Introduction
Ducks are waterfowl that belong to the Anatidae family. Other waterfowl such as loons, grebes, gallinules and coots may be confused with this species (Idahor et al., Reference IDAHOR, AKINOLA and CHIA2015; Ericson et al., Reference ERICSON, QU, BLOM, JOHANSSON and IRESTEDT2017; Farghly et al., Reference FARGHLY, ABD EL-HACK, ALAGAWANY, SAADELDIN and SWELUM2018a) but do not represent a monophyletic group; therefore, swans and geese are not considered ducks. They are distributed throughout the world, except in Antarctica (Idahor et al., Reference IDAHOR, AKINOLA and CHIA2015), due to their adaptability to different environments. Because of the variable market demand for a one-day-old chick by the duck industry, fertility and hatchability are important economic factors which represent the major components of reproductive performance and are sensitive to environmental and genetic factors (Widiyaningrum et al., Reference WIDIYANINGRUM, LISDIANA and UTAMI2016).
Fertility is defined as the percentage of embryonated eggs after three days placement in the incubator; meanwhile, hatchability is the percentage of fertile eggs that hatch (King'ori, Reference KING'ORI2011; Taplah et al., 2018). Fertility and hatchability are affected by genetic and non-genetic factors. For a particular variety, non-genetic factors have a greater influence on fertility and hatchability in production (King'ori, Reference KING'ORI2011), Including management of the breeder, egg quality, and incubation processes. The management of breeding stock includes genetic selection (Drouilhet et al., Reference DROUILHET, BASSO, BERNADET, CORNUEZ, BODIN, DAVID and MARIE-ETANCELIN2014), age of the breeders (King'ori, Reference KING'ORI2011), season and feeding (King'ori, Reference KING'ORI2011), breeding system and breeding technology (Weis et al., Reference WEIS, HRNČÁR, PÁL, BARAŇSKA, BUJKO and MALÍKOVÁ2011) and egg quality (Hester, Reference HESTER2017). The objective of the following review was to provide advanced information about the factors that affect fertility and hatchability of duck eggs and ways to improve them.
Fertility
Fertility can be affected by the quality of broodstock, male:female ratio, environmental temperature, storage time and housing systems. Brillard (Reference BRILLARD2003) stated that fertility depends on the ability of females to ovulate, store sperm and provide an appropriate environment for the formation and development of the egg. Likewise, Brillard (Reference BRILLARD2003) and Mohan et al. (Reference MOHAN, SHARMA, KOLLURI and DHAMA2018) reported that the quality and quantity of semen deposited by the male were important to obtain good fertility. However, there are differences between duck breeds.
Muscovy ducks age influences the reproductive characteristics of both females and males (Yakubu, Reference YAKUBU2013). For flocks from the same breed, differences in fertility have been reported for different batches of eggs. In Muscovy ducks, the highest fertility is obtained at the peak of posture, compared to the moments before or after the peak of posture (Nickolova, Reference NICKOLOVA2005). Genitalia of waterfowl is more complicated than chickens, making the fertility issue more prevalent in ducks than in chickens, as well as the issue of sexual dimorphism in body size in some breeds of ducks and crossbreeding (Yakubu et al., Reference YAKUBU, MUHAMMED, ARI, MUSA-AZARA and OMEJE2015).
The male:female ratio in all poultry species plays an important role in achieving greater fertility. For Muscovy ducks a ratio of one drake to five ducks is used according to Idahor et al. (Reference IDAHOR, AKINOLA and CHIA2015) and one to four according to Nickolova (Reference NICKOLOVA2005) and Alonso-Alvarez (Reference ALONSO-ALVAREZ2006). In Muscovy ducks, a ratio of one to six has been used satisfactorily where there is a shortage of males (Banerjee, Reference BANERJEE2013). However, larger sexual ratios of 1:4.3 to 1:10 cause a reduced egg fertility from 75.9% down to 49.6% (Nickolova, Reference NICKOLOVA2005), which corresponded with a loss in fertility from 97.09% to 93.41% when the sex ratio was increased from 1:5 to 1:8. In Khaki Campbell ducks, Giri et al. (Reference GIRI, KUMAR, JAISWARA, BAIS, SAHOO, SASTRY and SARAN2014) studies the ideal mating sex ratio for production of fertile eggs under intensive rearing and concluded that a 1:5 sex mating ratio was ideal for better fertility and hatchability.
