Availability of ten inorganic phosphorus sources of reagent grade, i.e., three monobasic, three dibasic and three tribasic phosphates of sodium, potassium and calcium, and phosphoric acid, were determined by bioassay on toe ash content of chicks recommended previously, using tribasic calcium phosphate of special grade as a standard.Availabilities of monobasic, dibasic and tribasic calcium phosphates were 126, 114, and 96, respectively, and the difference in availabilities was significant. Availabilities of phosphorus in three sodium and three potassium phosphates and in phosphoric acid were identical within biological error variance, being between 116 and 130. Average availability of phosphorus in these seven phosphorus sources and in monobasic calcium phosphate was 124.It is recommended to use monobasic calcium phosphate as a standard in bioassay of phosphorus availability. By multiplying 0.806, availabilities of phosphorus in various feed ingredients reported previously using tribasic calcium phosphate as a standard can be converted to availabilities on new standard of monobasic calcium phosphate. The converted availabilities of sixteen feed ingredients including feed grade dibasic calcium phosphate are presented in Table 2.
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Yoshida et al. (1979) studied this question.