|dc.description.abstract||Specifications for the amount of calcium to be included in feeds for pullets and laying
hens vary considerably in the scientific literature. Much of this variation is due to the
fact that genetic selection has changed the growth rate, age at sexual maturity and
potential egg production of laying hens over the years, and these changes will continue
in the future. Husbandry techniques have also changed, so that pullets are now
sometimes encouraged to start producing eggs at an earlier age with the use of
appropriate lighting programmes. The supply of calcium to pullets in the pre-laying
period depends critically on the age at which the pullets reach maturity and start to lay
eggs, as pullets should be given an opportunity to deposit calcium in their medullary
bone just before laying commences. The difficulty is knowing how much calcium
should be included in the feed before sexual maturity, and for how long before this
event a higher calcium content should be included, in order to minimise problems
associated with mineral deficiencies and excesses during the laying period.
The aim of the present study was to determine whether, by offering pullets a choice of
two feeds varying in calcium content, the choice they made in the period leading up to
sexual maturity could be used to determine the amount of calcium that should be fed to
them during this period. Two experiments were conducted with Hyline pullets: in the
first, 384 pullets were used, starting at 14 weeks of age, and in the second, 144 birds
were used, starting at 10 weeks of age. In both experiments, equal numbers of Hyline
Brown and Hyline Silver pullets were used.
The first experiment consisted of four dietary treatments: Two basal diets were
formulated to contain high (30 g calcium/kg) and low (8 g calcium/kg) calcium
contents, with all other nutrients being the same. These two basal diets were fed alone
and as a 1:1 blend to produce an intermediate calcium diet (19 g calcium/kg). The
fourth treatment consisted of the low calcium basal and limestone grit as a choice diet.
At 18 weeks of age six pullets from each treatment were killed for analysis of tibia
breaking strength and 144 of these pullets (72 Silver and 72 Brown) were selected
randomly and kept on the same treatment as before, but individually so that age at
sexual maturity could be determined. There was no significant difference observed in
age at sexual maturity or mortality, but pullets that were on the low calcium feed
consumed significantly more feed and consequently, attained higher body weight gain
than the other treatments. The opposite occurred for pullets that were on the high
calcium diet. There was no significant effect of dietary calcium content on tibia
breaking strength at 17 weeks.
For the second experiment, pullets (n = 144) were reared on a lighting regime of
8L:16D to 10, 14 or 18 weeks, at which ages the photoperiod was increased to 14 hours.
This had the effect of altering the age at sexual maturity, so that the effects of age and
attainment of sexual maturity could be separated when determining the choice made by
pullets in the amount of calcium consumed in the pre-laying period. In all cases,
pullets increased their intake of calcium approximately two weeks before attaining
sexual maturity, this increase being independent of the age of the pullets at the time.
The study revealed that commercial laying-type pullets increase their intake of calcium,
when given the opportunity to do so, approximately two weeks prior to the onset of lay.
Where they do not have a choice between two sources of calcium this increased
requirement for calcium causes pullets on low calcium feeds to increase their intake of
feed and consequently simultaneously increase the intake of all nutrients other than
calcium, resulting in an increased body weight. Where birds are fed a high calcium feed
only, food intake does not increase to the same extent during this period, but the
increase observed is likely to be to satisfy the increased demand for nutrients other than
calcium in this pre-laying period.
On the basis of the choices made by pullets in this study, these birds should be reared on
low calcium feeds until two weeks before the onset of lay, at which stage the calcium
should be increased to enable the pullets to deposit calcium in their medullary bone in
preparation for the increased demand for calcium in the laying period.||en