An increase of the environmental temperature above the optimal ranges of thermal comfort in poultry affects productive performance of breeders. According to Chowdhury et al. (Reference CHOWDHURY, ASHRAF, MONDAL, MONDOL and HASAN2004), heat stress causes a decrease in the number of germ cells, ovular release, fertilisation and survival capacity of the embryo and abnormalities in sperm of the Northern Pintail duck (Penfold et al., Reference PENFOLD, WILDT, HERZOG, LYNCH, WARE, DERRICKSON and MONFORT2000). The latter decreases deposition of sperm in the host gland in the reproductive system of female (Brillard, Reference BRILLARD2003; Abd El-Hack et al., Reference ABD EL-HACK, ALAGAWANY and NORELDIN2018). Fertility was significantly higher at months from January to May as compared to June to August months and was significantly lowered by 9.70%, 12.72% and 14.29% in Summer as compared to Autumn, Winter and Spring season, respectively (Awad, Reference AWAD2013)
The storage time before the incubation of Pekin duck eggs has a significant effect on their fertility, hatchability and early embryo mortality (Waehner et al., Reference WAEHNER, PINGEL and HAIDONG2015). Reyna and Burggren (Reference REYNA and BURGGREN2017) reported a decrease in the fertility of duck eggs stored for more than six days from laying to incubation.
Housing system influences the fertility of ducks, allows mass mating and provides access to swimming, which significantly increases the fertility rate as ducks are waterfowl by nature and prefer to mate in water (Ojewola, Reference OJEWOLA2006). However, egg weight linearly improves fertility; in Khaki Campbell ducks, the egg weight increased from 60 g to 75 g when they had water access, which positively affected egg fertility (Giri et al., Reference GIRI, KUMAR, JAISWARA, BAIS, SAHOO, SASTRY and SARAN2014).
Hatchability
Factors attributed to the breeding birds, such as genetic selection, management and feeding, handling and storage of eggs and conditions inside the incubator can influence the hatchability of duck eggs. Archer et al. (Reference ARCHER, JEFFREY and TUCKER2017) and Ramli et al. (Reference RAMLI, WAHAB, ZAIN, RAUS, RAJA and PAHAT2017) stated that the temperature, relative humidity, ventilation and turning of the eggs throughout incubation and hatching were environmental factors that can modify hatchability.
breeder management
Breeders must use breeds with good genetic characteristics at optimal ages to obtain high fertility, hatchability and growth performance of progeny. The time of year and the feeding conditions play an important role in the development of the embryo, before and after incubation.
Genetic characteristics can influence the productive indicators of different breeds of ducks in varying ways. The Muscovy lineages, compared to the Pekin duck, show higher mortality due to adhesion to the egg shell and for normal ducklings, however, Pekin duck eggs have better hatchability than Muscovy eggs (Rashid et al., Reference RASHID, KAWSAR, MIAH and HOWLIDER2009). The eggs of the Muscovy such have shown less than 22.7% of hatchability (Ali et al., Reference ALI, RAHAMAN, WAHID and SINGH1989). The fertility and hatchability of laying Brown Tsaiya ducks were increased when a genetic selection was made and one artificial insemination per week was combined with Muscovy semen (Cheng et al., Reference CHENG, ROUVIER, POIVEY, TAI, TAI and HUANG2002).
age
The age of female ducks has more influence on fertility than age of males (Brommer and Rattiste, Reference BROMMER and RATTISTE2008). After 24 days of incubation, the relative weight of the yolk sac, with respect to total egg weight, increased in hens at 36 weeks compared with Pekin ducks at 26 and 31 weeks of age (Applegate et al., Reference APPLEGATE, HARPER and LILBURN1998). On the other hand, increasing the age of the female decreases the functioning of the sperm storage tubules, leading to problems in the reproductive system which influences egg quality.
