Zinc oxide, when added to weanling pig diets in pharmacological quantities of up to 2,500 mg/kg, can help prevent diarrhea during the post-weaning period. However, adding large quantities of zinc to diets has drawbacks. Zinc can interfere with calcium digestibility because it competes for the same transport pathway in cells lining the small intestine. Zinc may also reduce calcium digestibility by forming complexes with calcium and phytate.

The standardized total tract digestibility (STTD) of calcium in various ingredients has only recently been determined, and possible interactions between zinc and phytase on the STTD of calcium have not yet been reported. Therefore, an experiment was conducted to determine the effects of addition of zinc oxide and microbial phytase on STTD of calcium in diets fed to weanling pigs.

Experimental design

A total of 56 pigs with an average initial body weight of 15.4kg were randomly allotted to seven dietary treatments. Three diets were based on corn, potato protein isolate, cornstarch, and soybean oil and contained 0 mg zinc oxide and 0, 1,000, or 3,000 phytase units (FTU) per kg. Three additional diets were identical to the initial 3 diets with the exception  that they also contained 2,400 mg of zinc provided by 3,000 mg of zinc oxide per kg. A calcium-free diet was used to measure endogenous losses of calcium for use in the calculation of STTD. All diets contained 0.70% total Ca and 0.33% STTD P, and the Ca:STTD P ratio was 2.10:1.

Zinc reduces digestibility of calcium and phosphorus

The ATTD and STTD of calcium increased linearly (P < 0.01) as the concentration of phytase increased (Table 1). However, ATTD and STTD of calcium decreased linearly (P < 0.01) in diets containing zinc oxide compared with diets containing no zinc oxide; STTD of calcium was 4% less in diets containing zinc oxide than in diets with no zinc oxide added. No interactions between zinc oxide inclusion and phytase inclusion were observed.

ATTD of phosphorus increased linearly (P < 0.01) and quadratically (P < 0.05) as the concentration of phytase increased. For diets with no added phytase, ATTD of phosphorus was approximately 9.6% less in diets containing zinc oxide than in diets containing no zinc oxide. The increase in phosphorus digestibility when phytase was added was greater in diets that did not contain zinc oxide than in diets with zinc oxide indicating a reduced response to phytase by addition of zinc oxide to the diets (interaction; P < 0.05).

Key points

• Use of pharmacological quantities of zinc in diets fed to weanling pigs can reduce the digestibility of calcium and phosphorus.
• Addition of microbial phytase to diets improved calcium and phosphorus digestibility.
• To counteract the reduction in digestibility, producers who use pharmacological quantities of zinc in pig diets should increase the concentrations of calcium and phosphorus in the diets by 4% and 9.6%, respectively
• The ability of phytase to increase the STTD of Ca or the ATTD of P is reduced if zinc oxide is added to the diet.

Table 1. Apparent total tract digestibility (ATTD) and standardized total tract digestibility (STTD) of calcium and ATTD of phosphorus for pigs fed diets containing different levels of microbial phytase (0, 1,000 or 3,000 FTU phytase) without or with ZnO addition

^a-d Values within a column without a common superscript are different (P <0.05).
¹Linear effect of phytase in diets without ZnO (P <0.05).
²Quadratic effect of phytase in diets without ZnO (P <0.05).
³Linear effect of phytase in diets with ZnO (P <0.05).
⁴Quadratic effect of phytase in diets with ZnO (P <0.05).

This report is based on unpublished research by Laia Blavi, David Sola-Oriol, José Francisco Pérez, and Hans H. Stein.