| Summary: | <p> The objective of this research was to compare different expressions for the digestibility of Ca in both pigs and chickens, and to determine the true total tract digestible (TTTD) Ca requirement for 10- to 20-kg and 20- to 40-kg pigs. </p><p> In the first study, two experiments were conducted to compare apparent ileal digestibility (AID) and apparent total tract digestibility (ATTD) of Ca in pigs. In Exp. 1, three semi-purified diets with soybean meal, canola meal or sunflower meal as the sole source of Ca were formulated. Eighteen cannulated pigs (initial BW = 66 ± 5 kg) were assigned in a randomized complete block design to 3 treatments and 6 replicates per treatment. Results indicated that for either Ca or P, the ATTD was not different from AID in three diets. In Exp. 2, diets with four Ca concentrations were formulated with calcium carbonate as the Ca source. Sixteen cannulated pigs (initial BW = 73 ± 4 kg) were assigned in a randomized complete block design to 4 treatments in 2 experimental periods. The results indicated that the ATTD of Ca was not different from AID for all diets, and the Ca digestibility was not affected by the dietary Ca concentration. The results of these two experiments indicated that both AID and ATTD can be used to describe the digestibility of Ca for growing-finishing pigs. Total tract digestibility was used to express digestibility of Ca in later studies. </p><p> A second series of studies was conducted to decide whether ATTD or TTTD should be used to estimate Ca digestibility of limestone and dicalcium phosphate (DCP). Two pig experiments were conducted to determine the ATTD and TTTD of limestone and DCP, and their additivity in a semi-purified diet for pigs. In Exp. 1, forty eight barrows with an average initial BW of 19.2 ± 1.1 kg were assigned to 1 of 6 dietary treatments in a 2 × 3 factorial arrangement of two Ca sources, including limestone or DCP, with three dietary Ca concentrations each. Diets were fed for a 5-d adjustment period followed by a total collection period of 5 d. The results indicated that the increased dietary Ca concentration linearly increased Ca intake, digested and retained, but did not affect the ATTD of Ca when using limestone and DCP as Ca sources. In Exp. 2, seventy-two barrows with an average initial BW of 20.8 ± 1.3 kg were used to test the additivity of TTTD for Ca in limestone and dicalcium phosphate (DCP) in pigs. All pigs were assigned to 1 of 9 dietary treatments in a 3 × 3 factorial arrangement of three Ca sources, including limestone, DCP, or a mixed diet at a ratio of 1:1; and three dietary Ca concentrations. Feeding and sample collection procedures were as in Exp. 1. The results indicated that the average ATTD were 66.46, 70.34, and 69.32% for limestone, DCP, and their mixed diet, respectively. By regressing daily digested Ca against daily Ca intake, the TTTD of Ca was determined at 70.06, 76.42, and 73.72% for limestone, DCP, and their mixed diet, respectively. The endogenous losses of was estimated to be between 0.217 to 0.321 g/kg DM intake. The predicted TTTD for Ca in the mixed diet of limestone and DCP was calculated to be 72.67% based upon the Ca contribution coefficient of 0.59 for limestone and 0.41 for DCP. The predicted Ca TTTD (72.67%) in the mixed diet was not statistically different from the determined Ca TTTD (73.72%). It is concluded that although ATTD of limestone and DCP were not affected by the Ca concentration in the diet, TTTD is recommended for evaluation of Ca digestibility because of its additivity in a mixed diet. </p><p> A similar additivity experiment was conducted on Ross 708 broiler chickens from d 22 to d 27 post hatch, using true ileal digestibility (TID) for Ca in limestone and dicalcium phosphate (DCP). The birds were fed a standard broiler starter diet from d 1 to 11 post hatching. A total of 504 birds were grouped in a 3 × 3 factorial arrangement of 3 Ca sources including limestone, DCP, and their mixed diet at a ratio of 1:1, and 3 dietary Ca concentration at 3.3, 4.3, and 5.3 g/kg. The results showed that by regressing digested Ca per bird against Ca intake per bird, the TID of Ca was determined at 63.73, 67.14, and 67.79% for limestone, DCP, and the mixed diet, respectively. (Abstract shortened by ProQuest.) </p><p>
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