How to incubate eggs taking into account flock age and egg size

There is a certain distribution of egg masses within any batch of eggs from the same flock and of the same age. It is common to find that hatching eggs range in mass at about ± 20% about the mean. However, mean egg mass is not fixed for a given hen line. It moves with the age of the flock and can be influenced by nutrition, season, management, moulting, etc. (e.g. Romanoff & Romanoff, 1949; French & Tullett, 1991; Meir & Ar, 1991). It is also well known that, in comparison to 'normal' eggs, the relatively small eggs of a young flock and the relatively large eggs of an ageing flock, do not hatch well (e.g. Carter & Freeman, 1969). Past experience has shown that eggshell conductance for water vapour determines the rate of water vapour loss from eggs at a given incubator humidity and the rates of exchange of respiratory gases of the embryo (Landauer, 1967; Rahn, 1981; Meir et al., 1984). A water loss of about 12 to 13% of initial egg mass, on the day of transfer to hatchery, leads to optimal hatches and chick quality (Visschedijk, 1957; Meir & Ar, 1986; Ar, 1991). Since shell conductance changes with flock age, the humidity inside the incubator has to be changed in a way that would bring about the chosen water loss from the eggs (Rahn et al., 1981; Tullett, 1982; Meir & Ar, 1991). Nevertheless, in some studies, in which a large number of eggs over relatively long periods is considered, hatchability peak in relation to water loss is blunt and the range of 'permitted' water loss can be rather wide without seriously affecting it. This may stem either from insensitivity to water loss within a certain range (Carey, 1986; Davis & Ackerman, 1987) or from the 'need' of different eggs to lose different amounts of water due to their different initial water concentration (Tullett & Burton, 1982). In addition, it has been reported that gross composition of eggs is changed as a function mass. This is true both within the natural variability of a single batch as well as a function of flock age. The bigger is the egg the higher is the fraction of albumen and the lower is the fraction of the yolk (Nestor et al., 1972; Peebles & Brake, 1987; French & Shaw, 1989). Since initially, the albumen contains a relatively high and the yolk a relatively low fraction of water, the total fraction of water in the egg increases with egg mass (Romanoff, 1967). We asked ourselves if the different amount of water present in different eggs require different fractional water loss for optimal hatching and whether this is achieved 'automatically' since eggshell conductance seems to increase too with egg mass. Our partial results indicate so far: (1) There is an increase in albumen fraction and a decrease in yolk fraction with egg mass and with flock age. (2) There is an increase of 0.1-0.2% in water fraction per gram fresh egg mass as egg mass increases. (3) There is an increase in water fraction of eggs with flock age. (4) There is an increase in mean mass-specific shell conductance of about 0.002-0.003 mg of vapour per gram egg per day for each week of flock age. (5) These changes are apparent in all flocks and hen lines tested. (6) Re-initiation of lay after forced moulting reduces the starting point for mass-specific shell conductance but not the following age tendencies listed above. (7) Eggshell fraction is reduced very little with egg mass and flock age. Using these facts and taking the 'prescribed' water loss in common use for an 'average' egg as a reference, it is now possible to calculate specific optimal incubation water loss and thus specific incubator humidity for eggs, depending on their flock age and/or their mass. Experiments testing this hypothesis are now under way and a model, which takes into account the above parameters and allows for more precise determination of optimal incubator humidity, is being constructed.