|dc.description.abstract||Ovulation rate in laying hens is determined by the interaction of two biological systems;
namely, a circadian rhythm that restricts the release of luteinising hormone to an eight- to
ten-hour period of the day, and the process of follicle maturation. Etches and Schoch
(l984) used a two-compartmental model to represent the circadian rhythm and a Gompertz
equation for follicle maturation. In doing so, they were able to predict ovulation times for
two- to nine-egg sequences. This model has been improved by replacing their table of
values with continuous functions that predict the values for each parameter in the ovulatory
model, for any ovulation rate. Consequently, ovulation times may be predicted for any
A population model that simulates annual egg production has been developed in Visual
Basic. Each parameter in the model is allocated a mean and standard deviation, so that
variation is introduced into the flock. Mean age at first egg is predicted from the age at
photostimulation and the lengths of the photoperiods applied during rearing.
Quadratic-by-linear functions are used to predict changes in the hen's internal cycle length
over time, which in turn determine changes in the ovulation rate and rate of lay. Short egg
sequences, frequently observed at onset of lay in experimental flocks, are simulated
initially, followed by the prime (or longest) sequences, which are produced at the time of
peak rate of lay, before gradual increases in the internal cycle length cause the egg
sequences to become shorter once more.
In view of the fact that the interval between oviposition and the subsequent ovulation is
about 30 minutes, time of lay may be predicted from ovulation time for all eggs other than
the last egg of a sequence, because in this case there is no associated ovulation. A
curvilinear function is used to predict the value of the last interval from the ovulation rate,
because experimental data show that short sequences have longer intervals between the last
two eggs than long sequences. The circadian rhythm of LH release is linked to the onset of
darkness, so that mean time of lay occurs 13 to 14 hours after sunset. The distribution of
oviposition times is unimodal for young flocks and bimodal for older flocks.
Yolk weight is predicted from hen age using a function appropriate for the genotype.
Allometric functions are used to predict albumen weight from yolk weight and shell weight
from the weight of the egg contents. Egg.weight is given by the sum of the three
components. With advancing hen age, the proportion of yolk in the egg increases at the
expense of both albumen and shell.
Random events, such as internal ovulations, and the production of soft-shelled and double-yolked
eggs, are accounted for in the model. Their incidence is linked to the genotype and
to the age of the hens and their occurrence is restricted to a proportion of the flock.
Internal ovulations cause interruptions to egg sequences, thereby reducing overall mean
This model could be of benefit to a producer wanting to know how a change to the lighting
programme would affect the laying performance of the strain, or to a nutritionist desiring
to determine changes in voluntary feed intake and to the nutrient requirements of the birds
over the laying period. It may also be used as a teaching aid, so that students gain a
thorough understanding of the process of egg production and are able to test the response
of layers to different environmental stimuli. The user has control over a number of inputs,
thereby making it a generalised model that can be used for different strains. With a few
modifications, the model may be used to simulate the erratic and variable laying behaviour
of broiler breeders.||en