An experiment was conducted to determine the bioavailability of calcium in mineral oyster shell as dietary complement for laying hens. Eighty-one hens of commercial layer strain (Hy-line W-36) were allocated to 9 dietary treatments with three replicates of three hens per replicate in a completely randomized design. Treatment one as the basal diet, without any supplemental calcium, contained 0.37 % calcium. Diet 2 to 5 contained 1, 2, 3, or 4 % calcium from calcium carbonate as a standard source. Diet 6 to 9 contained 1, 2, 3, or 4 % calcium from the mineral oyster shell as the test source. All diets were isoenergetic, isonitrogenous and equal in amino acid content. Data were subjected to ANOVA procedures appropriate for completely randomized designs. Difference between individual treatment means were assessed using Duncan multiple range test. The slope ratio assay was used to determine bioavailability of calcium in mineral oyster shell. GLM procedure of SAS was employed to obtain parameter estimates for calculation of relative bioavailability. Hens fed diets based on mineral oyster shell and calcium carbonate, exhibited very similar performances. However, hens fed 0.37 % calcium did not perform as well as the others and had lower egg productions. Effect of calcium level on egg specific gravity, eggshell thickness and shell weight was significant (P< 0.01). In most cases, no significant differences were observed between slopes of test and standard calcium source regression lines. In the case of egg production and egg shell thickness, slopes of test and standard lines were significantly different (P<0.05). In these cases relative bioavailabilities were 138 and 120 % respectively. It is concluded that mineral oyster shell is a good calcium source comparable in bioavailability to calcium carbonate for laying hens.
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