Evaluation of growth performance on family breeding of the Leizhou Black Duck: A preliminary study

Abstract This study determined the growth performance, correlations of linear body dimensions, slaughter performance and the fitting model of two generations of Leizhou Black Ducks. Fifteen (15) male and 225 female parents forming generation 0 were selected from the Leizhou duck population. Fifteen (15) families were created in the ratio of 1 male:15 females. Eggs from all the families were collected and numbered according to the family. Generation 1 ducklings were selected and grouped into families in the same ratio. Body weights (BWs) and linear body dimensions were recorded every 2 weeks from weeks 0 to 16. Logistic, Gompertz and Von Bertalanffy models were employed to ascertain the growth model of both sexes of Leizhou Black Ducks. Except for weeks 0–2, generation 1 families had greater BWs than the generation 0 families (p < .05). Males from both generations had significantly higher (p < .01) BWs than females at 16 weeks old. Significant positive correlations were observed between BWs and measured body dimensions for both sexes except for between BW and pelvis width (PW) where negative correlations (p > .05) were observed for males at week 10 and females at 16 weeks old. There was a significant negative correlation (p < .01) between body length and PW in males at 10 weeks. The accuracy rate of Logistic, Gompertz and Von Bertalanffy models was at 0.972 and similar was obtained by all three models with Von Bertalanffy being the best model. Live weight of generation 1 before slaughter was significantly higher (p < .01) than generation 0, whereas no significant differences were seen in the other carcass traits. These findings provide insights into breeding Leizhou Black Duck to enhance growth performance and hybrid production.

except Leizhou Black Duck regardless of its reliable quality traits.
Leizhou Black Duck is a duck breed widely distributed in the Leizhou Peninsula in China where there are coastal areas and tidal flats with high-quality shallow sea biological communities such as fish, shrimps, crabs and shellfish. It has a 'snake-like' head with relatively full eyes, thin neck with a long black beak and a relatively smaller but compact, long and well-balanced body structure. Comparatively, the body size of adult female ducks is slightly smaller than the male ducks, but the body structure of the females is relatively firm and well-proportioned and has a breeding age of 90-120 days (Figure 1).
Leizhou Black Ducks have been raised on the beach for a long time and freely eat fish, shrimps, crabs and other aquatic animals. Thus, they have attained characteristics such as strong adaptability, strong disease resistance, long egg peak duration, early egg age, rich trace elements in eggs and coarse feeding tolerance .
As a high-quality local duck population, genetic diversity is an excellent genetic material to improve meat and egg performance and environmental adaptability. However, because the extensive free-range breeding of farmers is based on natural selection, the production of commercial ducks is carried out randomly. The technical content of breeding ducks is low, and there is a lack of artificial breeding system which has led to differences in individual varieties and uneven feather colour. Irregularities, alienation of genetic resources and decreased egg production are at a disadvantage in the production competition. Meanwhile, a few research has been done to identify genes involved in muscle development, functions of protein P94 and egg quality of Leizhou Black Ducks Qi, 2016;Qi-ming et al., 2013). However, to the best of our knowledge, there is no literature on the breeding of Leizhou Black Ducks on growth performance.
Therefore, this research focused on the breeding of two generations (0 and 1) of Leizhou Black Ducks using family breeding selection method. This was done to analyse the growth performance, the correlations between the linear body dimensions, slaughter performance and to ascertain the analytical power of the Gompertz, Logistic and Von Bertalanffy for growth estimations of the body traits in Leizhou Black Ducks.

| MATERIAL S AND ME THODS
All the animals were maintained and studied following the National Institute of Health (NIH) guidelines for care and use of laboratory animals, and all protocols were approved in advance by the Animal Care and Ethics Committee of Guangdong Ocean University of China (No. NXY20160172).

| Test materials
The test duck was derived from F4 selected by Guangdong Ocean University and Hengcheng Breeding Professional Cooperative in Potou District, Zhanjiang city.

| Duck selection
The parents used in this study comprised 15 male and 225 female Leizhou Black Ducks which were healthy for breeding. Fifteen (15) families were made from the selected animals in the ratio 1 male:15 females and placed in separate pens to form the generation 0 (G0) group. One hundred eggs were collected from each family, making 1,500 eggs in total. All the eggs were labelled according to their families. During the breeding period, 1 male and 15 female physically healthy ducks (1:15) were chosen from each family, making 15 males and 225 females. In all, 15 families were selected to form the first generation (G1) (Figure 2).

| Housing and feeding management
From hatching to 4 weeks of age, ducklings were kept in closed buildings with windows where wood chips were used as padding and indoor raising. Heat preservation lamps as a source of heat and light were provided for more than 23 hr per day for the first 4 weeks. The temperature inside the building and rearing area (heat lamps) were kept at 24°C and 30-32°C, respectively, for the first week. The temperatures were reduced later by 1°C inside the room and 2-3°C under the heat lamps and the relative humidity was at 60%. At 4-12 weeks old, the ducklings were kept in pond tidal flats. There were duck sheds, land playgrounds and surface activities. The duck pens were naturally ventilated, and the natural light was supplemented with artificial light. There were 15 pens each with an area of 20 m 2 containing 16 ducks each (1 male:15 females). At 13 weeks of age, eggs were ready for collection.

