Effects of Egg Weight on Egg Quality Traits of Potchefstroom Koekoek Chicken Genotype

This study was conducted at the University of Limpopo which is in the Capricorn District Municipality in the Limpopo province of South Africa. Annually, an average temperature of about 24.6 ◦C is received with a minimum average of 18.9 ◦C in June, and a maximum average of 28.2 ◦C in January. In the study area, January is mostly the wettest month with about 420 mm annual rainfall. July is the driest month with only about 2 mm. An average annual relative humidity of about 77.4% (Tyasi et al., 2020).

Two hundred eggs used in the study were collected from Potchefstroom Koekoek chicken genotype hens which were raised in a traditional cage system. The hens were thirty-three (33) weeks old and six (6) hens were kept per cage (60 × 60 × 45.5 cm). The average relative humidity was 56% and air temperature was 14.3°C during egg collection period. Moreover, feed and water were given unlimited. Hens were fed by the layers’ mash containing 16% crude protein. The eggs were weighed and grouped into the following categories: small weight (< 60 g), medium weight (60 to 69 g) and large weight (> 69 g). Egg weight was measured using a sensitive digital weighing balance (Mettler Toledo, PL203 CE) with accuracy of 0.001 g. Egg length and egg width were taken using a Vernier calliper. Egg length, egg weight diameter, yolk weight, albumen weight, shell weight, shell index, shell ratio and albumen ratio were measured and computed as described by Ukwu et al. (2017).

Statistical Package for Social Sciences (IBM SPSS, 2019) version 26 was used for data analysis. Pearson’s correlation was used to determine the correlation coefficient between measured traits. Analysis of Variance (ANOVA) was used to determine the effect of egg weight group on egg quality traits. The following model was used for ANOVA.

Yij= μ+Ti+eij

where; Yij is jth observation in the ith egg weight group, µ is overall mean, Ti is effect of the ith egg weight group (i is small, medium and large) and eij is residual error.

The descriptive statistics of egg quality traits (egg length, egg diameter, yolk weight, albumen weight, shell weight, shell index, yolk ratio, albumen ratio and shell ratio) in different egg weight groups (small, medium and large) are presented in Table I. It appears that small egg group had higher mean numerical value of yolk ratio (3.02) than the other egg weight groups, medium egg group had a higher mean numerical values in egg weight diameter (4.44 cm), yolk weight (1.85 g), albumen weight (3.50 g), shell weight (0.90 g), shell ratio (1.44) and albumen ratio (5.60) while large egg group had a longer numerical value of egg length (5.73 cm) than the other egg weight groups. These findings are similar to the study of Abanikannda et al. (2007) on Harco chicken layer genotype. However, these results are higher than of Ukwu et al. (2017) on Isa Brown layer genotype of Nigeria. This variation might be due to genotype differences between Potchefstroom Koekoek layer and Isa Brown layer.

Table II shows the relationships between egg weight and egg quality traits of Potchefstroom Koekoek layer genotype. The results indicated that EW had a highly

SD, standard deviation, Std. Error, Standard error of means; CV, coefficient of variation.

statistical significant positive correlation (P < 0.01) with EL (r = 0.82), AW (r = 0.67) and SW (r = 0.62) while EW had a statistically positive significant correlation (P < 0.05) with EWD (r = 0.40), YW (r = 0.33), SR (r = 0.27) and AR (r = 0.31) while negatively statistical correlation with SI (r = -0.44) and YR (r = -0.48), respectively. Positive significant (P < 0.01) correlation were found between egg weight diameter and albumen weight and albumen ratio, while a positive significant (P < 0.05) correlation was observed between egg weight diameter and yolk weight, shell weight and shell index. However, high negative significant (P < 0.01) correlation observed between egg weight diameter and yolk ratio. Positive correlations were found between yolk weight and shell weight, shell ratio (P < 0.01) and yolk ratio (P < 0.05). Moreover, moderate negative significant (P < 0.05) correlation was observed between yolk weight and shell index, albumen ratio. The correlation between albumen weight and shell weight, albumen ratio was highly positive significant (P < 0.01) and low significant (P < 0.05) with shell index in the study. However, high negative significant (P < 0.05) between albumen weight and yolk ratio was found. This revealed that as albumen weight increases the yolk ratio decreases. Negative significant correlation between shell weight and shell index (P < 0.05) and yolk ratio (P < 0.01) was found in the study, although high positive correlation was observed between shell weight and shell ratio (P < 0.01). The correlation between shell index and shell ratio (P < 0.01), yolk ratio (P < 0.05) were negative significant in the study. Even though, positive significant (P < 0.01) correlation was found between shell index and albumen ratio. A negative significant (P < 0.01) correlation between shell ratio and albumen ratio was observed in the study, while low positive significant (P < 0.05) was found between shell ratio and yolk ratio. Moreover, a negative significant (P < 0.01) correlation was observed between albumen ratio and yolk ratio. These findings are in agreement with the report of Ukwu et al. (2017) who reported that egg weight had a highly statistical significant correlation (P < 0.01) with egg length (r = 0.773), egg diameter (r = 0.888) and shell weight (r = 0.680), and concluded that increasing egg length, egg diameter and shell weight might increase egg weight of Isa Brown layer genotype of Nigeria. Aktan (2004) and Alkan et al. (2013) reported that egg weight has a statistical positive correlation with albumen weight. However, Alkan et al. (2010) reported that egg weight has a statistical significant correlation with shell weight, yolk weight and albumen weight. Our findings suggest that increasing egg length, albumen weight and shell weight might also improve egg weight of Potchefstroom Koekoek. Therefore, egg length, albumen weight and shell weight might be employed in the selection criteria during breeding to improve egg weight.

