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A breeding stock hen normally starts producing eggs at the age of 23 to 24 weeks and produces about 183 hatchable eggs out of 199 eggs produced in 65 weeks of its laying life period (Ali et al, 2003). However the process from the time of egg formation, embryo development up to hatching is very cumbersome (North, 1984), and may be influenced by various factors. These factors may include all developed by breeding stock, breeding house conditions, pre-incubation management practices, temperature, humidity, egg orientation and aeration during the incubation time (Katule, 2008; Ayo, et al 2011).
However each factor has great significant effects on egg hatchability and embryo development. Thus hatchability of fruitful eggs is simply aborted by various management practices from the breeding stock house, pre-incubation and incubation period. The pre-incubation period involves all activities from where fertilized eggs are being collected, transported, fumigated, sorted, trayed and stored before setting for incubation.
Ali, et al (2003) reported that, the current incubation process and management practices have been described by Wilson (1991). These practices comprise setting eggs broad end up, frequently egg rotation at 450 and provision of reasonable temperature of 37.5°C with a relative humidity of 50 - 60% during the first eighteen days (Katule, 2008). According to Ayo, et al (2011) reported that incubation temperature of 37.8°C gave the best results in terms of embryo growth and hatchability. However constant supply of high temperature of about 38.9°C during incubation period initially will facilitate embryo growth, utilization of nutrients from the egg yolk and albumen reserves, but as time goes reduces embryonic development which results into imperfect metabolic process because of inadequate gaseous exchange (Ayo, et al 2011).
Thus; the temperature should be reduced to about 36.5°C during the last three days of incubation and the relative humidity increased up to 70 - 75% (North, 1984; Katule, 2008), with no further need for eggs rotation in order to ensure heat distribution. Even though all the precise incubation management practices are being followed to ensure successful hatching of fertile eggs, it is very essential to be aware of how these factors affect egg hatchability and embryo development.
5.2 Factors affecting hatchability of chicken eggs
Hatchability is the percentage of fertile eggs that hatch. It is therefore important to understand the factors that influence fertility and hatchability of eggs. For the hatchability traits, breed has little effect on hatchability of poultry eggs, although light breeds have been reported to have higher fertility and hatchability. The diet of breeder poultry should be adequate in both quality and quantity to meet the recommended levels set out in the feed standards for the category. The most influential egg parameters that influence hatchability are: weight, shell thickness and porosity, shape index (described as maximum breadth to length ratio) and the consistency of the contents (King'ori, 2011). In Tanzania, Minga et al (1989) reported a mean hatchability of 70% with a range of 50-100%. Inbreeding has been found to lower hatchability of fowl eggs as shown by Siltmann et al (1971). There are many factors contributing to the failure of a fertile egg to hatch which include lethal genes, insufficient nutrients in the egg and exposure to conditions that do not meet the needs of the developing embryo (King'ori, 2011)
5.3 Breeding stock and breeding house conditions
There are various ways in which factors generated from the breeding stock and breeding house, environmental conditions can lead into poor hatchability. These include the fertility of the eggs, physical characteristics of the eggs, health status of the breeding stock, pre-laying environment of the egg, and hygienic condition of the breeding house (Ernst, et al 1999; Katule, 2008). Breeder factors that affect hatchability include strain, health, nutrition and age of the flock, egg size, weight and quality, egg storage duration and conditions (King'ori, 2011)
5.4 Egg fertility and sex ratio
Fertility refers to the percentage of incubated eggs that are fertile (King'ori, 2011). There are number of factors which can influence egg fertility in the breeding house. These are such as cocks to hens mating ratio, diseases, cocks sterility, insufficient nutrients supply in the diet, age of the cock, frequency of eggs collection, nature and hygienic condition of the laying area and egg storage temperature (Ernst, et al 1999; Katule, 2008;). One factor which found to influence fertility of eggs is male to female ratio. Too many cocks will spend most of their time fighting instead of mating and this will lower the fertility. Also few cocks will select few hens to mate with and leave others (Sukuzi, 1987). This ratio ranges from 1 cock for 5-10 hens (Gonzalez et al (1999). For maximum fertility there should be at least one cock to ten hens for the meat-type breeding stock, while for the light breeds there can be as many as fifteen hens to a cock. In the case of medium heavy breeds about twelve hens to a cock would be optimum.
It should bear in mind that birds, like many other animals are animals of habit and order. Usually a particular hen would accept and mate with a specific male in the breeding pen, and would not accept any other male unless its mate has gone missing in the pen. This means therefore that if for some reason a cock is no longer able to mate then the hens it used to mate with will go unmated as long as their old mate is present in the pen. Soon or later such hens will start laying unfertilized eggs. To prevent this problem from occurring in the breeding house any male that shows signs of illness or weakness should be withdrawn from the breeding house so that its mates can accept and mate with other cocks.
One other important aspect to keep in mind is that, normally cocks do not prefer to mate with hens which are at the highest end of the peck order spectrum (i.e. extremely aggressive hens), nor would they regularly mate with hens which are very low in the social order. Therefore such hens are likely to lay a proportionately large number of unfertilized eggs. Also heat stress reduces the external and internal egg qualities. Heat stress affects all phases of semen production in breeder cocks (King'ori, 2011)
5.5 Egg production and Body weight
Age at first lay ranged between six and eight months with a hen having an average of three laying cycles per annum (Mwalusanya, 1998). Local chicken adult live weight ranges from 1.471 to 2.3 kg and 2.26 to 3.0 for hens and cocks respectively (Msofe et al) cited by ( Lugendo, 2010). Stage of embryonic development has also been correlated with body weight.
Coleman and Siegel (1966) reported that hens selected for low body weight at 8 wks of age produced eggs with advanced embryonic development at oviposition and an increased hatchability when compared to eggs from hens that were selected for high body weight. They suggested that advanced embryonic development at oviposition is perhaps beneficial in helping the embryo to withstand storage.
Egg production is the function of feed consumed, age at point of lay, age at peak of lay, peak percent lay, percent hen day egg production, laying period, rearing environment, health care and overall management of the flock. Thus, any variation in the aforementioned traits will result in a wide variability. Therefore, care must be exercised to fulfill all the production requirements in appropriate way thereby ensuring better productivity ( Murad et al, 2003).
Apart from the number of eggs produced per clutch, egg production can increase through increased number of clutches per year, shortening the age at first egg and improving hatchability of eggs (Lugendo, 2010).
A study by Minga et al.,(1989) revealed the annual egg production of about 60, and mature body weights of 1.2 kg and 2.2 kg for hens and cocks respectively.(cited by Mwalusanya, 1998).