In addition, the age of ducks affects the external and internal quality of eggs, influencing both hatchability and fertility. The ratio between yolk and albumin decreased with increasing age of the birds (Peebles et al., Reference PEEBLES, DOYLE, ZUMWALT, GERARD, LATOUR, BOYLE and SMITH2001). Larger eggs have comparatively less shell area per unit of interior weight than smaller ones, therefore, those from older broodstock have less weight when calculated in grams and more in percentage than the eggs of younger broodstock (Onasanya et al., 2013). Age affects the deposition of calcium and minerals in the shell (Onbaşılar et al., 2014). The shell allows gaseous exchange and loss of moisture from the egg, and poor shell quality results in excessive loss of relative humidity during incubation (Peebles et al., Reference PEEBLES, DOYLE, ZUMWALT, GERARD, LATOUR, BOYLE and SMITH2001).
season
The season when laying takes place affects fertility of ages and hatchability of ducklings, due to differences in temperature, wind speed, rainfall and relative humidity (Buhr, Reference BUHR1995; Farghly et al., Reference FARGHLY, ABD EL-HACK, ALAGAWANY, SAADELDIN and SWELUM2018b; Li et al., Reference LI, WEN, ZHANG, LIU, JIN, YAN and WANG2018). Chowdhury et al. (Reference CHOWDHURY, ASHRAF, MONDAL, MONDOL and HASAN2004) observed a high significance between the month and hatchability of Pekin duck eggs, where the hatchability in January was 59.54 ± 0.79%, but in July it decreased to 48.27 ± 0.79%. The best hatchability of Pekin duck eggs is observed during the winter (57.68 ± 0.59%), but during the summer it decreases to 54.14 ± 0.59%, and the lowest values are found in the monsoon or rainy season (49.13 ± 0.59%) (Chowdhury et al., Reference CHOWDHURY, ASHRAF, MONDAL, MONDOL and HASAN2004). Hatchability of duck eggs showed the highest value in March (65.21%), but the lowest value (34.96%) was observed in August (Awad, Reference AWAD2013). Fertility and hatchability were lower in the summer months compared to other seasons due to high environmental temperature that causes a decrease in the reproductive efficiency in both male and female ducks.
nutrition
During egg formation, essential nutrients are deposited for the development of embryo during the incubation period and form reserves for the developing embryo until the first meal becomes available to the young birds (El-Kholy et al., Reference EL-KHOLY, IBRAHIM, EL-MEKKAWY and ALAGAWANY2019; Saeed et al., Reference SAEED, BABAZADEH, NAVEED, ALAGAWANY, ABD EL-HACK, ARAIN, TIWARI, SACHAN, KARTHIK, DHAMA, ELNESR and CHAO2019). The nutrients are used for tissue formation, heat generation and muscle activity, and are stored in both albumin and yolk (Uni et al., Reference UNI, YADGARY and YAIR2012; Onbaşılar et al., 2014). Egg albumin from eggs from Pekin ducks is mainly water (85.7 to 88.1%) containing 10.9% to 13.1% water-soluble proteins and represents from 53.0% to 55.6% of the total egg content. Although the yolk only represents 31.3% to 33.9% of the egg content and 41.7% to 44.1% is water, it has higher protein reserves (17.3% to 17.8%), lipids (35% to 38.4%) and minerals (Yair and Uni, Reference YAIR and UNI2011).
Nutrient deficiencies impede the proper development of embryos, reduce hatchability and increase embryonic death, in addition to causing disorders of the musculoskeletal system, immune system and cardiovascular system (Uni et al., Reference UNI, YADGARY and YAIR2012). The nutrients in the egg determine the weight of the duckling at birth, body size and hatchability. Therefore, feeding ducks correctly can increase deposition of nutrients in the egg, without causing any change in weight of the broodstock, which are important factors to consider to avoid low quality ejaculation and ovulation and, at an extreme, early ovarian and testicular regression (Brillard, Reference BRILLARD2003).
