Effect of the Ratio of Phosphorus to Calcium on Zinc Availability to Rainbow Trout in High Phosphorus Diet

Abstract

A feeding experiment was conducted to determine the effect of the ratio of phosphorus (P) to calcium (Ca) in the diet on zinc (Zn) availability to rainbow trout. Phosphoric acid and calcium carbonate were supplemented to a semipurified diet in order to obtain various combined levels of P (10, 18 and 24mg/g) and Ca (1.9 ?? 37.7mg/g). Growth was not affected by the supplementation of additional Ca alone. However, the growth was reduced significantly by the supplementation of additional P at dietary levels of 18 and 24mg/g, and retarded growth was weakened by a combined supplementation of Ca. The highest growth was obtained when the Ca supplementation gave a dietary level equal to P. Zn absorption was also reduced by the addition of P, and was recovered by the supplementation of Ca.
The results of this study have demonstrated that P contained in a diet at twice or thrice the requirement level reduced Zn availability to rainbow trout. The deleterious effect of an excess amount of P was effectively weakened by the supplementation of Ca. It is suggested that the optimum ratio of P to Ca may be regulated at 1:1 in order to weaken the influence of an excess amount of P mainly derived from hard tissues or of inorganic origin.