The Effect of Eggshell Thickness on the Hatchability of Guinea Fowl and Pheasants

I Department of Animal Science, Agricultural Faculty, Ondokuz Mayis University, Samsun, Turkey, usyamak@omu.edu.tr, 00903623121919/1389 II Department of Animal Science, Agricultural and Natural Science Faculty, Bozok University, Yozgat, Turkey, m.akif.boz@bozok. edu.tr, 00903542161094/4483 III Department of Animal Science, Agricultural Faculty, Ankara University, Ankara, Turkey, ucara@ankara.edu.tr, 00903125961370


IntRoduCtIon
Pheasant (Phasianus colchicus) and guinea fowl (Numidia meleagris) are some species used in commercial poultry production for different reasons in various parts of the world.In most countries, pheasants are bred mainly as game birds (Caglayan et al., 2010), as a source of animal protein (meat) for humans (Ozbey et al., 2011), or are released in the wild to prevent the depletion of species' population (Yamak, 2015).Guinea fowl, on the other hand, are raised both for meat and egg production and as a hobby.In Africa, guinea fowl production has cultural significance as a traditional activity (Konlan et al., 2011), and guinea fowl meat and eggs are second to chicken eggs and meat in terms of poultry products consumed (Bernacki et al., 2013).
The most important step in poultry production is incubation.In the northern hemisphere, pheasants start laying in early spring and continue until mid-summer; however, total egg production, fertility, and hatching rates vary and tend to be unsatisfactory (Ozbey et al., 2011;Kozuszek et al., 2009), with reported hatchability rates of fertilized eggs ranging between 41 and 79% (Ozbey et al., 2011;Kontecka et al., 2014).Similar variations have been observed in guinea fowl, with reported hatchability rates ranging between 45 and 86% (Yamak et al., 2015a;Bernacki et al., 2013;Royter & Arutyunyan, 1990).Considering the value of pheasant chicks and guinea fowl keets, together with the low egg numbers and variations in fertility and hatchability, successful incubation of all eggs is particularly desirable in these poultry species (Demirel & Kirikci, 2009;Yamak et al., 2015a).
Studies investigating methods of improving pheasant and guinea fowl egg production have focused on nutrition (Jones et al., 2010;Nahashon et al., 2007) and on the effects of housing system on egg production and hatching (Kontecka et al., 2013;Avornyo et al., 2007).Egg storage prior to incubation -a common practice in these species The Effect of Eggshell Thickness on the Hatchability of Guinea Fowl and Pheasants due to their relatively low egg production -has been shown to have a negative effect on hatchability (Moreki & Ditshupo, 2012;Demirel & Kirikci, 2009).
Hatchability may also be affected by eggshell thickness; however, most studies examining the relationship between eggshell thickness and hatchability have focused on chickens (Tsarenko, 1988;Bennet, 1992;Yamak et al., 2015b), with only limited studies evaluated other poultry species, namely turkeys, geese, ostriches, and partridges (Koneva, 1968;Tsarenko et al., 1978;Gonzalez et al., 1999;Yamak et al., 2015c).Therefore, the present study aimed at evaluating the relationship between eggshell thickness and hatchability of pheasant and guinea fowl eggs using an ultrasound gauge to measure thickness.

MAteRIAl And MethodS
Within the framework of this study, two separate experiments were conducted in June 2015.Experiment 1 was conducted at the Yozgat Guinea Fowl Breeding Station of the Turkish Ministry of Forest and Water Affairs.In total, 964 guinea-fowl eggs laid by a flock of 46-wk-old breeders were collected.The breeding flock consisted of 350 female and 150 male guinea fowls.Breeders were fed ad libitum a commercial corn-andsoybean-based chicken layer feed (145g CP, 11.5 MJ ME, 10g Ca, 4g P, 6.5g lysine, 3g methionine per kg).
Eggs were placed in a storage machine and kept at 18°C and 70-80% relative humidity for up to 7 days.All eggs were numbered, and eggshell thickness was measured on the blunted edge with an eggshell thickness gauge (ORKA Tech.Ltd., Israel) that uses precision ultrasound to measure thickness without breaking the egg and is accurate to within 0.01 mm.
The total incubation period was 28 days.Eggs were incubated for 25 days in an incubator set at 37.5°C and 60% relative humidity and then transferred to a hatcher set at 36.5°C and 70% relative humidity.After the 28-day incubation period, all unhatched eggs were broken and opened, and infertile eggs and embryonic deaths were identified.Infertile eggs (n=227) were not used in the calculation of hatching rates.
Experiment 2 was conducted at the Samsun Pheasant Breeding Station of the Turkish Ministry of Forest and Water Affairs.In total, 1,728 pheasant laid by a flock of 52-wk-old breeders were collected.The breeding flock consisted of 900 female and 128 male pheasants.Breeders were fed ad libitum a commercial corn-and-soybeanbased chicken layer feed (145g CP, 11.5 MJ ME, 10g Ca, 4g P, 6.5g lysine, 3g methionine per kg).
Eggs were placed in a storage machine and kept at 18°C and 70-80% relative humidity for up to four days.All eggs were numbered, and eggshell thickness was measured on the blunted edge with an eggshell thickness gauge (ORKA Tech.Ltd., Israel) that uses precision ultrasound to gauge thickness without breaking the egg and is accurate to within 0.01 mm.
The total incubation period was 25 days.Eggs were incubated for 21 days in an incubator set at 37.7°C and 62% relative humidity and then transferred to a hatcher set at 37.7°C and 85% relative humidity.After the 25-day incubation period, all unhatched eggs were broken open, and infertile eggs and embryonic deaths were identified.Infertile eggs (n=237) were not used in the calculation of hatching rates.

