Zinc (Zn) is an essential trace element that is needed for growth, bone development, and immune competence. Inorganic Zn sources such as Zn oxide and Zn sulfate (ZnSO₄) are most commonly used in swine diets. However, because of low bioavailability of Zn the inorganic Zn sources chelated Zn sources may be used instead because these sources have greater bioavailability of Zn. Chelated zinc may also reduce reactiveness with other components of the diet. A new chelated Zn source, zinc bis-glycinate, in which Zn is bound to two glycine molecules, was recently developed, but there is limited information about effects of this new chelated Zn source on digestibility and Zn retention in pigs. Therefore, an experiment was conducted to test the hypothesis that the relative bioavailability by weanling pigs of Zn in Zn bis-glycinate is greater than in Zn mono-glycinate and in ZnSO₄.

Animals, Housing, Experimental design, and diets

A corn-soybean basal diet was formulated without Zn supplementation, and 6 additional diets were formulated by supplementing the basal diet with 25 or 50 mg/kg of Zn supplied from ZnSO₄, Zn-mono-glycinate, or the new source of Zn-bis-glycinate. Thus, there were a total of 7 diets.

Seventy weaned pigs with an average initial BW of 15.40 ± 2.10 kg were fed the low-Zn basal diet for 10 d (depletion period). Pigs were housed individually and allowed ad libitum access to feed during the depletion period. On d 11, pigs were moved to metabolism crates and allotted to a randomized complete block design with 7 diets, 2 blocks of 35 pigs, and 5 pigs per diet in each block for a total of 10 replicate pigs per diet. Feed was supplied in meal form and pigs were limit fed daily at 3.4 times the maintenance energy requirement (i.e., 197 kcal of ME/kg of BW^0.60; NRC, 2012) of the smallest pig in each replicate. Pigs were fed experimental diets for 12 days. The initial 5 d were considered an adaptation period to the diet. Fecal markers were fed on d 6 and d 11. On the last day of the experiment, a blood sample was collected from each pig. All pigs were subsequently euthanized and the left femur was collected. At the conclusion of the experiment, fecal and urine samples were analyzed for Zn and apparent total tract digestibility (ATTD) of Zn and retention of Zn from each diet were calculated. Femur samples were dried, ashed and concentrations of Zn were analyzed. A relative bioavailability of Zn was determined in Zn mono-glycinate and Zn bis-glycinate using ZnSO₄ as the standard.

Results

Daily Zn intake was greater (P < 0.01) for pigs fed diets supplemented with Zn-mono-glycinate than for pigs fed diets supplemented with ZnSO₄ or Zn-bis-glycinate (Table 1). In addition, daily Zn intake increased (P < 0.01) as the supplementation of Zn increased in the diets, regardless of the source of Zn used. Pigs fed diets supplemented with Zn-mono-glycinate had greater (P < 0.05) fecal excretion of Zn than pigs fed diets supplemented with ZnSO₄. Additionally, fecal excretion of zinc increased as the supplementation of zinc increased in the diet, regardless of the source of Zn in the diet. There was no difference among diets supplemented with Zn-mono-glycinate or Zn-bis-glycinate for ATTD of Zn, but ATTD of Zn in both of these sources was greater than in diets supplemented with ZnSO₄. The urinary Zn excretion increased (P < 0.05) as supplementation of Zn-mono-glycinate increased in the diets. The Zn absorbed was less (P < 0.05) for pigs fed diets supplemented with ZnSO₄ than for pigs fed diets supplemented with Zn-mono-glycinate or Zn-bis-glycinate and Zn absorbed increased (P < 0.01) as the supplementation of Zn-mono-glycinate or Zn-bis-glycinate increased. However, Zn retention as a percentage of Zn intake was not different among zinc sources, but there was an increase (P < 0.05) as the supplementation of Zn-mono-glycinate or Zn-bis-glycinate increased in the diets. Neither the source of Zn nor the concentrations of Zn influenced concentration of bone ash in gram per femur or bone Zn in milligram per femur. However, bone Zn concentration in milligram per kilogram bone ash was greater (P < 0.01) in pigs fed diets supplemented with ZnSO₄ than in pigs fed diets supplemented with Zn-bis-glycinate. No differences among Zn sources were observed for plasma Zn concentration on the last day of the experiment.

The relative bioavailability of Zn in Zn-mono-glycine as determined by Zn absorbed, and using ZnSO₄ as the standard, was 298%, with 95% confidence intervals from 243 to 352% (Fig. 1). The relative bioavailability of Zn in Zn-bis-glycinate using ZnSO₄ as the standard was 264%, with confidence intervals from 232 – 295%.

Key points

• Sources of chelated Zn used in this experiment, increased Zn absorbed and ATTD of Zn by pigs.
• The Zn-mono-glycinate and Zn-bis-glycinate had greater relative bioavailability than ZnSO₄, indicating that chelated Zn may resist interferences from other ligands in the intestinal tract, and therefore, arrive at the absorptive site in the small intestine as an intact structure.
• These results indicate that less chelated Zn is needed in diets for weanling pigs than if ZnSO4 is used, and therefore, may reduce excretion of Zn in manure.

Table 1. Apparent total tract digestibility (ATTD) of Zn, retention of Zn, and bone Zn and plasma Zn concentrations in pigs fed diets supplemented with 0, 25, or 50 mg/kg of Zn from different sources¹

¹Data are least squares means of 10 observations

²Plasma Zn d 1, Zn concentration in plasma after the depletion period; Plasma Zn d 12, Zn concentration in plasma on the last day of experiment.

Figure 1. Bioavailability of Zn in 2 sources of Zn relative to Zn sulfate (ZnSO₄) using Zn absorbed as the response criteria.

P-value of the supplemental Zn level < 0.003; P-value of the Zn source = 0.081.