Development of an experimental model to assess the bioavailability of zinc in practical piglet diets

Sufficient zinc (Zn) supply is a key element of successful animal husbandry. Proper use of dietary Zn sources, however, demands knowledge of Zn requirement and bioavailability, reflecting practical feeding systems. In this study, an experimental model is presented where 48 fully weaned and individually housed piglets received a fine differentiated alimentary Zn supply. The basal diet consisted mainly of corn and soybean meal (native Zn: 28.1 mg/kg feed) and was fortified with Zn from Zn sulphate at eight levels (0, 5, 10, 15, 20, 30, 40 and 60 mg Zn/kg). All animals were pretreated uniformly with the highest Zn supply (88 mg total Zn/kg feed) for two weeks (feeding ad libitum). Subsequently, animals were switched to the eight experimental diets (six animals per group, restricted feeding at 450 g/d). This period was limited to 8 d in order to avoid clinical Zn-deficiency symptoms. Measurements included amounts of apparently digested Zn, final levels of plasma Zn, plasma Zn-binding capacity, plasma alkaline phosphatase activity, femur Zn, liver Zn as well as hepatic metallothionein (Mt) 1a and Mt2b gene expression and hepatic Mt protein abundance. Clinical signs of Zn deficiency were completely absent through the entire study. All the analysed parameters except for Mt protein abundance responded sensitively to graduations in dietary Zn contents and indicated the presence of Zn deficiency at lower dietary Zn additions. Amounts of apparently digested Zn, liver Zn as well as hepatic Mt1a and Mt2b gene expression indicated transition from deficient to sufficient Zn supply between 47.5 and 58.2 mg of total Zn per kg of diet as assessed by broken-line response techniques. Analysed blood and bone parameters responded linearly to graduations in dietary Zn supply even within sufficient Zn supply levels. Taken together, the results indicate the suitability of our experimental model to determine Zn requirement in piglets and hence to also assess bioavailability of dietary Zn sources. The latter may be done by comparing the slope of the amounts of apparently digested Zn as well as by determining the response of blood and bone parameters to graduations in dietary Zn at insufficient Zn supply.