The current study also focused on the effect of egg weight into egg quality traits (Table III). The egg length of small, medium and large egg weight groups was significantly (P > 0.05) different. Numerically, the large egg weight group had the highest egg length (5.70 cm) while small had the lowest egg length (5.41 cm). Egg weight diameter of the different egg weight groups differ significantly (P < 0.05). However, the medium egg weight group had the highest egg weight diameter (4.44 cm) while, large weight group had the lowest egg weight diameter (4.24 cm). Yolk weight for small, medium and large egg groups were 16.74, 18.52 and 17.67 g respectively. Egg yolk weight from the three groups differ significantly (P < 0.05). However, medium egg weight group had the better yolk weight (18.52 g) than eggs in the other groups. Mean albumen weight indices were 31.76, 34.99 and 33.32 g for small, medium and large egg weight groups respectively. Albumen indices among the different egg weight groups differ significantly (P < 0.05). However, the small egg weight group had the lowest albumen weight (31.76 g) compared to other groups. The egg shell weight of small, medium and large egg weight groups was significantly (P < 0.05) different and were observed to be 7.37, 8.98 and 8.14 g, respectively. The egg shell index indices of small, medium and large egg weight groups were significantly (P < 0.05) different. The small egg weight group had the highest shell index (79.59 %) while the large weight group had the lowest index (74.55 %), but statistically significant.

*, Correlation is significant at the 0.05 level; **, Correlation is significant at the 0.01 level; EL, Egg length; EWD, Egg weight diameter; YW, Yolk weight; AW, Albumen weight; SW, Shell weight; Shell index; SR, Shell ratio; AR, Albumen ratio; YR, Yolk ratio.

a, b, c: means in the same row with different superscripts are significantly (P < 0.05) different. SEM: standard error of the mean.

The shell ratio of the egg weight groups was significantly (P < 0.05) different, with numerical values of 13.19, 14.37 and 13.76 for small, medium and large respectively. The medium egg weight group had the better mean shell ratio (14.37), while the small egg weight group had the lowest mean shell ratio (13.19). The albumen ratio of small, medium and large egg weight groups did not differ significantly (P > 0.05). Numerically, the small egg weight group had the highest albumen ratio (56.62) while medium had the lowest mean albumen ratio (55.97) but statistical insignificant. The yolk ratio indices for small, medium and large egg weight groups were 30.18, 29.64 and 29.89, respectively. Egg from the three groups were not significantly (P > 0.05) affected by yolk ratio. However, eggs from the small egg weight group had better mean yolk ratio (30.18) than other groups. Our results are in agreement with the study of Alkan et al. (2015) who reported that egg quality traits are affected by the egg weight in Partridge (Alectoris chukas). However, Sekeroglu and Altuntas (2009) also reported that egg weight influences the egg characteristics in laying hens. The findings of our study are in disagreement with the study of Alkan et al. (2015) who reported that albumen weight improve as the egg weight increase. This variation might be due to bird variation since Khan et al. (2004) indicated that bird breed has an effect of egg quality traits.

The study was carried out to investigate the effect of egg weight on egg quality traits of Potchefstroom Koekoek as indigenous chicken layer chicken. Our study firstly determined the relationship between egg weight and egg quality traits and indicated that egg weight is statistically significant correlated with egg length, albumen weight and shell weight. ANOVA findings recognised that there is a statistically significant difference among egg weight groups viz small, medium, and large on all measured egg quality traits except albumen ratio and yolk ratio. The study might be helpful to chicken farmers focusing on egg production during selection to improve egg weight inbreeding. Further studies need to be done on the effect of egg weight on egg quality traits in a bigger sample size of Potchefstroom Koekoek eggs or different chicken layer breeds.

The authors express appreciation to the University of Limpopo Experimental Farmworkers for their assistance during data collection.

The authors have declared no conflict of interest.