| Data collection
The body weights (BWs) of individuals were recorded at 0, 2, 4, 6, Twenty males at 10 weeks from both generations were selected for slaughter performance. The traits measured were live weight as weight of animals before slaughter, whole net carcass rate as the rate of the carcass immediately after slaughter, half net carcass rate as the rate of the carcass after removing the digestive and reproductive organs, dressing percentage measured as the rate of duck after slaughter removing all internal organs, head and inedible parts of tails and legs, breast and leg muscle rate measured as the share of muscle on the breast and leg, and sebum rate as the amount of fat on the body. All parameters for slaughter performance were weighed using an electronic balance at a precision of 0.1 g.

| Equations of growth models
Logistic model: where Y is the live weight at a particular age, A is the asymptotic body weight, B is a scale parameter, k is the intrinsic growth rate, t is the age in weeks and e is exponent.

| Statistical Analysis
Test forms, field records, and data were compiled by Microsoft Excel and SPSS19.0 was used to analyse the data. Data on growth performance, BW and linear body traits and slaughter performance between the two generations were analysed using t test to reveal the differences. Correlation analysis among the measured body dimensions and significant tests were done using Pearson correlation coefficient method. p-value < .05 was considered statistically significant.

| Progress in growth performance of G0 and G1 families
The BW of each generation increased each week of age ( Figure 3).
Comparatively, the BWs of G1 were higher than the G0 families except for a few weeks in which the G0 were higher than the G1, but the differences were not significant (p > .05) ( Table 2). The weight of the first generation at 0-2 weeks was slightly lower than the zero generation. At 16 weeks old, there was a significant difference (p < .01) as the weight of males increased from 1258.69 g in G0 to 1333.52 g in G1 with 74.83 g increase, and the females increased from 1235.00 g in G0 to 1261.96 g in G1 with a significant increase (p < .01) of 26.96 g (Table 2). Except for week 0 of G1, the BWs of females in both G0 and G1 at 0-6 weeks old were greater than the males but were not significant; nonetheless, a change in trend occurred from weeks 8 to 16 where weights of males became higher than females. At week 16, the BWs of males were significantly higher (p < .01) than females in all generations of Leizhou Black Ducks.

| Body growth curve of Leizhou Black Duck
To comprehensively analyse the developmental characteristics of the body traits of Leizhou Black Ducks, the most commonly used three models of Logistic, Gompertz and Von Bertalanffy, which can describe the growth of poultry based on the fitting results of the curve models, were used. Table 3 shows that the three models of Logistic, Gompertz and Von Bertalanffy of the Leizhou Black Duck body traits have a good fit, above 0.972, which coincides with the actual curve. However, Von Bertalanffy had the highest correlations in most of the body traits.

| Correlation of body traits of males and females in the first generation
The  (Table 5). In male and female ducks at 16 weeks old, the BW had a significant positive correlation (p < .01, .05) with BL, CW, CD, KL and SDL. CD had a significant positive correlation (p < .01) with PW and CW in male and female ducks, respectively, whereas CW significantly had a positive correlation (p < .01) with SG only in female ducks (Table 6).

| Slaughter performance
Except mean live weights, which were significantly different between the two generations (p < .01), all other variables including dressing percentage, half net carcass rate, whole net carcass rate, share of leg muscle, share of breast muscle and share of sebum were not significant (p > .05). The mean live weight (g) of generations 0 and 1 were 960.50 ± 43.20 and 1030.80 ± 27.11 (p < .01) respectively (Table 7).

| Growth performance
Growth and development are two closely related processes; the latter dominates in embryo formation and the former during the postnatal stage of life (Hyánková, Novotná, Knížetová, & Horáčková, 2004 (Damaziak et al., 2014;Kim et al., 2012;Önk et al., 2018;Sari, Tilki, Önk, & Isik, 2013;Steczny et al., 2017). In this study, sex did not affect the weights of ducks in both generations at week 0. However, from the second to the sixth week, female ducks had higher BWs than their male counterparts for the two generations, which is similarly revealed in the results of a previous study (Kokoszyński et al., 2019).
The trend changed from the 8th to the 16th week, where males in both generations had a significantly higher BW than females. The change and increased difference in weights of both sexes from 8th to 16th week may be due to the intensive growth of males than females at the latter stage. It is reported that the difference could be due to the effects of androgenic hormones on protein breakdown and the growth rate of skeletal muscles in males .
It has been revealed that male ducks have higher BWs than females especially in the latter stages of growth (Kim et

