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Abstract
1. In a 6×7 factorial experiment using 2688 22‐week‐old laying hens of the Lohmann‐SL strain kept in cages (4 birds/cage), diets containing six calcium (20, 25, 30, 35, 40, 45 g calcium/kg) and seven phosphorus concentrations (3.2, 4.2, 5.2, 6.2, 7.2, 8.2, 16.2 g total phosphorus/kg (P t )) were combined orthogonally. The resulting 42 treatments were replicated 8 times so that a replicate consisted of a double cage of 2×4 hens. The experiment lasted 40 weeks (10×28 days).
2. The experimental diets, based on maize and soyabean meals contained 11.5 MJ metabolisable energy/kg and 175 g/kg protein. Different dietary calcium and phosphorus contents were obtained by substituting oat hulls with limestone and dicalcium phosphate.
3. Mortality, egg production, egg weight, egg mass, food intake and food conversion efficiency were determined as well as the breaking strength, thickness of shells and the percentage of eggs with defective shells.
4. All responses measured were significantly influenced by the variance sources (calcium, phosphorus, interaction). Most of the production traits responded asymptotically to increasing dietary phosphorus concentration, the greatest increases or decreases generally being seen between 3–2 and 5'2 g P</kg. Further but weaker increases were seen between 5–2 and 8–2 or 16–2 g P</kg.
5. Increases in dietary calcium content always resulted in curvilinear responses. In all cases optimal effects were obtained with diets containing 25 g calcium/kg and the worst values at 45 g calcium/kg. The interaction between calcium and phosphorus was recognised by strong performance depressions and a high mortality at combinations of the lowest phosphorus concentration (3.2 g/kg) with high calcium contents (35 to 45 g/kg). These were largely offset by increasing dietary phosphorus. Thus, between 7.2 and 16.2 g Pt/kg and 25 and 45 g Ca/kg a plateau was formed where only small differences in egg production were observed.
6. From the three egg shell characteristics measured, breaking strength and shell thickness responded differently to the percentage of eggs with defective shells. While breaking strength and shell thickness were respectively negatively and positively influenced by increasing dietary phosphorus and calcium contents, both elements affected the proportion of eggs with defective shells. Thus the diet with the lowest phosphorus concentration (3.2 P t /kg) and the highest calcium content (45 g/kg) resulted in the thickest egg shells as well as in the highest percentage of eggs with defective shells while inversely the diet with the highest phosphorus content (16.2 g/kg) and the lowest calcium concentration (20 g/kg) produced eggs with the fewest shell defects.
7. It is concluded that the minimum phosphorus requirement for the laying hen is either 3.2 g P,/kg diet or 360 mg P t hen/day when the calcium concentration must be restricted to 25 g/kg. However, to achieve the highest egg production and lowest mortality 7.0 to 8.0 g/kg or 880–1020 mg P,/hen/day are needed. In order to keep the percentage of damaged or shell‐less eggs as low as possible at the same time the calcium content of the food should generally not exceed 30 g/kg.