Highlights  Discrimination of reaction-limited and diffusion-limited dissolution is done from the dependence on physicochemical parameters  Variability in ZnO dissolution kinetics is accounted for through statistical analysis  Dissolution kinetics of coarse ZnO powders depends on specific surface area  Dissolution kinetics of fine ZnO powders depends on aggregation and agglomeration

Seleno-Yeasts (SY) used as feed additives are known to contain different Selenium (Se) species. Seleno-Yeasts has been shown, on previous analytical methods, to contain selenomethionine (SeMet), selenocysteine (SeCys), selenate (SeIV) and selenite (SeVI), and various other organic and inorganic Se forms identified but rarely quantified. A new advanced method has allowed elemental Se (Se0), an inorganic Se species, to be quantified, thereby obtaining better insight into the proportion of inorganic Se in SY products. The study aimed to quantify the Se0 in SY products and assess the proportion of inorganic Se in SY. The Se speciation of 13 fresh commercials SY from different suppliers and batches, was assayed for the total Se, inorganic Se species (SeIV, SeVI and Se0), and organic Se species (SeMet and SeCys). Results on total Se were in line with the expected Se concentrations for all evaluated samples. The proportion of Se present as Se0 ranged from 3.6% to 51.8%. The quantity of Se0 in the SY products, added to SeIV and SeVI, indicated an average proportion of inorganic Se of 14.2% for the 13 analyzed SY products. The proportion of Se as SeMet ranged from 19.0% to 71.8%, (average of 55.8%), and a large variability in the SeMet content was observed. The SeCys content was also variable, with an average of 3.8%, relative to the total Se. In conclusion, advances in the analytical characterization have revealed that SY products can have a significantly high proportion of inorganic Se, which could affect the bioavailability of Se from SY supplements and explain their variable and lower bio-efficacy than pure SeMet supplements, such as hydroxy-selenomethionine.

The bioavailability of a trace mineral source is related to its intestinal solubility (bioavailability), which in turn is determined by its physicochemical properties. It is still not clear which characteristics are more relevant in affecting solubility and bioavailability of mineral sources. Zinc oxide (ZnO) is a common feed additive used to supplement zinc in the diet of monogastric animals. However, different sources have shown different responses on animal bioavailability. This study hypothesized that different sources of feed grade ZnO have various physicochemical features that lead to distinct bioavailability values. Feed grade ZnO samples collected from the feed industry worldwide and characterized for their physicochemical features were tested in broilers and their zinc bioavailability determined. A total of 135 male Cobb broiler chickens were fed a standard starter diet from day 1 after hatching up to day 7. At day 8, animals were allocated in individual cages and fed one of the following dietary treatments during 15 days: a basal diet with 23.5 ppm of zinc and 7 diets with supplemented ZnO or zinc sulphate (ZnSO4) at 6 or 12 ppm. Different sources of ZnO showed an effect of Zn solubility in the stomach and Zn supplementation influenced total Zn in the ileum. The bioavailability of the different sources varied from 49 to 160% compared to the ZnSO4 reference. Aggregate size of particles seems to explain a large variability in the bioavailability of the different sources tested in broilers. In conclusion, physicochemical properties of Zn oxide can partly explain the variability observed in terms of Zn biological value.