The calcium requirement of commercial layer pullets in the pre-laying period.
Themeli, Lufuluvhi Reginald.
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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.