Statistical analysis
All statistical analyses were performed using SPSS Software Version 20.0 licensed to Ondokuz Mayis University.Frequency analysis was performed using Tukey's Hinges test, and eggshell thickness groups were formed according to percentiles.Kruskal-Wallis tests were used to examine the effects of eggshell thickness (by percentile group as well as by thickness value) on hatchability.Kendal's Tau Correlation Analysis was used to assess relationships between eggshell thickness and hatchability.The effect of eggshell thickness value on hatchability was analyzed using the Chi-square test.A difference of p<0.05 was considered statistically significant.

ReSultS
The distribution and hatching rates of guinea fowl eggs by shell thickness value are given in Table 1.Eggshell thickness value did not influence guinea fowl hatching rates (p=0.107).Eggshell thickness of fertilized guinea fowl eggs (n=737) ranged between 0.27-0.47mm.Eggs were classified into groups according to shell thickness, and 124 eggs were classified as thin-shelled (<0.31 mm), 468 as medium-shelled (0.31-0.35 mm) and 145 as thick-shelled (>0.35).Hatching rates for thin-, medium-and thick-shelled guinea fowl eggs were 86.3%, 89.1% and 85.5%, respectively (Table 2).The differences in hatching rates among groups were not significant (p=0.425).
The distribution and hatching rates of pheasant eggs by shell thickness value are given in Table 3. Eggshell thickness value had no effect on pheasant hatching rates (p=0.236).Eggshell thicknesses of fertilized pheasant eggs (n=1474 eggs) ranged between 0.24 and 0.49 Sarica M, Onder H The Effect of Eggshell Thickness on the Hatchability of Guinea Fowl and Pheasants mm.Eggs were classified into groups according to shell thickness, with 279 eggs classified as thin-shelled (<0.31 mm), 865 as medium-shelled (0.31-0.35 mm) and 330 as thick-shelled (>0.35).Hatching rates for thin-, medium-and thick-shelled pheasant eggs were 78.5%, 78.7% and 80.0%, respectively (Table 4).Differences in hatching rates among groups were not significant (p=0.390).

dISCuSSIon
Poultry eggshell quality and thickness are affected by numerous factors such as nutrition, stress, disease, and production system (Roberts, 2004).Eggshell thickness has been reported to range between 0.30 and 0.45 mm in guinea fowl eggs (Adeyemo & Oyejola, 2004;Bernacki et al., 2013) and between 0.253 and 0.343 mm in pheasant eggs (Nowaczewski et al., 2013;Kozuszek et al., 2009).The average eggshell thickness of guinea fowl and pheasant eggs determined in this study were 0.33 mm and 0.36 mm, respectively.In order to determine the effect of eggshell thickness on hatchability, this study grouped eggs into thin-shelled, medium-shelled and thickshelled groups based on calculations made using fertilized eggs only.Accordingly, 16.83% of guinea fowl eggs were classified as thin-shelled, 63.5% as medium-shelled and 19.67% as thick-shelled, whereas 18.93% of pheasant eggs were classified as thin-shelled, 58.68% as medium-shelled and 22.39% as thick-shelled.

