) b

74.1 a

65.6 (-11.5) b

Egg specific gravity, g/cm3

1.062 b

1.066 (+0.4) a

1.061 b

1.065 (+0.4) a

a,bMeans between genotypes within a row with different superscripts are significantly different (p<0.05)

When compared to LW and NH breeds, the shell thickness was stronger (p<0.05) by 9.1 % and 6.5% for NaxLW and NaxNH crosses, respectively. The NaxLW cross produced eggs with better breaking strength and percentage shell (p<0.05) compared to LW breed. The NaxNH cross also produced eggs with the best breaking strength being stronger by 28.6% (p<0.001) compared to NH breed. Egg deformation was better (p<0.05) for NaxLW and NaxNH crosses compared to LW and NH breeds. As shown in Figure 2, both NaxLW and NaxNH crosses showed better (p<0.05) egg specific gravity than LW and NH breeds. In general, except deformation and breaking strength, eggshell quality traits declined with increasing age for all genotypes.

Figure 2.  Comparison of egg specific gravity between native Na, commercial
chicken breeds and their F1 crosses exposed to elevated temperature

The F1 crosses kept under heat stress environment showed less depression in yolk diameter and yolk colour compared to commercial breeds. In general, heat stress reduced yolk height and yolk diameter (p<0.05) by 3.9 and 4.1%, respectively compared to control group. The individual reduction in yolk height due to heat stress was 5.1, 3.8, and 3.6% for Na, commercial chicken breeds and F1 crosses, respectively. Furthermore, heat stress reduced yolk diameter (p<0.05) of LW, NH and NaxNH genotypes. Yolk colour decreased (p<0.05) in Na, NH and NaxNH genotypes. Under heat stress environment, the NaxLW cross showed significant improvements (p<0.05) in albumen height, shell density, HU, yolk diameter and shell thickness compared to the NaxNH cross (Table 4).

Table 4.  Average values of albumen and yolk quality traits for Na, LW, NH and their F1 crosses under heat stress environment (mean ±s.d; n=240x4 measurements)

Egg quality traits






Albumen height, mm






Yolk height, mm






Yolk diameter, mm






Haugh Unit






Yolk index






Yolk colour






a-eMeans between genotypes within a row with different superscripts are significantly different (p<0.05)

The NH and F1 crosses produced eggs with better (p<0.05) yolk colour than other genotypes. With increasing age, yolk height, yolk index, albumen height and HU score declined in both heat stressed and control chickens. Conversely, the yolk diameter and colour slightly increased, as the chickens grew older. Within heat stressed genotypes, the LW had a higher mortality of 11.1% and 16.7% before and after sexual maturity, respectively (Table 5). During the laying period, no mortality was noted in heat stressed NaxLW genotype.

Table 5.  Effect of long-term heat stress on body weight and survival of local Na, exotic chicken breeds and their F1 crosses (mean ± s.d.)

Body weight





Na x LW

Na x NH

At 20 weeks of  age,  g


952aE ±12






818bD ±16





At 68 weeks of age,  g













Mortality rate, %













A-EMeans between genotypes within a temperature with different superscripts are significantly different (p<0.05)

a,bMeans between temperatures within each genotype with different superscripts are significantly different (p<0.05)

Temp.= Temperature N= Normal temperature H= High temperature

Exposure to long-term heat stress at the age of 20 weeks, which corresponds to sexual maturity, resulted in body weight loss of 9.4% in all genetic groups except in NaxLW crosses (Table 5). The decrease in body weight for individual genotypes was 14.1, 7.8, 13.7 and 12.6% for Na, LW, NH and NaxNH, respectively. Conversely, no significant effect of heat stress on body weight was found at the 68th week of age for all genotypes, except for the NH breed, which showed a significant body weight loss of 16.6%. The body weight of NaxLW (1189 g) at 20 weeks of age was comparable to LW (1248 g), but considerably heavier than the native Na chicken (818 g) and NaxNH hens (1101g).


As presented in Table 6, a significant positive correlation was observed between shell thickness, breaking strength, shell percentage and specific gravity.

Table 6.  Correlation analysis of selected egg shell quality parameters

Shell quality traits

Shell thickness

Breaking strength

Shell percentage

Egg specific gravity

Egg deformation

Shell thickness






Breaking strength






Shell percentage






Egg specific gravity






Egg deformation






*** Significantly different at p<0.001

Egg deformation was correlated negatively with other external egg quality parameters. Although not significant, a negative correlation between specific gravity and internal egg qualities (yolk index and haugh units) was noted. On the other hand, yolk index and haugh units were positively correlated with shell thickness, breaking strength and shell percentage (data not shown).



The relatively low egg weight for native naked neck chicken breed may be related to late sexual maturity or to selection for high egg weight in the commercial breeds. Eggs with thick and strong shells are usually the most marketable, so this trait is very important from the economic point of view. Number of cracked eggs and storage life of the eggs directly depends on the shell thickness and strength. Egg quality depression due to heat stress in LW and NH breeds is consistent with the findings of previous studies (Sauveur and Picard 1987; Grizzle et al 1992; Chand and Sinha 1995; Deeb and Cahaner 2001). The decline in shell quality was particularly noticeable in NH breed, indicating low tolerance to heat stress for this particular trait. Pech-Waffenschmidt (1992) found a significant depression of the eggshell portion due to heat stress just after two experimental weeks and the decline remained throughout the trial. The heat-induced eggshell quality reduction could be explained by reduced blood flow through shell gland, respiratory alkalosis (as a result of panting), reduced blood ionic calcium content, reduced carbonic anhydrase in shell gland and kidneys and reduced calcium mobilisation from bone store (De Andrade et al 1977; Wolfenson et al 1979; Daghir 1995). The reduced feed intake during the long-term heat exposure might have also attributed to a decreased mineral intake, which could result in reduced availability of calcium ions for the eggshell formation (Pech‑Waffenschmidt 1992; Rama and Nagalakshmi 1998; Deeb and Cahaner 2001). At heat stress environment, native naked neck chickens demonstrated higher eggshell quality traits than commercial chicken breeds and their crosses because of better thermoregulation in dissipating heat stress more efficiently (Deeb and Cahaner 1999). 


