Calcium and phosphorus levels in the diet of laying birds

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Abstract
Two hatches of crossbred pullet chicks were brooded and reared to 18 weeks of age on diets which conformed to the recommendations of the National Research Council (N.R.C.) of America for calcium and phosphorus. The pullets of each hatch were then divided at random into six laying groups. Three groups were fed diets containing 2.25, 3.14 and 5.0 per cent respectively of calcium without supplementary phosphorus and the other three groups were fed diets of the same graded calcium levels but with supplementary phosphorus to give a total inorganic phosphorus content of 0.45 per cent. When the rate of egg production in the groups receiving 2.25 per cent calcium exceeded 70 per cent an ad libitum supply of limestone grit was provided and the intake recorded. Over the twelve months of lay this resulted in an average total dietary intake level of nearly 2.65 per cent calcium. Egg yield was not significantly influenced by the diets, except that the production of birds fed on the diet containing 5 per cent calcium without supplementary phosphorus was significantly depressed. Food intake was not adversely affected by the higher levels of ground limestone; in fact, there was a positive regression of food consumption on the percentage of calcium in the diet. Because of the greater food intake and the lower egg production, food consumed per dozen eggs and cost of food per dozen eggs were higher in the groups receiving 5 per cent dietary calcium without phosphorus supplementation, compared with groups which received lower dietary levels of calcium. There were, however, no significant differences between the various groups in income or in the margin of income over costs, per bird or per dozen eggs. The data on egg grading showed that over the two experiments and at all levels of calcium there was no evidence of any influence of phosphorus on the combined percentage of large and standard eggs. The average percentage for the two diets with the 5 per cent level of calcium was significantly greater than the combined average for all the other diets. The average percentage of cracked eggs for the groups with diets containing 5 per cent calcium was 9.4 per cent compared with an average of 11.1 per cent for all the other diets, but the difference was not significant. The results suggest that there is no economic advantage in raising the calcium content of the basal diet used in these experiments beyond the level of 3.14 per cent and that at this level there appears to be no need with this basal diet to supplement the phosphorus content.