The Effect of Eggshell Thickness on the Hatchability of Guinea Fowl and Pheasants
Fertility rates of guinea fowl and pheasant eggs were 76.45% and 86.28%, respectively.Fertility may be affected by various factors, including the general condition of the parents, mating rate, age, eggstorage duration and condition, weather conditions, and geographical location (Yamak et al., 2015a;Agbolosu et al., 2012).Given that previous studies reported fertility rates of between 43 and 91.7% for guinea fowl eggs (Yamak et al., 2015a;Bernacki et al., 2013;Agbolosu et al., 2012) and 55.3 and 95.8% for pheasant eggs (Ozbey et al., 2011;Caglayan et al., 2010;Kozuszek et al., 2009), the fertility rates found in the current study should be considered to be within acceptable ranges.
Hatchability rates were assessed as the ratio of hatched chicks to fertilized eggs.Overall hatching rates were 87.9% for guinea fowl eggs (Table 1) and 79.0% for pheasant eggs (Table 3).Reported hatchability rates of guinea fowl and pheasant eggs widely vary, with rates of between 45 and88% reported for guinea fowl (Royter & Arutyunyan, 1990;Saina, 2005;Moreki & Mothei, 2013;Yamak et al., 2015a) and between 41.54 and 96.7% for pheasant eggs (Esen et al., 2010;Demirel & Kirikci, 2009;Kozuszek et al., 2009).Factors that affect hatchability have been well described in previous studies and include male-female ratio and nutrition of parents, as well as egg-storage conditions (Yamak et al., 2015a;Kozuszek et al., 2009).Moreover, any abnormalities in egg physical characteristics can cause a collapse in embryo development and prevent successful hatching (Narushin & Romanov, 2002).This study calculated hatching rates separately for each shell thickness value of both guinea fowl and pheasant eggs.Hatching rate differences were not statistically influenced by eggshell thickness values neither for guinea fowl eggs (Table 1, p=0.107) or pheasant eggs (Table 3,p=0.236).
The present study grouped both guinea fowl and pheasant eggs according to eggshell thickness as either thin-, medium-, or thick-shelled eggs (Tables 2 and Table  4).Whereas hatching rates of thin-shelled pheasant eggs were 1.5% lower than hatching rates of thickshelled pheasant eggs, hatching rates of thin-shelled guinea fowl eggs were 1% higher than hatching rates of thick-shelled guinea fowl eggs.In both cases, the differences in hatching rates among eggshell thickness groups were not statistically significant (p>0.05).Bennet (1992) also reported hatchability rates of thinshelled chicken eggs to be 3 to 9% lower than those of thick-shelled eggs.However, most previous studies have shown significant hatching rates differences between thin-and thick-shelled egg, with some studies reporting higher hatching rates in thin-shelled eggs while others in thick-shelled eggs.Tsarenko (1988) reported hatchability rates of thin-shelled eggs to be 30% higher than those of thick-shelled eggs.Tsarenko et al. (1978) found hatchability rates of thinshelled goose eggs to be 20-40% lower than those of thick-shelled goose eggs.Koneva (1968) reported similar findings in turkey eggs, whereas Andrews (1972), in contrast, reported higher hatchability rates in thin-shelled than in thick-shelled turkey eggs, and Gonzalez et al. (1999) found hatchability rates of thinshelled ostrich eggs to be higher than those of thickshelled ostrich eggs.
The huge differences in the reported findings regarding the relationship between eggshell thickness and hatchability may be related to the methodology used to determine eggshell thickness.Voisey & Hamilton (1976) showed that eggshell thickness is closely related to egg specific gravity, and most studies since then have assessed eggshell thickness according to egg specific gravity.However, other studies determined eggshell thickness using logarithms that rely mainly on egg weight to calculate eggshell thickness (Tyler & Geake, 1961;Ar et al., 1974;Shafey, 2002).In contrast with the direct measurement by ultrasound used in the present study, calculations based on specific gravity or egg weight all rely on indirect methods of measurement.In a study comparing various indirect methods for measuring eggshell thickness, Yamak et al. (2014) showed that the same chicken egg could be identified as thin-shelled by one indirect method and as thick-shelled by another indirect method.These findings highlight the importance of direct measurement.Previous studies conducted using the same direct ultrasound measurement method used in the present study, but with different poultry species, namely chickens (Yamak et al., 2015b) and partridges (Yamak et al., 2016), also found hatching rates to be unaffected by eggshell thickness.Moreover, the hatching time of partridge eggs was not affected by eggshell thickness (Yamak et al., 2016).

ConCluSIon
This study measured eggshell thickness directly using an ultrasound gauge and found no significant differences in hatching rates as a function of eggshell thickness.Although these findings differ from those obtained in some previous studies using indirect measurement methods, direct measurement with an ultrasound gauge has been shown to provide more accurate results (Yamak et al., 2014).Accordingly, it

Table 1 -
Hatching rates of guinea fowl eggs according to eggshell thickness value.

Table 2 -
Hatching rates of guinea fowl eggs according to eggshell thickness group.

Table 3 -
Hatching rates of pheasant eggs according to eggshell thickness value.

Table 4 -
Hatching rates of pheasant eggs according to eggshell thickness group.