TA B L E 3 (Continued)
Yakubu, 2011). The higher BW of males than females could also be because males have lower feed conversion ratio than females, thus more efficient in feed conversion (Dong & Ogle, 2003;Etuk, Abasiekong, Ojewola, & Akomas, 2006;Ihuoma & Okata, 2016). The feeding standards during this growth stage are conducive to improving the growth efficiency of Leizhou Black Ducks.
Generally, the BW performance of generation 1 ducks for both sexes at the 16th week was significantly higher than the generation 0 ducks. This could be due to the proper selection of ducklings from generation 0 family ducks for breeding. The BWs recorded in the current study was lesser than the values recorded for male and female native ducks raised under different systems (Önk et al., 2018), Aylesbury ducks (Ihuoma & Okata, 2016) and Pekin ducks (Mazurowski et al., 2015;Steczny et al., 2017). These dissimilarities could be ascribed to the differences in breed, raising systems, care and feeding.

| Analysis of body growth curve
Modelling of growth curves is essential as it enhances the visualization of growth patterns with time, that is, the relationship Notes: The lower left corner is the correlation coefficient of the male duck, and the upper right corner is the correlation coefficient of the female duck.
between the age and BW of animals. The equations generated are useful for predicting the expected BWs and measurements of animals at a particular age (Onba & Erdem, 2011). In this study, three models (Logistic, Gompertz and Von Bertalanffy) were used to analyse the growth patterns and body measurements of Leizhou Black Ducks to identify the best model for describing the growth curves of ducks. The body measurements (BL, CW, CD, KL, PW, SL, SG, SDL and NL) increased in both males and females with increasing age as similarly reported in Pekin ducks (Onba & Erdem, 2011).
Male ducks had higher measurements of the body traits than females except for the CW, which was higher in females than males.
CW and depth are the essential traits that reflect the growth of animals (Saatci & Tilki, 2007).
The results showed that all three models could completely fit the overall growth of Leizhou Black Ducks, but Von Bertalanffy was the best as it gave the highest correlations in most of the body traits. However, Gompertz was seen to be the best fitting model in previous studies. Gompertz model was most fit for live weight records in chickens when compared with Logistic and Richard growth curve models (Tufarelli et al., 2015). Also, Gompertz exhibited less bias when compared to Logistic and Von Bertalanffy models in the growth of three chicken breeds (Zhao et al., 2015). Another study also concluded that the Gompertz model was more appropriate for BWs and linear body traits of male and female native ducks raised in different systems (Önk et al., 2018).

| Correlation analysis of body traits of males and females in the first generation
In the breeding process of ducks, body size trait is an important

| Slaughter performance
According to the results, the mean live weight of ducks selected for slaughter was higher in generation 1 than generations 0. However, there were no significant differences in the other traits taken for both generations. Muscle tissue is most valued in the carcass of all animals and its weight is mainly determined by leg muscle and breast muscle weight in poultry (Górska & Mróz, 2014;Kokoszyński et al., 2015). In this study, the leg muscle values recorded were greater than the values recorded in 7 weeks old (males, 12.1% and females, 9.9%) Pekin ducks, but lower than ducks in another report (males, 13.5% and females, 13.2%) (Biesiada-Drzazga, Charuta, Janocha, & Łeczycka, 2011; Steczny et al., 2017). The breast muscle rate recorded in this study is lower than those reported earlier in Pekin ducks (Kokoszyński & Bernacki, 2011;Steczny et al., 2017). Ducks are considered to have high body fat but the sebum rate recorded in this study for Leizhou Black Duck is lower than those recorded for Pekin ducks (Kokoszyński et al., 2015;Kowalczyk, Łukaszewicz, Adamski, & Kuźniacka, 2012;Steczny et al., 2017), which may be attributed to the differences in breed, raising systems, care and feeding.

| CON CLUS ION
Leizhou male ducks had better BW than females in both generations at the end of 16 weeks. Even though the females performed better than the males in the early stage, the males outperformed them in the latter stages. Comparatively, generation 1 families had higher BWs and live weights than the generation 0 families. Von Bertalanffy was the best fitting model for the overall growth of Leizhou Black Ducks. Furthermore, BWs and measured body dimensions of generation 1 males and females were usually positively correlated. This implied that BW was dependent on the other body dimensions, thus an upsurge in BW is related to the equivalent upsurge in body dimensions. These findings will serve as a basis to improve upon Leizhou Black Duck breeding and hybrid production.

ACK N OWLED G EM ENT
The authors acknowledge the immeasurable support of Ernest Amponsah Asiamah and the financial supporters.

CO N FLI C T O F I NTE R E S T
All authors declare no conflict of interest.

E TH I C A L S TATEM ENT
The authors confirm that the ethical policies of the journal, as noted on the journal's author guidelines page, have been adhered to and the appropriate ethical review committee approval has been received. All the animals were maintained and studied following the National Institute of Health (NIH) guidelines for care and use of laboratory animals, and all protocols were approved in advance by the Animal Care and Ethics Committee of Guangdong Ocean University of China (No. NXY20160172).