Certain nutrients and food ingredients have adverse effects on the quality and quantity of eggs produced by ducks. Anti-nutritional factors, such as gossypol from cotton seed meal, cause infertility in males manifested as immobility of sperm caused by damage to the mitochondria located in the tail of sperm, and extensive damage to the germinal epithelium (Randel et al., Reference RANDEL, CHASE and WYSE1992). Increased consumption of gossypol causes increased permeability of the yolk sac membrane which leads to excessive pigmentation of the albumin, known as ‘pink disease’ or white albumen which is mottled or gummy. It is necessary for producers to constantly update feeding standards and include new recommendations to ensure the quantity and quality of food necessary to the layer ducks.
Egg factors
Eggs are normally fertile four days after the deposition of the semen inside the female, and in this period all necessary components for the development of embryo until the birth should be present in the egg. Under normal circumstances, infertility is due to different factors related to the management of the broodstock and environmental conditions, since these are related to modifications in the physical and chemical characteristics that reduce embryonic development and hatching of good quality ducklings (Narushin and Romanov, Reference NARUSHIN and ROMANOV2002).
The management of broodstock plays an important role to ensure a good hatchability, breed used, season, health, and nutrition, also size, weight and quality of egg, in addition to the duration and storage conditions (Wilson, Reference WILSON1991; Onasanya et al., 2013). The quality of eggs for incubation is determined by both external and internal characteristics, however, there are other factors that prevent hatching, such as the inheritance of lethal genes, lack of nutrients and sudden variations in incubation conditions (King'ori, Reference KING'ORI2011).
egg quality
Egg quality has a significant effect on hatchability, as conditions of the microenvironment during the storage process and early incubation modify both the external and internal quality of eggs (Narushin and Romanov, Reference NARUSHIN and ROMANOV2002). External characteristics such as weight, index form (described as maximum breadth to length ratio), shell thickness, porosity, surface quality of the shell and resistance to breakage have been measured (Toro et al., Reference TORO, AGUILAR, BERTOT, HURTADO and NAVA2015). Internal quality is determined by the height of the dense albumen and the yolk, in addition to Haugh Units. Haugh units are calculated as:
HU=100 log (H+7.57−1.7W0.37)
where HU=Haugh units, H=albumin height and W=egg weight (Toro et al., Reference TORO, AGUILAR, BERTOT, HURTADO and NAVA2015).
Heavier eggs are more likely to hatch than those of low weight; however, there are contradictions about the weight of eggs that are not within the average values for incubation (King'ori, Reference KING'ORI2011). An increase in weight, shell thickness and internal contents of eggs leads to a higher total weight, which in turn reflects more reserves of nutrients and energy (Toro et al., Reference TORO, AGUILAR, BERTOT, HURTADO and NAVA2015). Light weight eggs have relatively shorter incubation times (Weis et al., Reference WEIS, HRNČÁR, PÁL, BARAŇSKA, BUJKO and MALÍKOVÁ2011) and the weight of hatchlings is higher from large eggs. It is important to note that, during incubation, larger embryos produce a greater amount of heat, which requires good ventilation to maintain the correct temperature (Jibrin et al., Reference JIBRIN, IDIKE, AHMAD and IBRAHIM2011).
Likewise, egg size affects hatchability. Wilson (Reference WILSON1991) and Weis et al. (Reference WEIS, HRNČÁR, PÁL, BARAŇSKA, BUJKO and MALÍKOVÁ2011) found that medium-sized eggs from Muscovy ducks had better hatchability than small eggs. In this sense Demirel and Kırıkçi (2009), found a greater increase in yolk compared to albumin as the egg size increased, which could be a major influence.