Early studies documented the negative effects of high environmental temperature on shell thickness and egg specific gravity (Nordstrom 1973; El Jack and Blum 1978; Ahvar, 1979; Vo et al 1980). Wilson et al (1972) reported better shell thickness in a cyclic environment than at a constant hot environment. The increase in shell deformation due to heat exposure in this experiment agrees with the results of Miller and Sunde (1975). Under heat stress environments, the white eggshell of LW was comparatively thicker than brown shells of NH breed which agrees with the previous findings of Potts and Washburn (1974).


The introduction of the naked neck gene into a layer population was reported to compensate for the deteriorating effect of heat stress on shell quality (Pech‑Waffenschmidt 1992). This was thought to be caused by better sensible heat loss of the naked neck hens with a concomitant metabolic exoneration. In this study, the F1 crosses were superior to commercial chicken breeds for most of eggshell quality traits under heat stress conditions. Within heat stressed group, eggs produced by NaxLW cross were much better in shell thickness than either of the parents (Na, NH and LW). This superiority of F1 crosses over commercial chicken breeds is certainly due to the introduction of the native Na gene into the exotic chicken breed genome, which attributes to better tolerant against chronic heat stress.


This study is in line with previously reported findings that egg and eggshell quality deteriorate as the hens getting older (Campo and Escudero 1984; Shalev and Pasternak 1993; Roberts and Nolan 1997; Fassill et al 2009). Various explanations have been suggested for the deterioration of eggshell quality of fowl with increasing age. According to Etches et al (1995) the calcium carbonate secretion of older hens is insufficient to overcome the simultaneous increase in egg size resulting in a decline of shell thickness and shell stability. Verheyen and Decuypere (1991) also suggested that age related decrease in eggshell quality might be caused by the reduced capacity of the shell gland to take up calcium from the blood and/or deposit it at the calcification centres in the shell membranes. Earlier study conducted by Decuypere and Verheyen (1986) suggested that a decline in shell quality with age could be associated with decreased calcium retention with increasing age, decreased absorption, increased excretion and an incomplete replacement of calcium mobilised from the medullary bone. The increase in egg length and width and its shape index with age is in accordance with the findings of Roberts and Nolan (1997) and Fassill et al (2009).


A general heat stress induced reduction in albumen and yolk quality was noted in this experiment. On the contrary, Ahvar (1979) and Vo et al (1980) reported an increase of HU as the environmental temperature raised. The observed better albumen quality and HU of white eggs from LW than brown eggs of NH agrees with findings by Verheyen and Decuypere (1991) and Förster and Flock (1997). The latter authors reported that white-egg strains have better albumen quality than brown-egg strains in German Random Sample Tests. The depression in yolk height and diameter due to heat stress is consistent with reports by Rauen (1985) and Haaren-Kiso (1991). The decline in albumen height and HU with increasing age in this data accords with the findings of Williams (1992), Silversides et al (1993) and Danilov (1997). The decline in albumen quality and HU during the hen’s first laying year could be related to the higher egg weight along with high water content (Verheyen and Decuypere 1991).


The F1 crosses between Ethiopian local chicken ecotypes and White Leghorn raised at tropical environment reached their sexual maturity at 167 days (Mekonnen 1998), which was comparatively longer by 13 and 8 days for NaxLW (154 d) and NaxNH (159 d), respectively. According to the same author, age at first egg of F1 crosses was not significantly different from that of the White Leghorn. On the other hand, the average heat stress induced reduction in body weight at adult stage was low (5.6%) compared to that at sexual maturity (9.4%), which is in agreement with the reports of Renden and McDaniel (1984) and Proudfoot and Hulan (1987). This might partly be explained that hens at the stage of sexual maturity not being fully adapted to the heat stress, while the thermo-tolerance has been improved with age resulting in a slight reduction of body weight at adult stage.


Interestingly, in this study long-term heat stress did not depress the body weight of NaxLW crossbreed combination; instead, there was an increase of 1.7 and 2.9% at sexual maturity and adult stage, respectively, suggesting a gradual adaptation to long-term heat exposure with age for this particular trait. This could be explained by the small body size of the NaxLW cross contributed by the genetic background of their LW parents, which accounts for better heat tolerance. This finding is supported by Deeb and Cahaner (2002) who reported that light bodied breeds are less affected by heat stress in growth performance. Moreover, the body weight of heat stressed F1 crosses was larger than Na, but comparable to the average of pure breed lines, which is in accordance with previous findings by Nwosu (1992). A significant correlation between shell qualities parameter were obtained in our investigation. This is in agreement with findings by Voisey (1976). However, egg deformation was negatively correlated with other shell quality traits.




The authors are grateful to Dr. S. Götze and all working staff of Nutztierwissenschaftliches Zentrum Merbitz (Martin-Luther University Halle-Wittenberg, Germany) for their immense technical support during the course of this experiment. We further express our sincere gratitude to the Catholic Academic Exchange Service (KAAD) of Germany for its generous financial support during the study period.


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Received 10 December 2009; Accepted 8 February 2010; Published 1 April 2010

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