With longer storage time, HU decreases and pH of the albumin increases, apparently due to loss of water (Martínez et al., Reference MARTÍNEZ, GUERRA, RODRÍGUEZ and BETANCUR2014). Elibol et al. (Reference ELIBOL, PEAK and BRAKE2002) stated that the low pH in fresh eggs did not affect fertility and hatchability, however, other authors indicated the opposite, and perhaps this difference may be due to different flock age (Benton and Brake, Reference BENTON and BRAKE1994).
embryo survival
According to Kuurman et al. (Reference KUURMAN, BAILEY, KOOPS and GROSSMAN2003), mortality describes a curved, diphasic Weibull distribution, due to the peaks of mortality in the first and last third of incubation. The survival of the embryo does not only depend on environmental conditions before and during incubation, factors related to the genotype of the dam and the sire, mainly the dam, affects the survival of the duckling within the egg. Chromosomal aberrations and lethal genes acquired from the sire and dam can cause high percentages of early embryonic mortality (Liptoi and Hidas, Reference LIPTOI and HIDAS2006). The stage of development decreases the heritability of the susceptibility to embryonic death, from 0.09 for early mortality to 0.05 for late mortality, based on the parent component and from 0.25 to 0.18 based on the dam component (Beaumont et al., Reference BEAUMONT, MILLET, LE BIHAN-DUVAL, KIPI and DUPUY1997). In this sense, Bennewitz et al. (Reference BENNEWITZ, MORGADES, PREISINGER, THALLER and KALM2007) reported low direct heritability of hatchability, based on linear and threshold models respectively, with hatching capacity treated as a trait of the dam.
egg storage
The microenvironment conditions during storage, storage time and position of the eggs on the incubation tray are important to guarantee a profitable production of ducklings, especially for production in the tropics and subtropics. To avoid the effect of heat stress on the embryo during storage, temperature should be 13°C (55°F) and never higher than 24°C, and relative humidity should be less than 75% but higher than 40% (Buhr, Reference BUHR1995). Eggs stored under refrigeration should be placed for 4 h at room temperature before incubating (Onbaşılar et al., 2007). Onbaşılar et al. (2007) reported that hatchability was lower and early embryonic death rate was the highest in eggs stored for 11 d. Saha et al. (Reference SAHA, CHOWDHURY and HAMID1992) reported that hatchability of Khaki Campbell duck eggs was greatest in eggs stored for 3 d compared to 7 d pre-incubation.
It is advisable not to store eggs for more than two weeks, as hatchability decreases significantly after 14 d. Researcher have shown that storage period is related to both early and late embryonic deaths (Onbaşılar et al., 2007). The hatchability of total and fertile eggs decreased with increasing storage period, since early and late embryonic death was higher due to loss of water and degradation of albumin during storage.
However, the optimal storage period is not fixed. It varies according to the age of the batch and the strain and species, due to the differences in the quality of the albumin. Keeping eggs with the small end down and the large end upwards can result in more successful hatching than when doing the opposite, perhaps because the incorrect orientation of the head of the embryo towards the larger pole prevents correct elimination of water. In this sense, Bauer et al. (Reference BAUER, TULLETT and WILSON1990) reported a 17% decrease in the hatchability of embryos of broilers located with the small pole up. Apparently, this happens because the embryo cannot find the air chamber that is located in the round pole, especially at the end of incubation (Bauer et al., Reference BAUER, TULLETT and WILSON1990).
Conclusions
Success of the duck sector at small or large scale depends on a regular supply of one-day-old chicks. The production of ducklings is influenced by the fertility and hatchability of the eggs, which are important economic factors representing the major components of reproductive performance. Fertility and hatchability are most sensitive to the environmental and genetic influences. There are many factors related to producers, breeders and environmental conditions during incubation that can influence these parameters, including managerial, nutritional and genetic factors as well as eggs quality and incubator factors. There are different methods used to improve the hatchability percentage of waterfowl eggs - cooling and periodic spraying with water are some of the most commonly used. Dipping eggs and in ovo injection with nutrients such as ascorbic acid and vitamin E during the incubation period is one of the tools used to improve hatchability percentage.
